Neuromuscular responses to mechanical foot stimulation: the influence of loading and postural context.

INTRODUCTION Recent work indicates mechanical stimulation of soles may attenuate muscle atrophy initiated by gravitational unloading, including that experienced during spaceflight. The aim of the present study was to determine the modulating effect of unloading and body configurations on the neuromuscular response to mechanical foot stimulation. METHODS A solenoid (2.5-cm2 surface area) embedded within a platform provided non-noxious stimulation to the lateral foot sole: 100 ms duration, 3-mm protrusion. Stimulation was applied while measuring root mean square electromyography of the soleus and lateral gastrocnemius. Experiment 1 compared seated and standing conditions, as well as different levels of gravitational unloading created by suspension. Experiment 2 altered postural stability by varying leg stance widths during a static stepping posture. Either the foot of the support leg or the nonsupport leg was stimulated. Reduced levels of loading further altered the level of postural challenge and support while maintaining the same body configuration. RESULTS In both experiments, loading was not a modulating factor to the response, supporting the use of mechanical foot pressure as a countermeasure for spaceflight. Body configuration and postural instability both modulated the response, independently of load. DISCUSSION In conclusion, an application of dynamic foot stimulation could be used to elicit neuromuscular activity without the need of background muscle activity or gravitational loading. However, the body configuration of the user with respect to postural stability needs to be considered in the application, and may provide further scope of benefits extending to the activation of postural synergies.