The relationship between the sensory responses to ankle-joint loading and corticomotor excitability.

PURPOSE: Maintaining joint stability is dependent on the ability of the nervous system to sense and react to potentially injurious loads. In attempts to understand the neurophysiologic mechanisms underlying joint stability, this afferent and efferent activity has been quantified separately at the cortical, segmental and peripheral levels using various electrophysiologic techniques in vivo. However, no studies have attempted to quantify sensory and motor activation at multiple levels of the nervous system in a single subset, to understand potential adaptations for optimizing joint stability.

MATERIALS AND METHODS: Muscle spindle afferent activity and sensory cortex event-related desynchronization were quantified during ankle-joint loading; and motor excitability was assessed through transcranial magnetic stimulation and the Hoffmann reflex in a subset of 42 able-bodied individuals. Microneurography and electroencephalography were used to collect the muscle spindle afferent and sensory cortex activation, respectively, as joint load was applied using an ankle arthrometer. Separately, motor-evoked potentials were obtained from the tibialis anterior (TA) and soleus (SOL) using transcranial magnetic stimulation over the motor cortex, and compared to the reflexive responses evoked via sciatic nerve electrical stimulation.

RESULTS: Correlation coefficients revealed significant correlations between the motor threshold of the soleus and early muscle spindle afferent activity (r = -0.494) and early cortical event-related desynchronization (r = 0.470), as well as tibialis anterior motor-evoked potential size and late muscle spindle afferent activity (r = 0.499).

CONCLUSIONS: The results of this study highlight the nervous system's capability to offset motor output based on the volume of sensory input at the segmental and cortical levels.