Facilitation of the soleus stretch reflex induced by electrical excitation of plantar cutaneous afferents located around the heel

Location-specific cutaneous electrical stimulation of the footsole modulates corticospinal excitability to the plantarflexors and dorsiflexors during standing

Non‐noxious electrical stimulation to distinct locations of the foot sole evokes location‐specific cutaneous reflex responses in lower limb muscles. These reflexes occur at latencies that may enable them to be mediated via a transcortical pathway. Corticospinal excitability to the plantarflexors and dorsiflexors was measured in 16 participants using motor evoked potentials (MEPs). Spinal excitability was measured in eight of the original participants using cervicomedullary motor evoked potentials (CMEPs). Measurements were collected with and without preceding cutaneous stimulus to either the heel (HEEL) or metatarsal (MET) locations of the foot sole, and evoked potentials were elicited to coincide with the arrival of the cutaneous volley at either the motor cortex or spinal cord. Plantarflexor MEPs and CMEPs were facilitated with cutaneous stimulation to the HEEL for MEPs (soleus p = 0.04, medial gastrocnemius (MG) p = 0.017) and CMEPs (soleus p = 0.047 and MG p = 0.015), but they were unchanged following MET stimulation for MEPs or CMEPs. Dorsiflexor MEPs were unchanged with cutaneous stimulation at either location, but dorsiflexor CMEPs increased with cutaneous stimulation (p = 0.05). In general, the increase in CMEP amplitudes was larger than the increase in MEP amplitudes, indicating that an increase in spinal excitability likely explains most of the increase in corticospinal excitability. The larger change observed in the CMEP also indicates that excitability from supraspinal sources likely decreased, which could be due to a net change in the excitability of intracortical circuits. This study provides evidence that cutaneous reflexes from foot sole skin are likely influenced by a transcortical pathway.

Soleus responses to Achilles tendon stimuli are suppressed by heel and enhanced by metatarsal cutaneous stimuli during standing

KEY POINTS

We examined the influence of cutaneous feedback from the heel and metatarsal regions of the foot sole on the soleus stretch reflex pathway during standing. We found that heel electrical stimuli suppressed and metatarsal stimuli enhanced the soleus vibration response. Follow-up experiments indicated that the interaction between foot sole cutaneous feedback and the soleus vibration response was likely not mediated by presynaptic inhibition and was contingent upon a modulation at the ⍺-motoneuron pool level. The spatially organized interaction between cutaneous feedback from the foot sole and the soleus vibration response provides information about how somatosensory information is combined to appropriately respond to perturbations during standing.

ABSTRACT

Cutaneous feedback from the foot sole provides balance-relevant information and has the potential to interact with spinal reflex pathways. In this study, we examined how cutaneous feedback from the foot sole (heel and metatarsals) influenced the soleus response to proprioceptive stimuli during standing. We delivered noisy vibration (10-115 Hz) to the right Achilles tendon while we intermittently applied electrical pulse trains (five 1-ms pulses at 200 Hz, every 0.8-1.0 s) to the skin under either the heel or the metatarsals of the ipsilateral foot sole. We analysed time-dependent (referenced to cutaneous stimuli) coherence and cross-correlations between the vibration acceleration and rectified soleus EMG. Vibration-EMG coherence was observed across a bandwidth of ∼10-80 Hz, and coherence was suppressed by heel but enhanced by metatarsal cutaneous stimuli. Cross-correlations showed soleus EMG was correlated with the vibration (∼40 ms lag) and cross-correlations were also suppressed by heel (from 104-155 ms) but enhanced by metatarsal (from 76-128 ms) stimuli. To examine the neural mechanisms mediating this reflex interaction, we conducted two further experiments to probe potential contributions from (1) presynaptic inhibition, and (2) modulations at the ⍺- and γ-motoneuron pools. Results suggest the cutaneous interactions with the stretch reflex pathway required a modulation at the ⍺-motoneuron pool and were likely not mediated by presynaptic inhibition. These findings demonstrate that foot sole cutaneous information functionally tunes the stretch reflex pathway during the control of upright posture and balance.

Limb Segment Load Inhibits the Recovery of Soleus H-Reflex After Segmental Vibration in Humans

We investigated the effects of vertical vibration and compressive load on soleus H-reflex amplitude and postactivation depression. We hypothesized that, in the presence of a compressive load, limb vibration induces a longer suppression of soleus H-reflex. Eleven healthy adults received vibratory stimulation at a fixed frequency (30 Hz) over two loading conditions (0% and 50% of individual's body weight). H-reflex amplitude was depressed ∼88% in both conditions during vibration. Cyclic application of compression after cessation of the vibration caused a persistent reduction in H-reflex excitability and postactivation depression for > 2.5 min. A combination of limb segment vibration and compression may offer a nonpharmacologic method to modulate spinal reflex excitability in people after CNS injury.

Effect of cooling foot sole skin receptors on achilles tendon reflex

INTRODUCTION

This study investigated whether a controlled reduction of foot sole temperature affects the Achilles tendon stretch reflex and plantar flexion. Methods Five stretch reflexes in 52 healthy subjects were evoked by Achilles tendon taps. Short latency responses of 3 muscles of the lower limb and maximal force of plantar flexion were analyzed. Foot sole hypothermia was induced by a thermal platform at various foot temperature conditions: Stage I (25°C), Stage II (12°C), Stage IIIa (0°C), and Stage IIIb (0°C). Results Reduction of plantar cutaneous inputs resulted in a decrease in amplitude of medial gastrocnemius and soleus as well as delays in time to maximal force of plantar flexion. Medial gastrocnemius, lateral gastrocnemius, and soleus were affected differently by induced cooling. No inhibition effects in reflexes were observed at 12°C. Conclusions The results suggest that input on the plantar foot sole participates complementarily in the Achilles stretch reflex Muscle Nerve, 2015. Muscle Nerve 53: 965-971, 2016.

Phase dependent modulation of soleus H-reflex in healthy, non-injured individuals while walking with an ankle foot orthosis

OBJECTIVE

To examine the dynamic modulation of the soleus H-reflex while walking with a posterior leaf spring ankle foot orthosis (PAFO).

METHODS

Soleus H-reflexes were evoked on randomly chosen lower limb of fourteen healthy individuals (age range of 22-36 years, 7 women) while walking on a treadmill with and without a PAFO. In order to capture excitability across the duration of the gait cycle, H-reflexes were evoked at heel strike (HS), HS+100ms, HS+200ms, HS+300ms, HS+400ms in the stance phase and at toe-off (TO), TO+100ms, TO+200ms, TO+300ms, TO+400ms in the swing phase respectively.

RESULTS

H-reflex excitability was significantly higher in the form of greater slope of the rise in H-reflex amplitude across the swing phase (p=0.024) and greater mean H-reflex amplitude (p=0.014) in the swing phase of walking with a PAFO. There was no change in the slope (p=0.25) or the mean amplitude of H-reflexes (p=0.22) in the stance phase of walking with a PAFO. Mean background EMG activity between the two walking conditions was not significantly different for both the tibialis anterior (p=0.69) and soleus muscles (p=0.59).

CONCLUSION

PAFO increased reflex excitability in the swing phase of walking in healthy individuals. Altered sensory input originating from joint, muscle and cutaneous receptors may be the underlying mechanism for greater reflex excitability. The neurophysiological effect of PAFOs on reflex modulation during walking needs to be tested in persons with neurological injury. The relationship between the sensory input and the reflex output during walking may assist in determining if there exists a neurological disadvantage of using a compensatory device such as a PAFO.

Limb compressive load does not inhibit post activation depression of soleus H-reflex in indiviudals with chronic spinal cord injury

OBJECTIVE

We investigated the effect of various doses of limb compressive load on soleus H-reflex amplitude and post activation depression in individuals with/without chronic SCI. We hypothesized that SCI reorganization changes the typical reflex response to an external load.

METHODS

Ten healthy adults and 10 individuals with SCI received three doses of compressive load to the top of their knee (10%, 25%, and 50% of the body weight, BW). Soleus H-reflexes were measured before (baseline) and during the loading phase.

RESULTS

With persistent background muscle activity across all testing sessions, segment compressive load significantly decreased post activation depression in the control group, but did not change the post activation ratio in the SCI group. Normalized H2 amplitude significantly increased according to load (50%> 25%> 10%) in the control group whereas was minimally modulated to load in those with SCI.

CONCLUSIONS

Segment compressive load inhibits post activation depression in humans without SCI, but minimally modulates the reflex circuitry in people with chronic SCI. These findings suggest that spinal cord reorganization mitigates the typical response to load in people with chronic SCI.

SIGNIFICANCE

Early limb load training may impact the reorganization of the spinal cord in humans with acute SCI.

Limb segment load inhibits post activation depression of soleus H-reflex in humans

OBJECTIVE

We investigated the effect of various doses of limb segment load on soleus H-reflex amplitude and post activation depression in healthy individuals. We also explored the influence of limb segment load on spinal circuitry in one individual with chronic SCI.

METHODS

Twenty-eight healthy adults and one SCI subject received compressive loads applied to the top of their knee at varied doses of load (10%, 25%, and 50% of the body weight). Soleus H-reflexes were measured before (baseline) and during the loading phase.

RESULTS

There were no significant differences in H-reflex amplitudes during the 50% BW load-on phase as compared to either baseline session or the load-off phase. However, the post activation depression was decreased over 9% (p<0.05) during the load-on phase compared to the load-off phase and scaled according to load (50%>25%>10%). The post activation depression ratio also appears less responsive to varying loads after chronic SCI.

CONCLUSIONS

Limb segment load decreases post-activation depression in humans. These findings suggest that the mechanism associated with post activation depression is modulated by limb segment load, and may be influenced by spinal reorganization after SCI.

SIGNIFICANCE

Future studies will determine if various levels of spasticity modulate the response of limb segment load on post activation depression in those with acute and chronic SCI.

The effect of kinesiotape on function, pain, and motoneuronal excitability in healthy people and people with Achilles tendinopathy

OBJECTIVE

To investigate the effect of kinesiotape on hop distance, pain, and motoneuronal excitability in healthy people and people with Achilles tendinopathy (AT).

DESIGN

Within-subject design.

SETTING

An academic health science center, which is an acute London National Health Service trust.

PARTICIPANTS

With ethical approval and informed consent, a convenience sample of 26 healthy people and 29 people with AT were recruited. Seven participants were lost after functional testing, leaving 24 participants in each group.

INTERVENTIONS

Kinesiotape applied over the Achilles tendon.

MAIN OUTCOME MEASURES

The single-leg hop test and visual analog scale were measured with and without the tape. Using the Hoffman (H) reflex, change in motoneuronal excitability of calf muscles was measured before tape application, with the tape on and after its removal.

RESULTS

There were no changes to hop distance when tape was applied (P = 0.55). Additionally, there were no changes to pain (P = 0.74). The H reflex amplitude of soleus and gastrocnemius increased in the healthy group after its removal (P = 0.01 and P = 0.03, respectively), whereas the H reflex remained unchanged in people with AT (P = 0.43 and 0.16, respectively).

CONCLUSIONS

Calf muscles were facilitated by kinesiotape in healthy participants. Despite this, there was no change to hop distance. Kinesiotape had no effect on hop distance, pain, or motoneuronal excitability in people with AT. These results do not support the use of kinesiotape applied in this way for this condition.

A novel approach to mechanical foot stimulation during human locomotion under body weight support

Input from the foot plays an essential part in perceiving support surfaces and determining kinematic events in human walking. To simulate adequate tactile pressure inputs under body weight support (BWS) conditions that represent an effective form of locomotion training, we here developed a new method of phasic mechanical foot stimulation using light-weight pneumatic insoles placed inside the shoes (under the heel and metatarsus). To test the system, we asked healthy participants to walk on a treadmill with different levels of BWS. The pressure under the stimulated areas of the feet and subjective sensations were higher at high levels of BWS and when applied to the ball and toes rather than heels. Foot stimulation did not disturb significantly the normal motor pattern, and in all participants we evoked a reliable step-synchronized triggering of stimuli for each leg separately. This approach has been performed in a general framework looking for "afferent templates" of human locomotion that could be used for functional sensory stimulation. The proposed technique can be used to imitate or partially restore surrogate contact forces under body weight support conditions.

Body weight support by virtual model control of an impedance controlled exoskeleton (LOPES) for gait training

The feasibility of an alternative method to support body weight in a powered exoskeleton is demonstrated. Instead of using an overhead suspension system, body weight is supported by augmenting the joint moments through virtual model control. The advantages of this novel method is that it allows for independent support of the left and right leg, and does not interfere with the excitation of cutanous afferents and balance of the body or trunk. Results show that after a short familiarization period the activity of muscles during initial stance reduces and kinematics become close to normal.