34
Neuromuscular Control of the Human Leg Extensor Muscles in Jump Exercises Under Various Stretch-Load Conditions A. Golihofer, H. Kyröläinen / Institute for Sport Sciences University of Freiburg, D 7800 Freiburg, West Germany; Department of Biology of Physical Activity, Universtiy of Jyvaskylä, SF4100 JyvAskylä, Finland
Introduction A. Golihofer, H. Kyröläinen, Neuromuscular Control of the Human Leg Extensor Muscles in Jump Exercises Under Various Stretch-Load Conditions. mt J Sports Med, Vol 12,No l,pp34—4O, 1991. Accepted after revision: April 17, 1990
Ten active males performed reactive drop jumps from a height of 40 cm in six experimental conditions: jumps with additional loads of 100 N (BW+ 100 N) and 200 N (BW+ 200 N), an ordinary jump with body weight (BW) and three jumps in which the body weight was artificially reduced (BW— 172 N, BW—337 N and BW—495
N). The vertical ground reaction forces, the angular displacement in the knee and ankle joints as well as the surface electromyogram (EMGs) of the triceps surae muscles and tibialis ant, muscle were recorded.
When compared to the control condition (BW) in the jumps with extra load and in the jumps with reduced body weight, both the take-off velocity as well as the mean vertical ground reaction force were decreased during
the push-off phase. The integrated EMG before ground contact as well as the duration of the preactivation phase was significantly reduced as a function of the load condition. Upon the touchdown, the coactivation of the muscles acting around the ankle joint was greatest in the control jump. Through all experimental conditions, the mean activation amplitude remained rather constant both for the impact as well as for the push-off phase of the contact.
The control of movement in jump exercises has been analyzed extensively in humans (1, 17, 18) as well as in
animals (6, 20). In jump performances, the leg extensor muscles are activated before ground contact of the feet (8, 19).
During the first part of the contact, the muscles are actively stretched prior to the subsequent shortening phase. This kind of activation pattern is typical for so-called stretch-shortening cycle exercises (11). In stretch-shortening cycle (SSC) contractions, it could be demonstrated that a major part of the elastic energy can be stored during the impact phase of the ground contact in the tendomuscular system and utilized in the subsequent push-off phase either to improve performance capability in conditions with maximal effort (2, 11) or to increase the efficiency of the contraction quality in submaximal contraction conditions (2, 16). Both regulations can be achieved only if the time between impact and push-off phase is short (12).
In several studies, the neuromuscular control in reactive movements (5, 14) was investigated. It could be de-
monstrated that the electromyographic (EMG) activity as a centrally generated part prior to contact represents an essential prerequisite for a powerful output of the muscular work. The purpose of this study was to investigate the neuromuscular control of the leg extensor muscles with vary-
ing loads in drop jump exercises from identical heights. In order to change the stretch-load condition systematically and individually, various jump conditions were performed either with extra loads or with a specific system which allowed to unload the body weight artificially. The modification of the neuromuscular control was assessed by recording the superficial
EMG.
It is concluded that the centrally programmed activity prior to the contact can be seen as the decisive mechanism in the regulation of the stiffness behavior of the tendomuscular system. The extent of the preprogrammed activity determines mainly the physical output of the entire jump exercise.
Int.J.SportsMed. 12(1991)34—40 Georgmieme Verlag Stuttgart New York
Ten physically active males volunteered for the study. Their age was 29.9 (SD 7.6) years, the mean height was 179 (SD 5) cm and the mean body weight was 75.8 (SD 8.0) kg.
All of them were fully informed of the procedures and appraised of all possible risks involved in the study. They gave
Key words
stretch-shortening-cycle, control, stiffness, EMG
Methods
their written consent.
neuromuscular
The subjects performed six drop jumps from the height of 40 cm in six different experimental conditions: jumps with extra loads of 200 N and 100 N ("BW + 200 N", "BW+ 100 N"), ordinary jumps with normal body weight ("BW"), and jumps with reduced body weight ("BW-'- 172 N",
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Abstract
mt. J. Sports Med. 12(1991) 35
Neuromuscular Control of the Human Leg Extensor Muscles pre
Fig. I The schematic presentation of
impact
push-off
the system to reduce the body weight of the subjects artificially f 2000 N force
0.25 mV
m. castes m.
a control condition. Using a special belt system, the body weight of the subjects was artificially reduced (Fig. 1). One end of a rope was connected to the subjects, the other through rolls on the ceiling to different weights. The reduction was allowed
only during the impact phase of the ground contact. In the transition between the impact and the push-off phases, two assistants blocked the effects of the lightening weights. With this arrangement the stretch load for the impact could be varied in-
dividually in the specific experimental conditions, whereas the load during the push-off phase was kept constant. In the extra load conditions ("BW + 100 N" and "BW + 200 N") the subjects wore different kinds of load-vests. These extra loads were used during the whole contact phase. In all conditions the subjects were instructed to jump reactively with only a little bending in the knee joints and returning to the starting position.
The vertical ground reaction forces were measured using a force platform system (Kistler). Determination of the body weight of the subjects and the force-time integral during the push-off phase of contact allowed the calculation of the take-off velocity of the center of gravity. The dis-
placements in the angles of the ankle and knee joints were measured by electrogoniometers. EMG signals from the muscles of gastrocnemius (GA), soleus (SO), tibialis anterior (TA) and vastus medialis (VM) were registered with bipolar surface electrodes (Beckman) fixed with a constant interelectrode distance of 20 mm longitudinally over the muscle belly. The determination of the motor point area was omitted. All EMG registrations were checked for mechanical artifacts. The EMG signals were full-wave rectified and recorded together with signals of the electrogoniometers and the ground reaction forces via A/D conversion in a microcomputer (Victor-Sirius, sampling frequency 1kHZ per channel). Regarding the early touchdown of the feet on the platform system (steep rise of vertical ground reaction force), six jumps from each experimental condition were averaged for each individual subject.
From ankle an knee goniometer recordings, the total muscle-tendon length of the triceps surae muscle including the achilles tendon was calculated according to the
model of Frigo and Pedotti (7). From the differentiated muscle-tendon length curve, the time phases of the impact and
push-off phases of the contact were determined (Fig. 2). Despite the fact that the transition of lengthening and shorten-
f 0.25 mV m. gastrocnemius
f 500 mm/s dS/dT
muscle-tendon-length
f 15 mm
too Subj F40—0
Fig. 2 An example of the analyzed signals during the drop jump exeriments. The contact phase has been divided into impact and pushoff phases according to the stretch velocity of the total tendomuscular system of M. triceps and achilles tendon.
ing of the entire muscle-tendon length can only be used to de-
termine impact and push-off phases of the triceps sureae
muscle, this instant was also expanded to express both phases for VM. The rational background for the distribution of the
ground contact into impact and push-off phases was to get more detailed information about the regulation of the neu-
ronal quantities in the various test conditions. With the methodological approach employed in this study, our definition of both phases for the various muscles should be regarded only as a first approximation in order to separate generally both phases for the whole system: the contractile and the serious elastic part of the muscle-tendon complex.
Recently it has been shown that the phases of
shortening and lengthening of the human tendomuscular complex of biceps brachii muscle can be related simultaneously to the tendinous as well as to the contractile appara-
tus without substantial phase shift with the movement observed in the joint (1).
All EMGs and forces were integrated over different functional time phases: (a) Preactivity (starting from the first rise of the activation pattern above the zero line until the instant of touchdown), (b) neuronal input to the muscle during the impact phase and (c) during the push-off phase of the contact. Furthermore, all of the EMG quantities were time-nor-
malized in order to get the mean activation amplitudes (AEMG). Additionally, SO, GA and TA were integrated 50 ms before to 50 ms after contact. Integration of the EMG patterns and their relation to the ground reaction forces require the as-
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"BW—495 N"). The jumps with body weight ("BW") served as
36 mt. J. Sports Med. 12 (1991) 2'
A. Go1lhoJr, H. Kyrolainen Table 1 The mean angular velocities (deg x s1) of the ankle and knee joints during both the impact and push-off phases of the conI
tact
r.
__________
Ankle Ecc
BW+200N BW+ lOON
SW BW-172N BW-337N BW-495N
Knee
—417±88 —399±98 —455±72 —298±88 —218±92 — 90
65
Cone
Ecc
Conc
430± 93
—230±85 —217±95 —218±57 —173±65 —133±58
303± 105 329± 126 329± 118
487± 104
521± 91 421± 83 329± 91 329 75
— 87
50
327±114 320± 98 298 102
I
Results
ities (Fig. 3C), which was the highest in the BW condition (2.2 0.2 m x s 5. In the "BW—495 N" condition the take-off velocities were decreased by 18.2% (1.8 0.3 m x s5. In the load condition ("BW + 200 N") the respect decreases were 13.6% (1.9 0.3 mx s 5. All changes were nonsignificant.
SW —495N
SW —337 N
SW
BW
8W + lOON
BW +
In the experimental conditions, where the body weight was reduced, the mean angular velocities of the ankle and knee joints decreased significantly for the impact phase. The respective changes for the push-off phase were nonsignificant (Tab. I). Fig. 4 indicates that during the impact phase of the ground contact the displacements in the ankle and knee joints decreased significantly with the reduction of the bodyweight. For the push-off phase these regulations could not be observed.
Fig. 5A and B present the activation charac-
Fig. 3 The push-off time of the take-off (3A), the mean vertical force during the push-off phase of the contact (3B), and the take-off velocity (3C) in the various test conditions.
teristics in the different experimental conditions. Both the duration of the preactivation phase as well as the integrated EMG activity (IEMG) before touchdown demonstrated similarities with respect to the loading of the subjects. In the extra load and
sumption that the electromechanical delay is either neglectible
in the control condition, only small variations could be observed. The activation characteristic changed markedly when the body weight was reduced. The mean amplitudes of the
or constant throughout the test conditions. According to the results from Cavanagh and Komi (3), there exists a significant
difference between the electromechanical delay in eccentric condition (49.5 ms) and in concentric condition (55.5 ms). Nevertheless, in the present study the phase lag between EMG and mechanical responses was assumed to be the same for all subjects in all tested conditions. Therefore, the differences in electromechanical delay were treated as systematic errors.
All individual recordings of one specific ex-
muscle acitivities (AEMG) were almost on the same level in all conditions during both the impact and the push-off phases of the contact (Fig. 6). There was, however, a slight non-significant increment in the EMG amplitudes from the heavy to the light load conditions during the push-off phase of the contact. EMG-force-ratio curves (Fig. 7) demonstrate high sensitivity
to the stretch load of the jump condition. For the push-off phase the increment was linear with the reduction of the load condition. For the impact phase, however, these changes were exponential.
perimental condition were graphically and numerically summed with respect to the early onset of the ground contact, which was used as a trigger signal.
The IEMG 50 ms before and after touchdown of the toes was calculated in order to get an estimation of the level of "cocontraction" of the triceps surae and TA muscles. The degree of "cocontraction" was assessed calculating the sum of the IEMGs of both analyzed triceps surae muscles and TA. Fig. 8 represents the distribution of the mean values of the
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*
2
0
—
In the extra load as well as in the unloading conditions, the contact times were lengthened, which was primarily an effect of the elongation of the push-off phase (Fig. 3A). Similarily, the mean ground reaction forces during the push-off phase decreased compared to the BW condition (Fig. 3B). Analogous changes were also observed in the physical performance of the subjects expressed by the take-off veloc-
Neuromuscular Control of the Human Leg Extensor Muscles
Tnt. J. Sports Med. 12(1991) 37
I
mVi's
IEMG
lpreactivation phasel
deg
40
20
22 o 000 ++
00
2 Z 22 Z 0 0 0 C N U)
Z 22 C' N U)
N C) C) C., I
N C) — C)
00
000
++
I
I
00
000
I
— 495 N
C)
I
I
200
ms
duration lpreactivation phasel
80
80
deg
deg
150
60
60
100
40
40
2 ZZ C' N U) p
0 o0 o NJ
++
00
N C.) C') I
I
I 20
C)
50
i
C..i I
000
p
p
Zz z z U) 2 0 0 00.- 0 CN— N C.) C) ++
00
p
+ 200 N + 100 N
BW
— 172 N —337 N —495 N
p
N.
C')
I
I
I
000
Fig. 4 Displacements in the knee (left) and ankle (right) joints during the impact (upper) and the push-off (lower) phases of the contact.
Fig. 5 Integrated EMG5 (MEAN and SD) from mm. soleus (SO), gastrocnemius (GA) and vastus medialis (VM) during the preactivation phase. On the lower part of the figure the respective duration of the preactivation phase is represented. BW indicates the jumps with body weight, the numbers refer to the loading and unloading of the body weight, respectively.
individual muscles plus the parameter assessing "cocontrac-
and decreased in extra load or in reduced body weight condi-
tion" in the various load conditions. The level of "cocontraction" was the highest in the BW condition, whereas in extra
tions.
load and reduced body weight conditions "cocontraction" was decreased. Discussion
The results of the physical parameters in the drop jumps under various stretch-load conditions can be summarized as follows: In jump conditions with extra loads as well as in reduced body weight conditions, the duration of the total
ground contact is gradually increased in comparison to the BW conditions. Even in the experimental conditions with extra loads, the mean vertical load during the push-off phase of the contact was decreased. Also the take-off velocity was the
highest in BW condition. In the extra load condition the experimental set-up did not allow to remove the loads during the push-off phase. Nevertheless, the main physical parameters demonstrate that the basic physical performance was maximal in that condition where no load manipulation was performed,
Some explanations for these conclusions can be found when analyzing the neuromuscular activation. The EMG recordings indicate that the electrical activity prior to contact is the primary sensitive part with respect to the various load conditions. In the impact as well as in the push-off phase of the ground contact, the mean amplitude of the activation varied only slightly under the influence of a specific load condition.
We have studied the leg extensor muscles under stretch-shortening cycle conditions. The preprogramming of the muscle, especially the extensor muscles, is well documented in the literature (4, 5, 9, 14, 18). Functionally, the preactivation is interpreted as a preprogrammed neuronal activation part, which provides the tendomusclular system with
adequate stiffness prior to the ground contact. Schmidtbleicher and Gollhofer (14) stated a clear relationship between both the duration and the amount of the integrated EMG activity for the preactivation phase and the dropping height the
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20
38 mt. J. Sports Med. 12(1991)
A. Golihofer, H. Kyrölainen
A EMG
1.2
mV
1.4
(impact phase)
1.0
VN1
EMG/force-ratio (impact phase)
1.2
1.0
0.8
0.8 0.6 0.6 0.4
0.4 0.2
0.2
GA
An
0.8 mV
+200N +100N
BW —172N —337N —495N
0.5 VN1
AEMG
EMG/force-ratio (push.off phase)
(push.off phase)
0.7
BW —172N —337N —495N
0.4
0.6 0.5
0.3
0.4
VM
0.3
GA
io
0.2
I
L I
i.---.-.--..-..-.--1
I
I
0.1
Iso
J
J
----f
fin
+200N +100N
BW
—172N —337N —495N
Fig. 6 Mean amplitudes of muscle activities (AEMG) (MEAN and SD) during the impact (upper part) and push-off (lower part) phases of the contact from mm. soleus (SO), gastrocnemius (GA) and vastus media)is (VM). BW indicate the jumps with body weight, the numbers refer to the loading and unloading of the body weight, respec-
+200N +100N
BW —172N —337N —495N
Fig. 7 EMG-force-ratio curves (MEAN and SD) during the impact (upper part) and push-off (lower part) phases of the contact from mm. soleus (SO), gastrocnemius (GA) and vastus medialis (VM). BW indicates the jumps with body weight, the numbers refer to the loading and unloading of the body weight, respectively.
tively.
subjects jumped from. When the height of the drops is kept constant, but the compliance properties of the landing sur-
tivation amplitude during the impact phase of the contact remained constant, this relative late activation cannot compen-
faces are increased, the preactivation is systematically reduced
sate the reduced stiffness properties resulting from the
(8). Both observations emphasize the concept that the expected stretch load regulates the extend of the preactivation. However, in jumps from different heights conducted with
decreased preactivation. According to Fig. 4, the angular displacement in the knee and ankle is decreased when the loads are reduced. This regulation can be interpreted as a mechanical compensatory effect of the neuromuscular system to keep the tension in the lengthening (impact) phase of the muscles as high as possible. If the stretch-load demands are low, the preparatory EMG is decreased, therefore, the amplitude for amortization of the kinetic energy can also be reduced.
monkeys (6), aconstantpreactivation time of 8Oms is reported.
Except in the extra load condition the subjects had to perform their push-off work constantly against their
body weight. In the reduced body weight conditions the stretch load before impact was systematically diminished and in consequence the preactivity. From these observations it is suggested that in the reduced body weight conditions the preparatory EMG was not sufficient to provide the tendomuscu-
lar system with adequate stiffness. Therefore, the subjects' ability to jump reactively was decreased. Komi (11) emphasized that elastic energy stored in the tendomuscular system during the impact phase can be utilized for the subsequent push-off phase. If the preprogramming does not prepare the tendomuscular system before ground contact, no recoil of elastic energy can be expected. Considering the mechanical consequences, it must be pointed out that, even if the mean ac-
Fig. 9 summarizes another effect which could be observed in the individual as well as in the grand mean averages. With a higher load condition, a progressive reduction in the EMG activity of the two-joint muscle gastrocnemius is ob-
vious. This reduction already starts 30—50 ms before touchdown of the toes and lasts over 200 ms. The occurrence of this EMG reduction has already been reported by Greenwood and Hopkins (10), Gollhofer (8) and Schmidtbleicher (15). These EMG reductions are closely connected to the amount of the stretchload to which the tendomuscular system is exposed. Accordingly, they are clearly expressed in the high-load condi-
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+200N +100N
mt. .1. Sports Med. 12(1991) 39
Neuromuscular Control of the 1-Juman Leg Extensor Muscles vert. ground reaction forces
mVs
20 10
0
_
—a iaa n
______
+100 N
BW
7: c.Ili
Coactivity
F41kt%
+200 N
+200 N
—172
=
body weight
— 172 N
-:
I. .:ii
—337 N
so —495 N
GA
N —337 N —495 N
Fig. 8 Integrated EMG (50 ms before and 50 ms after touchdown of the feet) of mm soleus, gatrocnemius and tibialis anterior. Coactivity" was calculated as the sum of all three muscles.
10.25
2000 N
200 ms 200 ms
Fig. 9 Grand mean curves of the vertical ground reaction forces and the averaged EMG patterns of the M. gastrocnemius. The vertical line represents the touchdown; the numbers in the different traces indicate the conditions. For clarity the variation of the mean curves was omitted.
Fig. 10 Representative example (sub). WR) of the EMG pattern of the M. Soleus and M. Gastrocnemius together with the records from the displacement in the knee and ankle joint from a drop jump ("BW—172 N"). TO indicates the instant of touchdown; Ti —T4 are the latencies introduced from Lee and Tatton (1978) for the Ml, M2and M3 component of the stretch reflex activity of human wrist extensors
m. soleus
knee
m. gastrocnnmius
ankle
force
p40—172
tions, but abolished in the reduced body weigth conditions. Accordingly, the decreases in the parameter assessing cocontraction in the extra load conditions (Fig. 8) can be interpreted as a function of these increased inhibitory effects in the EMG. Functionally these reductions can be interpreted as a protection mechanism of the neuromuscular system to prevent intolerable stresses in the tendomuscular system. The observation that the reduction in the EMG already starts before ground contact supports the thesis that these inhibitory effects are part of a centrally generated motor program.
Numerous studies have demonstrated (4, 5, 8, 9, 14) that in stretch-shortening cycle contraction the afferent activity resulting from the stretch reflex system is highly active.
From the patterns presented in Fig. 9, distinct peaks in the
EMG are abolished because of the interindividual variances in
the latencies. Therefore Fig. 10 was constructed to demonstrate the powerful presence of the various components of the reflectory activation. As a representative example, the averaged EMG patterns of SO and GA are drawn together with the latencies of the short-, medium- and long-latency component suggested in the literature by Lee and Tatton (13) for the Ml, M2, and M3 components of the human wrist extensors. Both the latency and the shape of the responses in the EMG pattern match well the data presented in the literature.
In conclusion, two major innervation characteristics can be observed in our experiments. If the stretch load expected by the system is below the body weight, the main regulation already takes place before ground contact. The dur-
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lU! :: W//S 70
m. gastrocnemiUs
mt. .1. Sports Med. 12 (1991)
ation and the amount of the preactivity phase is decreased. If the stretch load is high, the neuromuscular activation before and during the early part of the ground contact is reduced in order to protect the tendomuscular system from high impact loads. Both characteristics have mechanically similar conse-
A. Golihofer, H Kyrölainen 10
quences: the reduction of stiffness, which leads to a diminished reactive movement capability.
12
Acknowledgment 13
This research was supported by the "Deutsche Forschungsgemeinschaft" SFB 325 "Modulation und Lernvorgange in Neuronensystemen". The authors thank Dr. M. Trippel and Mr. A. Kibele for providing us with appropriate software.
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Viitasalo J. T.: Neuromuscular function and mechanical effi-
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applications. In: Desmedt J. E. (ed): Cerebral motor control in man: Long loop mechanisms, Progr C/in Neurophysiol 4. Basel, Karger, l978,pp334—34l. Schmidtbleicher D., Gollhofer A.: Neuromuskuläre Untersuchungen zur Bestimmung individueller Belastungsgröl3en für em Tiefsprungtraining. Leistungssport 12:298—307, 1982. Schmidtbleicher D., Gollhofer A., Frick U.: Auswirkungen eines Tiefensprungtrainings auf die LeistungsfPhigkeit und das Inner-
vationsverhalten der Beinstreckmuskulatur. Dl Z Sportmed 38: 17
18 ciency of human leg extensor muscles during jumping exercises. ActaPhysiolScandl 14:443—550,1982. " Cavanagh P. R., Komi P. V.: Electromechanical delay in human 9 skeletal muscle under concentric and eccentric contractions. Eur J ApplPhysio/42: 159—163,1979. 20 Dietz V., Noth J.: Spinal stretch reflexes of triceps surae in active and passive movements. JPhysiol (Lond) 284: 180—181, 1978. 6 Dietz V., Noth J., Schmidtbleicher D.: Interaction between preactivity and stretch reflex in human triceps brachii during landing 21 fromforwardfalls.JPhyso/(Lond)31 1:113—125,1981. Dyhre-Poulsen P., Mosfeld Laurensen A.: Programmed electromyographic activity and negative incremental muscle stiffness in monkeys jumping downward. J Physiol (Lond) 350: 121—136,
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Gollhofer A., Schmidtbleicher D.: Muscle activation patterns of human leg extensors and force-time-characteristics in jumping exercises under increased stretching loads. In: Groot G., Hollander A. P., Huijing P. A., Van Ingen Schenau G. (eds.): Tnt. Series on Biomechanics, Biomechanics XI-A. Amsterdam, Free Uni Press, 1988,pp143—l48. Greenwood R., Hopkins A.: Landing from an unexpected fall and avoluntarystep.Brain99: 375—386,1976.
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Institut für Sport und Sportwissenschaft Universität Freiburg Schwarzwaldstr. 175 D-7800 Freiburg, W.-Germ any
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40
Neuromuscular Control of the Human Leg Extensor Muscles in Jump Exercises Under Various Stretch-Load Conditions A. Golihofer, H. Kyröläinen / Institute for Sport Sciences University of Freiburg, D 7800 Freiburg, West Germany; Department of Biology of Physical Activity, Universtiy of Jyvaskylä, SF4100 JyvAskylä, Finland
Introduction A. Golihofer, H. Kyröläinen, Neuromuscular Control of the Human Leg Extensor Muscles in Jump Exercises Under Various Stretch-Load Conditions. mt J Sports Med, Vol 12,No l,pp34—4O, 1991. Accepted after revision: April 17, 1990
Ten active males performed reactive drop jumps from a height of 40 cm in six experimental conditions: jumps with additional loads of 100 N (BW+ 100 N) and 200 N (BW+ 200 N), an ordinary jump with body weight (BW) and three jumps in which the body weight was artificially reduced (BW— 172 N, BW—337 N and BW—495
N). The vertical ground reaction forces, the angular displacement in the knee and ankle joints as well as the surface electromyogram (EMGs) of the triceps surae muscles and tibialis ant, muscle were recorded.
When compared to the control condition (BW) in the jumps with extra load and in the jumps with reduced body weight, both the take-off velocity as well as the mean vertical ground reaction force were decreased during
the push-off phase. The integrated EMG before ground contact as well as the duration of the preactivation phase was significantly reduced as a function of the load condition. Upon the touchdown, the coactivation of the muscles acting around the ankle joint was greatest in the control jump. Through all experimental conditions, the mean activation amplitude remained rather constant both for the impact as well as for the push-off phase of the contact.
The control of movement in jump exercises has been analyzed extensively in humans (1, 17, 18) as well as in
animals (6, 20). In jump performances, the leg extensor muscles are activated before ground contact of the feet (8, 19).
During the first part of the contact, the muscles are actively stretched prior to the subsequent shortening phase. This kind of activation pattern is typical for so-called stretch-shortening cycle exercises (11). In stretch-shortening cycle (SSC) contractions, it could be demonstrated that a major part of the elastic energy can be stored during the impact phase of the ground contact in the tendomuscular system and utilized in the subsequent push-off phase either to improve performance capability in conditions with maximal effort (2, 11) or to increase the efficiency of the contraction quality in submaximal contraction conditions (2, 16). Both regulations can be achieved only if the time between impact and push-off phase is short (12).
In several studies, the neuromuscular control in reactive movements (5, 14) was investigated. It could be de-
monstrated that the electromyographic (EMG) activity as a centrally generated part prior to contact represents an essential prerequisite for a powerful output of the muscular work. The purpose of this study was to investigate the neuromuscular control of the leg extensor muscles with vary-
ing loads in drop jump exercises from identical heights. In order to change the stretch-load condition systematically and individually, various jump conditions were performed either with extra loads or with a specific system which allowed to unload the body weight artificially. The modification of the neuromuscular control was assessed by recording the superficial
EMG.
It is concluded that the centrally programmed activity prior to the contact can be seen as the decisive mechanism in the regulation of the stiffness behavior of the tendomuscular system. The extent of the preprogrammed activity determines mainly the physical output of the entire jump exercise.
Int.J.SportsMed. 12(1991)34—40 Georgmieme Verlag Stuttgart New York
Ten physically active males volunteered for the study. Their age was 29.9 (SD 7.6) years, the mean height was 179 (SD 5) cm and the mean body weight was 75.8 (SD 8.0) kg.
All of them were fully informed of the procedures and appraised of all possible risks involved in the study. They gave
Key words
stretch-shortening-cycle, control, stiffness, EMG
Methods
their written consent.
neuromuscular
The subjects performed six drop jumps from the height of 40 cm in six different experimental conditions: jumps with extra loads of 200 N and 100 N ("BW + 200 N", "BW+ 100 N"), ordinary jumps with normal body weight ("BW"), and jumps with reduced body weight ("BW-'- 172 N",
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Abstract
mt. J. Sports Med. 12(1991) 35
Neuromuscular Control of the Human Leg Extensor Muscles pre
Fig. I The schematic presentation of
impact
push-off
the system to reduce the body weight of the subjects artificially f 2000 N force
0.25 mV
m. castes m.
a control condition. Using a special belt system, the body weight of the subjects was artificially reduced (Fig. 1). One end of a rope was connected to the subjects, the other through rolls on the ceiling to different weights. The reduction was allowed
only during the impact phase of the ground contact. In the transition between the impact and the push-off phases, two assistants blocked the effects of the lightening weights. With this arrangement the stretch load for the impact could be varied in-
dividually in the specific experimental conditions, whereas the load during the push-off phase was kept constant. In the extra load conditions ("BW + 100 N" and "BW + 200 N") the subjects wore different kinds of load-vests. These extra loads were used during the whole contact phase. In all conditions the subjects were instructed to jump reactively with only a little bending in the knee joints and returning to the starting position.
The vertical ground reaction forces were measured using a force platform system (Kistler). Determination of the body weight of the subjects and the force-time integral during the push-off phase of contact allowed the calculation of the take-off velocity of the center of gravity. The dis-
placements in the angles of the ankle and knee joints were measured by electrogoniometers. EMG signals from the muscles of gastrocnemius (GA), soleus (SO), tibialis anterior (TA) and vastus medialis (VM) were registered with bipolar surface electrodes (Beckman) fixed with a constant interelectrode distance of 20 mm longitudinally over the muscle belly. The determination of the motor point area was omitted. All EMG registrations were checked for mechanical artifacts. The EMG signals were full-wave rectified and recorded together with signals of the electrogoniometers and the ground reaction forces via A/D conversion in a microcomputer (Victor-Sirius, sampling frequency 1kHZ per channel). Regarding the early touchdown of the feet on the platform system (steep rise of vertical ground reaction force), six jumps from each experimental condition were averaged for each individual subject.
From ankle an knee goniometer recordings, the total muscle-tendon length of the triceps surae muscle including the achilles tendon was calculated according to the
model of Frigo and Pedotti (7). From the differentiated muscle-tendon length curve, the time phases of the impact and
push-off phases of the contact were determined (Fig. 2). Despite the fact that the transition of lengthening and shorten-
f 0.25 mV m. gastrocnemius
f 500 mm/s dS/dT
muscle-tendon-length
f 15 mm
too Subj F40—0
Fig. 2 An example of the analyzed signals during the drop jump exeriments. The contact phase has been divided into impact and pushoff phases according to the stretch velocity of the total tendomuscular system of M. triceps and achilles tendon.
ing of the entire muscle-tendon length can only be used to de-
termine impact and push-off phases of the triceps sureae
muscle, this instant was also expanded to express both phases for VM. The rational background for the distribution of the
ground contact into impact and push-off phases was to get more detailed information about the regulation of the neu-
ronal quantities in the various test conditions. With the methodological approach employed in this study, our definition of both phases for the various muscles should be regarded only as a first approximation in order to separate generally both phases for the whole system: the contractile and the serious elastic part of the muscle-tendon complex.
Recently it has been shown that the phases of
shortening and lengthening of the human tendomuscular complex of biceps brachii muscle can be related simultaneously to the tendinous as well as to the contractile appara-
tus without substantial phase shift with the movement observed in the joint (1).
All EMGs and forces were integrated over different functional time phases: (a) Preactivity (starting from the first rise of the activation pattern above the zero line until the instant of touchdown), (b) neuronal input to the muscle during the impact phase and (c) during the push-off phase of the contact. Furthermore, all of the EMG quantities were time-nor-
malized in order to get the mean activation amplitudes (AEMG). Additionally, SO, GA and TA were integrated 50 ms before to 50 ms after contact. Integration of the EMG patterns and their relation to the ground reaction forces require the as-
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"BW—495 N"). The jumps with body weight ("BW") served as
36 mt. J. Sports Med. 12 (1991) 2'
A. Go1lhoJr, H. Kyrolainen Table 1 The mean angular velocities (deg x s1) of the ankle and knee joints during both the impact and push-off phases of the conI
tact
r.
__________
Ankle Ecc
BW+200N BW+ lOON
SW BW-172N BW-337N BW-495N
Knee
—417±88 —399±98 —455±72 —298±88 —218±92 — 90
65
Cone
Ecc
Conc
430± 93
—230±85 —217±95 —218±57 —173±65 —133±58
303± 105 329± 126 329± 118
487± 104
521± 91 421± 83 329± 91 329 75
— 87
50
327±114 320± 98 298 102
I
Results
ities (Fig. 3C), which was the highest in the BW condition (2.2 0.2 m x s 5. In the "BW—495 N" condition the take-off velocities were decreased by 18.2% (1.8 0.3 m x s5. In the load condition ("BW + 200 N") the respect decreases were 13.6% (1.9 0.3 mx s 5. All changes were nonsignificant.
SW —495N
SW —337 N
SW
BW
8W + lOON
BW +
In the experimental conditions, where the body weight was reduced, the mean angular velocities of the ankle and knee joints decreased significantly for the impact phase. The respective changes for the push-off phase were nonsignificant (Tab. I). Fig. 4 indicates that during the impact phase of the ground contact the displacements in the ankle and knee joints decreased significantly with the reduction of the bodyweight. For the push-off phase these regulations could not be observed.
Fig. 5A and B present the activation charac-
Fig. 3 The push-off time of the take-off (3A), the mean vertical force during the push-off phase of the contact (3B), and the take-off velocity (3C) in the various test conditions.
teristics in the different experimental conditions. Both the duration of the preactivation phase as well as the integrated EMG activity (IEMG) before touchdown demonstrated similarities with respect to the loading of the subjects. In the extra load and
sumption that the electromechanical delay is either neglectible
in the control condition, only small variations could be observed. The activation characteristic changed markedly when the body weight was reduced. The mean amplitudes of the
or constant throughout the test conditions. According to the results from Cavanagh and Komi (3), there exists a significant
difference between the electromechanical delay in eccentric condition (49.5 ms) and in concentric condition (55.5 ms). Nevertheless, in the present study the phase lag between EMG and mechanical responses was assumed to be the same for all subjects in all tested conditions. Therefore, the differences in electromechanical delay were treated as systematic errors.
All individual recordings of one specific ex-
muscle acitivities (AEMG) were almost on the same level in all conditions during both the impact and the push-off phases of the contact (Fig. 6). There was, however, a slight non-significant increment in the EMG amplitudes from the heavy to the light load conditions during the push-off phase of the contact. EMG-force-ratio curves (Fig. 7) demonstrate high sensitivity
to the stretch load of the jump condition. For the push-off phase the increment was linear with the reduction of the load condition. For the impact phase, however, these changes were exponential.
perimental condition were graphically and numerically summed with respect to the early onset of the ground contact, which was used as a trigger signal.
The IEMG 50 ms before and after touchdown of the toes was calculated in order to get an estimation of the level of "cocontraction" of the triceps surae and TA muscles. The degree of "cocontraction" was assessed calculating the sum of the IEMGs of both analyzed triceps surae muscles and TA. Fig. 8 represents the distribution of the mean values of the
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*
2
0
—
In the extra load as well as in the unloading conditions, the contact times were lengthened, which was primarily an effect of the elongation of the push-off phase (Fig. 3A). Similarily, the mean ground reaction forces during the push-off phase decreased compared to the BW condition (Fig. 3B). Analogous changes were also observed in the physical performance of the subjects expressed by the take-off veloc-
Neuromuscular Control of the Human Leg Extensor Muscles
Tnt. J. Sports Med. 12(1991) 37
I
mVi's
IEMG
lpreactivation phasel
deg
40
20
22 o 000 ++
00
2 Z 22 Z 0 0 0 C N U)
Z 22 C' N U)
N C) C) C., I
N C) — C)
00
000
++
I
I
00
000
I
— 495 N
C)
I
I
200
ms
duration lpreactivation phasel
80
80
deg
deg
150
60
60
100
40
40
2 ZZ C' N U) p
0 o0 o NJ
++
00
N C.) C') I
I
I 20
C)
50
i
C..i I
000
p
p
Zz z z U) 2 0 0 00.- 0 CN— N C.) C) ++
00
p
+ 200 N + 100 N
BW
— 172 N —337 N —495 N
p
N.
C')
I
I
I
000
Fig. 4 Displacements in the knee (left) and ankle (right) joints during the impact (upper) and the push-off (lower) phases of the contact.
Fig. 5 Integrated EMG5 (MEAN and SD) from mm. soleus (SO), gastrocnemius (GA) and vastus medialis (VM) during the preactivation phase. On the lower part of the figure the respective duration of the preactivation phase is represented. BW indicates the jumps with body weight, the numbers refer to the loading and unloading of the body weight, respectively.
individual muscles plus the parameter assessing "cocontrac-
and decreased in extra load or in reduced body weight condi-
tion" in the various load conditions. The level of "cocontraction" was the highest in the BW condition, whereas in extra
tions.
load and reduced body weight conditions "cocontraction" was decreased. Discussion
The results of the physical parameters in the drop jumps under various stretch-load conditions can be summarized as follows: In jump conditions with extra loads as well as in reduced body weight conditions, the duration of the total
ground contact is gradually increased in comparison to the BW conditions. Even in the experimental conditions with extra loads, the mean vertical load during the push-off phase of the contact was decreased. Also the take-off velocity was the
highest in BW condition. In the extra load condition the experimental set-up did not allow to remove the loads during the push-off phase. Nevertheless, the main physical parameters demonstrate that the basic physical performance was maximal in that condition where no load manipulation was performed,
Some explanations for these conclusions can be found when analyzing the neuromuscular activation. The EMG recordings indicate that the electrical activity prior to contact is the primary sensitive part with respect to the various load conditions. In the impact as well as in the push-off phase of the ground contact, the mean amplitude of the activation varied only slightly under the influence of a specific load condition.
We have studied the leg extensor muscles under stretch-shortening cycle conditions. The preprogramming of the muscle, especially the extensor muscles, is well documented in the literature (4, 5, 9, 14, 18). Functionally, the preactivation is interpreted as a preprogrammed neuronal activation part, which provides the tendomusclular system with
adequate stiffness prior to the ground contact. Schmidtbleicher and Gollhofer (14) stated a clear relationship between both the duration and the amount of the integrated EMG activity for the preactivation phase and the dropping height the
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20
38 mt. J. Sports Med. 12(1991)
A. Golihofer, H. Kyrölainen
A EMG
1.2
mV
1.4
(impact phase)
1.0
VN1
EMG/force-ratio (impact phase)
1.2
1.0
0.8
0.8 0.6 0.6 0.4
0.4 0.2
0.2
GA
An
0.8 mV
+200N +100N
BW —172N —337N —495N
0.5 VN1
AEMG
EMG/force-ratio (push.off phase)
(push.off phase)
0.7
BW —172N —337N —495N
0.4
0.6 0.5
0.3
0.4
VM
0.3
GA
io
0.2
I
L I
i.---.-.--..-..-.--1
I
I
0.1
Iso
J
J
----f
fin
+200N +100N
BW
—172N —337N —495N
Fig. 6 Mean amplitudes of muscle activities (AEMG) (MEAN and SD) during the impact (upper part) and push-off (lower part) phases of the contact from mm. soleus (SO), gastrocnemius (GA) and vastus media)is (VM). BW indicate the jumps with body weight, the numbers refer to the loading and unloading of the body weight, respec-
+200N +100N
BW —172N —337N —495N
Fig. 7 EMG-force-ratio curves (MEAN and SD) during the impact (upper part) and push-off (lower part) phases of the contact from mm. soleus (SO), gastrocnemius (GA) and vastus medialis (VM). BW indicates the jumps with body weight, the numbers refer to the loading and unloading of the body weight, respectively.
tively.
subjects jumped from. When the height of the drops is kept constant, but the compliance properties of the landing sur-
tivation amplitude during the impact phase of the contact remained constant, this relative late activation cannot compen-
faces are increased, the preactivation is systematically reduced
sate the reduced stiffness properties resulting from the
(8). Both observations emphasize the concept that the expected stretch load regulates the extend of the preactivation. However, in jumps from different heights conducted with
decreased preactivation. According to Fig. 4, the angular displacement in the knee and ankle is decreased when the loads are reduced. This regulation can be interpreted as a mechanical compensatory effect of the neuromuscular system to keep the tension in the lengthening (impact) phase of the muscles as high as possible. If the stretch-load demands are low, the preparatory EMG is decreased, therefore, the amplitude for amortization of the kinetic energy can also be reduced.
monkeys (6), aconstantpreactivation time of 8Oms is reported.
Except in the extra load condition the subjects had to perform their push-off work constantly against their
body weight. In the reduced body weight conditions the stretch load before impact was systematically diminished and in consequence the preactivity. From these observations it is suggested that in the reduced body weight conditions the preparatory EMG was not sufficient to provide the tendomuscu-
lar system with adequate stiffness. Therefore, the subjects' ability to jump reactively was decreased. Komi (11) emphasized that elastic energy stored in the tendomuscular system during the impact phase can be utilized for the subsequent push-off phase. If the preprogramming does not prepare the tendomuscular system before ground contact, no recoil of elastic energy can be expected. Considering the mechanical consequences, it must be pointed out that, even if the mean ac-
Fig. 9 summarizes another effect which could be observed in the individual as well as in the grand mean averages. With a higher load condition, a progressive reduction in the EMG activity of the two-joint muscle gastrocnemius is ob-
vious. This reduction already starts 30—50 ms before touchdown of the toes and lasts over 200 ms. The occurrence of this EMG reduction has already been reported by Greenwood and Hopkins (10), Gollhofer (8) and Schmidtbleicher (15). These EMG reductions are closely connected to the amount of the stretchload to which the tendomuscular system is exposed. Accordingly, they are clearly expressed in the high-load condi-
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+200N +100N
mt. .1. Sports Med. 12(1991) 39
Neuromuscular Control of the 1-Juman Leg Extensor Muscles vert. ground reaction forces
mVs
20 10
0
_
—a iaa n
______
+100 N
BW
7: c.Ili
Coactivity
F41kt%
+200 N
+200 N
—172
=
body weight
— 172 N
-:
I. .:ii
—337 N
so —495 N
GA
N —337 N —495 N
Fig. 8 Integrated EMG (50 ms before and 50 ms after touchdown of the feet) of mm soleus, gatrocnemius and tibialis anterior. Coactivity" was calculated as the sum of all three muscles.
10.25
2000 N
200 ms 200 ms
Fig. 9 Grand mean curves of the vertical ground reaction forces and the averaged EMG patterns of the M. gastrocnemius. The vertical line represents the touchdown; the numbers in the different traces indicate the conditions. For clarity the variation of the mean curves was omitted.
Fig. 10 Representative example (sub). WR) of the EMG pattern of the M. Soleus and M. Gastrocnemius together with the records from the displacement in the knee and ankle joint from a drop jump ("BW—172 N"). TO indicates the instant of touchdown; Ti —T4 are the latencies introduced from Lee and Tatton (1978) for the Ml, M2and M3 component of the stretch reflex activity of human wrist extensors
m. soleus
knee
m. gastrocnnmius
ankle
force
p40—172
tions, but abolished in the reduced body weigth conditions. Accordingly, the decreases in the parameter assessing cocontraction in the extra load conditions (Fig. 8) can be interpreted as a function of these increased inhibitory effects in the EMG. Functionally these reductions can be interpreted as a protection mechanism of the neuromuscular system to prevent intolerable stresses in the tendomuscular system. The observation that the reduction in the EMG already starts before ground contact supports the thesis that these inhibitory effects are part of a centrally generated motor program.
Numerous studies have demonstrated (4, 5, 8, 9, 14) that in stretch-shortening cycle contraction the afferent activity resulting from the stretch reflex system is highly active.
From the patterns presented in Fig. 9, distinct peaks in the
EMG are abolished because of the interindividual variances in
the latencies. Therefore Fig. 10 was constructed to demonstrate the powerful presence of the various components of the reflectory activation. As a representative example, the averaged EMG patterns of SO and GA are drawn together with the latencies of the short-, medium- and long-latency component suggested in the literature by Lee and Tatton (13) for the Ml, M2, and M3 components of the human wrist extensors. Both the latency and the shape of the responses in the EMG pattern match well the data presented in the literature.
In conclusion, two major innervation characteristics can be observed in our experiments. If the stretch load expected by the system is below the body weight, the main regulation already takes place before ground contact. The dur-
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lU! :: W//S 70
m. gastrocnemiUs
mt. .1. Sports Med. 12 (1991)
ation and the amount of the preactivity phase is decreased. If the stretch load is high, the neuromuscular activation before and during the early part of the ground contact is reduced in order to protect the tendomuscular system from high impact loads. Both characteristics have mechanically similar conse-
A. Golihofer, H Kyrölainen 10
quences: the reduction of stiffness, which leads to a diminished reactive movement capability.
12
Acknowledgment 13
This research was supported by the "Deutsche Forschungsgemeinschaft" SFB 325 "Modulation und Lernvorgange in Neuronensystemen". The authors thank Dr. M. Trippel and Mr. A. Kibele for providing us with appropriate software.
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40
