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. 2004;7(1):15–22. doi: 10.1298/jjpta.7.15

Task Dependent Motor Strategy of Human Triceps Surae Muscle

Kiyokazu Akasaka 1,✉, Hideaki Onishi 2, Kouji Ihashi 3, Masayoshi Ichie 4, Yasunobu Handa 5
PMCID: PMC4316516  PMID: 25792933

Abstract

Even though many investigators have analyzed the functional difference of the three heads of triceps surae in human, none of them succeeded to clarify the distinctive functional difference of those three muscles. The aim of this study was to investigate whether the integrated EMGs (IEMGs) of the triceps surae muscle, gastrocnemius and soleus, were task dependent. IEMGs of the medial head of the gastrocnemius (GM), lateral head of the gastrocnemius (GL), and soleus (SO) were investigated at three different knee joint angles, at four different duration of ramp contraction, with the generation of a single ongoing force, from 0 to the maximum voluntary contraction (MVC). Three-way ANOVAs for repeated measures were used to estimate differences in IEMG values in each of the GM, GL, and SO, taken at four different durations of ramp contraction (5, 10, 15 and 20 s), at three different knee joint angles (0 deg, 30 deg and 90 deg), across ankle plantar flexion levels of force (10, 20, 30, 40, 50, 60 and 70% MVC). According to three-way ANOVAs for repeated measures, IEMG of the GM muscle showed a first-order interaction between force and knee joint angle. In addition, IEMG of the GL muscle showed first-order interactions between the level of force and knee joint angle, and between the level of force and duration of ramp contraction. Furthermore, IEMG of the SO showed a main effect only on level of force. These results suggest that the each head of the triceps surae may work task dependently.

Keywords: duration of contraction, joint angle, force, triceps surae muscle, isometric contraction

The functional distribution of motoneurons in the central nervous system may enable smoother muscle force generation and less fatigue during voluntary movement. The order in which muscles are recruited on the basis of the size principle and firing rate of motoneurons is reflected in the contraction property1–3). While muscles group such as the prime movers almost always consist of more than two different skeletal muscles, the combination of activities within individual muscles remains an exciting question in motor control.

The ankle plantar flexors consist of the triceps surae, plantaris, flexor hallucis longus, flexor digitorum longus, peroneus longus, and tibialis posterior4). Among these muscles, the triceps surae muscle is an interesting object for experimental investigation as it is viewed to constitute three unique muscles, the medial head of the gastrocnemius (GM), lateral head of the gastrocnemius (GL), and soleus (SO), all of which have been well identified histochemically and confirmed to function as agonists for ankle plantar flexion5–9). While the SO is composed mainly of slow twitch and fatigue-resistant muscle fibers, both the GM and GL have nearly the same proportions of slow and fast twitch muscle fibers. Because of these histochemical differences in the triceps surae muscle, the GM and GL should show fatigue characteristics different from those of the SO. Moreover, unlike the SO, a monoarticular muscle, the GM and GL are biarticular ones that cross the knee joint, hence the angle of the knee joint affects their contributions to plantar flexion torque. In addition to these difference, our previous study suggests that GL may have greater proportion of fast-fatigable twitch muscle fibers than GM10). Given these differences in the triceps muscles, their strategies in voluntary movements may differ11–14).

Even though many studies indicate that the IEMG value of the prime mover is linearly increase with an increase in force under an isometric condition15–18), some recent studies object to this opinion because the EMG amlitude increased nonlinearly with ensemble activation rate of a pool of motor units19)20). However, the increase in the IEMG value is commonly believed to due to increases in the amplitude and density of EMG signals resulting from the mutual influences of recruitment and firing rate. For this reason, many studies have used ramp contraction, defined as the generation of a single ongoing force from 0 to MVC10,21–26). To our knowledge, however, there have been no investigations on the activities of the prime mover at ramp contractions of various durations under an isometric condition and variable muscle length conditions. Within the human triceps surae, a muscle extensively studied by many investigators, the characteristic functional differences in the respective constituent muscles are not clearly distinguished. We hypothesized that activities of the triceps surae would be more clearly defined by three parameters of the constituent muscles, namely, duration of contraction, length of muscle, and plantar flexion performance. To look into this hypothesis, the present study investigated the behavior IEMG of each head of the triceps surae muscle during various ramp contraction durations (5, 10, 15 and 20 s), at three different knee joint angles (0 deg and flexed at 30 deg and 90 deg), across different ankle plantar flexion levels of force (10, 20, 30, 40, 50, 60 and 70% MVC).

Materals and Methods

Seven healthy men, (27.7 ± 5.6 years old; 168.4 ± 3.4 cm in height; 63.9 ± 12.8 kg in weight [mean ± S.D.]), volunteered for the present study. None had any history of musculoskeletal injury or orthopedic abnormality in the lower extremities. All of the subjects were fully informed of the nature of this research and gave their informed consent before participating in the experiments.

The subjects were positioned and secured on a dynamometer (KinCom AP; Chattanooga, TE, U.S.A.) for measurement of isometric ankle plantar flexion. The body was placed in a prone position on a test bench with the pelvis stabilized by belts. With the right foot firmly attached to the dynamometer, the knee angle was locked in a previously determined position with a turn-buckle type knee brace and attached to the bench (Fig. 1). To prevent the entire body upward, the pelvis of the subject was further stabilized by another belt, of the type usually used for indirect lumbar traction. EMG activity was recorded using Ag-AgCl surface electrodes (Blue Sensor, type NF-50-F, specially modified to a 16 mm diameter, MEDICOTEST, Ølstykke, Denmark). Before applying the electrodes, the skin was shaved and prepared to reduce skin resistance. Two pairs of electrodes were placed over each of the GM, GL and SO muscles of the right leg in a bipolar configuration parallel to the muscle fiber over the apparent belly of the muscles according to Delagi et al.27). Each pair of electrodes was positioned so that the central distance of the electrodes was set at 30 mm and held in place with adhesive tape to limit movement artifacts. Next, the ground electrode was placed over the greater trochanter of the femur. All EMG signals were pre-amplified with a gain of 2 by differential amplifiers (DPA-10P; Diamond Medical, Tokyo) and then amplified with a gain of 200 (Biotop 6R12; NEC San-ei, Tokyo). The best S/N ratio of either channel of EMG signals for each muscle was used for the subsequent filtering process.

Fig. 1.

Fig. 1.

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The experimental arrangement for measurement of ankle plantar flexion force. The center of the ankle joint was set at the rotational center of the lever arm of KinCom (dynamometer). For a better view, the two belts to stabilize the pelvic are not shown. a. the head of KimCom; b. transducer of KimCom; c. adjustable parts of ankle device; d. elastic band; e. pads; f. turn-buckle knee brace.

Before the experiments, isometric ankle plantar flexion forces with the maximal effort were measured twice for a 5-sec period at an ankle joint angle of 0 deg (neutral position, the footplate of the dynamometer perpendicular to the tibia) and a knee angle of 0, 30, or 90 deg. The higher force between two maximal efforts was determined as the MVC at each knee joint angle. Subjects were allowed a 3-min rest between maximal efforts. For experimental measurements, we used a force trajectories system (KM-EMS101; Henry Japan, Hasuda) consisting of both hardware and software components. Because the force output of KinCom has linearity, we adjusted the gain with a weight of 100 N using the dial on the hardware controller after calibrating it to 0 without any weight. The hardware received force signals from the KimCom system and output four different signals, i.e., raw force, amplified force, target force, and trigger signals.

The experimental protocol consisted of four different ramp contraction durations (5, 10, 15 and 20 s) at three different knee joint angles. A ramp was defined as a single ongoing contraction with a linear increase in force from 0 to MVC. A 3-min rest period was allowed between each contraction. The subjects were provided visual feedback of their elicited force trajectories and a target force line on a computer monitor during all contractions. Twelve trials for each maneuver were repeated for each subject if the subject made good force trajectories compared to a target line on the computer monitor (Fig. 2). If the force elicited by the subject deviated more than 5 % of the maximal force from the target line upto 80% MVC, the subject was asked to repeat the trial after a 3-min rest.

Fig. 2.

Fig. 2.

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Typical data from one subject: ankle planter flexion force and target line with 5% deviation lines during 5-s ramp contraction.

The raw EMG signals and force signals from a dynamometer were simultaneously recorded on a 16channel 8 mm data recorder (RX-800; TEAC, Tokyo) with a frequency range of DC-20 kHz. All signals were also continuously monitored on a pen-recorder, with a paper duration of 5 mm/sec. EMG signals were sampled at a frequency of 1.0 kHz and analogue-to-digital conversion with 12-bit accuracy over the range ± 5 V (AD12-16U(PC)EH; Contec, Osaka). The data were stored on an 8 mm tape for subsequent analyses.

The EMG signals were digital bandpass filtered between 10 Hz and 350 Hz (Bimtas2; Kissei Comtec; Matsumoto). IEMG values of the muscles were computed for 512 points of EMG signals, and each of its center was corresponding to the following levels of force: 10, 20, 30, 40, 50, 60 and 70% MVC, respectively. The IEMG values obtained from the four ramp contractions at three different knee joint angles at the same levels of force were used for the statistical analysis and graphical representation of the data.

Statistical analyses

Three-way ANOVAs for repeated measure were used to estimate differences in IEMG values in each of the GM, GL, and SO, taken at four different ramp contraction durations (5, 10, 15 and 20 s), at three different knee joint angles (0, 30, and 90 deg), across levels of force (10, 20, 30, 40, 50, 60 and 70% MVC). As post hoc tests, Tukey's honestly significant difference (HSD) test and multiple comparison tests were performed. p<0.05 was assumed to indicate statistical significance.

Results

The amplitude and density of raw EMG in each head of the triceps surae muscles were increased in all ramp contractions trials at increasing forces. Figure 3 shows the relationship between IEMG and force (% of MVC) for each of the three muscles at four different ramp contraction durations (5, 10, 15 and 20 s), at three different knee joint angles (0, 30, and 90 deg), at 10, 20, 30, 40, 50, 60, 70 % MVC were shown for GM, GL, and SO (Fig. 3). According to three-way ANOVAs, IEMG of the GM muscle showed a first-order interaction between the level of force and knee joint angle (df=12; F=8.96; p<0.05; Table 1). In addition, IEMG of the GL muscle showed first-order interactions between the level of force and knee joint angle (df=12; F=2.84; p<0.05), and between the level of force and duration of ramp contraction (df=18; F=1.93; p<0.05). Furthermore, IEMG of the SO showed a main effect only on the level of force (df=6; F=49.0; p<0.05).

Fig. 3.

Fig. 3.

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The Relationship between the force (in % of MVC) and integrated EMG (mVflms) at three different position of knee (extended, 30 deg and 90 deg in flexion) during four different durations of ramp contraction (5, 10, 15 and 20 s). a. medial head of the gastrocnemius (GM), b. lateral head of the gastrocnemius (GL) and c. soleus (SO). Durations of ramp contraction; 5 s (Inline graphic), 10 s (Inline graphic), 15 s (Inline graphic) and 20 s (Inline graphic) in rise time. The values represent the mean ± S.E. (n=7).

Table 1. Summary of three-way ANOVAs for each head of the triceps surae muscle.

C A × C B × C
GM *
GL * *
SO *
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A: 4 different durations of ramp contraction; B: knee joint angle; C: level of force. × indicates interaction between two factors; * indicates statistical significance (p<0.05).

For the multiple comparison tests for the GM, we combined and compared the ramp contraction durations as means because there were no significant interactions involving durations of contraction. Multiple comparison tests for the GM revealed that IEMG with the knee at 30 deg flexion showed the most sensitive increases as the ankle plantar flexion force was increased (Fig. 4). IEMG with the knee extended also increased under the same condition, but to a lesser extent. IEMG at 90 deg knee flexion was less sensitive to the increases in the levels of force. When IEMG of the GM was compared among all three knee joint angles, that obtained at 90 deg flexion was significantly smaller than those obtained at the knee extended and at 30 deg flexion positions at all levels of force investigated (Fig. 4). IEMG values at 30 deg knee flexion at 60 and 70% MVC of ankle plantar flexion force were significantly larger than those at 0 deg and at 90 deg. In addition, IEMG values at 0 deg knee flexion at 60 and 70% MVC ankle plantar flexion force were significantly larger than those at 90 deg.

Fig. 4.

Fig. 4.

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The relationship between the force (in % of MVC) and integrated EMG (mV·ms) of medial head of the gastrocnemius muscle with the knee at 0, 30, and 90 deg flexion.

For the multiple comparison tests for the GL, the ramp contraction durations were combined and compared as means for a first-order interaction between level of force and knee joint angle, whereas the knee joint angles were treated as they were in the calculation for first-order interactions between the level of force and duration of ramp contraction. Multiple comparison tests on the first-order interaction between the level of force and knee angle for the GL showed that the IEMG of GL had an order of sensitivity to ankle plantar flexion force comparable to that of the IEMG of the GM (Fig. 5). As for angular differences of the knee, IEMG of the GL between at 90 deg knee flexion differed from that at the other two knee angles (Fig. 5). IEMG values at 0 deg knee flexion at 50 and 60% MVC ankle plantar flexion force were significantly larger than those at 30 deg and 90 deg, while IEMG values at 30 knee flexion at 60 and 70% MVC were significantly larger than those at 90 deg. Multiple comparison tests on another first-order interaction between the level of force and ramp contraction duration for the GL revealed almost the same results among the four ramp contraction durations (Fig. 6). In the comparison among ramp contraction durations, IEMG obtained at an ankle plantar flexion force of 60% MVC was significantly different between ramp contraction durations of 10 and 20 s (Fig. 6). In addition, IEMG values obtained during 20 s ramp contraction at 70 % MVC were significantly different from those obtained during 5 and 10 s ramp contractions.

Fig. 5.

Fig. 5.

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The relationship between the force (in % of MVC) and integrated EMG (mV·ms) of lateral head of the gastrocnemius muscle with the knee at 0, 30, and 90 deg flexion.

Fig. 6.

Fig. 6.

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The relationship between the force (in % of MVC) and integrated EMG (mV·ms) of lateral head of the gastrocnemius muscle during four different ramp contraction durations of 5, 10, 15, 20 s.

To analyze the main effect of levels of force by multiple comparison tests for the SO, we combined and compared the ramp contraction duration and knee joint angle as means since there were no significant interactions involving ramp contraction durations and knee joint angles. IEMG values at 60 and 70% MVC were larger than those at weaker ankle plantar flexion levels of force (Fig. 7).

Fig. 7.

Fig. 7.

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The relationship between the force (in % of MVC) and integrated EMG (mV·ms) of the soleus muscle.

According to our results, the constituent muscles within the triceps surae were clearly distinguished from each other with respect to the duration of contraction, length of muscle, and plantar flexion force.

Discussion

The triceps surae is generally considered to be the prime mover of ankle plantar flexion because of its muscular mass and the length of the lever arm of the moment on the ankle joint28). Many investigators have reported on different aspects of this muscle.

Positions of knee angle

As the GM and GL are biarticular muscles while the SO is a monoarticular one, differences of muscle activation in the former depend upon the angular positions of the knee29). According to their M-wave study on these three muscles, when subjects moved from a knee extended position to 90 deg knee flexion, the M-wave amplitudes decreased in the GM and GL (0.64 ± 0.14, 0.66 ± 0.16, respectively), while that of the SO (0.96 ± 0.05) remained unchanged. Our results indicated that the biarticular muscles, the GM and GL, showed a significant decrease in IEMG at higher levels of ankle plantar flexion force when the knee was flexed at 90 deg, whereas the monoarticular muscle, the SO, showed no significant difference at any angular position of the knee. Several factors could explain these results. First of all, neuromuscular transmission failure is likely to bring about decreases in IEMG values and the amplitude of M-wave. As the ramp contractions at three different knee angles were based on the MVC at each knee angle in our experiments, the central drive from the spinal motor neurons was considered to be unchanged. Second, the muscle length and tension also influenced the force production. Several differences have been reported in the activation of the gastrocnemius muscle (GA) and SO30–33). When the knee is flexed at 90 deg, both heads of the GA are placed at a mechanical disadvantage due to the shortened length of the muscle. On the other hand, during isokinetic ankle plantar flexion with the knee extended, the GA and SO were suspected to have different optimal lengths at different ankle joint angles30). As the ankle plantar flexion force was measured at three different knee angles with the ankle at a neutral angle in our experiment, the GA might have been subject to mechanical disadvantages as the knee angle increased. Third, as muscle itself serves as a conductor, the EMG signal characteristics depend on the number of muscle fibers within the recording volume of electrodes34). When we looked at the relationship between the diameter of a muscle fiber and muscle length in our study, the diameter of a muscle fiber decreased when the muscle was stretched, and vice versa. In other words, the numbers of muscle fibers of the GM and GL obtained by surface electrodes with the knee extended and flexed at 30 deg, may have far outnumbered those obtained with the knee flexed at 90 deg. Another factor that might have influenced the EMG of the triceps surae muscle was the thickness of skin layer. According to an earlier report, the skin layer thickness could act as a low-pass filter35), hence the EMG of the triceps surae muscle obtained by surface electrodes might be influenced by the knee joint angle. This factor remains unclear in our experiment, as we did not measure thickness of the skin layer at any of the knee joint angles.

Ankle plantar flexion levels of force

Relating to the increase of IEMG during ramp contraction, EMG signals from all of the triceps increased in amplitude and firing rate with increasing levels of force. This result agrees with other literature that has dealt with prime movers15–18). In short, muscle force is graded primarily by increases in the number of active motor units and firing rate modulation1–3).

Duration of ramp contraction

While IEMG of the GL exclusively showed significant increase during the longer duration of ramp contraction, this result can be imputed to be several factors. The first factor is the size of motor unit action potential. The progressive enlargement in the IEMG during submaximal contraction is thought to primarily reflect an increase in the number of recruited motor units and in their firing rates36–38). Compared to the GM, the GL has a significantly shorter mean contraction and half-relaxation time39)40). In addition to these contractile properties, spectrum analysis of EMG signals obtained by the GL showed that median frequency during step contraction exclusively increased as ankle plantar flexion force increased in our previous study10). These two findings may indicate that the GL consists larger proportion of fast-fatigable muscle among the type 2 fiber subgroups than GM. A recent study also indicated the possibility that the size of motor unit action potentials enlarge, especially on fast-fatigable muscles41). Compared to smaller sizes of motor unit action potentials, increased firing rate of the larger size of motor unit action potential by fast-fatigable muscle at higher force of ankle planter flexion during longer duration of contraction, may cause significantly increase the IEMG.

In addition to the number of recruited motor units and in their firing rates, the decreased conduction velocity of muscle fiber during prolonged contractions may influence the IEMG42–44). Muscle fiber conduction velocity was decreased mainly by peripheral mechanisms such as changes in the membrane excitability presumably due to hydrogen ion and metabolic accumulation45)46) , K+ accumulation in the extracellular space47) and the changes of pH48). A steeper decrease in the muscle fiber conduction velocity may account for the increase in IEMG.

Motor control

The last factor relating to muscle activation is motor control. Previous reports have speculated that the EMG signals may differ as a result of the actual role played by a muscle in a particular task49)50). It has been hypothesized that motor neuron pools are organized into task groups to help the CNS coordinate the activity of motor units in other synergistic muscles. Within the triceps surae muscle, the EMG of the GA increased while that of the SO decreased during faster isotonic contraction. In cases where the prime movers consist of more than two muscles, selective recruitment strategy to control the duration of contraction has been suggested. In addition to these recruitment strategy, a recent study relating to motor control, suggested that shortened GA muscle may be influenced by peripheral affarents capable of reducing the excitability of the motoneurone pool51).

In conclusion, the present results suggest that each of the constituent muscles in the triceps surae has three distinctive functional characteristics, namely, the duration of contraction, muscle length, and plantar flexion force. These task-dependent motor strategies of human triceps surae muscle may contribute to the high functional precision of ankle plantar flexion performance.

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