37 Branko [kof, Vojko Strojnik (1999). Local – central fatigue  after continuous and interval running.   KinSI, 5(1–2) : 37–44
LOCAL – CENTRAL FATIGUE AFTER
CONTINUOUS AND INTERVAL RUNNING
LOKALNO CENTRALNA UTRUJENOST PO
NEPREKINJENEM IN INTERVALNEM TEKU
Branko [kof
Vojko Strojnik
Abstract
The research objective was to establish the influen-
ce of central fatigue on the efficiency of muscular
function after two different cyclic running loads.
Seven well-trained runners carried out two running
loads, differing in method and intensity: an intensi-
ve interval training 5 x 300 m (INT) and a continu-
ous 6 km run (CON) at sub-maximal speed. The sub-
jects carried out the test protocol for establishing the
influence of central fatigue in the state of resting and
after running. The index of central fatigue (In CF) was
defined as the difference between the index of de-
crease in torque of voluntary muscular contraction
(In T
MVC
) and the index of decrease of torque of vo-
luntary muscular contraction with additional electri-
cal stimulation (In T
MVC + ES
).
After INT T
MVC +ES
decreased by 6.6 ± 3.9 % (P <
0.05), after CON by 8.3 ± 10.1 % (P < 0.05), while
T
MVC
decreased much less. The decrease in muscu-
lar contractile ability was the consequence of perip-
heral fatigue for both running loads. In CF increased
after both loads (after INT by 52 ± 78 % - P < 0.05,
after CON by 48 ± 83 % - P < 0.05) but remained
less than 0 after both loads. This research showed
that central fatigue can be an important cause of de-
crease in the efficiency of muscular function also for
well-trained individuals.
Keywords: central fatigue, interval runs, continuous
tempo running, electrical stimulation
Izvle~ek
Cilj raziskave je bil ugotoviti vpliv centralne utrujeno-
sti na u~inkovitost mi{i~ne funkcije po dveh razli~-
nih cikli~nih dinami~nih obremenitvah. 
Sedem dobro treniranih teka~ev je opravilo dve po
metodi in intenzivnosti razli~ni teka{ki obremenitvi:
intenzivni intervalni trening 5 x 300m (INT) in ne-
prekinjen 6km dolg tek (CON) v submaksimalni hi-
trosti. V mirovanju in po teku so preiskovanci opra-
vili testni protokol ugotavljanja vpliva centralne utru-
jenosti. Indeks centralne utrujenosti (In CF) smo de-
finirali kot razliko med indeksom upadanja navora
zavestne mi{i~ne kontrakcije (In T
MVC
) in indeksom
upadanja navora zavestne mi{i~ne kontrakcije ob
dodatni elektri~ni stimulaciji (In T
MVC +ES
). 
Po INT se je navor T
MVC +ES
zni`al za 6,6 ± 3,9 % (P
0,05), po CON pa za 8,3 ± 10,1 % (P 0,05), med-
tem je bil padec navora T
MVC
izrazito ni`ji. Upad mi-
{i~ne kontraktilne sposobnosti je bil po obeh teka{-
kih obremenitvah posledica periferne utrujenosti. In
CF se je po obeh obremenitvah pove~al ( po INT za
52 ± 78 % (P  0,05), po CON pa za 48 ± 83 %
(P < 0,05), vendar je po obeh obremenitvah ostal
manj{i od 0. Raziskava je pokazala, da je tudi pri do-
bro treniranih posameznikih lahko centralna utruje-
nost pomemben vzrok za padec mi{i~ne kontraktil-
ne u~inkovitosti.
Klju~ne besede: centralna utrujenost, intervalni teki,
neprekinjen tempo tek, elektri~na stimulacija
Received: 22. 02. 1999 – Accepted: 14. 12. 1999
University of Ljubljana - Faculty of Sport, Gortanova 22, SI-1000 Ljub-
ljana, Slovenia
Tel: +386 61 140-10-77
Fax: +386 61 448-148
E-mail: branko.skof@sp.uni-lj.si

38 Branko [kof, Vojko Strojnik (1999). Local – central fatigue  after continuous and interval running.   KinSI, 5(1–2) : 37–44
INTRODUCTION
The processes of fatigue development are the result of
a reduced efficiency of an individual link/individual
links in the chain of the regulation of muscle function -
the activation - contraction chain (Gibson and Ed-
wards, 1985; Mc Comas, 1996). Thus, the decline in
muscle function can be the consequence of peripheral
fatigue which denotes disturbances in the various mec-
hanisms from the neuromuscular junction up to the ac-
tin-myosin bond and central fatigue, which is primarily
the consequence of the reduced effect of cortical mo-
tor centres and subcortical areas in the central nervous
system (Gandevia, Allen and Mc Kenzie, 1995; Gibson
and Edwards, 1985; Bigland-Ritchie, Jones, Hosking
and Edwards, 1978). Gandevia, Allen and Mc Kenzie
(1995) defined central fatigue as the difference bet-
ween a decrease in voluntary and in electrically stimu-
lated contractile muscle force. Gibson and Edwards
(1985), however, define the presence of central fatigue
as a condition in which the force of voluntary contrac-
tion is lower than the force of contraction obtained
with additional electrical stimulation.
The mechanisms of central fatigue are numerous and
various. The common consequence of disturbances in
individual mechanisms of central fatigue is the reduc-
tion in output (central drive) of the -motor neuron (Bi-
gland-Ritchie, Furbush and Woods, 1986; Gandevia,
Allen and Mc Kenzie, 1995; Mc Comas, 1992). This
means disturbances in the recruitment and frequency
modulation of motor units. These disturbances can be
a consequence of lower excitation of the CNS (lower
output of the motor cortex) (Gandevia, Allen and Mc
Kenzie, 1995) as a result of afferent effects from the pe-
riphery or mental processes. The reduced efficiency of
the -motor neurone can also be the consequence of di-
sturbances in the transmission of electrical impulses
from the motor cortical centres over subcortical cen-
tres and spinal cord into the muscle (Astrand and Ro-
dahl, 1986; Gibson and Edwards, 1985; Guyton,
1980).
From the researches dealing with the role that the mec-
hanisms of central fatigue play in the decrease in musc-
le contractile function, above all during the voluntary or
electrically stimulated isometric loads, it can be conc-
luded that the involvement and level of the influence of
the mechanisms of central fatigue in the decrease in
muscle function depends on the type of load, its dura-
tion and intensity (Bigland-Ritchie, Jones, Hosking and
Edwards, 1978; Gandevia and Mc Kenzie, 1987; Lin-
namo, Hakkinen and Komi, 1988; Newham, Mc
Carthy and Turner, 1991); the muscle involved (Bi-
gland-Ritchie, Furbush and Woods, 1986; Gandevia,
Allen and Mc Kenzie, 1995; Mc Kenzie, Bigland-Ritc-
hie, Gorman and Gandevia, 1992), and the
psychophysical abilities of the tested subjects (the trai-
ning status attained by an individual athlete, gender).
(Bigland-Ritchie, Jones, Hosking and Edwards, 1978;
Bigland-Ritchie, Furbush and Woods, 1986; Hakkinen,
1994; Hakkinen, 1995).
Cyclic loads of long duration differ from short-term iso-
metric and dynamic loads in movement structure, du-
ration and intensity. In long-term cyclic loads it is ne-
cessary to maintain the activity of the CNS at a certain
submaximal level also in the state of destroyed internal
homeostasis. To improve endurance abilities by suitab-
le training means to reduce fatigue - it means to exert
influence on all possible forms of fatigue development,
thus also on the reduction of the influence of central
fatigue (Bigland-Ritchie, Furbush and Woods, 1986).
In endurance sports those athletes have an advantage
who are able to maintain a satisfactory level of the ac-
tivity of the CNS also in extremely demanding physio-
logical states of the organism. A higher tolerance to pain
further contributes to the said advantage (Shephard
and Astrand, 1995).
The objective of the research has been to establish if -
and to what extent - is fatigue after continuous dynamic
loading also the consequence of a lowered efficiency of
the neural drive – central fatigue. We were interested in
whether these loads exert influence on the level of ac-
tivation and the ability of maintaining the activation le-
vel; at the same time we were also interested in how
the signs of central fatigue show in individual athletes
who are well trained in endurance.
METHODS
Sample of subjects. The sample of subjects consisted
of 7 well-trained middle and long distance runners.
The average result of the test subjects in running over
1500m was 3 minutes and 54.5 s (from 3:43 to
4:02). Their average age was 25.3 years ± 4.1 years
(mean ± SD), the body weight 62.5 kg ± 4.1 kg, and
body height 176 cm ± 3.6 cm.
The study has been approved by the National Com-
mittee for Medical Ethics.
Experimental procedure. The subjects participated
in trial measurements performed in a laboratory,
over the period of one week. The aim of the trial
measurements was that the subjects become used
to the measurement procedures in which percuate
electrical stimulation is used. These trial measure-
ments were followed by the tests for the determina-
tion of the intensity - speed of running in both expe-
rimental running tasks. The experimental protocol
comprised of two experimental tasks - different long-
duration dynamic (cyclic): (1) interval runs 5 x 300 m
(INT) at a sub-maximal speed (10% lower than in the

39 Branko [kof, Vojko Strojnik (1999). Local – central fatigue  after continuous and interval running.   KinSI, 5(1–2) : 37–44
400-m-test ) with a recovery period of one minute
between individual runs and (2) continuous running
at a steady pace over 6 km (CON) at the speed of
anaerobic threshold (criterion V
OBLA
).
Both running loads were performed on an athletic
track in the time before competition season. Before
each test loading, the test subjects performed a
warm-up: 10 minutes of easy running and stretc-
hing.
Determination of the intensity of continuous running.
The test was carried out on a treadmill. It consisted of
6 to 8 runs lasting 5 minutes each. The speed of an
individual run was steady and constant, and it in-
creased from one run to the next by 0.2 m . s
-1
. Bet-
ween individual runs there was a break of up to 45
seconds for taking blood samples. Using the method
devised by Beaver, Wasserman and Whipp (1985),
the running speed at the anaerobic threshold (OBLA
criterion) was calculated on the basis of the lactate
kinetics.
Determination of the intensity of interval runs. The
speed of interval runs was determined on the basis of
a test run over 400 m at the maximal possible speed.
The test on a treadmill and the test run over 400 m
were carried out in two consecutive days. 
Between the tests for the determination of the speed
of running and the first experimental task, namely
continuous running over 6 km, four or five days elap-
sed, and after a subsequent three days, the test sub-
jects also performed interval training. The test sub-
jects were asked not to carry out intensive training
loads at least two days before the experimental task.
To establish the initial state (after the warm-up), the
tests were conducted in the following order: taking a
blood sample, recording the heart rate and maximal
voluntary isometric extension at the knee with an ad-
ditional electrical stimulation of the quadriceps fe-
moris muscle. The same protocol was used after the
completed running load. The last measure – measu-
rement of the maximal voluntary isometric exten-
sion at the knee with an additional electrical stimu-
lation was finished 4 minutes after the completed
running load.
Electrical stimulation. A custom-made computer-
controlled four-channel electrical stimulator was
used. During measurements with electrical stimula-
tion and maximal voluntary isometric extension at
the knee, the test subjects were in a lying position fi-
xed at the pelvis and over the distal part of the thigh
to a specially adapted bench, so that the trunk and
the thigh of the fixed leg could not be moved. The
distal part of the shank was fixed to a bar connected
to a force transducer. A strain-gauge transducer was
used with linear properties inside 0 – 5000 N, with
hysteresis less than 1%. Stiff lower leg – bar connec-
tion was secured with an intermediate plate (as used
by football players to secure tibia and fibula) and a
bandage that prevented any movement between
the bar and lower leg. The angle at the knee of the fi-
xed leg was 45°.
Direct electrical stimulation of the muscle was used.
Self-adhering neurostimulation electrodes (6x8 cm;
Axelgaard, USA) were placed over the vastus latera-
lis and vastus medialis. Distal electrodes were pla-
ced over the distal part of the muscle belly, and pro-
ximal electrodes were placed over the middle part
of the muscle belly. Electrodes remained fixed for the
whole time of the experimental procedure by the
addition of a medical net.
Data was sampled at 1kHz using a 12 bit AD conver-
ter (Burr-Brown, ZDA) and stored in a computer.
Measurement of the level of activation (AL ) and dyna-
mic changes the torque of voluntary and electrical sti-
mulated contractions. During maximal isometric
contraction of the quadriceps femoris muscle in the
duration of 25 seconds we additionally stimulated
the vastus lateralis muscle and the vastus medialis
muscle with a short 0.8 s long train of electrical im-
pulses with a frequency of 100 Hz (Figure 1). The first
impulse was triggered after three seconds of volun-
tary concentric muscle contraction and the second
at 24
th
second isometric contraction. The strength
of electrical impulses was determined separately for
each test subject with respect to his level of ability of
tolerating electrical stimulation and was constant du-
ring the entire time of measurements. The amplitu-
de was sufficiently large to completely activate the
quadriceps femoris muscle (Strojnik, 1995).
Fig. 1. Torque in the knee during a 25 s maximal isometric con-
traction with additional electrical stimulation.

40 Branko [kof, Vojko Strojnik (1999). Local – central fatigue  after continuous and interval running.   KinSI, 5(1–2) : 37–44
From the analysis of the torque at the knee (Figure 1)
the following bio-mechanical parameters were se-
lected and calculated:
1) Magnitude of peak torque at the knee in the 3
rd
second (T
MVC1
) and in the 24
th
second (T
MVC2
) du-
ring 25-second maximal voluntary muscle con-
traction.
2) The index of the decrease of the magnitude of the
maximal torque at the knee between the 3
rd
and
24
th
second of maximal voluntary muscle contrac-
tion (equation 1) is
In T
MVC
= (T
MVC1
- T
MVC2
)/ T
MVC1
100 equation 1
3) Magnitude of torque at the knee during maximal
voluntary muscle contraction with additional elec-
trical stimulation after the 3
rd
second (T
MVC1 + ES1
),
and in the 24
th
second (T
MVC2 + ES2
).
4) The index of the decrease of the magnitude of the
maximal torque at the knee for an electrically sti-
mulated contraction between the 3
rd
and 24
th
se-
cond (equation 2) is
In T
MVC + ES
= (T
MVC1 + ES1
- T
MVC2 + ES2
) /
(
TMVC1 + ES1
) · 100 equation 2
5) The level of muscle activation (proportion of T
MVC
relative to the total muscle force capacity T
MVC
+ES
) in the 3
rd
(AL1) and 24
th
second (AL2) of the
test protocol (equation 3 and equation 4) is
AL 1 = T
MVC1
/T
MVC1 + ES1
· 100 equation 3
AL 2 = T
MVC2
/T
MVC2 + ES2
· 100 equation 4
On the basics of above parameters we defined the
index of central fatigue as:
In CF = In T
MVC
– In 
TMVC+ES
equation 5
Central fatigue was defined for the cases when In
T
MVC
> In T
MVC+ES
or when In CF > 0. This would
occur in the case when the decrease in the torque of
voluntary muscle contraction was larger than the de-
crease in the torque of electrically stimulated musc-
le contraction (D1 < D2 or AL1 > AL2) (Figure 2).
Heart rate. The heart rate frequency was measured
with heart rate meters of the type Polar PE 3000
(Oulu, Finland).
Concentration of lactate in blood (LA). The concen-
tration of lactate in blood was measured with a Kon-
tron 640 lactate analyser (Vienna, Austria). A sample
of 20 ìl of blood was taken from the hyperaemic ear-
lobe before (after warm-up) the running load and 3
minutes after completed running loads. The accu-
racy of the measurement of lactate concentration in
fresh blood was ± 0.2 mmol . l
-1
.
Statistical methods. For the calculation of statistical
significance of the differences in the individual pa-
rameter before and after the running load, the t-test
for dependent samples was used. For the calculation
of statistical significance of the differences between
individual test subjects as regards the presence of
central fatigue, a one-way ANOVA - analysis of va-
riance was used. The statistical significance was ac-
cepted with an alpha error of 5% in two-tailed te-
sting. To calculate the correlation between the indi-
vidual parameters, the Pearson correlation coeffi-
cient was used.
RESULTS
Dynamics of the heart rate frequency and the kine-
tics of lactate in blood before and during interval runs
and continuous running. The dynamics of the heart
rate frequency and the kinetics of lactate in blood in
the state of resting, during and after continuous run,
and during and after interval runs is shown in Figure
3a,b.
The speed attained by the test subjects during run-
ning over 6 km on an athletic track amounted to
4.96 ± 0.29 m . s
-1
. The concentration of lactate in
blood at the end of the run was 5.8 ± 1.8 mmol. 1
-1
and differed statistically significantly from the value
before the loading (P < 0.001). The average value
of HR was 192 ± 8 beats . min-1. The average speed
of five 300-m runs with recovery intervals of one mi-
nute in-between was 6.8 ± 0.4 m . s
-1
. The concen-
tration of lactate in blood immediately after the com-
Fig. 2. Schematic representation of the model defining the pre-
sence of central fatigue. Legend: D = differences between
TMVC±ES and TMVC (D1 - measures at 3
rd
second and D2 -
measures at 24
th
second of 25-second isometric contraction; AL
= level of muscular activation (AL1 - measures at 3
rd
second and
AL2 - measures at 24
th
second of 25-second isometric contrac-
tion 

41 Branko [kof, Vojko Strojnik (1999). Local – central fatigue  after continuous and interval running.   KinSI, 5(1–2) : 37–44
pleted interval runs was 11.9 ± 2.3 mmol. 1
-1
, whi-
le in three minutes after the completed loading it in-
creased to 12.9 ± 2.7 mmol .1
-1
. The heart rate af-
ter interval runs was 189 ± 11 beats . min
-1
. The dif-
ference (6.1 mmol.1
-1
) in the concentration of lacta-
te in blood after different running loads is statistically
significant (P < 0.001). The differences in the value
of the heart rate after completed running loads do
not, however, reach the level of statistical significan-
ce (P = 0.102).
The influence of running loads on the decrease of vo-
luntary and electrically stimulated muscle contrac-
tion. The changes in the torque T
MVC
and T
MVC + ES
during a voluntary isometric contraction in the du-
ration of 25 seconds in the state of resting and after
various running loads are shown in Table 1.
In all measurements, except in resting - before inter-
val runs, the torque T
MVC
decreased during the 25-
second isometric contraction. The changes in the
torque T
MVC
attained the level of statistical signifi-
cance in no measurement. In contrast to the dyna-
mics of the torque T
MVC
, the decrease in the torque
T
MVC + ES
during the isometric contraction lasting 25
seconds was more pronounced. The decrease in the
torque by 6,6 ± 3,9 after INT , 7.7 ± 6.3 % before,
and by 8.3 ± 10.1 % after the continuous load were
also statistically significant (P < 0.05). In all measure-
ments, the dynamics of the decrease of the torque
T
MVC + ES
was more pronounced than the decrease
in the torque of voluntary muscle contraction.
The differences between the influence of the vari-
ous running loads on the level of decrease T
MVC
in
T
MVC+ES
during of 25 second isometric contraction
were very small and statistically non-significant.
The influence of running loads on the level of muscle
activation. The influence of INT and CON on the le-
vel of muscle activation AL1 and AL2 is shown in
Table 1. 
Increase of AL1 after both running loads was very si-
milar and close to statistical significance. After INT
AL1 increased by 5 ± 12% (P = 0.067) and after
CON by 4.6 ± 13.5% (P = 0.087). The level of acti-
vation AL2 in all measurements was higher than AL1.
The increase in the level of muscle activation AL2
(compared with AL1) was statistically significant (P
< 0.05) in both measurements before load, while
after the runs the rise of muscle activation during iso-
metric contraction was lower and statistically non-
significant.
The increase in the activation level of AL1 after both
running loads and higher values of AL2 in compari-
son with AL1 are the consequence of a more rapid
Fig. 3. Dynamics of the heart rate frequency and the kinetics of
lactate in blood in continuous run (left) and in interval runs (right).
Values are means ± SD
INTERVAL RUNS (INT)
T
MVC1
(Nm) T
MVC2
(Nm) InT
MVC
(%) T
MVC+ES1
(Nm) T
MVC+ES2
(Nm) InT
MVC+ES
(%) AL1(%) AL2(%) InCF
Before 132 ± 27 137 ± 31 -4 ± 6.9 186 ± 36 175 ± 43 5.6 ± 4.1 70.9 ± 11 78.2 ± 12P -9.6
After 131 ± 33 129 ± 29 1.8 ± 3.1 176 ± 32 164 ± 32± 6.6 ± 3.9* 74.4 ± 7 78.1 ± 9 -4.4
CONTINUOUS RUN (CON)
T
MVC1
(Nm) T
MVC2
(Nm) InT
MVC
(%) T
MVC+ES1
(Nm) T
MVC+ES2
(Nm) InT
MVC+ES
(%) AL1(%) AL2(%) In CF
Before 134 ± 31 132 ± 23 1.3 ± 2.1 187 ± 29 174 ± 32± 7.7 ± 6.3* 71.7 ± 9 75.9 ± 13 
P
-6.4
After 135 ± 26 128 ± 27 4.8 ± 2.9 180 ± 33 165 ± 32± 8.3 ± 10.1* 75 ±  6 77.6 ± 9 -3.5
TABLE 1: The influence of interval runs (INT) and continuous run (CON) on changes of torque of voluntary
(T
MVC
) and electrically stimulated muscle contraction (T
MVC+ES
), on level of muscle activation (AL) and in-
dex of central fatigue (In CF).
Values are means  SD; n = 7 subjects; ± = significant differences ( P 0,05) between T
MVC±ES1
and T
MVC±ES2
; * = significant differences ( P 0,05) bet-
ween In T
MVC±ES1
and T
MVC±ES2
(relative changes); P = significant differences ( P 0,05) between AL1 and AL2.

42 Branko [kof, Vojko Strojnik (1999). Local – central fatigue  after continuous and interval running.   KinSI, 5(1–2) : 37–44
decrease of torque TMVC±ES in comparison with
the dynamics of T
MVC
decrease.
The influence of running loads on central fatigue. 
The influence of INT and CON on the index of cen-
tral fatigue is shown in figure 4 and Table 1. Before
the loads, the differences between the decrease in
In T
MVC
and In T
MVC + ES
were 9.6 % and 6.4 %, res-
pectively (P < 0.05), while after the runs they redu-
ced to 4.4 % and 3.5 %, respectively, and thus fell
below the threshold of statistical significance. After
the running loads, the drop in the torque of volun-
tary and electrically stimulated muscle contraction
increased; however, the differences did not attain
the level statistical significance. The index of central
fatigue was smaller than 0 in the state of resting and
after both loads. Compared with the values before
the loading, In CF increased after the loads; howe-
ver, the differences did not attain the level of statisti-
cal significance. The differences in the influence of
the role of central fatigue in neuromuscular fatigue
after the various running loads did not reach the
threshold of statistical significance as well.
Central fatigue in individual test subjects. The values
of the index of central fatigue for individual test sub-
jects and individual measurements are given in Tab-
le 2. Although the average In CF of the sample of test
subjects does not show the presence of central fati-
gue, it is possible to establish, by comparison of the
results of individual test subjects, statistically signifi-
cant (P < 0.05) differences in the role of central fati-
gue on the efficiency of their neuromuscular system.
In two subjects, central fatigue did not occur in any
measurement; in three subjects it occurred in one
measurement; in two subjects it was possible to
speak about the influence of central fatigue in two
measurements. A pronounced presence of central
fatigue after both running loads could be established
in one test subject; in two subjects only after a pro-
longed continuous loading.
DISCUSSION
The influence of running loads on the role of central
fatigue on the efficiency of neuromuscular system.
On the basis of comparisons between the dynamics
of the decrease in voluntary and electrically stimu-
lated contraction and the dynamics of the changing
of muscle activation during an isometric effort lasting
25 seconds it was possible to establish that the force
during electrical stimulation was falling faster after
running loads than the voluntary force and that du-
ring maximal isometric contraction of 25 seconds,
the level of muscle activation increased. On the ba-
sis of these results it is possible to conclude with cer-
tainty that the decline in muscle function after both
running loads was the consequence of peripheral fa-
tigue.
Since the recuperation of the muscle’s contractile
abilities (twitch torque, MVC, torque of muscular
contraction at high frequency stimulation) after in-
tensive loads is quick (the twitch torque achieves the
state before the intensive cyclic loading already after
6-7 minutes) ([kof, 1993; Vollestad, Sejersted and
Saugen, 1997), is should be taken into account that
the decrease in torque of voluntary muscular con-
traction, as well as the decrease of T
MVC + ES
during
the 25 s isometric contraction immediately after the
running loads, would be greater than that measured
in this study (in the 4
th
minute after the running). It is
also possible to conclude that the ratio between both
parameters would not change significantly in that
case.
The values of decrease of T
MVC
in endurance-trai-
ned test subjects in our study point to a different res-
ponse than in untrained test subjects of various ages
Fig. 4. The influence of interval (INT) and continuous (CON) loa-
ding on the relative change (means ± SD) of the import of central
fatigue on the muscular function.
MEASUREMENT In CF In CF In CF In CF AVERAGE P
SUBJECT Before After Before After In CF
CON CON INT INT
1. -12.5 11.59 -9 -7.54 -5.27
2. -6.44 4.36 7.22 -9.43 -1.00
3. 3.22 -10.1 -7.5 -14.56 -7.52
4. -4.08 8.34 -2.53 14.79 4.13 0.042
5. 1.08 -9.9 -15.31 -8.78 -9.53
6. -8.5 -14.12 -17.11 -6.73 -11.65
7. -17.52 -12.09 -17.41 -0.57 -11.89
Table 2. Values of the index of central fatigue (In CF) for
individual test subjects.

43 Branko [kof, Vojko Strojnik (1999). Local – central fatigue  after continuous and interval running.   KinSI, 5(1–2) : 37–44
in the studies (Bigland-Ritchie, Jones, Hosking and
Edwards, 1978; Bigland-Ritchie, Furbush and
Woods, 1986; Gandevia, Allen and Mc Kenzie,
1995; Mc Kenzie, Bigland-Ritchie, Gorman and
Gandevia, 1992).
The average decrease in T
MVC
in our research
amounted to 1.7 %, and in untrained subjects at the
same duration it was 8 to 15 %. An even more essen-
tial difference between the results of our research
and the mentioned ones shows the fact that the de-
gree of decreasing of TMVC in trained subjects was
by 6 % (in individual subjects even up to 15 %) lower
than the degree of decreasing of electrically stimu-
lated contraction. In the research by Bigland – Ritc-
hie, Furbush and Woods (1986), the fall in the force
of voluntary and electrically stimulated contraction
of the quadriceps muscle was the same in untrained
subjects.
It is possible to conclude that athletes trained in en-
durance are able to activate more adequately the
CNS in the state of fatigue, thereby at least partly
compensating the influences of the mechanisms of
peripheral muscle fatigue. This hypothesis is confir-
med (even if not statistically significantly) by the in-
crease in muscle activation after both running loads. 
A relatively good maintenance of the level of volun-
tary muscle contraction (high endurance, which
shows through the low decrease in T
MVC
during 25-
second maximal isometric contraction) can, howe-
ver, be explained with the »muscle wisdom« (Enoka,
1991) – the mechanism of regulation of the fre-
quency of triggering - excitation of motor units (Bi-
gland-Ritchie, Jones, Hosking and Edwards, 1978;
Jones, Bigland-Ritchie and Edwards, 1979). From
the results of the research it is possible to conclude
that test subjects well trained in endurance are able
to control the triggering of nervous impulses which
ensures better maintenance of sub-maximal muscle
contraction.
Although the decline in muscle function was also af-
ter running loads of a local nature - the consequen-
ce of peripheral fatigue, the increased dynamics of
drop of T
MVC
after running loads (decrease of T
MVC
is
bigger than T
MVC + ES
) can also be connected with
the increase in the influence of central fatigue.
Intensive loads which cause a pronounced increase
of acidosis and a pronounced decrease in peripheral
function of muscle contraction cause at the same
time negative influences on the central mechanisms
of the control of movement. A high acidosis, the
function III and IV of afferent paths weakens, which
can produce a decrease in the frequency in activa-
tion of motor units (Gandevia, Allen and Mc Kenzie,
1995; Mc Comas, 1992). At high-intensive loads,
the oxidation of branched amino acids BCAA in
muscles increases as an energy substitute for exhau-
sted glycogen reserves in prolonged high-intensity
loads due to which the quantity of tryptophane in
the CNS increases. This can reduce the activity of
thalamus and reticular formation (Newsholme,
Leech and Duester, 1994; Snyder, 1998). A prolon-
ged or intensive flow of nervous impulses can pro-
duce a decline in the efficiency of the transfer at
synaptic places – central fatigue due to the various
biochemical reasons (drop in pH, exhaustion of tran-
smitters) (Astrand and Rodahl, 1986; Guyton, 1980).
Due to the transgressed threshold of acidosis in both
running loads, it was not possible - despite a relati-
vely high difference in the speed of running between
the two running loads - to prove the differences in
the influence of two different cyclic loads on the le-
vel of muscle activation, the ability of preserving iso-
metric muscle contraction and the mechanisms of
fatigue.
Central fatigue in individual test subjects.
Although a highly selected sample of well-trained
runners was selected there was a significant differen-
ce between the subjects in the ability of maintaining
a high muscle efficiency during test measurement
causing fatigue before and after demanding running
loads. From the analysis of the results of individual
test subjects it is possible to establish a different inf-
luence of the mechanisms of central fatigue. The in-
crease in the influence of central mechanisms on the
fall of muscle function in our research could not be
simply attributed only to elevated acidosis since the
connection between the CF index and LA is low (r =
0.15 and 0.14). The differences in the threshold of
occurrence of central fatigue are affected by a who-
le cluster of mechanisms (some are described at the
beginning of this chapter), also of a psychological na-
ture, especially the level of the threshold of sustai-
ning pain and discomfort (Shephard and Astrand,
1995).
CONCLUSIONS
On the basis of the results of the research it has been
possible to conclude:
1) The decline in muscle contractile abilities after
running loads was a consequence of peripheral
fatigue.
2) The influence of central fatigue increased after the
running loads, but there were no differences bet-

44 Branko [kof, Vojko Strojnik (1999). Local – central fatigue  after continuous and interval running.   KinSI, 5(1–2) : 37–44
ween INT and CON in their influence on central fa-
tigue. 
3) The research has also shown that central fatigue
can be the cause of the decrease in the efficiency
of muscular function in a well-trained individual.
Sports training and psychological preparation of
competitors in endurance sports develop the abi-
lity of maintaining high activity of central mecha-
nisms also in the state of fatigue. Therefore the
checking of the efficiency or the diagnosing of this
kind of effects of training, and especially the iden-
tification of athletes with poorer performance (ef-
ficiency) of central mechanisms is appropriate
and useful.
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