Comparison of Both Metabolic and Functional Effects Induced by Two Neuromuscular Electrical Stimulation Protocols: A Comprehensive Approach Using Magnetic Resonance Imaging and Spectroscopy

Neuromuscular electrical stimulation (NMES) has emerged as a suitable tool for restoring, maintaining and/or enhancing muscular performance. From a practical point of view, NMES can elicit contractions either by direct activation of motor axons (i.e. peripheral mechanism) or by the recruitment of motoneurons in the spinal cord through the depolarization of sensory axons (central mechanism). It is noteworthy that NMES parameters widely affect the balance between transmission along these two pathways. Conventional NMES is usually delivered using short pulse duration (0.05-0.4 ms), low stimulation frequency (15-40 Hz) and high stimulus intensities so that the large antidromic volley in motor axons ensures that the evoked contraction will be driven largely by the direct depolarization of motor axons beneath the stimulation site with no or little involvement of central nervous system. On the contrary, when NMES is delivered using wide pulse widths (1 ms) and high frequency (up to 100 Hz) (WP-HF NMES), a portion of the evoked contraction arises from a central mechanism and the corresponding force (recently referred to as "extra force") is significantly (three times) larger than the conventional NMES-induced force. This extra force is supposed to represent the central contribution from the recruitment of spinal motoneurons by the evoked afferent volley given that no additional force was observed during a complete anesthetic block of the nerve proximal to the stimulation site. Despite the obvious differences in terms of activation of the neuromuscular system between conventional and WP-HF NMES, the functional, metabolic and cortical responses associated to both protocols remain to be determined