SKELETAL MUSCLE GROWTH FACTOR RESPONSE TO CUTANEOUS STIMULATION OF THE PLANTAR SURFACE OF THE FOOT

Skeletal muscle atrophy, deconditioning, and decreased function are consequences of skeletal muscle unloading induced by reduced physical activity, bed rest, and space flight. Reduced sensory input, particularly from the plantar surfaces of the feet, is one of the earliest consequences of unloading. This has been hypothesized to be a key factor in modulating the negative effects of unloading, especially those observed in the anti-gravity muscles of the lower limbs. Mechanical stimulation of the plantar surface during unloading to provide sensory input has been investigated as a potential means of attenuating unloading-induced muscle deconditioning. Results from human and animal models, including space flight, dry immersion, and hindlimb suspension, demonstrated that plantar stimulation evokes an increase in neuro-sensory activity and attenuates some elements of skeletal muscle deconditioning. The physiological mechanism(s) responsible for the protective effects of plantar stimulation in unloaded muscle, especially its anti-atrophic effects, have yet to be fully elucidated. Previous studies suggest that plantar stimulation might result in the release of growth hormone (GH) in response to increased afferent sensory nerve traffic induced by plantar stimulation. Since increased levels of circulating GH results in the activation of several different hypertrophic signaling pathways within skeletal muscle, it is possible that the anti-atrophic effects of plantar stimulation during unloading are mediated by GH. The purpose of this dissertation project was to develop a system that delivers plantar stimulation to human subjects in a controlled manner and to investigate whether plantar stimulation results in the release of GH during acute skeletal muscle unloading (supine rest). Specifically, we hypothesized that GH concentrations would increase during acute unloading when subjects received 10 min of plantar stimulation to the right forefoot. Further, we hypothesized that the GH response would be augmented when subjects ...