The Role of Lower Limb Load Receptors in Modulating Functional Motor Responses
Extended unloading conditions, such as the microgravity environment encountered in Low Earth Orbit, result in neuromuscular deconditioning that affects the activation and motor control of the antigravity muscles in the lower limbs. In turn, this loss of neuromuscular function causes problems with balance and locomotor control upon returning to normal gravitational conditions.
Furthermore, the case of long-duration space missions, such as those to Mars, would present new challenges because the neuromuscular deconditioning would presumably cause serious performance issues in such important tasks as landing, driving a rover vehicle, or any daily physical activity requiring motor coordination and manipulation of robotic systems.
The activation and motor control of the antigravity muscles of the lower limbs and trunk is modulated by load receptors in the soles of the feet and the muscles and joints of the lower extremities, and mechanical foot stimulation has been proven to trigger soleus and tibialis anterior muscle activation. This study attempts to provide more information about the independent roles of muscle, joint and plantar load receptors in the activation, coordination and control of the lower limb muscles.
Twelve subjects with similar physical characteristics to those of astronauts will perform a set of position matching trials that consist of both leg and ankle extensions/flexions with and without leg loading (provided by a dynamometer system) and with and without plantar loading (provided by mechano-stimulating boots). Surface Electromyography (EMG) and muscle transverse stiffness data will be collected and later analyzed to assess the independent contributions to muscle activation from the muscles, joints, and plantar load receptors.
The results from this study will help determine whether or not mechanical stimulation of the plantar receptors can be used to reduce, or even reverse, the neuromuscular deconditioning of the lower limbs due to gravity unloading during spaceflight. The benefit from these results could be the development of new passive countermeasures that, combined with current exercise protocols, could be used during long-duration space missions to ameliorate the deleterious neuromuscular changes and associated balance and locomotor performance issues.