A strong coordination between the two legs is important for maintaining a symmetric gait pattern and adapting to changes in the external environment. In humans as well as animals, receptors arising from the quadriceps muscle group influence the activation of ipsilateral muscles. Moreover, strong contralateral spinal connections arising from quadriceps and hamstrings afferents have been shown in animal models. Therefore, the aims of the present study were to assess if such connections also exist in humans and to elucidate on the possible pathways. Contralateral reflex responses were investigated in the right leg following unexpected unilateral knee joint rotations during locomotion in either the flexion or extension direction. Strong reflex responses in the contralateral biceps femoris (cBF) muscle with a mean onset latency of 76 ± 6 ms were evoked only from ipsilateral knee extension joint rotations in the late stance phase. To investigate the contribution of a transcortical pathway to this response, transcranial magnetic (TMS) and electrical (TES) stimulation were applied. Motor evoked potentials (MEPs) elicited by TMS, but not TES, were facilitated when elicited at the time of the cBF response to a greater extent than the algebraic sum of the cBF reflex and MEPs elicited separately, indicating that a transcortical pathway likely contributes to this interlimb reflex. The cBF reflex response may therefore be integrated with other sensory input, allowing for more flexible responses. We hypothesize that the cBF reflex response may be a preparation of the contralateral leg for early load bearing, slowing the forward progression of the body in order to maintain dynamic equilibrium during walking.
Journal of Physiology, 2013, Vol 591, Issue 19, p. 4921-4935