8 KINESIOLOGIA SLOVE NICA 3 (1997) 1 • 8--12 EFFECTS OF SUPERIMPOSED SUBMAXIMAL ELECTRICAL STIMULATION ON EXPLOSIVE MOVEMENTS DURING FATIGUE Vojko Strojnik* Dieter Strass * * VPLIV DODATNE PODPRAŽNE ELEKTRIČNE STIMULACIJE NA EKSPLOZIVNA GIBANJA PRI UTRUJENOSTI Abstract The aim of the study was to evaluate the influence of superimposed electrical stimulation (ES) of mus- cles on movements during fatigue. Ten male students of physical ed ucation performed a series of maximal explosive pushes with the right arm against weight in a vertical direction. Aspecial lyconstructed appa- ratus was used to control both the loads and direc- tion of the movements. The pushes against 66 % of maximal voluntary isometric force (MVC) load were executed every two seconds. When fatigue was ob- served (after 8 to 12 pushes) a superimposed ES of m. triceps brachii (MTB) was performed forthe next two pushes. The vertical forces acting on the weight were recorded forthe first push, the last push before ES, the push in the middle between these two and for both pushes with ES. Results showed that dy- namic force peaks, as well as force impulses in the first 200 msec after the force onset, were strongly in- fluenced by fatigue. On the other hand, the maxi- mal vertica l velocity of the weight at the first push with ES was significantly greater (p < 0.01) than for those before. The greatest effect was observed dur- ing the second half of the movement at high elbow extension. In the first pushes (with no fatigue) the first part of the workout seemed to be more important, while with fatigue and especially during ES, the force impulses were distributed more equally throughout the whole path which caused the greater maximal velocity at the end. It was concluded that superim- posed ES of MTB effected maximal explosive move- ments. Key-words: arm extension, electrical stimulation, in- termuscular coordination, movement velocity, fa- tigue *University of Ljubljana, Faculty of Sport, SI-1000 LJUBLJANA, Slovenia ''Institute of Sport and Sport Sciences University of Freiburg, D-7800 FREIBURG, Germany Izvleček Namen naloge je bil ugotoviti vpliv dodatne elek- trične stimulacije (ES) mišic na gibanje v utrujenosti. Deset študentov športne vzgoje je izved lo serijo naj- bolj eksplozivnih iztegovanj desne roke z obre- menitvijo v navpični smeri. Uporabljena je bila pose- bej izdelana upornica za kontrolo bremena in smeri gibanja. Breme je znašalo 66% največjega izo- metričnega bremena, iztegovanja pa so bila izvede- na vsaki 2 sekundi. Ko je nastopila utrujenost (po 8 do 12 ponovitvah), so bili merjenci pri naslednjih dveh iztegnitvah stimulirani (m. biceps brachii - TB). Vertikalne sile, delujoče na breme, so bile izmerjene pri prvi in zadnji iztegnitivi , iztegnitvi na sredi med njima in pri obeh z ES. Rezultati so pokazali, da sta bili največja dinam ična sila ter impulz sile v prvih 200 ms močno pod vplivom utrujenosti, medtem ko se je največja vertikalna hitrost bremena pri uporabi ES povečala (p<0.001 ). Največj i vpliv ES je bil opazen v drugi polovici giba, pri iztegnjeni roki. Pri prvih iztegnitvah (brez utrujenosti) se zdi prvi del gi- ba pomembnejši, medtem ko je bila pri utrujenosti in še posebej pri ES porazdelitev impulza sile enakomernejša, kar je prispevalo k večji končni hitrosti giba. Mogoče je zaključiti, da dodatna ES mišic vpliva na najbolj eksplozivne gibe v utrujenosti. Ključne besede: iztegovanje roke, električna stimu- lacija, medmišična koordinacija, hitrost gibanja, utru- jenost ~~~~ 9 EFFECTS OF SUPERIMPOSED SUBMAXIMAL ELECTRICAL STIMULA TION ON EXPLOSIVE MOVEMENTS OURING FATIGUE lntroduction The inability to sustain the same force leve! during continuous muscular cont raction, or decreasing maximal force leve! during repeated contractions are the most typical signs of fatigue. There are nu- merous mechanisms underlying fatigue wh ich can be considered as a command chain for muscular contraction (5) . According to their positions in that chain, they have been classified into central and pe- ripheral mechanisms (2). It is not only the force lev- e! that is subject to fatigue but also the slope of the force-time curve in explosive movements is affect- ed (17) . In brief, explosive muscular contractions, motor units are activated for a short period of tirne with a burst of neural impulses with a firing frequency which may exceed 100 Hz (6). It is not important just to reach as high a fi ring frequency as possible, butal - so to spend the minimum possible t irne to reach it (13). Another important feature is the synchroniza- tion of motor units, to contract in parallel and not in their normal sequence (8, 9). In movements where the goal isto achieve the highest speed at the end of the path, great attention should be given to inter- muscular coordination, especial ly when more mus- cle groups are involved (13, 11 ). Duchateau and Hainaut (3) stated that intracellular processes play the major role in contractile fa ilure duringsustained and intermittentcontractions. They found also that the nerve conduction velocity is not reduced in intermittent contractions. However, in volu ntary contractions there are not only physiolog- ical factors that influence the activation of motor units, butalso psychological ones, which acton mo- tor centres in the cortex. The purpose of this study was to examine the influ- ence of superimposed electrical stimulation of mus- cles on the explosive movements during fatigue to observe the effect of improved central neural drive to muscle. In order to explore this effect, a submax- imal cutaneous ES of m. triceps brachii (MTB) was employed during a vertical push against a weight. Methods Ten male students of physical education (age: 25 ± 3.8 years, height: 179 ± 4.2 cm, weight: 74 ± 5.5 kg) volunteered for the study. Ali subjects were ful ly informed of the procedures and possible risks in- volved in the study. They gave a conscious consent to their engagement. Each subject performed a series of maximal explo- sive vertical pushes against a weight in a specially constructed apparatus (16). They laid on their backs w ith a 90 degree angle between the trunk and the upper arm and the lower arm perpendicular to the upper arm. Loads were adj usted to 66 % of the sub- jects' maximal voluntary isometric force. They exe- cuted asi ngle push every two seconds. After fatigue was observed as an inabi lity to start moving the weight with the same starting acceleration as in the beginning (after 8 to 12 pushes) a superimposed sub- maximal electrical stimulation of MTB was intro- d uced for the next two pushes. The subjects were instructed to give their maximum for every single push and were encouraged during the whole workoutto do so. The return of the weight into the starting position was done with the same arm, slowly and without any external help. The start of the workout was controlled by an electronic counter, set to send a beep sound every two sec- onds. An experimenter encouraged each subject duri ng the whole series. Every subject was i nformed about the start of ES. Before testing, a warm-up pro- cedure was performed. A FES electrical impulse generator (Gorenje, Velenje, Slovenia) was employed for direct electrical stimu- lation of MTB. The anode (15 X 5 cm) was placed proximally and the cathode (8 X 5 cm) distal ly over the muscle. Unimodal, square wave, constant cu r- rent impulses, at a frequency of 50 Hz were given. The train of impulses lasted for 1 sec. The ampl itude (in mA) was set individually to be a little over the mo- tor th reshold. The impulse generator was manually triggered at the beep sound. The vert ica l force acting against the weight was recorded with a piezo sensor placed into the load's handle. The force signal was amplified, digitized and processed with a PC (sampl ing frequency 500 Hz). From force-time curves (F(t)) the maximal dynamic force during each single push (MDF) was evaluated. With a numerical integration of force-time cu rves over the fi rst 200 msec, after the force onset, the start impulses (SI) were calculated . The maximal vertical velocity during each single push (MVV) was calcu- lated, according to the following equation : MVV = 0.002 * m-1 *L (Fi - m) i = s, .. ,e s,e = startand end o f tirne interval, m = mass of the weight, Fi = vertical force Data was collected for the fi rst push (P1 ), the last push before ES (P3), the push between these two pushes (P2), the first push with ES (P4) and the sec- 10 Vojko Strojnik, Dieter Strass EFFECTS OF SUPERIMPOSED SUBMAXIMAL ELECTRICAL STIMULATION ON EXPLOSIVE MOVEMENTS DURING FATIGUE ond push with ES (PS) . The mean F(t) curves for the target pushes were also calculated. For every single push and every single variable, the mean and the standard deviat ion of all ten subjects were calculated, as well asa T-test for paired sam- ples between pushes for every variable. Statistical significance was set to the 5 % level of alpha error. Results Figure 1 shows the mean F(t) curves for the single pushes. Two clusters of curves could be seen, one representing P1 and P2 and the other P3 to PS. The starting impulses (Fig. 2) of the first two observed pushes differed significantly from the others. P1 had a lower, although nonsignificant, value than P2. There was a scarcely visible, nonsignificant differ- ence (46.75 and 46.27 Ns) between P3 and P4, which represented the voluntary activation and the voluntary activation with superimposed ES. Maximal dynamic forces during pushes (Fig. 3) showed slight- rlfT\8(ms) 900 Figure 1. Mean force-time curves for 1 O subjects synchronized at SN leve/ at the force onset. The symbols represent: P1 to PS are corre- sponding pushes (see text (or explanation), LW is load's weight. Figure 2. Mean starting impulses in Ns and their standard deviations at the first 200 ms after force onset. ly different relations than those in figure 2, since P2 was lower than P1 . An absolute decline of 7 % be- tween P1 and PS was observable. AII differences, ex- cept P1-P2 and P3-P4 were statistically significant. Completely different behaviour was seen in the maximal vertical velocities (Fig. 4). At P4 (first push with ES) subjects reached the highest velocity. The only statistically significant d ifference was that be- tween P3 and P4, where also an extremely high cor- relation was observable (r= O. 999). The statistical significance between pairs of pushes for all variables are shown in Table 1. Table 1. Levels of statistical significance of force pa- rameters between pairs of pushes SI MDF MVV Pl-2 0.75 0.34 0.89 P2-3 0.01 0.01 0.43 P3-4 P4-5 0.51 0.41 0.13 0.18 0.00 0.28 Table 1. 5tatistica/ significance (two-tailed P value) o( paired T-test be- tween pairs o( pushes for single variables (P1-2 represents the pair P1 and P2). SI - starting impulse, MDF - maxima/ dynamical force, MW - maximal vertical ve/ocity. 375,r---- - ----,---------, 355,+----t- ---ii----------- z Figure 3. Mean maximal dynamical forces in Newton and their standard deviations. 1.6.,------- -------- -~ 1.5+--- ----------- --1 U+------:;;----+----t----t---1 1.1 Figure 4. Mean maximal vertica/ velocities in m/s and their standard deviations. Vojko Strojnik, Oieter Strass EFFECTS OF SUPERIMPOSED SUBMAXIMAL ELECTRICAL STIMULA TION ON EXPLOSIVE MOVEMENTS DURING FATIGUE 1 1 Discussion ltwasexpected thatsuperimposed ES of MTB during fatigue could benefit al i the observed parameters. The results show, however, that superimposed ES had no significant effect on MDF and Sl. The be- haviour of the latter is slightly d ifferent from M DF, with a peak seen for the second measured push, which may be explained by neuromuscular faci lita- tion (14) and is also in agreement with other investi- gators (17). Both parameters were obtained at the very beginningof the movement. In this phase MTB is not the prime mover (7), so the eventual gain of its force may not be necessarily observable. On the oth- er hand, this early phase of the push shows a signifi- cant decrease in observed MDF and Sl. It is hard to say which mechanisms were dominant, causing fa- tigue, because there was no additional monitoring. There are many possible fatigue mechanisms (4), but it seems that the energy resources should not have a decisive role (1 ). However, this fatigue may be at- tributed tothe proximal partofthe kineticchain (m. pectoralis major, m. deltoideus). The most impressive results were obtained from the maximal velocities of the weights. They can bedi- vided into two parts: without and with superim- posed ES. For the first three pushes, relatively con- stant maxi mal velocities are observable, even though the other two parameters show sign if icant negative tendencies. Due to these changes the maximal ve- locity should decrease as well. Since the maximal ve- locities remain quite unchanged, it may be specu- lated that there is a velocity- limit of muscle short- ening, w hich may be responsible for a rested mus- cle being unable to overcome a certain velocity even ifthere isstill some acceleration path left. Th is means that the same velocity can be obtained in fatigue al- so, but with less acceleration and a longer accelera- tion path. It may also be concluded that the t irne needed to pass the distance, is more subject to fa- tigue, than the maximal velocity itself (13). The most interesting result of this study is the maxi- mal velocitygain during P4 and the lesserone during P5. This can be mainly contributed to the superim- posed ES of MTB. W ith the means of ES it is possible to activate more motor units and/or cause them to work at higher firing frequencies. Fast motor units have a h igher activation threshold and normally may not be fully voluntary activated. The superi mposed ES helps to activate primarily this type of motor un its and thus increase the ratio of activated fast to slow- twitch motor units. The result may be higher maxi- mal velocity of the weight (19). Of course, these re- sults may be also influenced by motivation, but this seems less likely. One reason is thatthe subjects were encouraged throughout the whole series of pushes and the other is an extremely high correlation in maximal velocity between P3 and P4. There is no strait explanation for decreasing in MVV from P4 to P5. It seems that both, an exhaustion as wel l asa fall of concentration after P4, might explain that. There is a considerable observable qualitative change in force production, between the first and the second half of the push, during fatigue and su- perimposed ES. The second half dominates, which implies changes in intermuscular coordination. It is possible to conclude that the role of MTB becomes a crucial one during fatigue. With the extension of the elbow, the effect of superimposed ES on MTB was increased (18). There are also b iomechanical reasons for greater force during the second half of the push. The veloc- ity at the end of the first half of t he push was smaller in each next push. According to Hill's equation (1 O), a muscle can produce greater force at lower veloci- ty. With greater force a MTB tendon is add itionally stretched, which may cause greater maximal veloc- ity of the weight (12) . But it does not seem to play a substantial role in velocity improvement, because of the relatively srna II force changes. It is also worth to men ti on that the contraction t i mes were of sufficient length (average 0.45 - 0.60 sec), to perm it application of full contracti le capabili ty, even in fatigue. With lower weights these times would be much shorter, which mightcause different courses of the maximal velocities. It is concluded that superimposed ES of MTB has a positive effect on maximal explosive vertical pushes in fatigue. There is no evidence of its influence on the first half of the push. The main result is a greater maximal velocity of the weight, as the superimposed ES of MTB was introduced. With superimposed ES, t he force level during the second part of the push was greater and took a longer tirne course. The rea- son for these improvements may lie in a greater ac- t ivation of MTB through superimposed ES. Since the muscles' activation is one of the factors that may be effectively trained in relatively short period of train- ing (15), this may fi nd its practical application in sports train ing, especially, when such artificial add i- t ional muscle's activation is employed asa diagnos- tical tool for the muscle's activation assessment. References 1. Bergstri:im M., Hultman E. Energy cost and fatigue during in- termittent electrical stimulation of human skeleta! muscle. J. Appl. Physiol. 1988; 65 :1500-1505 2. Bigland-Ritchie B. , Jones D.A., Hoskins G. P., Edwards RHT. Central and peripheral fatigue in maximum voluntary contrac- 12 Vojko Strojnik, Dieter Strass EFFECTS OF SUPERIMPOSED SUBMAXIMAL ELECTRICAL STIMULA TION ON EXPLOSIVE MOVEMENTS DURING FATIGUE tion of human quadriceps rnuscle. Ciin. Sci. Mol. Med. 1978; 54:604-614 3. Ouchateau J., Hainaut K. Electrical and rnechanical failures during sustained and interrnittent contractions in hurnans. J. Appl. Physiol. 1985; 58:942-947 4. Edwards R.H.T. Human muscle function and fatigue. In: Porter R., Whelan J. eds. Human muscle fatigue: Physiological mechanisms, Ciba foundations symposium No. 82. London: Pitman medical, 1981: 1-18 5. Gibson H., Edwards R.H.T. Muscle exercise and fatigue. Sports medicine 1985; 2 :120-132 6. Gydikov A., Kosarov O. Some features of different motor units in human biceps brachii. Pflueg. Arch. 1974; 347:75-88 7. Haberkorn-Butendeich E. Oynamometrische und elek- tromyographische Untersuchungen am musculus triceps brachii. In: Jahrbuch DSH Koln. Koln: Kolner Beitraege, 1973: 87-108 8. Henneman E., Samjen G., Carpenter O.O. Functional sig- nificance of celi size in spinal motoneurones. J. Neurophysiol. 1965a; 28:560-580 9. Henneman E., Sarnjen G., Carpenter O.O. Excitabi lity and inhibitabi lityof motoneurones of differentsizes. J. Neurophysiol. 1965b; 28:599-620 1 O. Hill A.V. The heat of shortening and dynamic constants of muscle. Proc. Roy. Soc. Buli. 1938; 126: 136-195 11. Hochmuth G., Marhold O. Further development of biome- chanical principles. In: Asmussen E., Joergensen K. eds. Biomechanics VI-B. Baltimore: University Park Press, 1978: 93- 106 12. ·Hof A.L., Geelen B.A., van den BergJ.W. Calf muscle mo- ment, work and efficiency in level walking: role of series elastic- ity. J. Biomech. 1983; 16:523-537 13. Muller K-J . Statische und dynamische Muskelkraft: eine empirische Grundlagenuntersuchung. Frankfurt am Main: Verlag Harri Oeutsch 1987 14. Romano C., Schieppati M. Reflex excitability of human soleus motoneurons during voluntary shortening or lengthen- ing contractions. J. Physiol. 1987; 390:271-284 15. Sale O.C. Neural responses to strength training. Med. Sci. Sports Exerc. 1988; 20: 134-1 42 16. Schmidtbleicher O. Maximalkraft und Bewegungsschnelligkeit. Bad H omburg: Limpert, 1980 17. Strass O. Effects of fatigue on isometric force tirne charac- teristics in human muscle, In: Jonsson B. eds. Biomechanics X- A. Champagne: Human Kinetics Publish , 1987 18. Strass O., Strojnik V. Oie Wirkung von Elektrostimulation und Willkorinnervation auf das maximale Kraftverhalten. Oeutsch. Z. Sportmed. 1991 ; 42:574-580 19. Tihanyi J. Oie physiologischen und mechanischen Grundlagen der Kraftventwicklung. Leistungssport 1987; 17:38- 44 Acknowledgements This research was supported by the Slovene M inistry of Science and Technology. Authors would like to thank to the students of the Institute of Sport and Sport Sciences in Freiburg for partici- pating in the study.