Unilateral displacement of lower limb evokes bilateral EMG responses in leg and foot muscles in standing humans

Abstract

During upright stance, foot dorsiflexion induced by the movement of a supporting platform elicits a short- (SLR) and a medium-latency response (MLR) in both the soleus and the flexor digitorum brevis muscles; foot plantarflexion elicits a MLR in the tibialis anterior. The SLR is the counterpart of the stretch reflex, but no general agreement exists about the origin of the MLR, though recent results suggest that it is transmitted through group II afferent fibres. Animal studies have shown that group II fibres impinge on interneurones projecting contralaterally as well as ipsilaterally, whereas group I fibres impinge on interneurones which project mainly ipsilaterally. Therefore, we compared the changes in amplitude and latency of the SLRs and MLRs in the right and left limb during postural perturbations induced while subjects maintained both feet on the platform (both-on condition) or while they maintained only one foot on the platform and the other on firm ground (one-on condition). Under the both-on condition, the pattern of EMG responses described above occurred bilaterally. Under the one-on condition, both SLRs and MLRs occurred in the displaced leg. However, whereas the SLRs did not change in amplitude compared with the both-on condition, the MLRs decreased in amplitude to about 50%. MLRs were also present in the non-displaced leg. They were not preceded by any SLR but showed a further decrease in size with respect to the corresponding responses in the perturbed leg. Latency of the MLRs of the perturbed leg increased by about 5 ms passing from the both-on to the one-on condition. In the latter condition, a further increase of 5 ms was observed in the nonperturbed leg with respect to the displaced one. The occurrence of the MLRs but not of the SLRs in the contralateral non-displaced leg is in keeping with the notion that crossed neural pathways fed by spindle group II afferent fibres subserve the MLRs. The changes in latency of the MLRs under the one-on condition compared with both-on give a cue about the synaptic delays along the neural circuit and the time taken by the afferent impulses to cross the spinal cord.

Interlimb Coordination During the Stance Phase of Gait in Subjects With Stroke

“…Recent studies involving healthy subjects have demonstrated that the degree of plantar flexor activity during propulsion depends on the degree of muscle activity 9 and on the magnitude of the ground reaction force 10 of the contralateral limb during heel strike. This interlimb relation observed during step-to-step transition of unimpaired walking 9,10 , and also in standing-related tasks 11,12 , can be justified by the bilateral influence of group II fibres on spinal interneurons 13 and by the importance of vestibulo-and reticulo-spinal pathways on group II fibres 14 .…”

Interlimb relation during the double support phase of gait: An electromyographic, mechanical and energy-based analysis

“…In fact, experiments on interlimb coordination of leg muscle activation have confirmed that unilateral leg displacement during gait evokes a bilateral response pattern, with a similar onset in both sides 3,4 but only when both limbs are performing a supportive role 5,6 . This is consistent with the evidence that a large majority of midlumbar interneurones recipient from group II input are influenced by afferent fibres from both ipsilateral and contralateral sides 7 and by vestibulo-and reticulo-spinal pathways 8 , and with the importance given to medium latency response from group II to feedback in the stance phase of gait 9 .…”

Changes in FDB and soleus muscle activity after a train of stimuli during upright stance

“…A minimal H reflex was considered when a consistent small H reflex was obtained for each stimulus pulse applied every 10 s. The stimulus pulse amplitude was set to 75% of the intensity required for a minimal soleus H reflex response to a single pulse. This value was smaller than what has been used in other studies 6,7,22 to avoid discomfort and to activate mainly large diameter sensory fibers. The sensation was perceived as an intense but painless vibration.…”

“…The flexor digitorum brevis (FDB) is a foot muscle that serves as a physiological extensor during quiet stance because its contraction acts against the force of gravity 2 . Some studies [3][4][5][6][7][8] have indicated that FDB activity is highly correlated with the position of the body's center of mass, suggesting that stabilization is among its functions. In addition, FDB activity is highly correlated with the soleus EMG pattern during upright posture, which may either be due to common descending commands that act on both MN pools or to common innervations of these pools by each muscle's afferents 3,5 .…”

“…The feet have to be considered as a specific sensory-motor sub-system due to their importance for signaling the position of the body' s center of gravity [27][28][29] and to muscles such as the FDB, which contribute to postural control [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]30 . That increased and sustained FDB activity can be triggered by appropriate stimuli reinforces the relevance of foot muscles for postural control.…”