Interlimb communication: Crossed responses in the human biceps femoris muscle

A continual coordination between the two legs is necessary for maintaining a symmetric walking pattern and adapting to changes in the external
environment. Recent evidence in animals and humans suggests that spinal interneuronal circuits under supraspinal control may mediate communication between the lower limbs. The overall objective of the present thesis was to further investigate and elucidate neural pathways underlying interlimb communication in humans, focusing primarily on the possible interlimb connections to the biceps femoris muscle. The major aims were 1) to investigate whether interlimb reflexes are present in sitting and walking following ipsilateral knee (iKnee) joint rotations (Studies I and III), 2) to elucidate the neural pathways involved in mediating the interlimb reflexes (Studies I, II and III), and 3) to investigate the functional role of the observed interlimb reflexes during walking (Study IV).
Study I demonstrated that short-latency (44 ms) crossed-spinal reflexes are present in the contralateral biceps femoris (cBF) muscle during sitting. The cBF reflexes were inhibitory following iKnee extension joint rotations, facilitatory following iKnee flexions, and intramuscular recordings revealed that the same population of cBF motor units were involved in the reversal in sign of the reflex. Study II indicated that velocity sensitive muscle spindle afferents likely contribute to the short-latency cBF reflexes. Study III showed that, while short-latency interlimb reflexes were not observed during walking, strong cBF reflex responses were evoked from iKnee extension joint rotations in the late stance phase with an onset latency of 76 ms. An increase in evoked responses in the cBF from transcranial magnetic stimulation (TMS) and not transcranial electrical stimulation (TES) following iKnee extensions provided evidence for a transcortical pathway contributing to this interlimb reflex. Study IV demonstrated that the functional role of the cBF reflex is likely a preparation for early load bearing, slowing the forward progression of the body to maintain dynamic equilibrium during walking. Therefore, the transcortical cBF reflex may be integrated with other sensory input, allowing for responses that are more adaptable to the environmental demands.
These results provide new insights into the neural mechanisms underlying human interlimb communication, as well as their functional relevance to human locomotion. Although it is difficult to propose the exact neural pathways mediating interlimb reflexes to the cBF muscle, this thesis provides the basis for future studies.

The results provide new insights into the neural mechanisms underlying human interlimb communication, as well as their functional relevance to human locomotion. Although it is difficult to propose the exact neural pathways mediating interlimb reflexes to the cBF muscle, this thesis provides the basis for future studies.