Why is it called triceps surae

Triceps surae muscle is another term used for the calf muscles , more specifically two of the three muscles of the superficial posterior compartment of the leg :. The plantaris muscle is not considered a part of this group but still contributes to the Achilles tendon.

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About Recent Edits Go ad-free. Edit article. View revision history Report problem with Article. In human walking, forces are necessary for propelling the body forward. At the same time, due to the gravitational attraction exerted by the Earth, upward-directed forces are obligatory for keeping the body in equilibrium and preventing it from falling.

Plantar flexor muscles are good candidates for generating both forces, since they exert their action at the interface between the human body and the ground. Debate has been surrounding the functional role of ankle flexors for some time.

Originally, based on the existing co-variation between triceps surae electromyographic EMG activity and velocity of progression, Winter [1] suggested that ankle plantar flexors provide the active push-off during the late part of the single stance phase. In contrast, Perry [2] had advised dropping the term push-off and postulated that the peak of the ground-reaction force in late stance phase is the result of the leverage put forth by the body alignment with respect to Earth-vertical axis rather than of an active downward thrust.

An introduction to these issues can be found in an influential paper by Sutherland et al. At the other temporal edge, Bogey et al. The push-off hypothesis has been endorsed by a series of articles modelling the net ankle moment [5] , [6] , [7] , [8] , [9]. However, other experiments have provided results that undermine this hypothesis.

Tibial nerve block with paralysis of triceps surae muscle along with plantaris, tibialis posterior, flexor hallucis longus, and flexor digitorum longus resulted in an increase in forward velocity of the centre of mass CoM during the late stance phase of gait, which was interpreted as defective control of CoM fall [3] , [10].

Replacement of one lower limb by a prosthesis did not affect the speed of progression, regardless of whether the stance limb was prosthetic or not [11]. Therefore, triceps surae would not provide the propelling thrust by pushing off the ground. Hence, the contribution of the plantar flexors to whole-body forward displacement during normal walking would primarily consist in restraining forward tibial rotation, thereby stabilising the knee joint [3] , and in controlling the braking of the fall of the CoM during the single support phase of gait, as recently suggested by Chastan et al.

The role of the ankle flexors in body support during gait has been agreed upon in the literature. The postural role of soleus in quiet standing is amplified during locomotion [14] , [15] , which imposes more body support as ankle torque increases [16] , [17]. Anderson and Pandy [18] studied the muscle contribution to body support using mathematical modelling based on a force-sharing problem algorithm.

They found that the ankle flexors generated nearly all the body support in early and late stance, in addition of being responsible for the second peak typically observed in the vertical ground reaction force. Neptune et al. They stressed however the fact that in late stance phase the energy produced by the soleus accelerates the trunk forward, while the gastrocnemii would deliver most of their energy to accelerate leg into swing.

On the other hand, Liu et al. Our aim was to unravel the functional role of ankle plantar flexors during human locomotion. We set out to see whether the triceps surae provides forward thrust by pushing off the ground or it controls body dynamic equilibrium, or does both at the same time.

The problem here lies in the fact that the coordination and synergy produced by the walking body make that several gait parameters are highly correlated. Propulsive force, forward velocity and triceps surae EMG during the stance phase are one example [1] , [20]. We posited that in order to properly assess the role of ankle plantar flexors in gait, one should increase propulsive forces for a constant forward velocity.

Since adding a load requires greater external force to move the body, then an increase in plantar flexor EMG activity for the greater antero-posterior force at the same walking velocity would verify the push-off hypothesis. Conversely, the absence of load-related increase in plantar flexor activity would discard the push-off hypothesis and support a functional role of these muscles in balance control. The gait initiation paradigm was used [21] , [22] , [23].

This choice was based on the fact that subjects initiating walking from a static upright position, when the initial forward-directed velocity is null, would actively produce the whole of the propulsive force according to the push-off concept. The push off might be less necessary if gait was already in the stationary state, when the body speed itself would produce part of the propulsive force. So, failure to support the push-off hypothesis during gait initiation would reinforce with stronger reason the alternative hypothesis.

We concurrently analysed the contribution of gravity in creating propulsive force by measuring the displacement of the CoM away from the support axis, which generates the forward disequilibrium torque. Ten healthy volunteers one female and nine males took part in the experiment. Their mean age, body mass and height were 34 years range 23—54 , 72 kg 61—83 and 1.

Written informed consent was obtained, as required by the Declaration of Helsinki and by the EA local Ethics Committee of University Paris-Sud, who specifically approved this study. A large force platform 0.

The force platform was embedded in the ground. The walkway was long enough 7 meters to allow subjects to carry out at least 6 steps, hence avoiding the interference of gait termination with the gait initiation motor programme [25].

The overall mass of the added load was 20 kg and consisted of 2 weight-lifting disks of equal mass and size 29 cm diameter and 3 cm thickness put in two backpacks carried by the subject at the abdominal and lumbar back level, i. The backpacks were firmly wrapped to the body to avoid unwanted displacement during stepping. For a subject weighting 83 kg, a total mass of 30 kg load was used instead. Electrode sites were prepared by cleansing and shaving the skin for optimal myoelectric impedance.

GM and GL were recorded from only 7 of the 10 subjects. EMG signals were amplified x and band-pass filtered 10— Hz. Before recording, we determined the preferential starting foot of the subjects. Subjects were asked to stand still eyes closed, and a small thrust was applied to their back forcing them to make a step forward. This was repeated 3 times. Then, subjects were instructed to initiate gait with the stepping leg that was used during this test.

In order to obtain a good reproducibility of the progression velocity during the experiment, subjects executed several blank trials to determine the steps lengths corresponding to their spontaneous S and fast F speed walking conditions, and two landmarks representing the step length for the S and F condition were then drawn on the force platform for each subject.

The average walking velocity proved to be about 1. Subjects stood still, barefoot on the force platform, looking straight ahead, and initiated gait following a verbal go-signal. They were instructed not to start walking in a reaction-time mode, but to start when they felt ready. This usually occurred within 2s from the go-signal.

Both S and F conditions were repeated with and without the added load L. All subjects started by performing the spontaneous unloaded series, following which the other conditions were performed in a pseudo-random order. Fifteen trials were acquired in each experimental condition. While walking, the body exerts a force on the ground, which in return applies on the subject an opposite force, the ground reaction force GRF that is measured by the force platform.

In addition, the force platform gave two moments with respect to the AP and ML axis of the platform. The coordinates of centre of pressure CoP were obtained by dividing these moments by the vertical GRF.

CoM velocity was obtained by integration of the CoM acceleration. The instantaneous position of the CoM was obtained by double integration of the CoM acceleration with respect to time [13] , [27]. All mechanical and EMG traces refer to walking at spontaneous S velocity.

The left panel shows the control condition no added load , the right panel shows the loaded L condition. The traces are assembled in four panels according to the type of recording. All traces start at time 0, corresponding to the onset of the anticipatory adjustment preceding the production of the first step, based on the magnified trace of the ML CoP position.

The vertical dotted lines are set at the instant of foot off FO and foot contact FC of the swinging leg. The period between FO and FC is the single stance phase of gait. The triceps muscles are active during this phase, starting shortly after FO and terminating around FC. The numbers and ticks on selected traces of the left panel indicate critical points for the analysis: 1—2, onset and offset of the propulsive force increase; 3—4, onset and offset of the braking action; 5, peak of AP CoM velocity; 6, AP CoP position used to determine step length.

The load also increases the difference of the AP component of the GRF, but has negligible effect on the other variables. During gait, CoM oscillates vertically while rotating around the CoP in the sagittal plane, and the CoM fall is braked during the single stance phase of gait. The braking of the CoM fall was evaluated as the variation of the vertical GRF between its minimum and maximum value within the single stance phase points 4 and 5 in Fig.

EMG activity of each muscle was calculated for three partly overlapping time windows: Wtot corresponds to the total duration of the burst, between the onset and termination of the EMG activity; Wb is the braking action phase and Wp is the propulsive phase.

The braking action goes from the instant when Ver GRF reverts and goes upwards until the time of foot contact FC , and the propulsive phase corresponds to the time interval initiating at the instant when AP GRF increases steeply and terminating just prior to FC points 1 and 2 in Fig. The integrated EMG activity of each trial was then divided by the time duration of each window in order to get the mean activity level.

For graphical representation, EMG activity was expressed as a percentage relative to the mean value of the unloaded spontaneous velocity condition for each muscle and for each of the three time-windows.

The focus of the present study was the effect of load on muscle activity for a same walking velocity as opposed to the effect of velocity itself. However, velocity was selected as independent variable in order to check the statistical difference between the velocities. Categorical factors were velocity and load. The EMG-related variables were the instants of onset and offset of the activity of each of the muscles with respect to time of gait initiation, and the mean level of the EMG surface of each muscle for each of the time windows Wtot, Wp and Wb.

Paired t-test was used to test the effect of load on the delay between the onset of SOL and the onset of the braking action. In Fig. The overall kinematics and lower limb muscle activity of the gait initiation process have been described in detail elsewhere [21] , [23] , [27].

Briefly, the gait initiation process includes two phases. The former is an anticipatory postural adjustment APA , which starts at the onset of the ground reaction force GRF variation t0 in Fig. Both events produce the backward displacement of the CoP. This produces a gap between the CoP position and the vertical projection of the CoM, causing a forward disequilibrium torque [23] , [28]. The latter phase is the step execution that follows the APA.

It goes from the time of FO of the swinging leg until the foot-contact FC of the same leg. Kinematics results Table 1 show that all subjects faithfully executed the experimental instruction, and thus maintained their walking velocity and step length regardless of the added load, thereby producing the same velocity condition.

For the progression velocity, the grand mean value was almost identical without and with load for the spontaneous velocity. The load had no effect on the progression velocity for the fast velocity, either. Adding the load had no effect on step length, for either the spontaneous or fast velocity conditions. The instants of FO and FC with respect to t0 did not change with added load, either, and this was true within the spontaneous and the fast velocity conditions Table 1. The onset of this increase point 1 was set at the minimal value of AP GRF during the single stance phase that occurred usually around mid-stance.

Ver GRF trace was valley-shaped. After attaining a minimum value point 3 , it increased again and reached a value well beyond body weight point 4. The variation of Ver GRF between points 3 and 4 is the braking action, or the vertical force opposing the CoM fall seen in the vertical CoM velocity trace.

Visual inspection of the two columns of Fig. Conversely, adding the load did not obviously change the braking action amplitude. The onset of the increase in Ver GRF always preceded the onset of the steep increase in AP GRF, while the duration of the braking action and that of the propulsive phase overlapped for a while during the single stance period from FO to FC. Velocity and position of the CoM are shown in Fig.

The negative value of the vertical CoM velocity until FC indicates that the CoM was always falling during the whole single support period, but with two phases, the first accelerating downward, the second decelerating, when the fall of CoM was being restrained.

The Ver CoM instantaneous position 6 th trace from top fell below its initial value during the single stance, stabilised slightly around foot contact and moved up again during the double support phase while the triceps surae muscles were being deactivated. The CoM curve matches well in profile and maximum value the curve calculated with an independent method a motion capture system by Jian et al. The third panel from top shows the CoP traces. The CoP underwent a displacement from the rear to the fore foot, where its forward movement was obviously halted due to foot length limitation, while the CoM continued to advance on the sagittal plane in a parabolic manner 5 th trace from top in Fig.

Triceps surae muscles were silent during the postural adjustment phase, while their synchronous bursts preceded shortly the vertical braking action and ended at around FC.

This was true under both unloaded and loaded conditions. The onset of the burst occurred in the interval between FO and the onset of the braking action and varied slightly between subjects and repeated measures. One source of variability between trials likely depended on the force platform registering the global resultant forces and not the local forces produced by the individual muscles.

This variation also affected the onset and termination of the muscle bursts. Another source of variability depended on the way the onset of the braking action was identified, since this was conservatively set at the lowermost point of the Ver GRF trace, in a region where the profile is not particularly sharp.

In spite of these sources of variations, consistent findings were observed in the EMG pattern. On the average, all muscles initiated their activity around FO, and ahead of point 4 the onset of the braking action , and terminated at or shortly after FC. In this example, but also for all muscles and subjects, the braking action never anticipated the EMG onset. The grand mean and standard deviation of the time-interval between onset of SOL and onset of braking action were 0.

The onset of the braking action has been plotted as a function of the onset of the SOL EMG activity for the unloaded left and loaded condition. The individual data points corresponding to spontaneous and fast velocities are superimposed in each plot.

The braking action regularly lags the onset of SOL activity, so that the data points identify a line parallel to the identity line dotted diagonal. In this subject, the points for spontaneous and fast velocity are almost confounded, and the points for the loaded condition indicate a small delay of the onset of the braking action with respect to the onset of the muscle activity. Such behaviour is only in part reflected in the other subjects, so that the mean intercept of the best fit lines are not significantly different between unloaded and loaded conditions.

On the same time scale, the mean instants of foot-off FO and foot contact FC of the swing leg are indicated by vertical dashed lines. The vertical dotted lines refer to the mean onset of the braking action. The two top graphs refer to spontaneous S walking velocity, unloaded left and loaded right.

The data of the fast F velocity conditions are reported in the bottom graphs. However, for both velocities, load increased the duration of the bursts, chiefly by anticipating the onset of their activity with respect to FO.

The histogram bars of Fig. The time-intervals between onset of EMG and onset of braking action BA, dotted line are broadly superimposed for the different muscles, with some anticipation for the SOL, inconsistent across trials and subjects.

To note is an advancement of the onset of the EMG bursts with load for both velocities, in the face of a substantial similarity in the time distribution of the EMGs between spontaneous and fast velocity. The values pertain to the time-window Wtot, from the onset to the termination of the EMG.

The left panel indicates that the subject generated two different propulsive forces during the unloaded and loaded trials, while the corresponding mean EMG values were essentially the same for the same walking velocity. The right panel shows that neither the amplitude of the braking action nor the EMG activity were affected by load within each velocity. This is summarized in Fig. The upper part of the Figure A contains two graphs, reporting the mean data from all trials of a representative subject.

The left part of the left graph spontaneous velocity, S shows that AP GRF the propulsive force is larger when load is added filled circle compared to no-load open circle. Notably, this increase occurs without changes in the SOL activity measured during Wtot. A similar pattern is shown in the right part of the same graph fast velocity, F.

Note that F velocity is associated with an increase in SOL activity with respect to S velocity abscissa : adding the load increases the propulsive force but does not further increase the amplitude of the burst. The lower part B contains two composite panels that summarize the results from all subjects. The left panel shows the mean and standard deviation of AP and Ver GRF, for the spontaneous top and fast velocity bottom. Open bars refer to no-load, filled bars to added-load condition.

The right panel shows the muscle activities for spontaneous top and fast velocity bottom conditions, unload and loaded, calculated within each time window Wtot: entire burst, Wb: braking action, Wp: propulsive force. The EMG is expressed in percentage of the mean value recorded in the normal unloaded condition. There is an effect of velocity on braking action, propulsive force and muscle activity all bars are higher in the bottom graphs , but no effect of load on any variable, except propulsive force at both velocities.

The right panel of Fig. However, within the same velocity, even when the amplitude of the propulsive force increased significantly as an effect of load, the grand mean of the EMG activity remained unchanged for SOL, GM and GL, regardless of the three time-windows used.

Worth noting is that, when the subjects performed the fast walking trials, EMG activity increased concurrently with the increase in the braking action at fast velocity, but again there was no increase in EMG activity when the load was added. Remarkably, there were no changes with load even in the Wp interval corresponding to the propulsive phase of the stance period, in any muscle and for either velocity, i. Therefore, it is fit to conclude that active recruitment of SOL, GM and GL muscle activity was not responsible for the increase in propulsive force required by the added load, but only for the increase in the braking action occurring with the increase in step length.

Worth noting is the striking similarity of the traces of the disequilibrium torque and AP GRF between unloaded and loaded conditions. This similarity verifies the fact that the energy generated by CoM rotation around CoP is transformed into forward propulsive force. Visual inspection of the individual traces in Fig. To better understand this, the CoM-CoP gap and the disequilibrium torque at the instant of foot contact were calculated.

The grand mean values and standard deviation of the gap and disequilibrium torque are reported in Fig. The traces are from one representative subject, during unloaded left and loaded condition right at spontaneous walking velocity.

The gap increases with velocity, but does not change with load. Conversely, the torque increases with both velocity and load. In turn, the Ver GRF is composed of body weight and variations with respect to it due to the vertical acceleration of CoM.

Since these variations CoM fall and braking action remained constant, then the increase in the disequilibrium torque was due to the absolute increase in body weight 4 th trace in Fig. Our findings are not in keeping with the notion that the triceps surae is responsible for the generation of propulsive forces for walking. They rather show that the triceps supports the body while it translates over the ankle joint, restraining it from falling. Indirectly, though, triceps surae activity controls step length and walking speed.

To challenge the push-off hypothesis we have used the gait initiation paradigm and compared the condition, in which the subject was loaded, to that without load while maintaining the same progression velocity between experimental conditions.

A thrust to the ground support would seem necessary to generate a forward-propulsive force to propel the body forward, and even more so with the added load. This push-off would necessarily come from ankle plantar-flexor force, because plantar flexors are suitably arranged and are active during single stance [18] , [23] , [30]. Since laws of motion dictate that more force is required to propel a heavier object, we added a weight to the subject to induce an increase in the propulsive force.

The significant increase of antero-posterior ground reaction force AP GRF that we observed during the loaded trials was indeed in accordance with the predicted effect of load. Because of the linear relationships between progression velocity and triceps surae EMG activity, and between progression velocity and propulsive force [1] , [31] , the effect of velocity had to be isolated in order to unambiguously assert that triceps activity is or is not responsible of AP propulsive force generation.

Thus, recording the triceps activity while imposing the same progression velocity between unloaded and loaded conditions permitted to extricate the postural from the propulsive role of the triceps. Further, the effect of load was tested at two velocities. Replicating the effect of load at different velocity conditions corroborated the strength of the results. Our findings were consistent within the two velocity conditions, where GRF, kinematics and EMG are modified by the effect of speed.

Therefore, we feel confident that our conclusions can be generalized to a range of walking velocities. GRF data alone undermine the push-off hypothesis.