36 KINESIOLOGIASLOVENICA 1995: 2 (1): 36-43 
INFLUENCE OF TWO DIFFERENT CYCLIC 
LOADS ON THE MUSCLE FATIGUE 
AND THE MUSCLE ENDURANCE ABILITY 
Branko Škof• 
UČINEK DVEH RAZLIČNIH CIKLIČNIH 
OBREMENITEV NA MIŠIČNO UTRUJENOST 
IN VZDRŽLJIVOST 
ABSTRACT 
On the basis of a selected sample of 8 middle and 
long distance runners and the chosen measurement 
method, we tried to determine the influence of a 6-
km long continuous run at the speed V08LA and in-
terval anaerobic lactate running loads of S x 300m 
in sub-maximal velocitywith one minute breaks, on 
the endurance ability of the neuromuscular system. 
The most significant conclusions are: 
1. The decrease in the endurance ability of the 
neuromuscular system is, after a continuous dura-
tion of aerobic load, greater than after a more inten-
sive anaerobic lactate interval loading. 
2. The effect of central mechanisms on the state of 
muscle fatigue increases after cyclic monostructural 
loading, but in well trained subjects the influence of 
central fatigue does not dom i nate u nder any cond i-
ti on. 
Key words: endurance, running, fatigue, neuromus-
cular system 
• Faculty of Sport, Ljubljana 
POVZETEK 
Na podlagi izbranega vzorca osmih kakovostnih 
tekačev na srednje in dolge proge in izbranega meril-
nega postopka smo ugotavljali vpliv 6km dolgega 
neprekinjenega teka pri hitrosti V08LA in intervalne 
anaerobno laktatne tekaške obremenitve S krat 
300m v submaksimalni hitrosti z minuto vmesnega 
odmora na vzdržljivostno sposobnost živčno­
mišičnega sistema. 
Najpomembnejša zaključka sta: 
1. Zmanjšanje vzdržljivostne sposobnosti živčno­
mišičnegasistema je po neprekinjeni dolgotrajni ae-
robni obremenitvi večje kot po intenzivnejši anae-
robno laktatni intervalni obremenitvi. 
2. Vpliv centralnih mehanizmov na mišično utru-
jenost se po ciklični monostrukturni obremenitvi 
poveča, toda pri dobro treniranih preiskovancih 
vpliv centralne utrujenosti v nobenem primeru ne 
prevladuje. 
Ključne besede: vzdržljivost, tek, utrujenost, živčno­
mišični sistem 
Branko Škof 
INFLUENCE OF1WO DIFFERENT CYCLIC LOADS ON THE MUSCLE FATIGUE ANO THE MUSCLE ENDURANCE AB!LITY 37 
1. INTRODUCTION 
The decrease in muscle function efficiency (de-
crease of different motoric abilities: strength, speed, 
endurance, etc.) during intensive sport activity is the 
result of biochemical changes in the central nervous 
system (central mechanisms of fatigue), biochemi-
cal and biop,hysical changes in efficiencyofotheror-
ganic systems (7,9, 12, 18). The condition of muscle 
fatigue is a complex biological phenomenon influ-
enced by various and numerous physiological and 
psychological factors. Different types of sport acti-
vities (regarding intensity, duration, technical com-
plexity, etc.) not only cause different degrees of mus-
cle efficiency (fatigue) decrease, but consideri ng da-
ta and finds of lab researches (2, 3, 4, 5, 9, 1 O, 11, 
15, 20), in which the principles of muscle fatigue 
were studied on the population (notselected) under 
non-typical sport loads, muscle fatigue, after various 
exertions, also has different origins (influence of cen-
tral and peri pheral mechanisms of fatigue is different 
under d ifferent loads). 
In this paper we want to establish the degree of the 
neuromuscular system endurance abilities drop 
caused by different cyclic monostructural (running) 
loads - a continuous 6-km long run at the speed of 
the anaerobic threshold (criterion VoBLA) and an in-
terval run of 5 times 300 m in sub-maximal speed 
with one minute breaks. We also wantto determine 
the effect of the neuromuscular activity of central 
mechanisms on the decrease in endurance abilities. 
Our pointof interest are also the possible differences 
between individual characteristics of muscle fatigue 
(degree and origin) aftertypical sport loads. 
II. METHODS 
Subject sample 
The subject sample comprised 8 well trained mid-
dle and long distance runners. Their average age was 
25.3 ± 4.1 years, bodyweight 62.5 ± 4.1 kgand 
body height 176 ± 3.6 cm. Ali are experienced ath-
letic runners, being in trainingfrom 5-15 years, in av-
erage 9 ± 3 years. 
Process and organisation of the experiment 
The experiment was executed in two phases. The 
initial phase of the experimental procedure was car-
ried out in a laboratory as preparation for further 
field measurements. 
The first task of the experiment was to acquaint the 
subjects with the measurement technology and 
methods, to come to know and get accustomed to 
the sensation of electrical excitation of the muscle as 
well as to determine the fundamental parameters of 
loads of current intensity stimulation impulses at the 
chosen frequency of 100 Hz. Determining the 
parameters of the current intensity of the high 
frequency electrical impulses trains was done by 
gradual increase of current intensity. Such intensity 
of electrical impulses of frequency 100 Hz was 
chosen that each individual subject could just tole-
rate. 
The second significant task in this phase of the ex-
periment was the establishing of the basic functional 
and motoric parametersof the runner's presentcon-
dition. The results of these measurements were used 
for an objective starting point in planning of indivi-
dual subject's suitable- relatively eq ual test loads. 
The second phase of the experiment presented the 
measurements on an athletic stadium. Each subject 
was measured by a series of experimental methods 
(functionally-biochemical and biomechanical) in the 
rest phase, after warm-up-preparation for loading 
- and 5 minutes after test loading on the athletic 
track (the functional and biochemical parameters 
were followed also duringthe test load itself and al-
so after its termination). Each subject had to perform 
two measurements; once with interval and once 
with continuous long lasting running test load. 
Between the two tests of the same subjects at least 3 
days passed. 
Tests, measurements and criteria for formation of 
individual parameters 
Determining the subject's running condition 
For the assessment of the present efficiency of run-
ners, two tests were used: 
a) 400-m run test: The test was performed on the 
athletic stadium on plastic tracks. The subjecttried to 
run the 400-m track as fast as possible. The run du-
ration was measured with accuracy of up to± 0,1 
second. 
b) testof repetitive S-minute runs with gradual speed 
increase. The testwas performed ona treadmill and 
consisted of 5-8 five minute runs. The speed of indi-
vidual runs was regular and constant, but gradually 
increased from run to run for 0,2 m/sec. With the 
help of the Beaver & collaborators (1,22) method , 
on the basi s of the lactate kinetics (LA) in the test of 
repetitive 5 minute runs, we can calculate the run-
ning speed which is defined by the lactate (LP) and 
the anaerobic (Onset of Blood Lactic Acid - OBLA) 
thresholds criterion. 
38 
Branko Škof 
INFLUENCE OFTWO DIFFERENT CYCLIC LOADS ON THE MUSCLE FATIGUE ANO THE MUSCLE ENDURANCE ABILITY 
For the experimental aerobic load, a continuous 
6 km long run, at the speed of the anaerobic thresh-
old (V 0 8LA) was selected, and measured for each in-
dividual subjecton a treadmill. 
Forthe test training of the lactate anaerobic load we 
selected repetitive 300-m runs. The load decided on 
was 5 x 300 min sub-maximal speed with short, one 
minute intervals. The tirne of repeated runs was 
longer for 1 O o/o than three quarters of the 400-m test 
run tirne. 
T300 m = (T 400 m / 4) x 3 + 1 O o/o (eq uation 1) 
Functional and biochemical parameters 
a) Heart rate 
The heart rate frequency was measured by pulsime-
ters PE 3000 (Pol ar Electro, Finland). The values were 
recorded every 1 5 seconds. 
b) Lactate concentration in the blood after the run-
ning load 
The concentration of lactate in the blood was mea-
sured by Konton 640 Lactate Analyser (Kanton, 
Austria) always directly after taking a blood sample 
from the earlobe. In interval loading the blood sam-
ple was taken after every 300-m run, whi le in the 6-
km long continuous run it was taken at 3600 m and 
atthe end of the run. 
Biochemical parameters of different muscle con-
tractions 
The efficiency of muscle contractions was measured 
by the torque in the knee joint with a suitable sup-
port. For measuring the torque with a constant han-
dle we used a compress-strain tensor and an alter-
nating voltage amplifier (both constructed by MES, 
Maribor) 
Electrical stimulation 
A two-channel currentstimulator (home made) was 
used. Ali impulses were bi-phasal and rectangular in 
shape. The followingexperimental procedure by use 
of electrical stimulation was performed: 
Procedure presentation: 
During maximal isometric contraction of the quadri-
ceps femuris muscle, which lasted for 25 seconds, 
the muscles vastus lateralis and vastus medialis were 
additionally stimulated by three short 0,8-second 
trains of electrical impulses of frequency 100 Hz. 
The first impulse was released in the 3rd second of 
the voluntary concentric muscle contraction, while 
the other two followed at intervals of 10.2 seconds. 
A contextual ly similar measurement procedure is 
known from the research of Biglad-Ritchie & colla-
borators, 1978 (2). 
Position of the subject during measurement 
During measurementthe subject lay on the back on 
a special bench in such a position that the shanks fell 
freely (Figure 1 ). The position of the pelvic girdle was 
additionally fixed with a special band. Prior to the 
start of the experiment the subject was only leaning 
the push-off leg on the support (knee angle was 45 
degrees) and on signal started to exert pressu re onto 
itwith all force, tryingto maintain the maximal force 
for 2 5 seconds. 
Figure 1: Position of subject during measurement 
The surface stimulation of the vastus lateralis and 
vastus medialis muscles was carried out by elec-
trodes of 5 x 8 cm in size. One electrode was at-
tached onto the motoric spot of the muscle, the 
other onto its distal part. Both electrodes were 
additionally fixed by a medica! net. The intensity of 
electrical impulses was determined for each subject 
separately according to his/her level of electrical 
stimulation tolerance ability and was constant all 
through the measurement. 
Analysis of the 25-second long voluntary isome-
tric contraction with intermediate periodical elec-
trical impulse trains 
Endurance abi I ity of the muscle contracti le system is 
described by two parameters: 
a) lndex of the decrease of voluntary contraction 
force Uz 
Uz = (FM3 - FM2) x FM2-1 x 100 (equation 2) 
The Uz index represents the dynamics of contrac-
tion force decrease of the muscle during the 25-sec-
Branko Škof 
INFLUENCE OFTWO OIFFERENT CYCLIC LOADS ON THE MUSCLE FATIGUE ANO THE MUSCLE ENDURANCE ABILITY 39 
A1 A2 ___ ____, __ _ 
\ ___ .,,.  
AJ 
M3 
Figure 2: Recording of the muscle force during the 25-second 
long maximum isometric contraction with an additiona/ period-
ica[ electrica/ stimulation (for symbo/s see text) 
ond maximal isometric contraction in the interval 
between the 12th and 25th second. 
b) Contraction force at the end of the 25-second 
isometric contraction (F M3): 
The average value of the force (FM2 and FM3) in the 
t irne interval of O.S seconds directly before the ad-
ditional stimulat ion of the muscle by a short electri-
cal impulse of 0.8 seconds and of frequency 100 Hz 
(A2 and A3), was selected (Figure 2). 
The influence of central mechanisms on muscle fa-
tigue is defined bythe difference between index Uz 
and index Us (equation 3), which represents the dy-
namics of the decrease of electrically stimulated 
muscle force during the 35-second period. 
Us= (FA3 -FA2) x F A2-1 x 100 (equation 3) 
lcu = Uz - Us (equation 4) 
In case of the drop of the central fatigue index bellow 
zero (lcu < O), in cases when the drop of voluntary 
muscle force is greater than the drop of the electri-
cally stimulated muscle force (in case of the presence 
of the decrease of muscle force - fatigue, the fatigue 
index has a negative fore-sign), ali points to the pre-
sence and predominant influence of central fatigue 
mechanisms on neuromuscular fatigue. 
Muscle activation index 
The muscle activation index contributes greatly to 
the elucidation of the problem of the muscle en-
durance ability as well as of the problem of assessing 
the presence of muscle fatigue central mechanisms. 
The index of muscle activation is expressed as the ra-
t io of the size of voluntary muscle force and the mus-
cle force stimulated by electrical impulse. 
(equation 5) 
Statistical methods 
For determiningstatistical significance among the re-
sults of individual parameters in various steps of 
measurement and among the results of parameters 
of different loads, the Student T-test was used. 
Ali results were relativised accordingto the parame-
ter value before loading-warm-up state, which we 
chose as criterial value. The value of the relativised 
parameter (Rp) was determined by the ratio of the 
difference in the values of the parameter at an indi-
vidual pointof measurement (Rx) and before loading 
(Rpred), and the value before loading (Rpred): 
Rp = (Rx - Rpred) x Rpred-1 x 100 (equation 6) 
III. RESULTS 
BIOCHEMICALAND FUNCTIONAL PARAMETERS 
BEFORE, DURING AND AFTER VARIOUS TYPES 
OF CYCLIC LOADS 
Continuous load 
The average speed of the 6-km run is 4.96 m/s, for 
the selected sample of subjects, which is in accor-
dance with the speed chosen on the basis of test 
loading. The average speed at the anaerobic thresh-
old by the V OBLA criterion was, for the selected sam-
ple of runners, 5.02 m/s. In spite of the fact that the 
subjects ran at the speed of the anaerobic threshold 
by the VasLA criterion, the concentration of lactate 
exceeded 4 mmol/1. After running 3600 m the aver-
age concentrat ion was between S and 5.5 mmol/I, 
at the end of the run it increased to 6.3 mmol/I 
(Figure 3). 
lactate ( mmol/1) 
14 
11 .2 
8.4 . 
5.6 
2.8 
pulse (b/min.) 
200 
160 
- - - - -. · · · 120 
80 
40 
o.._~-----,---....,....----.----.--'-o 
ready rest 3600m 6000m Srnin. po 
J -+- lactate - + - pulse J 
Figure 3: Lactate and heart rate kinetics before, during and after 
continuous running. 
Interval load 
The planned average running speed in this test load 
was 6.6 m/s, w hile the actual speed of interval runs 
was for 0.2 m/s faster. Lactate concentration in the 
blood increased linearly from run to run. Ateach fol-
lowing run the lactate increase was approximately 2 
mmol/1. In this way, at the end of the 5th run, the lac-
tate concentration reached the value of 11 , or 11 .5 
mmol/I (Figure 4). 
40 Branko Škof INFLUENCE DNO 0IFFERENT CYCUC LOADS ON THE MUSCLE FATIGUE ANO THE MUSCLE ENDURANCE ABIUTY 
)actate (mmol/1) pulse (b/min.) 
14 ..-----------------.-200 
. . ~ 
11.2 160 
8.4 120 
5.6 · • · · · · 80 
2.8 40 
1 +- lactate - + - pulse J 
Figure 4: Lactate and heart rate kinetics before, during and after 
interval load 
From theviewpointofthe intensityof the metabolic 
processes, the two experimental loads differ (al-
though the differences in lactate concentrations be-
fore and 5 minutes after different loading do not 
reach statistical significance P = 0.125), in regard to 
the exercise of the card iovascu lar system, i 11 ustrated 
by the heart rate, there were no differences between 
the interval lactate training and continuous running 
tempo (P = 0.807). The frequency of the heart rate 
in both types of exercise reached its maximal values 
(Figures 3,4). 
INFLUENCE OF THE CYCLIC MONOSTRUC-
TURAL EXERCISE ON THE ENOURANCE ABILITY 
OF MUSCULAR CONTRACTION 
The decrease dynamics of the muscle contraction 
force during the 25-second maximal isometric con-
traction Uz, after the 6 km long continuous run, is 
4. 7 % and is more distinct than after the interval run 
exercise, where the decrease in muscle endurance 
ability is only 2.7 % (Table 1 ). None of these diffe-
rences reach the threshold of statistical significance. 
Also the second parameter of the endurance ability, 
Fm3 (%) 
120 ,------------------
100 
80 
60 
40 
20 
o 
interval run (NS) continuous run 
I• before D after 
Figure 5: Effects of different types of loads on muscle force FM3 
force FM3 is, after continuous exercise running, for 
13.7 ± 12 % smaller than the force before exertion 
(the difference is statistically significant), while the 
interval load causes only 7.6 ± 21,4 % drop of the 
isometric force (Figure 5). 
The difference of 6.1 % between the influences of 
continuous and interval exercise on the voluntary 
muscle force FM3 is not statistically significant (P = 
0.239). 
AN ATTEMPT AT ASSESSING THE PRESENCE OF 
CENTRAL FATIGUE ANO THE EFFECT OF CYCLIC 
MONOSTRUCTURAL EXERCISE ON CENTRAL FA-
TIGUE MECHANISMS 
Dynamics of voluntary and electrically stimulat-
ed muscle force before loading 
The index of decrease in voluntary muscle force Uz, 
was in the phase before the interval workout load-
ing2,2 % (increase of force) and after the continuous 
run -4.1 % (Table 1 ). 
The dynamics of isometric muscle force in the inter-
val between two points of isometric contraction fol-
low-through, is very different from subject to sub-
ject. With some, this force decreases, with others it 
increases. It is therefore impossible to form any ge-
neral princi ples. 
More distinctive and much more consistent are the 
changes in forces of the complete muscle potential 
represented by the greatest volu ntary force together 
with the force caused by additional electrical im-
pulse of high frequency alongside the biggest volun-
tary contraction. With one exception, the stimula-
tion force decreased with all subjects. The index of 
the decrease in the complete contractile potential 
Us in a rested muscle was, before interval loading, 
-3.7 ± 4.3 % and -7.7 ± 4 % before continuous 
loading. From the analysis of results in table 1, it is 
possible to establ ish that the decrease in electrically 
Table 1: lndex of decrease in voluntary and electri-
cally stimulated muscle force Uz and Us (AS ± SO) 
and the difference of the state before and after dif-
ferent load types (AS ± SO) 
Load type before load afterload difference % p 
Parameter 
Uz(%) 2.2± 9.8 -2.7± 5.9 4 .9± 13.9 . 
Interval run 
Us(%) -3.7± 4.3 -6.5± 4.9 2.7± 5.9 NS 
Uz (%) -4 .1 ± 6.3 -4.7± 5.6 O.S± 5.5 NS 
Continuous run 
Us(%) -7.7± 4 -5.9± 2.5 1.9 ± 3.8 NS 
Branko Škof 
INFLUENCE OF TWO DIFFERENT CYCUC LOADS ON THE MUSCLE FATIGUE ANO THE MUSCLE ENDURANCE 41 
stimulated muscle force, duringthe period of 25 se-
conds, is greater in comparison to the dynamics of 
the decrease of voluntary isometric force. This 
means that the reasons for muscle fatigue are main-
ly of peripheral character. The analysis of individual 
subjects also shows that the signs of central fatigue 
do not predominate in one single case. Quite the 
opposite. The subjects, even when tired, were able 
to activate the muscle even more and in this way 
lower the activation deficit (increase of "muscle 
advantage") (Table 3). 
Dynamics of voluntary and electrically stimulat-
ed muscle force after loading 
The decrease in voluntary muscle force is, in the 
state after interval loading, 2.7 ± 5.9 %, after con-
tinuous loading4.6 ± 5.6 % (Table 1 ). 
From the analysis of the result dynamics of individu-
al subjects, we can establish that the trends of vo-
luntary as well as of stimulated muscle force, after 
termination of the running load, are more consistent 
compared to the results before exertion. With only 
one subject the voluntary isometric muscle force 
increased, whilewith alf others the voluntary as well 
as the stimulated force decreased as expected. The 
index of decrease in the stimulation force is, also 
after the running load, higher than the index of 
decrease of isometric muscle force, confirming that 
the basic reasons forthe decrease in contractile mus-
cle ability can be found mainly in peripheral mech-
anisms. But the results in Table 1 show that, after the 
end of the running load, the difference between 
indexes Uz and Us grewsmaller. Before loadingthis 
difference was 5.9, i.e. 3.6 %, after loading itwas 3.8 
(afterinterval runs) and 1.2 %aftercontinuousexer-
tion. The decrease in the differences between the 
degree of fatiguing of voluntary and of stimulated 
muscle contractions after loading signifies an 
increase in the influence of central fatigue mecha-
nisms. The index of central fatigue lcu decreased 
after both loads. But the values of (lcu) were, even 
after interval and continuous runs, distinctly greater 
than O, which means that the main reasons of the 
decrease in isometric muscle force are still mostly 
biochemical and biophysical changes in the muscle 
contractile system - therefore the peripheral muscle 
mechanisms (Table 2). 
Table 2: lndex of central fatigue (lcu) before and after loading 
and the difference in state before and after load of different type 
(AS ± SD) 
M easurment tirne. before load after load difference o/o p 
Load type 
1 nterval run 5.9± 6.4 3.8± 6.2 2.2± 11.8 NS 
Continuous run 3.6± 4.3 1.2± 4.8 1.4± S NS 
Difference 1.4±6.1 2.7 ± 8.4 
The confirmation of this assertion is also an increase 
in the index of muscle activation, the "muscle 
advantage" in the fatigue stale. (Table 3). 
IV. DISCUSSION 
The decrease in muscle endurance ability means an 
increased sensitivity of the neuromuscular system to-
wards the change of the homeostatic state - the 
worsening of biochemical and biophysical condi-
tions (fatigue) in individual organic systems (7, 14, 
18). The fatigue of the neuromuscu lar system is, af-
ter intensive sport burdening such as interval and 
continuous running, in our research, mainlythe re-
sult of the accumulation of metabolic products, in-
creased acidosis, also the exhaustion of the energy 
deposits, i.e. disturbance in the function ing of indi-
vidual energetic processes (7, 12, 13, 14, 17, 18, 19, 
21) lncreased acidosis (high leve! of H+ ion con-
centration) weakens the re-phosphorisation (pro-
cesses of glycolysis) and by this causes the obstruc-
tion of the restoration of ATP (14,19,21). The de-
crease in pH also causes, through the weaken ing of 
the Na+K+ATP-iasis functioning, a changed elec-
trolytic balance which provokes disturbances in 
circulation of action potential into the muscle cell 
(8, 16,21 ). lncreased concentration of H + ions, 
which develops during the formation of lactate, with 
glycogenolysis inhibits certain active parts of myosin 
and through this directly lowers the efficiency of 
muscle contraction (19, 21 ). 
In different intensity of run loading, the activity of 
various motoric units with different muscle tissues, 
differs. Due to this, the magnitude of biochemical 
and biophysical changes - disturbances, in indivi-
d ual m uscles varies. 
The resu lts of the research show that the decrease in 
the endurance ability of the muscle contractile sys-
tem, after long and continuous aerobic burden ing, is 
greater than after more intensive anaerobic lactate 
exertion. 
Table 3: lndex of muscle activation IMA (AS ± SD) and the d if-
ference in state before, after loadi ng and load of different type 
(AS ±SD) 
Loadtype before load after load difference p 
Parameter 
M 2/A2 68 8%± 21.8 68%± 23.7 1.1%± 4.4 NS 
Interval run 
M J/A3 71.9%± 21.3 71.8%± 26.2 0%± 7.7 NS 
Difference 3.2%± 4.1 3.8%± 4.6 
M2/A2 74.6%± 15.5 66.6%± 13.8 9.8%± 13.9 . 
Continuous run 
M3/A3 77.2%± 14.9 67.8%± 15.9 11.1 ± 12.1 . 
Difference 2.5o/o± 3.2 1.2%± 2.6 
42 Branko Škof INFLUENCE OF TWO DIFFERENT CYCUC LOAOS ON THE MUSCLE FATIGUE ANO THE MUSCLE ENOURANCE ABILITY 
O n the basis of the research resu lts and data from 
other studies (3 , 4, 5, 8, 15, 16), it appears that the 
continuous 6-km run and the sustainment of maxi-
mal isometric contraction over a longer period of 
ti me, cause fatigue of motoric units with a similar fre-
quency of triggering and those differing from the 
ones in more intensive interval runs. 
At the beginning, electrical impulses of h igh fre-
quency take care of maintaining the 25-second max-
i mal isometric contractions, at the end of the load, 
when the value of the muscle force FM3 and the 
decrease in the contraction force Uz have been 
measured, by lowering the frequency of the action 
potential, muscle force is sustained by electrical 
impulses of lower frequency. 
Jones and Bigland-Ritchie & collaborators, in thei r 
researches, found, with the aid of electromyogra-
phic technology (3, 4, 15, 16), that the maximal 
isometric contraction of the adductor pollicis mus-
cle is achieved by the nervous impulse with a fre-
quency of about 60 Hz. The frequency of nervous 
impulses rapidly diminishes afteronly a fewseconds 
of maximal contraction. Bigland-Ritchie (4, 5) dis-
covered that, during maximal isometric contraction, 
the frequency of motoric units' stretching decreased 
in less than one minute by 50 % (from 27 Hz to 14 
Hz). With the lowering of the excitation frequency, 
muscle force decreases as well, but not in complete 
interdependence. 
Maximal isometric forces are triggered by high fre-
quency nervous impulses. Lasting sustainment of 
maximal force on the same leve! is prevented by a 
relatively fast fall of the action potential (lower fre-
q uency) as well as by changes in the muscle cell 
(changes of the Ca2+ accumulation in the T-tubules 
and inter-tissue space of the muscle, changes of ion 
concentration in the extracellular fluid (12, 13, 14). 
Taking into account that in both types of running 
loads, the motoric units with low triggering frequen-
cy are probably more active, these motoric units, 
due to fatigue under special running loads, by the 
end of the 25-second isometric contraction, when 
the force FM3 and the index Uz are measured, con-
tribute a lesser part to the common contractile force. 
lf this hypothesis holds then the following question is 
in place: is the frequency of motoric units' stretch-
ing in both running loads and during the 25 second 
maximal isometric contraction really completely 
equal (about 20 Hz - low-frequency peripheral fa-
t igue)(20)? By use of available experimental tech-
nology, we could notassess these differences in our 
research. Regarding a greater decrease in voluntary 
muscle force after continuous lasti ng load, it is pos-
sible to conclude that, in continuous lasting run, as 
well as after the end of the 25 second maximal iso-
metric contraction, motoric units with similar fre-
quency of stretching, which is lower than the stretch-
ing of motoric units in faster interval runs, are pro-
bably activated. 
The improvement of the experimental method for 
assessing the endurance functions of the muscle 
contractile system is certainly in the extension of the 
follow-through period of isometric force dynamics 
and in searching for a possibility to follow the influ-
ences of fatigue of individual sport loads with atest 
exercise which is, regardingload method,similar or 
equal to the sportexercise. 
A more distinct decrease in voluntary muscle force 
after continuous lasting loading is also the result of 
an increased influence of central mechanisms. 
Muscle fatigue is a very complex state, which not on-
ly depends on biophysical, biochemical and other 
changes in the muscle itself, but also changes in oth-
er organic systems exert strong influence on this 
state, especially CNS (2, 9, 10, 12, 17). The decrease 
in muscle efficiency has, beside physiological and 
neurophysiological, also its psychological back-
ground (7, 14). By the mentioned experimental 
method we tried to assess the presence of central 
mechanisms (mechanisms of fatigue outside the 
muscle) and their influence on muscle fatigue after 
various running loads. The increase of central mech-
anisms influence upon muscle fatigue ensues from 
the physiological and psychological factors. The fun-
damental recognition of this paper is that the influ-
ence of central mechanisms on the degree of mus-
cle fatigue in fact increases after the runn ing load, 
but in the sample of selected runners it does not 
exceed the influence of peripheral local mecha-
nisms. 
Mechanisms of central fatigue are represented most-
ly by the decrease in activation abilities of motoric 
centres in the CNS (Central Nervous System) and the 
drop of efficiency of the corticospinal connect ion 
(conductivity speed of nervous impulses through the 
nerve tissue and across the synapses) (12). The con-
sequences of intensive or continuous running exer-
tion are reflected in the oversaturation of the central 
nervous system by very intensive flow of signals from 
the proprioreceptors and chemoreceptors from ac-
tive areas of the organism (12, 13, 14, 18, 19).Most 
probably also the high leve! of acidosis, that is trans-
ferred by the blood from the active muscle groups 
into the CNS, influences the decrease in the activa-
tion efficiency of the motoric centres of the CNS, as 
well as the speed of nervous impulses transfer (12, 
14, 19). Quite certainly the psychological barriers 
have a signif icant effect on the leve! of central fa-
J 
 
Branko Škof 
INFLUENCE OF1WO DIFFERENT CYCLIC LOADS ON THE MUSCLE FATIGUE ANO THE MUSCLE ENDURANCE ABILITY 43 
tigue. Ali these mechanisms presenta physiological 
and biochemical basis for certain psychological func-
tions and can cause motivation decrease. 
Although the differences in central mechanism's in-
fluence on muscle fatigue, after different running 
loads, do not reach the threshold of statistical signi-
ficance, they are stili interesting and worthy of com-
ment. The portion of central fatigue in the degree of 
muscle efficiency decrease is, after a continuous 
load (run), in regard to the value after interval runs, 
greater (Table 1,2). Also after lasting aerobic loading, 
the increase in muscle advantage at the end of the 
25-second isometric contraction, is smaller than 
after interval loading (Table 3). Although the acida-
tion of the organism after a 20 minute run, at the 
speed of Srn/s is, in comparison to the 45-second 
long run, at the speed of 6.8 m/s, with one minute 
break, quite lower, a lasting exertion and its 
monotony can cause a certain leve! of saturation 
("numbness" of the nervous system motoric centres), 
the decrease in the runner's motivation and by this a 
fall in muscle contractile ability. 
From the viewpoint of psychical demand, a lasting, 
continuous and intensive workout is more fatiguing 
than a repetition of shorter workouts even if these 
contain more intensive loads with interval breaks. In 
planning workout units, especially with young 
sportsmen/women, it is sensible to avoid frequent 
use of long, continuous and highly intensive loads. 
It is better to use interval form of workouts. 
The sustainment of 25-second long maximal iso-
metric contraction certainly demands, as every en-
durance activity , especially in enduring problems 
ensuing from the overcoming of physical exertion. 
Also from the analysis of the results of individual sub-
jects it is possible to conclude that the presence of 
central fatigue probably does not depend so much 
on the type of load, but more on the psychical abili-
ty of the subject (the ability to endure subjective 
problems and pain in overcoming physical exertion) 
and on his/her endeavour in performing test exer-
cises. 
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