Interlimb reflexes following ipsilateral knee joint rotations are suppressed in an unstable walking environment

Interlimb reflexes play an important role in human walking, such as when dynamic stability is threatened by external perturbations or changes in the walking surface. For example, we have previously shown that interlimb reflexes in the contralateral biceps femoris (cBF) following ipsilateral knee (iKnee) extension rotations during walking contribute to slowing the forward progression of the body in order to maintain dynamic stability following the perturbation (Stevenson et al., 2015). However, little is known about how such interlimb reflexes are modulated in an unstable walking environment. Based on experiments investigating intra- and interlimb cutaneous reflexes and soleus H-reflexes (Llewellyn et al., 1990; Haridas et al., 2005; 2006; Krauss and Misiaszek, 2007), we hypothesized that the amplitude of interlimb reflexes following iKnee perturbations is altered when walking in an unstable environment. To test this, interlimb reflexes were elicited in participants (n = 6) by either iKnee extension rotations during the late stance phase (50%) of the gait cycle or iKnee flexion rotations during the mid-swing phase (80%). The iKnee perturbations were applied while the participants walked normally on a treadmill (stable condition), or while random increases or decreases in velocity were applied to the treadmill (unstable condition). The abrupt treadmill velocity changes were applied every 3-8 steps, included three different velocity changes between ±1.12 m/s, and lasted for 1.5 seconds. They were strong enough to cause instability, but not enough to result in a fall. In the unstable condition, the iKnee perturbations were applied in steps without treadmill velocity changes. During normal walking, iKnee extension perturbations elicited facilitatory interlimb reflexes in the cBF and contralateral soleus (cSOL) muscles with onset latencies of 81 and 92 ms, respectively, while iKnee flexion perturbations elicited reflexes in the cBF (facilitatory) and cSOL (inhibitory) with onset latencies of 66 and 85 ms, respectively. The cBF reflex amplitudes were significantly suppressed (P’s < .05) in the unstable walking condition following both types of iKnee perturbations, while the cSOL reflexes remained unchanged. Consistent with previous studies, these preliminary results suggest that challenging dynamic stability during walking leads to specific changes in reflex amplitudes that are not related to a generalized change in reflex excitability. Descending cortical influences likely contribute to the specific modulations based on the environmental demands.