Soleus Hoffmann reflex amplitudes are specifically modulated by cutaneous inputs from the arms and opposite leg during walking but not standing

Heteronymous feedback from quadriceps onto soleus in stroke survivors

BACKGROUND

Recent findings suggest increased excitatory heteronymous feedback from quadriceps onto soleus may contribute to abnormal coactivation of knee and ankle extensors after stroke. However, there is lack of consensus on whether persons post-stroke exhibit altered heteronymous reflexes and, when present, the origin of increased excitation (i.e. increased excitation alone and/or decreased inhibition). This study examined heteronymous excitation and inhibition from quadriceps onto soleus in paretic, nonparetic, and age-matched control limbs to determine whether increased excitation was due to excitatory and/or reduced inhibitory reflex circuits. A secondary purpose was to examine whether heteronymous reflex magnitudes were related to clinical measures of lower limb recovery, walking-speed, and dynamic balance.

METHODS

Heteronymous excitation and inhibition from quadriceps onto soleus were examined in fourteen persons post-stroke and fourteen age-matched unimpaired participants. Heteronymous feedback was elicited by femoral nerve and quadriceps muscle stimulation in separate trials while participants tonically activated soleus at 20% maximum voluntary isometric contraction. Fugl-Meyer assessment of lower extremity, 10-m walk test, and Mini-BESTest were assessed in stroke survivors.

RESULTS

Heteronymous excitation and inhibition onsets, durations, and magnitudes were not different between paretic, nonparetic or age-matched unimpaired limbs. Quadriceps stimulation elicited excitation that was half the magnitude of femoral nerve stimulation. Femoral nerve elicited paretic limb heteronymous excitation was positively correlated with walking speed but did not reach significance because only a subset of paretic limbs exhibited excitation (n = 8, Spearman r = 0.69, P = 0.058).

CONCLUSIONS

Heteronymous feedback from quadriceps onto soleus assessed in a seated posture was not impaired in persons post-stroke. Despite being unable to identify whether reduced inhibition contributes to abnormal excitation reported in prior studies, our results indicate quadriceps stimulation may allow a better estimate of heteronymous inhibition in those that exhibit exaggerated excitation. Heteronymous excitation magnitude in the paretic limb was positively correlated with self-selected walking speed suggesting paretic limb excitation at the higher end of a normal range may facilitate walking ability after stroke. Future studies are needed to identify whether heteronymous feedback from Q onto SOL is altered after stroke in upright postures and during motor tasks as a necessary next step to identify mechanisms underlying motor impairment.

Heteronymous feedback from quadriceps onto soleus in stroke survivors

Background

Recent findings suggest increased excitatory heteronymous feedback from quadriceps onto soleus may contribute to abnormal coactivation of knee and ankle extensors after stroke. However, there is lack of consensus on whether persons post-stroke exhibit altered heteronymous reflexes and, when present, the origin of increased excitation (i.e. increased excitation alone and/or decreased inhibition). This study examined heteronymous excitation and inhibition from quadriceps onto soleus in paretic, nonparetic, and age-matched control limbs to determine whether increased excitation was due to excitatory and/or reduced inhibitory reflex circuits. A secondary purpose was to examine whether heteronymous reflex magnitudes were related to clinical measures of lower limb recovery, walking-speed, and dynamic balance.

Methods

Heteronymous excitation and inhibition from quadriceps onto soleus were examined in fourteen persons post-stroke and fourteen age-matched unimpaired participants. Heteronymous feedback was elicited by femoral nerve and quadriceps muscle stimulation in separate trials while participants tonically activated soleus at 20% max. Fugl-Myer assessment of lower extremity, 10-meter walk test, and Mini-BESTest were assessed in stroke survivors.

Results

Heteronymous excitation and inhibition onsets, durations, and magnitudes were not different between paretic, nonparetic or age-matched unimpaired limbs. Quadriceps stimulation elicited excitation that was half the magnitude of femoral nerve stimulation. Femoral nerve elicited paretic limb heteronymous excitation was positively correlated with walking speed but did not reach significance because only a subset of paretic limbs exhibited excitation (n = 8, Spearman r = 0.69, P = 0.058).

Conclusions

Heteronymous feedback from quadriceps onto soleus assessed in a seated posture was not impaired in persons post-stroke. Despite being unable to identify whether reduced inhibition contributes to abnormal excitation reported in prior studies, our results indicate quadriceps stimulation may allow a better estimate of heteronymous inhibition in those that exhibit exaggerated excitation. Heteronymous excitation magnitude in the paretic limb was positively correlated with self-selected walking speed suggesting paretic limb excitation at the higher end of a normal range may facilitate walking ability after stroke. Future studies are needed to identify whether heteronymous feedback from Q onto SOL is altered after stroke in upright postures and during motor tasks as a necessary next step to identify mechanisms underlying motor impairment.

Foot sole cutaneous stimulation mitigates neuromuscular fatigue during a sustained plantar flexor isometric task

Neuromuscular fatigue impairs motor coordination, movement stability, and proprioception, which further decreases performance. A neuromechanical coupling exists between foot sole cutaneous mechanoreceptors and motoneurons of the lower limb, however, the contribution of skin sensory input on muscle fatigue remains unclear. The purpose of this study was to determine if the presence of cutaneous stimulation could mitigate the effect of fatigue of the plantar flexor muscles during a sustained isometric task at 30% maximal voluntary contraction (MVC). Participants (N = 16, age 24.1 ± 2.6 yr) underwent a 30% isometric plantar flexor fatiguing task in a seated position with hip, knee, and ankle angle at 80°, 100°, and 90°, respectively, with intermittent MVCs until task failure. Failure was defined as when the participant could no longer maintain 30% MVC for a minimum of two seconds. Throughout the protocol, electrical stimulation was applied to either the right heel, right metatarsals, or no stimulation. A subset of participants (N = 6) underwent an additional condition with electrical stimulation applied to the left arm. MVCs were also conducted intermittently throughout recovery for 30 min. Foot sole cutaneous stimulation mitigated fatigue, as demonstrated by an ~15% increased time to task failure (TTF) compared with the control condition. When normalized to TTF, MVC torque amplitude was not different at each time epoch, which indicated that each %MVC was maintained longer into the fatigue task during the heel and metatarsal stimulation conditions However, there was no significant effect of cutaneous stimulation on recovery. The results indicate that cutaneous stimulation may serve as a feasible means to mitigate fatigue.NEW & NOTEWORTHY Cutaneous coupling with lower limb motor neurons has long been known. We set out to establish whether this pathway could serve a purpose other than muscular modulation during standing and walking. We found that during a submaximal contraction of the plantar flexor muscles, the addition of intermittent cutaneous stimulation to the skin of the foot sole resulted in an increase in time to task failure by 15%, which was over a minute longer in duration. We conclude that skin stimulation may serve as a mechanism to mitigate fatigue.

Robotic investigation on effect of stretch reflex and crossed inhibitory response on bipedal hopping

To maintain balance during dynamic locomotion, the effects of proprioceptive sensory feedback control (e.g. reflexive control) should not be ignored because of its simple sensation and fast reaction time. Scientists have identified the pathways of reflexes; however, it is difficult to investigate their effects during locomotion because locomotion is controlled by a complex neural system and current technology does not allow us to change the control pathways in living humans. To understand these effects, we construct a musculoskeletal bipedal robot, which has similar body structure and dynamics to those of a human. By conducting experiments on this robot, we investigate the effects of reflexes (stretch reflex and crossed inhibitory response) on posture during hopping, a simple and representative bouncing gait with complex dynamics. Through over 300 hopping trials, we confirm that both the stretch reflex and crossed response can contribute to reducing the lateral inclination during hopping. These reflexive pathways do not use any prior knowledge of the dynamic information of the body such as its inclination. Beyond improving the understanding of the human neural system, this study provides roboticists with biomimetic ideas for robot locomotion control.

Rhythmic wrist movements facilitate the soleus H-reflex and non-voluntary air-stepping in humans

Neural coupling between the upper and lower limbs during human walking is supported by modulation of cross-limb reflexes and the presence of rhythmic activity in the proximal arm muscles. Nevertheless, the involvement of distal arm muscles in cyclic movements and sensorimotor neuromodulation is also suggested given their step-synchronized activation in many locomotor-related tasks (e.g., swimming, skiing, climbing, cycling, crawling, etc.). Here we investigated the effect of rhythmic wrist movements, separately and in conjunction with arm swinging, on the characteristics of non-voluntary cyclic leg movements evoked by muscle vibration in a gravity neutral position and on the soleus H-reflex of the stationary legs. For the H-reflex modulation, five conditions were compared: stationary arms, voluntary alternating upper limb swinging, combined upper limb and wrist motion, wrist movements only and motion of the upper limbs with addition of load. Rhythmic wrist movements significantly facilitated the amplitude of non-voluntary leg oscillations, including ankle joint oscillations, and the H-reflex. The latter effect was related to rhythmicity of wrist motion rather than to a simple extra tension in the upper limb muscles (a kind of the Jendrassik manoeuvre) since adding resistance to arm oscillations (without flexion-extension in the wrist joint) had an opposite inhibitory effect on the H-reflex. Our results further support the existence of connections between the distal parts of the upper and lower extremities at the neural level, suggesting that wrist joint movements can be an important component of motor neurorehabilitation.

A soft ankle brace increases soleus Hoffman reflex amplitude but does not modify presynaptic inhibition during upright standing

External ankle supports, such as ankle braces, may improve postural stability by stimulating cutaneous receptors. It remains unknown whether these supports have an effect on the posture central regulation. The aim of this study was to determine the effect of wearing a soft ankle brace on soleus H-reflex amplitude and presynaptic inhibition during standing. Sixteen subjects stood on a rigid floor with their eyes opened, either barefoot or wearing a soft ankle brace. H-reflex amplitude was measured on the soleus muscle by stimulating the tibial nerve electrically. Modulation of presynaptic inhibition was assessed by conditioning the H-reflex with fibular nerve (D1 inhibition) and femoral nerve (heteronymous facilitation) electrical stimulations. The unconditioned H-reflex amplitude was significantly greater when wearing the ankle brace than barefoot, whereas D1 and HF conditioned soleus H-reflex did not differ significantly between bracing conditions. These results suggest that the ankle brace increased the soleus motoneuron excitability without altering presynaptic mechanisms, potentially because of increased cutaneous mechanoreceptors afferent signals provided by the soft ankle brace.