Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Original article 85 ABSTRACT This study evaluates the effectiveness of different resistance training intensities (high, moderate, and low) on quadriceps femoris muscle hypertrophy in healthy adults. A literature search was conducted in January 2023 using multiple databases, including Web of Science, SPORTDiscus, Embase, and PubMed. The methodological quality of the studies was assessed using the TESTEX scale. A total of 22 studies with 519 participants were included in this meta-analysis. Statistical analysis, performed using ReviewManager 5.2, showed that all resistance training intensities led to hypertrophy in the quadriceps femoris muscle compared to control or pre- test values. High-intensity training resulted in a muscle thickness increase of 2.3 mm (95% CI: 2.21-2.38), moderate-intensity training led to an increase of 1.88 mm (95% CI: 1.74-2.02), and low-intensity training showed an increase of 10.92 mm (95% CI: 10.77-11.08), all with p<.001. Meta-regression analysis revealed a significant relationship between training intensity and hypertrophy in the vastus intermedius (β=0.01, p=0.05, R²=0.56) and vastus lateralis (β=0.01, p=0.007, R²=0.34). However, no significant effect was found for the rectus femoris (β=0.03, p=0.417, R²=0.04) or vastus medialis (β=0.003, p=0.895, R²=0.002). In conclusion, resistance training at different intensities promotes hypertrophy across all quadriceps muscles, with variations depending on the specific muscle group. Meta-regression suggests that every 10% increase in training intensity corresponds to a 0.1 mm increase in vastus intermedius hypertrophy. No significant effect of training intensity was observed for the vastus lateralis, vastus medialis, or rectus femoris muscles. Keywords: Muscle hypertrophy, quadriceps femoris, knee extensors, muscle thickness 1Department of Physical Education and Sport, Faculty of Sport Sciences, Gaziantep University, Gaziantep, Turkey 2Department of Coaching Training, Faculty of Sport Sciences, Erzurum Technical University, Erzurum, Turkey 3Department of Coaching Training, Faculty of Sport Sciences, Ankara University, Ankara, Turkey IZVLEČEK Ta študija ocenjuje učinkovitost različnih intenzitet vadbe za moč (visoka, zmerna in nizka) na hipertrofijo mišične skupine kvadriceos femoris pri zdravih odraslih. Iskanje literature je bilo izvedeno januarja 2023 z uporabo več podatkovnih baz, vključno z Web of Science, SPORTDiscus, Embase in PubMed. Metodološka kakovost študij je bila ocenjena s pomočjo lestvice TESTEX. V to metaanalizo je bilo vključenih skupno 22 študij s 519 udeleženci. Statistična analiza, izvedena z uporabo programa ReviewManager 5.2, je pokazala, da so vse intenzitete vadbe za moč povzročile hipertrofijo mišice kvadriceps femoris v primerjavi s kontrolnimi ali predtestnimi vrednostmi. Visoko-intenzivni trening je povzročil povečanje debeline mišice za 2,3 mm (95% CI: 2,21–2,38), zmerno-intenzivni trening je privedel do povečanja za 1,88 mm (95CI: 1,74–2,02), nizko-intenzivni trening pa je pokazal povečanje za 10,92 mm (95 % CI: 10,77–11,08), vse z p<.001. Meta-regresijska analiza je pokazala pomembno povezavo med intenziteto treninga in hipertrofijo mišice vastus intermedius (β=0,01, p=0,05, R² = 0,56) ter vastus lateralis (β = 0,01, p = 0,007, R² = 0,34). Vendar pa ni bilo ugotovljenega pomembnega vpliva na mišici rectus femoris (β=0,03, p=0,417, R²=0,04) in vastus medialis (β=0,003, p=0,895, R²=0,002). Zaključno lahko rečemo da vadba za moč pri različnih intenzitetah spodbuja hipertrofijo vseh mišic kvadriceps, pri čemer obstajajo razlike glede na posamezno mišično skupino. Meta-regresijska analiza nakazuje da vsakih 10 % povečanja intenzitete vadbe ustreza 0,1 mm povečanju hipertrofije mišice vastus intermedius. Vendar pa ni bilo ugotovljenega pomembnega vpliva intenzitete vadbe na mišice vastus lateralis, vastus medialis in rectus femoris. Ključne besede: Mišična hipertrofija, kvadriceps femoris, iztegovalke kolena, debelina mišice Corresponding author*: Yusuf Buzdağlı Department of Coaching Training, Faculty of Sport Sciences, Erzurum Technical University E-mail: yusuf.buzdagli@erzurum.edu.tr https://doi.org/10.52165/kinsi.31.2.85-109 Cemre Didem EYİPINAR 1, Yusuf BUZDAĞLI 2* Raci KARAYİĞİT 3 HYPERTROPHIC RESPONSE OF LOWER EXTREMITY MUSCLES TO DIFFERENT RESISTANCE TRAINING INTENSITIES: A META-ANALYSIS AND META-REGRESSION HIPERTROFIČNI ODGOVOR MIŠIC SPODNJIH OKONČIN NA RAZLIČNE INTENZITETE TRENINGA ZA MOČ: META-ANALIZA IN META-REGRESIJA Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Lower Limb Hypertrophy by RT Intensity 86 INTRODUCTION Resistance training is a suggested form of exercise since it can enhance the ability to accomplish everyday tasks improve overall health and well-being indicators, including physical independence and decreased risk of all-cause death, and also increase sportive performance in the athletic population. Muscle hypertrophy is one of the primary responses seen with resistance training with chronic exposure (Brad Schoenfeld & Grgic, 2018). As a result of muscle hypertrophy and an increase in myofibrils, the principal alteration entails a rise in the cross- sectional area of the total muscle and specific muscle fibers. Early in the resistance exercise process, satellite cells become activated; their growth and fusion with preexisting fibers are crucial for the hypertrophic response. Additional potential architectural modifications in quadriceps muscles involve hyperplasia, modifications to muscle design, myofilament thickness, connective tissue composition, and tendon and connective tissue structure (Folland & Williams, 2007). In addition, many internal and external factors are also influential in muscle hypertrophy. Exercise characteristics like frequency, intensity, time/duration, and type are examples of various external factors, called the FITT principle (Campbell et al., 2019). However, there are multiple studies on the effect of resistance training intensity (Borde, 2015; Fry, 2004; Brad Schoenfeld, Grgic, Ogborn, & Krieger, 2017), frequency (Borde, 2015; Polito, Papst, & Farinatti, 2021; Brad Schoenfeld, Grgic, & Krieger, 2019), number of sets (Borde, 2015; Krieger, 2009), repetition (Hackett, Ghayomzadeh, Farrell, Davies, & Sabag, 2022; Nicholson, Ispoglou, & Bissas, 2016; Brad Schoenfeld, Peterson, Ogborn, Contreras, & Sonmez, 2015), and type (Henselmans & Schoenfeld, 2014; Roig et al., 2009; Brad Schoenfeld, Ogborn, Vigotsky, Franchi, & Krieger, 2017) on muscle hypertrophy. One of the most crucial variables is exercise intensity since high-intensity exercises, particularly in elderly and very young individuals, may lead to adverse outcomes like higher injury risk and psychological burden and lower training motivation. Therefore, if lowering exercise intensity will not reduce hypertrophic responses, which some studies suggest, engaging in low-intensity resistance training can be preferable. High-intensity resistance training (≥ 70% of 1 repetition maximum (RM)) has been promoted for many years as the primary method for promoting improvements in muscular hypertrophy. Recent findings, however, have questioned this concept in terms of hypertrophy, with multiple studies revealing equivalent increases in muscle hypertrophy across low (≤ 50% of 1RM) and Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Lower Limb Hypertrophy by RT Intensity 87 high (≥ 75% of 1RM) resistance training loads (Brad Schoenfeld et al., 2015). In contrast, the optimal resistance intensity regarding the quadriceps muscle group hypertrophic adaptations remains uncertain. Though in healthy people, high (Brigatto et al., 2022; Csapo & Alegre, 2016; Brad Schoenfeld, Contreras, et al., 2019) and low (Correa et al., 2012; Brad Schoenfeld et al., 2015) intensity resistance exercises were demonstrated to improve muscle mass for quadriceps femoris, and other studies highlighted no intensity-based differences (Amirthalingam et al., 2017; Carvalho et al., 2022; Correa et al., 2012; Mitchell et al., 2012; Brad Schoenfeld, Grgic, et al., 2017). Therefore, this study searches for the answer to the question, “what is the net range of intensity of resistance training one provides to increase quadriceps femoris muscle hypertrophy?”. It has not been directly examined in the literature separately for each muscle in the quadriceps femoris muscle in healthy adults. METHODS Registration The study was registered before the literature search in Open Science Framework (OSF) (https://doi.org/10.17605/OSF.IO/UWYSA), and it adhered to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) standards (Liberati, Tetzlaff, & Altman, 2009). Sources of Data And Search Strategy Seven electronic databases (Web of Science, SPORTDiscus, Embase, PubMed, Google Scholar), ClinicalTrials website (https://clinicaltrials.gov/), and grey literature database (https://opengrey.eu/) were scanned with (“Resistance training” OR “Strength training”) AND (“Muscle hypertrophy” OR “Muscle thickness”) keywords till January 2023. All randomized or non-randomized experimental studies are included. During the literature search, hand searching was performed by going to the reference’s reference and looking for relevant systematic reviews simultaneously. Inclusion and Exclusion Criteria The inclusion criteria were; (i) studies examining the effect of resistance training on muscle hypertrophy, (ii) studies in lower extremity muscle thickness is measured by ultrasonography, (iii) studies in which the measurements of muscle hypertrophy are clearly stated, (iv) studies with healthy individuals without chronic disease or disability, (v) had a minimum duration of 4 Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Lower Limb Hypertrophy by RT Intensity 88 weeks, (vi) carried out training to muscle failure, (vii) open access and randomized or non- randomized human trials were included. The exclusion criteria were; (i) studies that do not include resistance or strength exercises, (ii) studies in which measurements such as muscle volume and circumference (not taken by ultrasonography; since ultrasonography is accepted as the gold standard in determining muscle thickness (hypertrophy) in various studies (Amirthalingam et al., 2017; Brad Schoenfeld, Contreras, et al., 2019; Brad Schoenfeld et al., 2015). For this reason, studies conducted with this method have been preferred.), (iii) studies that include measurements other than the muscles in the lower extremity, (iv) studies that include individuals with chronic diseases or rats and not open access were excluded. Data Extraction From each trial, two authors independently retrieved descriptive and result data. These obtained data were as follows: author and year of studies, design, samples, resistance exercise protocols (intensity (% 1RM) and total sets (number of sets * repetitions) of exercises), duration of the study, hypertrophy area, and results. In addition, the raw mean and standard deviation of post- training muscular hypertrophy measurements and the number of participants were extracted. If data is given graphically, WebPlotDigitizer (https://automeris.io/WebPlotDigitizer/) was used to interpret data that could only be presented visually. The data was then imported into the meta-analysis tool from the excel file. Evaluation Of Methodological Quality A new proven method for evaluating the quality of the study and report in exercise training research, the “Tool for the assEssment of Study qualiTy and reporting in EXercise” (TESTEX) Scale, is used to determine the risk of bias in this study. This scale considers eligibility and allocation concealment and includes 12 criterion evaluations with a maximum score of 15. In exercise training research, subsequent blinding of participants and researchers is rarely possible and only affects quality assessment (Smart et al., 2015). Answers to every item on the TESTEX scala are "yes" or "no," with "yes" being connected with a point and "no" being related to a score if criteria are not satisfied. Studies are categorized as having "excellent quality" (12–15 points), "good quality" (9–11 points), "fair quality" (6–8 points), or "poor quality" (6 points) based on the summary scores (Nunes et al., 2021). Additionally, two researchers (C.D.E and Y.B) will independently assess the methodological quality. Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Lower Limb Hypertrophy by RT Intensity 89 Statistical Analysis For continuous data, meta-analyses were done using the post-intervention lower extremity muscle thickness measurements' raw mean value and standard deviation measurements for effect size calculation. The effect sizes were evaluated as “small” (≤0.2), “moderate” (0.21- 0.5), “large” (0.51-0.8), and “very large” (>0.8) (Cohen, 1992). A random effect model was used for all analyses. Meta-analysis was conducted for each result with RevMan 5.2 toll for lower extremity (Vastus Lateralis, Medialis, Intermedius, and Rectus Femoris) muscles. The I2 test was used to determine the degree of heterogeneity (Sutton & Higgins, 2008). TESTEX scale was used to detect the quality of the studies (Smart et al., 2015). Additionally, the relationship between resistance training intensity and change in quadriceps femoris muscle thickness was explored by conducting a meta-regression analysis with MedCalc (MedCalc software, version 16.1; MedCalc, Ostend, Belgium) program. For that, muscle thickness (mm) was the primary moderator as a continuous variable for meta-regression analysis. Ninety-five percent confidence intervals were employed, and the significance level was 5%. RESULTS Selection of Studies All of the studies that were considered reported comparing the effects of various resistance exercise intensities (low, moderate, high) on the development of muscular hypertrophy in the lower extremity in healthy adults. As a result of the literature review, a total of 607 studies were reached via various databases. Following the elimination of duplicate studies, 81 were kept for screening. Twenty-two of them were evaluated for eligibility, while 59 were eliminated. In total, 22 studies were evaluated (Figure 1). Characteristics of The Included Studies The total number of articles found was based on the type of analyzed variables: Eighteen articles for the RF, 20 for the VL, 9 for the VM, and 7 for the VI muscles. Twenty-two trials included 519 healthy participants in the existing meta-analysis. Every study had between 15 (Gonzalez et al., 2017; Ikezoe, Kobayashi, Nakamura, & Ichihashi, 2020) and 58 individuals (Correa et al., 2012). The duration of resistance training varies between 5 and 16 weeks, and the most common application period is eight weeks. Additionally, there are eight studies for high- intensity (≥ 80 % of 1 RM) resistance training, 11 for moderate-intensity (60-79% of 1 RM) Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Lower Limb Hypertrophy by RT Intensity 90 resistance training, and 6 for low-intensity (30-59 % of 1 RM) resistance training. The most commonly studied resistance training intensity is moderate (60-79%) (Carvalho et al., 2022). Categorization of Research Two researchers (C.D.E and Y.B) independently focused on studies. They recorded data relating to the following variables into a worksheet: year of study, author, design, participant group, the protocol of resistance exercises (determined as regions of maximal repetitions or by a percentage of the 1RM test), muscle area, TESTEX scores of studies and results. Resistance exercise loads are classified into three categories according to the latest meta-analysis (Carvalho et al., 2022): low (30-59 % of 1 RM or 16-35 RM), moderate (60-79% of 1 RM or 8-12 RM), and high (≥ 80 % of 1 RM or ≤ 7 RM). The table containing the studies accessed and categorized as a result of the literature review is as follows (Table 1). Figure 1. Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) Flow Chart of Study Selection Process (Page et al., 2021). Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Lower Limb Hypertrophy by RT Intensity 91 Table 1. All Included Papers' Methodological Features and Outcomes Author, year Design Sample Resistance exercise protocol Duration Muscle area Results **Carvalho et al. (2020) TESTEX: 10 Randomized parallel group Repeated measures 26 resistance-trained men 3-week 4 sets *1-3 RM with 3 min rest and 5-week 4 sets * 8-12 RM back squat and leg press training 8 weeks (2d/w) VL Resistance exercise protocol induced more significant muscle growth in VL (p <0.05) in 3 weeks. Alkan (2019) TESTEX: 10 Pre-post 20 young individuals Experiment-1: 20 Experiment-2: 20 3-4 sets*10 repetitions (80-100 % 1RM) Experiment-1: Resisted leg raise and knee extension Experiment-2: Resisted knee flexion 8 weeks (3d/w) VI RF After an 8-week training period, both groups had increased muscle thickness (p <0.05). Santos, Valamatos, Mil-Homens, and Armada-da-Silva (2018) TESTEX: 11 RCT 28 young male adults Experiment-1: 9 (Focused on eccentric exercise) Experiment-2: 11 (Focused on concentric exercise) Control: 8 (no exercise) Experiment: First 3 weeks 5 sets * 6 repetitions with 60°s-1 angular velocity. The number of contraction sets conducted at 60°s-1 reduced to only 2 during the next 12 weeks, but extra sets were performed at 90°s-1 (weeks 4-6), 120°s-1 (weeks 7- 9), 150 °s-1 (weeks 10-12), and 180 °s-1 (weeks 13-15). isokinetic knee flexion and extension training 15 weeks (3d/w) VL VI VM RF All QF muscle thickness increased with strength exercise (p <0.05) except VL muscle. Ikezoe et al. (2020) TESTEX: 10 Pre-post 15 healthy men Experiment-1: 7 Experiment-2: 8 Experiment-1: 12 sets * 8 repetitions (30% 1RM) with 90 s rest Experiment-2: 3 sets * 8 repetitions (80% 1RM) with 90 s rest concentric and eccentric contractions in biodex dynamometer 8 weeks (3d/w) RF The 8-week resistance training increased by 11.3% and 20.4% RF muscle thickness for both training conditions (p <0.01). Müller et al. (2020) TESTEX: 10 Pre-post 35 older men Experiment-1: 18 Experiment-2: 17 Experiment-1: 2-4 sets * 6-15 repetitions (65-80% 1RM) Experiment-2: 3-4 sets * 6-8 repetitions (40-60 %1 RM) bilateral leg press and bilateral knee extension exercises- 16 weeks (2d/w) VL VM RF Significant increases (p <0.05) were observed in QF muscle thickness in both groups with no differences between groups. Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Lower Limb Hypertrophy by RT Intensity 92 Table 1. Continued Boone, Stout, Beyer, Fukuda, and Hoffman (2015) TESTEX: 10 RCT 18 untrained young men Experiment: 9 Control: 9 (no exercise) 3 sets * 8-10 repetitions (80% 1RM) leg press and leg extension training 4 weeks (3d/w) RF VL Resistance training resulted in significant increases in RF (p <0.001) and VL (p <0.001) muscle thickness. *Cadore et al. (2014) TESTEX: 11 Pre-post 22 healthy individual (F: 14 M:8) Experiment-1: 11 Focused on eccentric exercise Experiment-2: 11 Focused on concentric exercise 2-5 sets * 8-12 repetitions- isokinetic exercise with 60°s-1 angular velocity. 6 weeks (2d/w) VL Both groups exhibited increased VL muscle thickness (p <0.05). *Yoshiko and Watanabe (2021) TESTEX: 11 Pre-post 16 healthy older (F:13 M:3) Experiment-1: 8 Experiment-2: 8 Experiment-1: 4 sets * 35 RM weight- bearing deep squat exercise Experiment-2: 4 sets * 35 RM weight- bearing shallow squat exercise 12 weeks (3d/w) VL VI RF No significant change was observed for QF muscle thickness in both groups. *Zaras et al. (2020) TESTEX: 10 Pre-post 16 healthy male Experiment-1: 8 Experiment-2: 8 Experiment-1: 4 sets * 6 repetitions (85 % 1RM) leg press exercise with 3 min rest Experiment-2:4 sets * 6 repetitions (85 % 1RM) leg press exercise with 3 min rest +20 s inter-repetition rest period between single repetitions. 7 weeks (2d/w) VL VI Following resistance training, the thickness of the VL muscle increased dramatically over time (p = 0.043). *Nakamura et al. (2021) TESTEX: 10 Pre-post 16 healthy young men Experiment-1: 8 Experiment-2: 8 Experiment-1: 3 sets * 10 RM parallel squat exercise with 3 min rest Experiment-2: Static stretching exercises 5 weeks (2d/w) VL VI VM RF QF muscle thickness increased in both groups, but no significant difference was observed. Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Lower Limb Hypertrophy by RT Intensity 93 Karsten, Fu, Larumbe-Zabala, Seijo, and Naclerio (2021) TESTEX: 11 Pre-post 18 resistance-trained men Experiment-1: 9 Experiment-2: 9 Chest and bench press, deadlift, lateral pull down, dumbell fly, barbell lying arm extension, barbell shoulder press, reverse grip bent-over row exercises Experiment-1: 4 sets * 10 repetitions (75 % 1RM) with 2 min rest Experiment-2: 8 sets * 5 repetitions (75 % 1RM) with 1 min rest 6 weeks (2d/w) VM The VM muscle thickness increased (+3.28 ±2.32 mm) only significantly (p <0.05) in the experiment-1 group. Korkmaz (2018) TESTEX: 11 Pre-post- randomised 23 footballer Experiment-1: 12 Experiment-2: 11 Experiment-1: 4 sets * 8-12 repetition (80% 1RM) knee extension exercise with 2 min rest Experiment-2:4 sets * 8-12 repetition (80% 1RM) knee extension exercise with 2 min rest and blood flow restriction 6 weeks (2d/w) VL RF Blood flow restricted training provides better benefits than traditional strength training to improve muscular hypertrophy. Pinto et al. (2014) TESTEX: 10 RCT 36 sedentary elderly women Experiment: 19 Control: 17 (no exercise) 2-3 sets * 12-15 RM knee flexion, leg press and knee extension exercises 6 weeks (2d/w) VL VI VM RF All measurements of the QF muscle thickness (vastus lateralis, medialis, intermedius, and rectus femoris) showed a significant increase only in the experiment group (p ≤0.05). **Correa et al. (2012) TESTEX: 10 RCT 58 healthy older woman Experiment:41 Control:17 (no exercise) First period (6 weeks) 2-3 sets * 12-20 RM Second period (6 weeks) 3-4 sets * 8-12 RM leg press, knee extension and flexion exercises with 2 min rest 12 weeks (2d/w) VL VM RF After 6 weeks of resistance training, a significant increase occurred in QF muscle thickness (p <0.05). Usui, Maeo, Tayashiki, Nakatani, and Kanehisa (2015) TESTEX: 11 Pre-post 16 healthy young men Experiment-1: 9 Experiment-2: 7 Experiment-1: 3 sets * 10 repetitions (50% 1RM) parallel squat training 3 s lowering and 3 s lifting without a pause phase Experiment-2: 3 sets * 10 repetitions (50% 1RM) parallel squat training 1 s lowering and 1 s lifting with 1 s pause phase 8 weeks (3d/w) VL VI VM RF In experiment-1 group, RF muscle thickness increased at +10% (p=0.026). No changes were observed experiment -2 group. Table 1. Continued Table 1. Continued Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Lower Limb Hypertrophy by RT Intensity 94 Brad Schoenfeld and Grgic (2018) TESTEX: 10 Pre-post 30 male volunteers Experiment-1: 15 Experiment-2: 15 4 sets * 8-12 RM standing barbell curl and the machine leg extension with 2 min rest. Experiment-1: focused on contracting the target muscle during training Experiment-2: focused on the outcome of the lift 8 weeks (3d/w) VL RF RF and VL muscle thickness showed small and insignificant effect sizes favoring experiment 1 and 2 groups (p=0.418 and p=0.999, respectively). Gonzalez et al. (2017) TESTEX: 11 Pre-post RCT 15 resistance trained men 4 sets * 10 RM bench press, barbell back squat and deadlift training with 2 min rest 8 weeks (3d/w) VL RF Significant improvements occurred with resistance training for all muscle thickness measurements (p <0.05) Zaroni et al. (2018) TESTEX: 10 Pre-post RCT 18 healthy men Experiment-1: 9 (training a muscle group 5 days per week) Experiment-2: 9 (training a muscle group 1 day per week Experiment: 3 sets * 10-12 RM bench press, hack squat, deadlift, machine lat pull down, biceps curl, nosebreaker, dumbbell hammer curl, leg press, parallel back squat, cable triceps 8 weeks (TOTAL routine training a muscle group 5 days per week) VL Total routine strength training significantly increases in VL muscle (p <0.05). *Nogueira et al. (2009) TESTEX: 11 Pre-post 20 healthy older men Experiment-1: 9 Experiment-2: 11 Experiment-1: 3 sets * 8 repetitions (40-60 % 1RM) knee and elbow flexion/ extension, leg and chess press exercise with 90 s rest interval Experiment-2: Power exercises 10 weeks (2d/w) RF RF muscle thickness increased solely in power training (p <0.05). *Evangelista et al. (2019) TESTEX: 12 Pre-post 29 sedentary healthy adults Experiment-1: 17 Experiment-2:12 Experiment-1: 4 sets * 8-12 RM bench press, knee flexion and extension, arm curl, seated row exercises with 90 s rest interval Experiment-2: 4 sets * 8-12 RM bench press, knee flexion and extension, arm curl, seated row exercises with 90 s rest interval + streching exercises 8 weeks (2d/w) RF VL RF muscle thickness increased (p ≤0.0001) in both groups. Table 1. Continued Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Lower Limb Hypertrophy by RT Intensity 95 Table 1. Continued Abbreviations: d/ w: days a week, QF: Quadriceps femoris, RCT: Randomised Controlled Trial, RDB: Randomise Double Blind, RF: Rectus Femoris, RM: Repetition Maximum, VI: Vastus Intermedius, VL: Vastus Lateralis, VM: Vastus Medialis *: Only the results of rresistance exercises from two different exercises or practices were evaluated. **: Only the measurements of the relevant weeks were evaluated. Brad Schoenfeld et al. (2016) TESTEX: 10 Pre-post 23 young resistance- trained men Experiment-1: 12 Experiment-2: 11 Experiment-1: 3 sets * 8-12 RM flat barbell press, plate-loaded leg press, plate- loaded seated cable row, barbell back squat exercise with 1 min rest Experiment-2: 3 sets * 8-12 RM flat barbell press, plate-loaded leg press, plate- loaded seated cable row, barbell back squat exercise with 3 min rest 8 weeks (3 times per day) VL Long interval resting group significantly increased QF muscle thickness from baseline to post by 5.4% and 7% (p <0.01). Bartolomei et al. (2021) TESTEX: 12 Pre-post 21 resistance- trained men Experiment-1: 10 Experiment-2: 11 5 sets* 6 RM deep squat, leg curl and extension, lunges, triceps extension, bench press, front raises exercises with 2 min rest. Experiment-1: Focusing on all muscle group Experiment-2: Focusing on one muscle group 10 weeks (4d/w) VL In experiment-2 group, changes in VL muscle thickness were substantially larger (p=0.037) than in experiment-1 group. Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Lower Limb Hypertrophy by RT Intensity 96 Evaluation of Quality The average score on the TESTEX scale was 10,6. Three studies (Bartolomei et al., 2021; Evangelista et al., 2019; Pinto et al., 2014) were classified as excellent, and 19 studies (Alkan, 2019; Boone et al., 2015; Cadore et al., 2014; Carvalho et al., 2020; Correa et al., 2012; Gonzalez et al., 2017; Ikezoe et al., 2020; Karsten et al., 2021; Korkmaz, 2018; Müller et al., 2020; Nakamura et al., 2021; Nogueira et al., 2009; Santos et al., 2018; Brad Schoenfeld & Grgic, 2018; Brad Schoenfeld et al., 2016; Usui et al., 2015; Yoshiko & Watanabe, 2021; Zaras et al., 2020; Zaroni et al., 2018) had good quality. None of the evaluated studies were assessed for their fair or poor methodological quality. The findings of the quality evaluation are shown (Table 2). Table 2. TESTEX Scala (Overall TESTEX score out of 15 points) Notes. *: Three points possible - 1 point if adherence is greater than 85%, 1 point if adverse events are reported, 1 point if exercise attendance is reported. #: Two points possible - 1 point if primary outcome is reported, 1 point if all other results are reported. Study Eligibility criteria specified Randomly allocated participants Allocation concealed Groups similar at baseline Assessors blinded Outcome measure assessed more than %85 of participants* Intention to treat analysis Reporting of between group statistical comparison Point measures and measures variability reported # Activity monitoring in control group Relative exercise intensity review Exercise volume and energy expenditure Overall TESTEX score Alkan, 2019 YES YES YES YES NO 2 NO YES 2 YES NO YES 10 Karsten et al., 2021 YES YES YES YES NO 2 NO YES 2 NO YES YES 11 Pinto et al., 2014 YES YES YES YES NO 2 NO YES 2 YES YES YES 12 Schoenfeld et al.,2018 YES YES NO YES NO 2 NO YES 2 YES NO YES 10 Correa et al., 2012 YES YES NO YES NO 2 NO YES 2 NO YES YES 10 Evangelista et al., 2019 YES YES YES YES NO 2 NO YES 2 YES YES YES 12 Bartolomei et al., 2021 YES YES YES YES NO 2 NO YES 2 YES YES YES 12 Zaras et al., 2020 YES NO NO YES NO 2 NO YES 2 YES YES YES 10 Cadore et al., 2014 YES YES NO YES NO 2 NO YES 2 YES YES YES 11 Usui et al., 2015 YES YES NO YES NO 2 NO YES 2 YES YES YES 11 Nogueira et al., 2009 YES YES NO YES NO 2 NO YES 2 YES YES YES 11 Santos et al., 2018 YES YES NO YES NO 2 NO YES 2 YES YES YES 11 Gonzalez et al., 2017 YES YES NO YES NO 2 NO YES 2 YES YES YES 11 Zaroni et al., 2018 YES YES NO YES NO 2 NO YES 2 NO YES YES 10 Schoenfeld et al., 2016 YES YES NO YES NO 2 NO YES 2 NO YES YES 10 Carvalho et al., 2020 YES YES NO YES NO 2 NO YES 2 NO YES YES 10 Ikezoe et al., 2020 YES YES NO YES NO 2 NO YES 2 NO YES YES 10 Müller et al., 2020 YES YES NO YES NO 2 NO YES 2 NO YES YES 10 Nakamura et al., 2021 YES YES NO YES NO 2 NO YES 2 NO YES YES 10 Boone et al., 2015 YES YES NO YES NO 2 NO YES 2 NO YES YES 10 Korkmaz, 2018 YES YES NO YES NO 2 NO YES 2 YES YES YES 11 Yoshiko & Watanabe, 2021 YES YES NO YES NO 2 NO YES 2 YES YES YES 11 Mean Score 10.59 Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Lower Limb Hypertrophy by RT Intensity 97 Resistance Training's Impact on Muscular Hypertrophy Based on the random effects model, high, moderate, and low-intensity resistance training significantly increased quadriceps muscle hypertrophy compared to the control or pre-test group (Table 3). Table 3. The effect of resistance exercise intensities on muscle hypertrophy (meta-analyses results). Subgroups ES (%95 CI) p-value Heterogeneity (I2) Weight High Intensity Resistance Training VL 2.45 [2.26, 2.64] p<.001 96% 20.3% VM 0.70 [0.27, 1.13] p = .001 Not applicable 4.1% VI 0.62 [0.46, 0.78] p < .001 99% 30.0% RF 3.48 [3.35, 3.60] p < .001 99% 45.6% Total 2.30 [2.21, 2.38] p < .001 99% 100% Moderate Intensity Resistance Training VL 2.45 [2.27, 2.63] p < .001 85% 60.0% VM 2.01 [0.84, 3.19] p = .0008 0% 1.4% VI 0.57 [-1.01, 2.14] p = .0048 85% 0.8% RF 0.99 [0.76, 1.22] p < .001 55% 37.8% Total 1.88 [1.74, 2.02] p < .001 87% 100% Low Intensity Resistance Training VL 14.21 [13.87, 14.56] p <.001 99% 20.3% VM 2.46 [0.66, 4.26] p = .007 0% 0.8% VI 15.59 [15.25, 15.93] p < .001 72% 21.0% RF 8.19 [7.98, 8.39] p < .001 100% 57.9% Total 10.92 [10.77, 11.08] p < .001 100% 100.0% Data demonstrated are presented as a standardized ES estimate (signifying the raw mean difference between experiment and pre or control groups) with 95% CI and p-value. Positive ES values favor resistance training performed in experiment groups. According to Table 3, high-intensity resistance training had a significantly increasing effect (p<.001) on quadriceps femoris muscle hypertrophy [Raw mean difference (RMD)= 2.30; confidence interval (CI) 95%: [2.21, 2.38]] with 99 % heterogeneity ratio compared to control/pre-test group. Moderate-intensity resistance training had a significantly increasing effect (p<.001) on quadriceps muscles hypertrophy [Raw Mean difference (RMD) =1.88; confidence interval (CI) 95%: [1.74 – 2.02]] with 87% heterogeneity ratio compared to control or pre-test group. Low-intensity resistance training had a significantly increasing effect (p<.001) on quadriceps muscles hypertrophy [Raw mean difference (RMD) =10.92; confidence interval (CI) 95%: [10.77 – 11.08)]] with 100% heterogeneity ratio compared to control or pre- test group. Shortly, resistance training is effective on the quadriceps muscles hypertrophy. Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Lower Limb Hypertrophy by RT Intensity 98 Meta-Regression Analysis Meta-regressions to examine the relationship between muscle thickness and resistance training intensity were carried out to recognize the sources of heterogeneity because they were statistically significant (p<.001) and had a large amount of unexplained heterogeneity. The dose-response relationship between resistance training intensity (% of 1RM) and change in quadriceps femoris muscle thickness (mm) is shown with scatter plots. The 95% confidence intervals are represented by dotted lines (Figures 2. and 3.). According to meta regression, it was determined that there was a non-significant direct relationship between resistance training intensity (% of 1RM) and change in RF (β=0.03; p = 0.417; R2 = 0.04) and VM muscle thickness (β = 0.003; p = 0.895; R2 = 0.002). It was also determined that there was a significant direct relationship between resistance training intensity and change in VI (β=0.01; p = 0.05; R2 = 0.56) and VL muscle thickness (β = 0.01; p = 0.007; R2 = 0.34). Figure 2. Scatter Plots of RF and VL Muscles Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Lower Limb Hypertrophy by RT Intensity 99 Figure 3. Scatter Plots of VM and VI Muscles Based on the scatter plots, when resistance exercise intensity increases by 10%, RF muscle thickness increases by 0.3 mm, and VM muscle thickness increases by 0.03 mm. When resistance exercise intensity increases by 10%, VI and VL muscle thickness increases by 0.1 mm. While the increase in RF and VM muscle thicknesses is not statistically significant, the rise in VI and VL is statistically significant (p = 0.417, p = 0.89, p = 0.05, p = 0.007, respectively). Additionally, R2 values indicate that the relationship between resistance training intensity and changes in RF, VM, and VL muscle thickness is insufficient except for the change in VI muscle thickness (R2 = 56%). Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Lower Limb Hypertrophy by RT Intensity 100 DISCUSSION This study searches for the answer to the question, “What is the net range of intensity of resistance training one provides to increase quadriceps muscle hypertrophy?”. To the best of the authors’ knowledge, there is no study in which the hypertrophic effects of resistance exercises are evaluated separately for each muscle in the quadriceps femoris. The main finding of this study is that resistance exercises, at all intensities (high-moderate-low), significantly enhance quadriceps femoris muscle size. Further, a significant direct relationship between resistance training intensity and change in VI and VL in which as intensity increases by 10%, muscle thickness increases by 0.1 mm. Given that it has been proven that resistance training’s acute and chronic effects diminish with time, it is necessary to manipulate resistance training components, including “intensity,” for consistent strength and morphological changes (Borde et al., 2015). Based on the results of this meta-analysis, athletes and coaches can maintain resistance exercise intensity in any range from low (30% of 1RM) to high (80% of 1RM) for various purposes at different periods of the training period without affecting the hypertrophic adaptation magnitude of quadriceps femoris. Parallel to this, in the meta-analysis by Carvalho et al. (2022), even when volume load is equalized across conditions, muscle hypertrophy is identical regardless of the level of resistance intensity from 30% to 80% of 1RM. Another meta-analysis, which examined the effects of high and low-intensity resistance exercises on muscle hypertrophy, determined no difference between the intensities (Brad Schoenfeld, Grgic, et al., 2017). Furthermore, according to these conclusions from three meta-analyses, one can perform resistance training with lower intensities (30-59% of 1RM), in turn, enlarge the training volume loads (Morton et al., 2019). The evidence suggests a dose-response relationship between training volume load and hypertrophic adaptations, therefore this might affect on changes in muscle size (Lasevicius et al., 2022). In other meta-analysis studies, dose response analysis was not evaluated. However, in this study, dose response connection was applied to investigate how resistance training intensity change would affect the hypertrophic response. Determining at what intensity there is significant hypertrophy may be useful to inform practitioners. On the other hand, it was proposed by three studies that higher training intensities induce greater increases in muscle fiber cross sectional area of vastus lateralis in “untrained” individuals (Campos et al., 2002; Holm et al., 2008; Schuenke et al., 2012). Supporting our and the other two meta-analyses abovementioned, Brad Schoenfeld et al. (2015) demonstrated that both low and high resistance intensities, even when training load volume equated, induce a similar increase in muscle size Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Lower Limb Hypertrophy by RT Intensity 101 after 8 weeks of resistance training in “resistance trained” men. It can be speculated that the resistance training experience may determine the size of hypertrophic adaptation depending on the intensity of resistance training, at least in the first few months. Even though low-volume, high-intensity resistance training is demonstrably beneficial to promoting muscle growth, the high intensities (80% 1RM) employed may be challenging for untrained individuals or remote training programs with limited access to equipment. In individuals with less resistance exercise experience, relatively greater volumes of lower intensity resistance training may be a more viable choice (Fyfe, Hamilton, & Daly, 2021). It could be the most effective approach to keep an elevated resistance intensity for the first several months and gradually reduce it. Lastly, it should be remembered that the “repetition to failure” regimen has been demonstrated as a key component for low intensity resistance training to be as effective as high loads for muscle growth (Lasevicius et al., 2022; Lim et al., 2019). The consensus is that an intensity of more than ≥60% of 1 RM is required to produce meaningful improvements in muscle size using conventional resistance training techniques. Nevertheless, there is mounting evidence that low-intensity resistance exercise, also using various techniques, can significantly improve muscle hypertrophy, often comparable to typical high-intensity exercise (B. J. Schoenfeld, 2013). In response to resistance training, three central components have been postulated to drive hypertrophic alterations: muscle damage, mechanical tension, and metabolic stress (B. J. Schoenfeld, 2010). Resistance training-related mechanical tension compromises the skeletal muscle's stability by prompting myofibers and satellite cells to respond mechanically and chemically (Toigo & Boutellier, 2006). Motor unit activation frequency, which determines both the intensity and length of excitation coupling, is thought to transmit signals for a variety of downstream pathways, including CaMKII (upregulates muscle atrophy) and CAMKIV (promotes mitochondrial biogenesis) (Chin, 2005). Conversely, it has been demonstrated that higher-intensity resistance training can regulate the acute secretion of growth hormone, particularly in routines meant to increase metabolic stress (Hoffman et al., 2003). Similarly, it has been suggested that glycolytic activity may elevate the acidic environment, accelerate muscle damage, and further stimulate sympathetic nerve function, resulting in a higher adaptation for muscle hypertrophy (Buresh, Berg, & French, 2009). On the other hand, it has also been hypothesized that the mechanisms of exercise-induced muscle growth are entirely intrinsic and unaffected by temporary changes in circulating hormones (West, Burd, Staples, & Phillips, 2010). Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Lower Limb Hypertrophy by RT Intensity 102 Another main finding is that as the intensity of resistance exercise increases, there is no significant increase in RF and VM muscle thicknesses; in contrast, a significant increase is observed in VI and VL muscle thicknesses. Since there is no meta-analysis where the hypertrophic response of four muscle heads (VL, RF, VM, and VI) to resistance training is investigated, it is impossible to make a direct comparison. More recently, the literature has offered contradictory findings about resistance training “intensity” and increases in VL-VI muscle size. Several researchers have seen larger gains in muscle growth with increasing intensity (Campos et al., 2002; Holm et al., 2008; Schuenke et al., 2012), whereas others revealed no statistically significant difference between low and high intensity (Tanimoto et al., 2008). A potential limitation in most of this research is that training volume differed between groups. When volume equated, all intensities (40%, 60% and %80 of 1RM) were found to enhance VL cross-sectional area similarly (Lasevicius et al., 2018; Brad Schoenfeld et al., 2015). For this reason, the result of this meta-analysis is that the muscle thickness in VL and VI increases in an intensity-dependent manner, which may be caused by the “volume” of resistance training. Moreover, the fact that almost all studies in this meta-analysis employed "closed chain" exercises, which predominantly leads to hypertrophy of vastus muscles (Ema, Sakaguchi, Akagi, & Kawakami, 2016), may have contributed to the failure of RF to respond even when "intensity" rises. VL, RF, VM, and VI are the four muscle heads that make up the quadriceps femoris. The surface head VL is located outside the thigh. RF is positioned in the center of the anterior side of the thigh, while VM is on the side. VI is positioned at the interior of the thigh because it is above the femur and beneath the three surface heads (Pasta, Nanni, Molini, & Bianchi, 2010). Physiological cross-sectional area (PCSA), the biggest cross-sectional area point of a pennate muscle perpendicular to its muscle fibers, volume, muscle length, fascicle length, and fascicle pennation angle are all distinctive architectural characteristics of each head (Lieber, 2002). Because of its location, the VL head of the quadriceps is the most investigated. It has been used as a substitute for the entire quadriceps muscle to assess muscle size, electromyographic activity, metabolic characteristics, and muscle fiber composition, whether for clinical or sports purposes (Coratella et al., 2020; El‐Ansary et al., 2021; Methenitis et al., 2016). According to a recent current experimental study, the best substitutes for the entire resistance training- induced hypertrophy of the quadriceps appear to be VL and VI (Spiliopoulou et al., 2022). VL and VI could offer accurate data on whole quadriceps muscle hypertrophic response lower- body resistance training when ultrasonography is employed. In the current study, the best Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Lower Limb Hypertrophy by RT Intensity 103 hypertrophic result against the increase in density was seen in VL and VI, which is attributed to the broader presence of PCSA. In addition, when the types of exercises performed based on the studies included in the analysis are examined, it is observed that the resistance exercises performed on the lower extremities are primarily aimed at the VL and VI muscles. The lack of training of the RF and VM muscles in exercise applications and the smaller PCSA will make it difficult to observe a hypertrophic effect on these muscles. Especially in recent years, the increase in the risk of lower extremity injuries in sportive activities and competitions has increased the number of studies on this subject. It has been suggested that the most critical issue that increases this risk is due to the power imbalance in the hamstring/quadriceps ratios (Cheung, Smith, & Wong, 2012; Padasala, Joksimovic, Bruno, Melino, & Manzi, 2020; Yoo, 2016). To minimize the risk of injury, the focus should be on hypertrophy of all quadriceps muscles. Based on the current results of the meta-analysis, special programs should be applied mainly for RF and VM muscles in lower extremity resistance exercise applications with lower intensities (≤ 80% of 1RM). CONCLUSION This meta-analysis indicated that resistance training had a significantly increasing effect on quadriceps femoris muscle hypertrophy. Meta-regression analysis identified a substantial relationship between resistance training intensity and change in VI and VL muscle thickness and a non-significant direct link between resistance training intensity and change in RF and VM muscle thickness. It can be said that the increase in VI muscle hypertrophy is dependent on the resistance training intensity based on its R2 value. Conversely, resistance training intensity is less likely to explain VL, VM, and RF muscle hypertrophy. By this perspective, analyses using multiple covariates such as frequency, type, time or total protein intake status, and type of sets (interset/drop set) in line with the FITT principle can be done in future studies. In addition, it was determined that the number of studies on VI and VM muscle hypertrophy was low. Therefore, future studies can focus on filling this gap in the literature. These results point to a potential advantage of including a wide range of loading values in a hypertrophy-focused exercise for quadriceps femoris muscles. Consequently, the meta-analysis results determined that the resistance training intensity gave similar results in the hypertrophy of the quadriceps femoris muscle. However, since the Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Lower Limb Hypertrophy by RT Intensity 104 percentage of muscle hypertrophy explained by exercise intensity is high (R2=56%), every 10% increase in resistance exercise provides a hypertrophic response of 0.1 mm in the VI muscle. This study is vital in performing muscle-specific strengthening exercises and detecting the intensity of a significant hypertrophy point. In addition, there will be benefits such as providing information to practitioners on this subject and indicating to researchers at what point there needs to be more literature. LIMITATIONS This meta-analysis has several limitations. Hypertrophic adaptations to resistance training are influenced by multiple variables beyond training intensity, including movement tempo, exercise execution tempo, repetition cadence, and eccentric-concentric loading patterns. However, due to the heterogeneity of training protocols across the included studies and incomplete reporting of these parameters, their independent effects on muscle hypertrophy could not be systematically analyzed. Moreover, factors such as total training volume, rest intervals, training experience of participants, and neuromuscular adaptations may have contributed to variability in hypertrophic responses. The lack of uniformity in measurement techniques (e.g., ultrasound vs. MRI) and differences in anatomical sites used for assessing muscle thickness could also introduce methodological inconsistencies. Additionally, variations in participant characteristics such as age, sex, baseline strength levels, and genetic predispositions may have influenced the results, limiting the generalizability of the findings. Future research should aim to standardize resistance training protocols, ensuring consistent reporting of movement tempo, contraction phases, and repetition cadences. Additionally, studies employing advanced imaging techniques with strict methodological controls are needed to isolate the effects of specific training variables on quadriceps hypertrophy. PRACTICAL APPLICATIONS • This study searches for the answer to the question, “What is the net range of intensity of resistance training one provides to increase quadriceps muscle hypertrophy?”. • Starting from here, this study aims to detect the effectiveness of different resistance training intensities (high, moderate, and low) for quadriceps femoris muscle group hypertrophic response in healthy adults. Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Lower Limb Hypertrophy by RT Intensity 105 • The meta-analysis results detected that the different resistance training intensities gave similar results in the hypertrophy of the quadriceps femoris muscle. • The meta-regression analysis showed that for every 10% greater difference in resistance exercise intensity, the hypertrophic response in the VI muscle was determined to be 0.1 mm greater significantly. However, no significant hypertrophic response was observed for VL, VM, and RF muscles. • This study may be helpful in performing muscle-specific strengthening exercises and detecting the intensity of a significant hypertrophy point for practitioners on this subject and athletes who want to track muscle hypertrophy. Declaration of Conflicting Interests The authors report no conflicts of interest and no source of funding. Acknowledgment We thank Prof. Dr. Gordon L. Warren for his contribution and suggestions for this study. REFERENCES Alkan, E. (2019). Effect of Dıfferent Izotonıc Quadrıceps Exercıse Traınıng on Muscle Strength, Muscle Thıckness and Balance ın Healthy Indıvıduals (Master’s thesis). 9 Eylül University, Amirthalingam, T., Mavros, Y., Wilson, G. C., Clarke, J. L., Mitchell, L., & Hackett, D. A. (2017). Effects of a modified German volume training program on muscular hypertrophy and strength. The Journal of Strength Conditioning Research, 31(11), 3109-3119. Bartolomei, S., Nigro, F., Lanzoni, I. M., Masina, F., Di Michele, R., & Hoffman, J. R. (2021). A comparison between total body and split routine resistance training programs in trained men. The Journal of Strength Conditioning Research, 35(6), 1520-1526. Boone, C. H., Stout, J. R., Beyer, K. S., Fukuda, D. H., & Hoffman, J. R. (2015). Muscle strength and hypertrophy occur independently of protein supplementation during short-term resistance training in untrained men. Applied Physiology, Nutrition, Metabolism, 40(8), 797-802. Borde, R. (2015). Dose-response relationships of resistance training in healthy old adults: A systematic review meta-analysis. Sports medicine, 45(12), 1693-1720. Brigatto, F. A., Lima, L. E. d. M., Germano, M. D., Aoki, M. S., Braz, T. V., & Lopes, C. R. (2022). High resistance-training volume enhances muscle thickness in resistance-trained men. Journal of strength conditioning research, 36(1), 22-30. Buresh, R., Berg, K., & French, J. (2009). The effect of resistive exercise rest interval on hormonal response, strength, and hypertrophy with training. The Journal of Strength Conditioning Research, 23(1), 62-71. Cadore, E., González‐Izal, M., Pallarés, J., Rodriguez‐Falces, J., Häkkinen, K., Kraemer, W., . . . Izquierdo, M. (2014). Muscle conduction velocity, strength, neural activity, and morphological changes after eccentric and concentric training. Scandinavian journal of medicine science in sports, 24(5), e343-e352. Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Lower Limb Hypertrophy by RT Intensity 106 Campbell, K. L., Winters-Stone, K., Wiskemann, J., May, A. M., Schwartz, A. L., Courneya, K. S., . . . Gerber, L. (2019). Exercise guidelines for cancer survivors: consensus statement from international multidisciplinary roundtable. Medicine science in sports exercise, 51(11), 2375. Campos, G. E., Luecke, T. J., Wendeln, H. K., Toma, K., Hagerman, F. C., Murray, T. F., . . . Staron, R. S. (2002). Muscular adaptations in response to three different resistance-training regimens: specificity of repetition maximum training zones. European journal of applied physiology, 88, 50-60. Carvalho, L., Junior, R. M., Barreira, J., Schoenfeld, B. J., Orazem, J., & Barroso, R. (2022). Muscle hypertrophy and strength gains after resistance training with different volume-matched loads: a systematic review and meta- analysis. Applied Physiology, Nutrition, Metabolism, 47(4), 357-368. Carvalho, L., Junior, R. M., Truffi, G., Serra, A., Sander, R., De Souza, E. O., & Barroso, R. (2020). Is stronger better? Influence of a strength phase followed by a hypertrophy phase on muscular adaptations in resistance-trained men. Research in Sports Medicine, 29(6), 536-546. Cheung, R., Smith, A., & Wong, D. (2012). H: Q ratios and bilateral leg strength in college field and court sports players. Journal of human kinetics, 33(2012), 63-71. Chin, E. R. (2005). Role of Ca2+/calmodulin-dependent kinases in skeletal muscle plasticity. Journal of applied physiology, 99(2), 414-423. Cohen, J. (1992). Statistical power analysis. Current directions in psychological science, 1(3), 98-101. Coratella, G., Longo, S., Rampichini, S., Limonta, E., Shokohyar, S., Bisconti, A. V., . . . Esposito, F. (2020). Quadriceps and gastrocnemii anatomical cross-sectional area and vastus lateralis fascicle length predict peak- power and time-to-peak-power. Research Quarterly for Exercise Sports medicine, 91(1), 158-165. Correa, C. S., LaRoche, D. P., Cadore, E. L., Reischak-Oliveira, A., Bottaro, M., Kruel, L. F. M., . . . Lacerda, F. (2012). 3 Different types of strength training in older women. International journal of sports medicine, 33(12), 962-969. Csapo, R., & Alegre, L. (2016). Effects of resistance training with moderate vs heavy loads on muscle mass and strength in the elderly: A meta‐analysis. Scandinavian journal of medicine science in sports, 26(9), 995-1006. El‐Ansary, D., Marshall, C. J., Farragher, J., Annoni, R., Schwank, A., McFarlane, J., . . . Zito, G. (2021). Architectural anatomy of the quadriceps and the relationship with muscle strength: An observational study utilising real‐time ultrasound in healthy adults. Journal of Anatomy, 239(4), 847-855. Ema, R., Sakaguchi, M., Akagi, R., & Kawakami, Y. (2016). Unique activation of the quadriceps femoris during single-and multi-joint exercises. European journal of applied physiology, 116, 1031-1041. Evangelista, A. L., De Souza, E. O., Moreira, D. C., Alonso, A. C., Teixeira, C. V. L. S., Wadhi, T., . . . Greve, J. M. D. A. (2019). Interset stretching vs. traditional strength training: effects on muscle strength and size in untrained individuals. The Journal of Strength Conditioning Research, 33, S159-S166. Folland, J. P., & Williams, A. G. (2007). Morphological and neurological contributions to increased strength. Sports medicine, 37, 145-168. Fry, A. C. (2004). The role of resistance exercise intensity on muscle fibre adaptations. Sports medicine, 34, 663- 679. Fyfe, J. J., Hamilton, D. L., & Daly, R. M. (2021). Minimal-dose resistance training for improving muscle mass, strength, and function: A narrative review of current evidence and practical considerations. Sports medicine, 1-17. Gonzalez, A. M., Sell, K. M., Ghigiarelli, J. J., Kelly, C. F., Shone, E. W., Accetta, M. R., . . . Mangine, G. T. (2017). Effects of phosphatidic acid supplementation on muscle thickness and strength in resistance-trained men. Applied Physiology, Nutrition, Metabolism, 42(4), 443-448. Hackett, D., Ghayomzadeh, M., Farrell, S., Davies, T., & Sabag, A. (2022). Influence of total repetitions per set on local muscular endurance: A systematic review with meta-analysis and meta-regression. Science Sports, 37(5- 6), 405-420. Henselmans, M., & Schoenfeld, B. J. (2014). The effect of inter-set rest intervals on resistance exercise-induced muscle hypertrophy. Sports medicine, 44(12), 1635-1643. Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Lower Limb Hypertrophy by RT Intensity 107 Hoffman, J. R., Im, J., Rundell, K. W., Kang, J., Nioka, S., SPEIRING, B. A., . . . Chance, B. (2003). Effect of muscle oxygenation during resistance exercise on anabolic hormone response. Medicine science in sports exercise, 35(11), 1929-1934. Holm, L., Reitelseder, S., Pedersen, T. G., Doessing, S., Petersen, S. G., Flyvbjerg, A., . . . Kjaer, M. (2008). Changes in muscle size and MHC composition in response to resistance exercise with heavy and light loading intensity. Journal of applied physiology, 105(5), 1454-1461. Ikezoe, T., Kobayashi, T., Nakamura, M., & Ichihashi, N. (2020). Effects of low-load, higher-repetition vs. High- load, lower-repetition resistance training not performed to failure on muscle strength, mass, and echo intensity in healthy young men: A time-course study. The Journal of Strength Conditioning Research, 34(12), 3439-3445. Karsten, B., Fu, Y. L., Larumbe-Zabala, E., Seijo, M., & Naclerio, F. (2021). Impact of two high-volume set configuration workouts on resistance training outcomes in recreationally trained men. The Journal of Strength Conditioning Research, 35, S136-S143. Korkmaz, E. (2018). The Effects of 6 Weeks Blood Flow Restriction Training on Muscle Strength and Evaluation of Muscle Architecture with Ultrasonography in U19 Male Soccer Players (Master’s ). Eskisehir Osmangazi University, Krieger, J. W. (2009). Single versus multiple sets of resistance exercise: a meta-regression. The Journal of Strength Conditioning Research, 23(6), 1890-1901. Lasevicius, T., Schoenfeld, B. J., Silva-Batista, C., Barros, T. d. S., Aihara, A. Y., Brendon, H., . . . Teixeira, E. L. (2022). Muscle failure promotes greater muscle hypertrophy in low-load but not in high-load resistance training. Journal of strength conditioning research, 36(2), 346-351. Lasevicius, T., Ugrinowitsch, C., Schoenfeld, B. J., Roschel, H., Tavares, L. D., De Souza, E. O., . . . Tricoli, V. (2018). Effects of different intensities of resistance training with equated volume load on muscle strength and hypertrophy. European journal of sport science, 18(6), 772-780. Liberati, M., Tetzlaff, J., & Altman, D. (2009). Preferred Reporting items for systematic reviews and meta analyses: THE PRISMA statement. Plos Medicine, 6(7), 1-6. Lieber, R. L. (2002). Skeletal muscle structure, function, and plasticity: Lippincott Williams & Wilkins. Lim, C., Kim, H. J., Morton, R. W., Harris, R., Philips, S. M., Jeong, T. S., & Kim, C. K. (2019). Resistance exercise-induced changes in muscle metabolism are load-dependent. Med Sci Sports Exerc, 51(12), 2578-2585. Methenitis, S., Karandreas, N., Spengos, K., Zaras, N., Stasinaki, A.-N., & Terzis, G. (2016). Muscle Fiber Conduction Velocity, Muscle Fiber Composition, and Power Performance. Medicine science in sports exercise, 48(9), 1761-1771. Mitchell, C. J., Churchward-Venne, T. A., West, D. W., Burd, N. A., Breen, L., Baker, S. K., & Phillips, S. M. (2012). Resistance exercise load does not determine training-mediated hypertrophic gains in young men. Journal of applied physiology, 113(1), 71-77. Morton, R. W., Sonne, M. W., Farias Zuniga, A., Mohammad, I. Y., Jones, A., McGlory, C., . . . Phillips, S. M. (2019). Muscle fibre activation is unaffected by load and repetition duration when resistance exercise is performed to task failure. The Journal of physiology, 597(17), 4601-4613. Müller, D. C., Izquierdo, M., Boeno, F. P., Aagaard, P., Teodoro, J. L., Grazioli, R., . . . Pinto, R. S. (2020). Adaptations in mechanical muscle function, muscle morphology, and aerobic power to high-intensity endurance training combined with either traditional or power strength training in older adults: a randomized clinical trial. European journal of applied physiology, 120(5), 1165-1177. Nakamura, M., Ikezu, H., Sato, S., Yahata, K., Kiyono, R., Yoshida, R., . . . Nunes, J. P. (2021). Effects of adding inter-set static stretching to flywheel resistance training on flexibility, muscular strength, and regional hypertrophy in young men. International Journal of Environmental Research Public Health, 18(7), 3770. Nicholson, G., Ispoglou, T., & Bissas, A. (2016). The impact of repetition mechanics on the adaptations resulting from strength-, hypertrophy-and cluster-type resistance training. European journal of applied physiology, 116, 1875-1888. Nogueira, W., Gentil, P., Mello, S., Oliveira, R., Bezerra, A., & Bottaro, M. (2009). Effects of power training on muscle thickness of older men. International journal of sports medicine, 30(03), 200-204. Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Lower Limb Hypertrophy by RT Intensity 108 Nunes, J. P., Grgic, J., Cunha, P. M., Ribeiro, A. S., Schoenfeld, B. J., de Salles, B. F., & Cyrino, E. S. (2021). What influence does resistance exercise order have on muscular strength gains and muscle hypertrophy? A systematic review and meta-analysis. European journal of sport science, 21(2), 149-157. Padasala, M., Joksimovic, M., Bruno, C., Melino, D., & Manzi, V. (2020). Muscle injuries in athletes. The relationship between H/Q ratio (hamstring/quadriceps ratio). Italian Journal Sports Rehabilitation and Posturology, 7(1), 1478-1498. Page, M. J., McKenzie, J. E., Bossuyt, P. M., Boutron, I., Hoffmann, T. C., Mulrow, C. D., . . . Brennan, S. E. (2021). The PRISMA 2020 statement: an updated guideline for reporting systematic reviews. International journal of surgery, 88, 105906. Pasta, G., Nanni, G., Molini, L., & Bianchi, S. (2010). Sonography of the quadriceps muscle: Examination technique, normal anatomy, and traumatic lesions. Journal of Ultrasound, 13(2), 76-84. Pinto, R. S., Correa, C. S., Radaelli, R., Cadore, E. L., Brown, L. E., & Bottaro, M. (2014). Short-term strength training improves muscle quality and functional capacity of elderly women. Age, 36, 365-372. Polito, M. D., Papst, R. R., & Farinatti, P. (2021). Moderators of strength gains and hypertrophy in resistance training: A systematic review and meta-analysis. Journal of sports sciences, 39(19), 2189-2198. Roig, M., O’Brien, K., Kirk, G., Murray, R., McKinnon, P., Shadgan, B., & Reid, W. (2009). The effects of eccentric versus concentric resistance training on muscle strength and mass in healthy adults: a systematic review with meta-analysis. British journal of sports medicine, 43(8), 556-568. Santos, R., Valamatos, M., Mil-Homens, P., & Armada-da-Silva, P. (2018). Muscle thickness and echo-intensity changes of the quadriceps femoris muscle during a strength training program. Radiography, 24(4), e75-e84. Schoenfeld, B., Contreras, B., Krieger, J., Grgic, J., Delcastillo, K., Belliard, R., & Alto, A. (2019). Resistance training volume enhances muscle hypertrophy but not strength in trained men. Medicine science in sports exercise, 51(1), 94. Schoenfeld, B., & Grgic, J. (2018). Evidence-based guidelines for resistance training volume to maximize muscle hypertrophy. Strength Conditioning Journal, 40(4), 107-112. Schoenfeld, B., Grgic, J., & Krieger, J. (2019). How many times per week should a muscle be trained to maximize muscle hypertrophy? A systematic review and meta-analysis of studies examining the effects of resistance training frequency. Journal of sports sciences, 37(11), 1286-1295. Schoenfeld, B., Grgic, J., Ogborn, D., & Krieger, J. W. (2017). Strength and hypertrophy adaptations between low-vs. high-load resistance training: a systematic review and meta-analysis. The Journal of Strength Conditioning Research, 31(12), 3508-3523. Schoenfeld, B., Ogborn, D. I., Vigotsky, A. D., Franchi, M. V., & Krieger, J. W. (2017). Hypertrophic effects of concentric vs. eccentric muscle actions: a systematic review and meta-analysis. The Journal of Strength Conditioning Research, 31(9), 2599-2608. Schoenfeld, B., Peterson, M. D., Ogborn, D., Contreras, B., & Sonmez, G. T. (2015). Effects of low-vs. high-load resistance training on muscle strength and hypertrophy in well-trained men. The Journal of Strength Conditioning Research, 29(10), 2954-2963. Schoenfeld, B., Pope, Z. K., Benik, F. M., Hester, G. M., Sellers, J., Nooner, J. L., . . . Ross, C. L. (2016). Longer interset rest periods enhance muscle strength and hypertrophy in resistance-trained men. Journal of strength conditioning research, 30(7), 1805-1812. Schoenfeld, B. J. (2010). The mechanisms of muscle hypertrophy and their application to resistance training. The Journal of Strength Conditioning Research, 24(10), 2857-2872. Schoenfeld, B. J. (2013). Is there a minimum intensity threshold for resistance training-induced hypertrophic adaptations? Sports medicine, 43(12), 1279-1288. Schuenke, M. D., Herman, J. R., Gliders, R. M., Hagerman, F. C., Hikida, R. S., Rana, S. R., . . . Staron, R. S. (2012). Early-phase muscular adaptations in response to slow-speed versus traditional resistance-training regimens. European journal of applied physiology, 112, 3585-3595. Kinesiologia Slovenica, 31, 2, 85-109 (2025), ISSN 1318-2269 Lower Limb Hypertrophy by RT Intensity 109 Smart, N. A., Waldron, M., Ismail, H., Giallauria, F., Vigorito, C., Cornelissen, V., & Dieberg, G. (2015). Validation of a new tool for the assessment of study quality and reporting in exercise training studies: TESTEX. JBI Evidence Implementation, 13(1), 9-18. Spiliopoulou, P., Methenitis, S., Zaras, N., Stasinaki, A.-N., Krekoukia, M., Tsitkanou, S., & Terzis, G. (2022). Vastus Lateralis and Vastus Intermedius as Predictors of Quadriceps Femoris Muscle Hypertrophy after Strength Training. Applied Sciences, 12(18), 9133. Sutton, A. J., & Higgins, J. P. (2008). Recent developments in meta‐analysis. Statistics in medicine, 27(5), 625- 650. Tanimoto, M., Sanada, K., Yamamoto, K., Kawano, H., Gando, Y., Tabata, I., . . . Miyachi, M. (2008). Effects of whole-body low-intensity resistance training with slow movement and tonic force generation on muscular size and strength in young men. The Journal of Strength Conditioning Research, 22(6), 1926-1938. Toigo, M., & Boutellier, U. (2006). New fundamental resistance exercise determinants of molecular and cellular muscle adaptations. European journal of applied physiology, 97, 643-663. Usui, S., Maeo, S., Tayashiki, K., Nakatani, M., & Kanehisa, H. (2015). Low-load slow movement squat training increases muscle size and strength but not power. International journal of sports medicine, 37(04), 305-312. West, D. W., Burd, N. A., Staples, A. W., & Phillips, S. M. (2010). Human exercise-mediated skeletal muscle hypertrophy is an intrinsic process. The international journal of biochemistry cell biology, 42(9), 1371-1375. Yoo, W.-g. (2016). Comparison of hamstring-to-quadriceps ratio between accelerating and decelerating sections during squat exercise. Journal of Physical Therapy Science, 28(9), 2468-2469. Yoshiko, A., & Watanabe, K. (2021). Impact of home-based squat training with two-depths on lower limb muscle parameters and physical functional tests in older adults. Scientific reports, 11(1), 1-10. Zaras, N., Stasinaki, A.-N., Spiliopoulou, P., Mpampoulis, T., Hadjicharalambous, M., & Terzis, G. (2020). Effect of inter-repetition rest vs. traditional strength training on lower body strength, rate of force development, and muscle architecture. Applied Sciences, 11(1), 45. Zaroni, R. S., Brigatto, F. A., Schoenfeld, B. J., Braz, T. V., Benvenutti, J. C., Germano, M. D., . . . Lopes, C. R. (2018). High resistance-training frequency enhances muscle thickness in resistance-trained men. The Journal of Strength Conditioning Research, 33, S140-S151.