Contributions of feed-forward and feedback strategies at the human ankle during control of unstable loads

The nervous system can regulate the mechanical properties of the human ankle through feed-forward mechanisms such as co-contraction and rapid feedback mechanisms such as stretch reflexes. Though each of these strategies may contribute to joint stability, it is unclear how their relative contribution varies when ankle stability is threatened. We addressed this question by characterizing co-contraction and stretch reflexes during balance of an inverted pendulum simulated by a rotary motor configured as an admittance servo. The stability of this haptic environment was manipulated by varying the stiffness of a virtual spring supporting the pendulum. We hypothesized that co-contraction and stretch reflex amplitude would increase as the stability of the haptic load attached to the ankle was reduced. Electromyographic activity in soleus, medial and lateral gastrocnemius, and tibialis anterior was used to characterize co-contraction patterns and stretch reflex amplitude as subjects stabilized the haptic load. Our results revealed that co-contraction was heightened as stability was reduced, but that the resulting joint stiffness was not sufficient to fully counteract the imposed instability. Reflex amplitude, in comparison, was attenuated as load stability was reduced, contrary to results from upper limb studies using similar paradigms. Together these findings suggest that the nervous system utilizes feed-forward co-contraction rather than rapid involuntary feedback to increase ankle stability during simple balance tasks. Furthermore, since the stiffness generated through co-contraction was not sufficient to fully balance the haptic load, our results suggest an important role for slower, volitional feedback in the control of ankle stability during balancing tasks.     

Stretch reflex excitability of the anti-gravity ankle extensor muscle in elderly humans

AIM: To examine whether the stretch reflex excitability of the soleus muscle changes with age, stretch reflexes at rest (REST) and during weak voluntary contractions (ACT) were elicited in 18 older and 14 younger subjects. METHOD: The amplitude of the stretch reflex responses and gain, defined as the gradient of the regression line for the relation between stretch reflex responses against the angular velocity of the applied perturbation, were evaluated in each short-latency (M1) and two long-latency components (M2 and M3). RESULTS: It was found that in the older group, both the amplitude and gain of the M1 component did not change from the REST to the ACT conditions, whereas in the younger group both variables significantly increased from the REST to ACT conditions. The latency of the M1 component was significantly shorter under the REST condition (older vs. younger: 51.8 +/- 7.37 vs. 55.1 +/- 8.69 ms), while no group differences were found in those variables under the ACT condition, suggesting that the muscle-tendon complexes of SOL muscles of the older subjects were less elastic and had less slack, probably due to age-related histochemical alterations. Further, the Hoffman reflex (H-reflex), elicited during the REST condition in 10 older and 11 younger subjects showed no significant differences, suggesting that the soleus motoneuron response to the Ia input was comparable between the two subject groups. CONCLUSION: The histochemical alterations occurring with the ageing process might augment the short-latency stretch reflex in the SOL muscle without enhancement of motoneuronal excitability, and this effect might be masked when the muscle is voluntarily activated.     

Spacing practice sessions across days earlier rather than later in training improves performance of a visuomotor skill

Reduced depression of transmitter release from Ia afferents following previous activation (post-activation depression) has been suggested to be involved in the pathophysiology of spasticity. However, the effect of this mechanism on the myotatic reflex and its possible contribution to increased reflex excitability in spastic participants has not been tested. To investigate these effects, we examined post-activation depression in Soleus H-reflex responses and in mechanically evoked Soleus stretch reflex responses. Stretch reflex responses were evoked with consecutive dorsiflexion perturbations delivered at different intervals. The magnitude of the stretch reflex and ankle torque response was assessed as a function of the time between perturbations. Soleus stretch reflexes were evoked with constant velocity (175°/s) and amplitude (6°) plantar flexion perturbations. Soleus H-reflexes were evoked by electrical stimulation of the tibial nerve in the popliteal fossa. The stretch reflex and H-reflex responses of 30 spastic participants (with multiple sclerosis or spinal cord injury) were compared with those of 15 healthy participants. In the healthy participants, the magnitude of the soleus stretch reflex and H-reflex decreased as the interval between the stimulus/perturbation was decreased. Similarly, the stretch-evoked torque decreased. In the spastic participants, the post-activation depression of both reflexes and the stretch-evoked torque was significantly smaller than in healthy participants. These findings demonstrate that post-activation depression is an important factor in the evaluation of stretch reflex excitability and muscle stiffness in spasticity, and they strengthen the hypothesis that reduced post-activation depression plays a role in the pathophysiology of spasticity.     

Soleus H-reflex excitability changes in response to sinusoidal hip stretches in the injured human spinal cord

Imposed static hip stretches substantially modulate the soleus H-reflex in people with an intact or injured spinal cord while stretch of the hip flexors affect the walking pattern in lower vertebrates and humans. The aim of this study was to assess the effects of dynamic hip stretches on the soleus H-reflex in supine spinal cord injured (SCI) subjects. Sinusoidal movements were imposed on the right hip joint at 0.2 Hz by a Biodex system. H-reflexes from the soleus muscle were recorded as the leg moved in flexion or extension. Stimuli were sent only once in every hip movement cycle that each lasted 5 s. Torque responses were recorded at the hip, knee, and ankle joints. A hip phase-dependent soleus H-reflex modulation was present in all subjects. The reflex was facilitated during hip extension and suppressed during hip flexion. There were no significant differences in pre- or post-stimulus soleus background activity between the two conditions. Oscillatory responses were present as the hip was maximally flexed. Sinusoidal hip stretches modulated the soleus H-reflex in a manner similar to that previously observed following static hip stretches. The amount of reflex facilitation depended on the angle of hip extension. Further research is needed on the afferent control of spinal reflex pathways in health and disease in order to better understand the neural control of movement in humans. This will aid in the development of rehabilitation strategies to restore motor function in these patients.     

Modulation of short latency stretch reflexes during human hopping

To gain insight into central and peripheral reflex control mechanisms in moving humans we have investigated short latency stretch reflex activity in m. tricgif surae during two legged hopping. The objectives were: (1) to compare movement induced short latency stretch reflexes in soleus and medial gastrocnemius (MG) muscles, (2) to determine the relationship between the size of these reflexes and the muscle spindle stretch velocities, and (3) to compare the size of the movement induced short latency stretch reflexes and the H-reflexes simultaneously. Six well-trained healthy male subjects participated and they hopped at three different work rates. Surface electromyogram (EMG) and H-reflexes were recorded during hopping. Muscle spindle length changes were estimated as the difference between estimated origin-to-insertion length changes and tendon length changes. The important findings were that during hopping: (1) movement induced short latency stretch reflexes were observed consistently in soleus, (2) the EMG amplitude of this stretch reflex was negatively correlated with the estimated peak muscle spindle stretch velocity (r_s=?0.52, P < 0.02), and (3) the amplitude of the soleus H-reflex at touchdown did not change in parallel with the stretch reflex. The negative correlation observed between the stretch reflex and the estimated peak muscle spindle stretch velocity in soleus is opposite to the basic velocity sensitive behaviour of stretch reflexes mechanically elicited during resting conditions. Possible control mechanisms are discussed. Additionally, muscle spindle length changes estimated from changes in the skeletal movements (joint angles) should be inferred cautiously because of tendon compliance, especially at high tendon forces.     

Evidence for reduced efficacy of the Ia-pathway during shortening plantar flexions with increasing effort

To determine whether the soleus (SOL) H-reflex is modulated during shortening contractions in a manner that has been observed for isometric contractions, SOL H-reflexes and M-waves were elicited via percutaneous electrical stimulation to the tibial nerve at an intensity that evoked an H-reflex at 50% of its maximum in 11 healthy subjects. Paired electrical stimuli were delivered as the ankle angle passed through 90° at an interval of 400 ms while the subject performed shortening contractions at levels of plantar flexion torque ranging between 2 and 30% of that during a maximal voluntary contraction (MVC). H-reflexes were also recorded during the performance of isomeric contractions of plantar flexors at similar levels of plantar flexion torque and at the same joint angle (muscle length) in an additional five healthy subjects. Correlations were examined between the peak-to-peak amplitude of the first H-reflexes, M-waves and plantar flexion torques in both protocols. It was revealed that no significant correlation was found between the SOL H-reflex and increasing plantar flexion torque during shortening contractions (ρ = −0.07, P = 0.15), while a strong positive correlation was observed for the isometric conditions (ρ = 0.99, P < 0.01). No significant change was observed in the SOL M-wave for either contraction type. Furthermore, the H-reflexes elicited via paired stimuli with the same background activity in voluntary shortening contractions showed almost identical amplitudes, suggesting that the level of homosynaptic post-activation depression did not change in response to the varying levels of activation in voluntary shortening contractions. Therefore, the lack of increase in the H-reflex during shortening contractions at increasing intensities is possibly due to a centrally regulated increase in presynaptic inhibition. Such a downward modulation of the reflex suggests that Ia-excitatory input onto the SOL motoneurone pool needs to be reduced during the performance of shortening contractions.     

Phase-dependent inhibition of H-reflexes during walking in humans is independent of reduction in knee angular velocity.

The purpose of this investigation was to investigate whether reduction in impulses arising from stretch of the quadriceps by restricting rapid knee flexion in early swing would affect inhibition of the H-reflex during swing. The contribution of afferent input arising from knee angular velocity to phase-dependent modulation of short-latency responses in the soleus was studied by simultaneously measuring joint velocity and soleus H-reflex responses at midstance and midswing phases of treadmill walking in 15 normal subjects. Stimulus strength was varied so that both maximal M and H waves were identified in each subject at midswing and midstance with the knee unrestricted (UK) and with knee movement restricted (RK), using a full leg bivalved cast to immobilize the knee joint. All subjects exhibited short-latency reflex responses in the soleus muscle. The H/M ratio at midswing was significantly reduced compared with midstance under both UK and RK walking conditions (P < 0.0001). When compared with UK walking, knee joint angular velocity during RK walking was significantly reduced at midswing (P < 0.001) and midstance (P < 0.005) compared with UK. There were, however, no significant differences in H/M ratios at midswing and midstance between UK and RK walking tests. Inhibition of the H-reflex in the soleus muscle during swing was not affected by significant reduction in knee angular velocity. These results indicate that the sensory input from changes in angular velocity at the knee does not lay the inhibitory foundation of phase-related reflex modulation in the ankle extensors during walking as suggested by Brooke and colleagues.     

Autogenetic reflex action in tibialis anterior compared with that in soleus muscle in the decerebrate cat

Summary The regulatory actions of autogenetic reflex pathways on the mechanical properties of an ankle flexor (tibialis anterior) and an extensor (soleus) in the premammillary decerebrate cat were studied. The two muscle were isolated in the same cat and each was stretched during its separate activation. The yield in stiffness shown by areflexive muscles during stretch was largely compensated for in tibialis anterior as well as in soleus by reflex action. Resultant (total) stiffness varied by less than a factor of two over a wide range of contractile forces in the two muscles. Further, resultant stiffness increased as stretch amplitude decreased in both muscles, but the variation was less for TA. In most preparations, the resultant stiffness in soleus was significantly larger than the resultant stiffness of tibialis anterior. It is concluded that autogenetic reflexes govern the mechanical properties of both flexors and extensors. In addition, the extensor bias in the decerebrate preparation is due not only to greater activation in extensors but to a greater resultant stiffness as well.     

Effects of changes in hip joint angle on H-reflex excitability in humans

We examined the amplitude modulation of the soleus (Sol) H-reflex during controlled variations of the hip joint angle in 21 healthy adult human subjects. Hip angle variations were imposed separately, or in combination either with stimulation of the plantar skin or with electrical activation of muscle afferents from the medial gastrocnemius (MG) nerve. We found that with subjects in the supine position, flexion of the hip significantly depressed Sol H-reflex excitability, by as much as 50% of control reflex values (Ho) recorded at 10 degrees of hip flexion. Conversely, significant facilitation of the H-reflex was observed when the hip joint was extended (10 degrees), with amplitudes reaching 200+/-15.3% of Ho. Changes in H-reflex amplitude were also observed during electrical stimulation of either the foot sole or the MG nerve, when stimuli were delivered at different hip angles. Foot sole stimulation resulted in facilitation of the H-reflex with the hip extended while depression of the reflex was recorded with the hip flexed. In contrast, MG nerve stimulation at group-I muscle afferent strength resulted in a significant increase in the Sol H-reflex magnitude with the hip flexed, while during hip extension, no significant effects were observed [corrected]. This study provides evidence for the existence of a spinal mechanism, determined principally by the hip joint angle, which promotes switching between inhibitory and facilitatory pathways during hip flexion and extension. The origins of such a spinal mechanism are discussed.     

Modulation of reflex responses in activated ankle dorsiflexors differs in healthy young and elderly subjects

During voluntary contractions, motor neurone activity is modulated by descending input and sensory feedback. Impaired excitatory afferent feedback with ageing may, therefore, alter motor control. This study investigated the age-related changes in afferent feedback through the recording of reflex responses during voluntary muscle activation. Short- and long-latency components of the stretch reflex and Hoffmann reflex (H-reflex) were recorded during voluntary contractions (10% of maximal voluntary contraction; MVC) of the ankle dorsiflexor muscles of young and elderly adults (≥70 years). Furthermore, the modulation of spinal reflex excitability was analyzed at different torque levels (10–50% MVC). The short-latency stretch reflex and the H-reflex areas were similar in the two age groups at 10% MVC whereas the area of the long-latency component of the stretch reflex augmented with ageing (P < 0.05). However, the area of the H-reflex increased linearly with the level of contraction up to 50% MVC in young adults, whereas it slightly increased to 30% MVC and plateaued thereafter in elderly adults. The absence of age-related changes in the short-latency stretch reflex and H-reflex areas suggests that the reflex circuitry and the sensitivity of the muscle spindles are not substantially affected by ageing. The modest increase in the H-reflex area with the contraction intensity in elderly adults, however, indicates that the modulation of afferent feedback is reduced with advancing age. This observation, associated with a greater long-latency stretch reflex, suggests that elderly adults rely more on central than peripheral mechanisms to regulate motor output of the dorsiflexor muscles.     

Short-term plasticity of human spinal inhibitory circuits after isometric and isotonic ankle training

The purpose of this study was to determine to what extent one session of isotonic and isometric ankle dorsi and plantar flexion training induces changes in the frequency-dependent depression of the soleus H-reflex. Further, adaptation of reciprocal Ia inhibition exerted from tibialis anterior flexor group I afferents on soleus motoneurons, and presynaptic inhibition of Ia afferent terminals induced by a conditioning afferent volley following stimulation of the antagonist nerve were established with subjects seated before and after training. The soleus H-reflexes evoked at the inter-stimulus intervals of 1, 2, 3, 5, and 8 s were normalized to the mean amplitude of the H-reflex evoked every 10 s. Conditioned H-reflexes were normalized to the associated control H-reflex evoked with subjects seated before and after training. Twenty-six subjects were randomly assigned to one or more of the 4 exercise groups. Isometric ankle dorsi flexion training decreased the reciprocal and presynaptic inhibition, while isotonic ankle dorsi flexion had no significant effects. Isotonic plantar flexion training decreased only the reciprocal inhibition, whilst isometric plantar flexion had no significant effects on the reciprocal or presynaptic inhibition. None of the training exercise protocols affected the amount of homosynaptic depression of the soleus H-reflex. Our findings support the notion that plastic changes of reciprocal and presynaptic inhibition due to exercise are transferrable to a resting state, and that homosynaptic depression remains unaltered after a single session of ankle training. Further research is needed to outline the time-course of plastic changes of spinal inhibitory mechanisms in humans.     

Intrinsic properties and reflex compensation in reinnervated triceps surae muscles of the cat: effect of activation level

The manner in which activation levels influence intrinsic muscular properties and contributions of the stretch reflex were studied in homogeneous soleus (SOL) and heterogeneous gastrocnemius (G) muscles in the decerebrate cat. Intrinsic mechanical properties were represented by the initial stiffness of the muscle, measured prior to reflex action, and by the tendency of the muscle to yield during stretch in the absence of the stretch reflex. Stiffness regulation by the stretch reflex was evaluated by measuring the extent to which reflex action reduces yielding and the extent to which stiffness depends on background force. Intrinsic mechanical properties were measured in muscles deprived of effective autogenic reflexes using the method of muscular reinnervation. Reinnervated muscles were recruited to force levels comparable to those achieved during natural locomotion. As force declined during crossed-extension reflexes in reinnervated and intact muscles, initial stiffness declined according to similar convex trajectories. The data did not support the hypothesis that, for a given force level, initial stiffness is greatest in populations of predominantly type I motor units. Incremental stiffness (Deltaf/Deltal) of both G and SOL increased in the presence of the stretch reflex. Yielding of SOL (ratio of incremental to initial stiffness) substantially decreased in the presence of the stretch reflex over the full range of forces. In reflexive G, yielding significantly decreased for low to intermediate forces, whereas at higher forces, yielding was similar irrespective of the presence or absence of the stretch reflex. The stretch reflex regulates stiffness in both homogeneous and heterogeneous muscles.     

Reflex response and control of the human soleus and gastrocnemius muscles during walking and running at increasing velocity

We measured the soleus and the gastrocnemius H-reflex modulation in seven subjects during walking at 4.5 km/h and during running at 8, 12 and 15 km/h. The recordings in the medial gastrocnemius were corrected for cross-talk from the soleus muscle. The gastrocnemius H-reflex was in general lower than the soleus H-reflex. In both muscles the H-reflex increased significantly from walking to running but also with increasing running speed. The peak of EMG activity increased in both muscles with increasing speed. The V-wave of both muscles was absent or rather low during walking, but it increased significantly from walking to running with increasing running speed in the soleus but not in the medial gastrocnemius. In both muscles the V-wave was highest just prior to heel strike. It is suggested that this was due to a high firing frequency of the motoneurones in this phase of the movement. It is concluded that a shift towards the faster gastrocnemius at higher running speeds on behalf of the soleus muscle did not occur. The fact that the physiological cross-sectional area of the soleus is much larger than that of the lumped gastrocnemii is most probably the reason why the soleus is important also at higher running velocities.     

Changes in the excitability of soleus muscle short latency stretch reflexes during human hopping after 4 weeks of hopping training

Changes in the excitability of the human triceps surae muscle short latency stretch reflexes were investigated in six male subjects before and after 4 weeks of progressive two-legged hopping training. During the measurements the subjects performed 2-Hz hopping with: preferred contact time (PCT) and short contact time. The following reflex parameters were examined before and after the training period: the soleus muscle (SOL) Hoffmann-reflex (H-reflex) at rest and during hopping, the short latency electromyogram (EMG) components of the movement induced stretch reflex (MSR) in SOL and medial gastrocnemius muscle (MG), and the EMG amplitude of the SOL and MG tendon reflexes (T-reflexes) elicited at rest. The main results can be summarized as follows: the SOL T-reflex had increased by about 28% (P < 0.05) after training while the MG T-reflex was unchanged; the SOL MSR (always evident) and the MG MSR (when observable) did not change in amplitude with training, and before training the SOL H-reflex in both hopping situations was significantly depressed to about 40% of the reference value at standing rest (P < 0.05). After training the H-reflex during PCT hopping was no longer depressed. As the value of the measured mechanical parameters (the total work rate, joint angular velocity and the ankle joint work rate) was unchanged after training in both hopping situations, the reflex changes observed could not be ascribed to changes in the movement pattern. To explain the observed changes, hypotheses of changes in the excitability of the stretch reflex caused by the training were taken into consideration and discussed.     

Patterns of muscle activation in human hopping

In the present study, we examined the electromyogram (EMG) patterns of the soleus and medial gastrocnemius (MG) muscles during rhythmical, two-legged hopping to investigate the contributions of the monosynaptic short- and long-latency stretch reflexes during such a natural movement in human. During rhythmical hopping, soleus muscle is activated reflexly at near-monosynaptic latency by stretch resulting from passive ankle flexion upon landing. Soleus muscle also contracts voluntarily in order to launch the body into the next hop. This is part of the rhythmical bursts of activity producing the hops. Depending on the hopping interval, this phase of activation can follow the short-latency phase or precede landing at very short hopping intervals. In MG, there is an initial phase of activity that stiffens the muscle in preparation for landing, and continues through the contact phase. The monosynaptic reflex response to landing is usually superimposed on this activity. Depending on the hopping interval, both of these responses may be overlaid with activity that is time-locked to the take-off into the next hop, and serves to launch the body into the next hop. However, no evidence for a long-latency stretch reflex was found. In addition, the preferred hopping frequency for all subjects was about 2 Hz. This frequency is associated with a pattern of EMG activity the timing of which indicates that it balances the requirement for a comfortable landing from a hop with the optimal muscle activation required for launching the following hop. Electronic Publication     

Rhythmic arm cycling differentially modulates stretch and H-reflex amplitudes in soleus muscle

During rhythmic arm cycling, soleus H-reflex amplitudes are reduced by modulation of group Ia presynaptic inhibition. This suppression of reflex amplitude is graded to the frequency of arm cycling with a threshold of 0.8 Hz. Despite the data on modulation of the soleus H-reflex amplitude induced by rhythmic arm cycling, comparatively little is known about the modulation of stretch reflexes due to remote limb movement. Therefore, the present study was intended to explore the effect of arm cycling on stretch and H-reflex amplitudes in the soleus muscle. In so doing, additional information on the mechanism of action during rhythmic arm cycling would be revealed. Although both reflexes share the same afferent pathway, we hypothesized that stretch reflex amplitudes would be less suppressed by arm cycling because they are less inhibited by presynaptic inhibition. Failure to reject this hypothesis would add additional strength to the argument that Ia presynaptic inhibition is the mechanism modulating soleus H-reflex amplitude during rhythmic arm cycling. Participants were seated in a customized chair with feet strapped to footplates. Three motor tasks were performed: static control trials and arm cycling at 1 and 2 Hz. Soleus H-reflexes were evoked using single 1 ms pulses of electrical stimulation delivered to the tibial nerve at the popliteal fossa. A constant M-wave and ~6% MVC activation of soleus were maintained across conditions. Stretch reflexes were evoked using a single sinusoidal pulse at 100 Hz given by a vibratory shaker placed over the triceps surae tendon and controlled by a custom-written LabView program. Results demonstrated that rhythmic arm cycling that was effective for conditioning soleus H-reflexes did not show a suppressive effect on the amplitude of the soleus stretch reflex. We suggest this indicates that stretch reflexes are less sensitive to conditioning by rhythmic arm movement, as compared to H-reflexes, due to the relative insensitivity to Ia presynaptic inhibition.     

Reduced short and long latency reflexes during voluntary tracking movement of the human wrist joint

In six healthy human subjects we compared changes in the strength of Hoffmann (H), short latency (30-55 ms) and long latency (55-100 ms) stretch reflexes of flexor carpi radialis (FCR) muscle during movement and isometric contractions. In one set of experiments, stretches were imposed to the wrist during voluntarily tracked sinusoidal movement and during matched isometric contractions to compare short and long latency stretch reflex responses. In the second set, H-reflexes were compared during similar matched conditions. All reflexes decreased significantly (P < 0.05) during the voluntary tracking movement. The H-reflex was reduced during the wrist flexion, on average, by 33% of its value obtained during the isometric condition. Compared with their values during isometric conditions, the short latency stretch reflex and long latency stretch reflex during movement were reduced by 52 and 40%, respectively. From the pattern changes of the stretch reflexes and the H-reflex, a movement-induced presynaptic inhibition combined with pronounced muscle spindle unloading is proposed to play an important role in decreasing the strength of the stretch reflexes during the tracking task as compared with a matched isometric contraction.     

Plasticity of reflexes from the foot during locomotion after denervating ankle extensors in intact cats

Although sensory feedback is important in regulating the timing and magnitude of muscle activity during locomotion few studies have evaluated how it changes after peripheral nerve lesions. To assess this, reflexes evoked by stimulating a nerve before and after denervating other nerves can be quantified to determine changes. The aim of this study was to investigate consequences of denervating ankle extensor muscles, the lateral gastrocnemius, and soleus (LGS) on reflexes from the plantar foot surface evoked by stimulating the tibialis (Tib) nerve. Three cats (n = 3) were trained to walk on a treadmill and chronically implanted with electrodes in 14 hindlimb muscles bilaterally to record EMG activity. A stimulating cuff electrode was placed around the left Tib nerve (Tib) nerve at the ankle to evoke reflexes. Several control values of EMGs, limb kinematics, and Tib nerve reflexes were obtained during locomotion for at least 3 wk before the left LGS nerve was cut. We found that the locomotor EMG bursts of several muscles was altered, with a large increase in amplitude in the early days postneurectomy followed by a gradual decrease toward intact values later on. There were changes in the stimulated locomotor EMG bursts (Tib nerve reflexes) of ipsilateral flexors and extensors and of contralateral ankle extensors, which dissociated from changes in baseline locomotor EMG (e.g., nonstimulated bursts during reflex trials). The functional significance of these changes in muscle activity and reflex pathways on the recovery of locomotion after denervating ankle extensors is discussed.     

Changes in muscle activity with increase in leg stiffness during hopping

While the spring-like leg behavior of legs in mammalian locomotion has been well documented, its neural basis remains ambiguous. The purpose of the present study was to examine leg stiffness control during hopping. Seven male subjects performed in place two-legged hopping at their preferred frequency with two different contact times of the stance phase, preferred and short ones (PCT and SCT, respectively). Based on a spring-mass model, leg stiffness was calculated from the subjects' body mass, ground contact and flight times. Surface electromyographic (EMG) activities of the medial gastrocnemius (MG), soleus (SOL) and tibialis anterior (TA) muscles were recorded. Leg stiffness was higher in the SCT condition than in the PCT condition. The SCT condition was characterized by high EMG activity of MG and SOL at both pre- and post-landing phases, which peaked at about 50 ms. On the other hand, the activity of TA was low throughout the contact phase as compared with those of MG and SOL, and its peak value around 50 ms after landing was significantly lower for the SCT condition than for the PCT condition. We conclude that (1) the leg stiffness is regulated by a change in centrally programmed muscle preactivation and probably also by a concomitant change in the short-latency stretch reflex response of the triceps surae muscles, and (2) the co-contraction of antagonistic TA does not play a major role in leg stiffness control.     

Effects of UTP on Na+, Cl− and K+ transport in primary cultures from human sweat gland coils

In six healthy human subjects we compared changes in the strength of Hoffmann (H), short latency (30–55 ms) and long latency (55–100 ms) stretch reflexes of flexor carpi radialis (FCR) muscle during movement and isometric contractions. In one set of experiments, stretches were imposed to the wrist during voluntarily tracked sinusoidal movement and during matched isometric contractions to compare short and long latency stretch reflex responses. In the second set, H-reflexes were compared during similar matched conditions. All reflexes decreased significantly (P < 0.05) during the voluntary tracking movement. The H-reflex was reduced during the wrist flexion, on average, by 33% of its value obtained during the isometric condition. Compared with their values during isometric conditions, the short latency stretch reflex and long latency stretch reflex during movement were reduced by 52 and 40%, respectively. From the pattern changes of the stretch reflexes and the H-reflex, a movement-induced presynaptic inhibition combined with pronounced muscle spindle unloading is proposed to play an important role in decreasing the strength of the stretch reflexes during the tracking task as compared with a matched isometric contraction.