On the Modulation of the Short Latency Reflex in the Locomoting Human

Gait is one of the most fundamental activities the human motor system is designed to perform. The human motor system utilizes sophisticated control strategies to enable seamless every day walking. Not only are we, humans, capable of efficiently controlling their gait, we are also capable of adapting how gait is controlled based on changes to the environment or to the body itself. Decades of evidence now suggest that the reflex circuitry of the spinal cord is directly involved in gait control, and that they adapt when gait is adapted. However, the exact role of spinal reflexes in gait control and adaptation remains vaguely understood. In this work, I set out to investigate the dynamics of spinal reflexes during gait adaptation using techniques from neurophysiology and computational and experimental gait biomechanics. I demonstrate for the first time, using a technique called the H-reflex, that the short latency spinal reflexes undergo significant modulation during gait adaptation. I show that this adaptation can occur in one leg but not the other during split-belt gait adaptation, and that it is likely of supraspinal origins. I then present a series of computational and experimental studies that establish a causal link between changes of spinal reflex gains and gait control. I show, computationally, that step length asymmetry can be directly controlled by preferentially tuning spinal reflex gains. I then present experimental evidence that inducing similar changes in the reflex gain during active locomotion produce reproducible transient patterns of gait asymmetry, hence establishing a causal link between the two. Collectively, my thesis work highlights tuning of spinal reflex gains as a potential mechanism the human motor system utilizes to control gait.