Electrical Stimulation and assessment of the induced force in the Denervated Muscle

Injury to lower motor neurons (LMN), results in flaccid paralysis of the affected limbs devastating muscle mass, bones and circulation. There is no existing standard treatment for denervated muscles. Denervated muscles have a greatly reduced response to electrical stimulation and do not respond to standard electrical stimulation. Effective therapy requires a portable and high strength stimulator suitable for home use. In this paper, we report both a novel stimulator and a novel muscle contraction sensor suitable for use in wards, at home and other nonspecialized settings. The present study shows the benefit, safety and usability of such a stimulator in three subjects with LMN injuries. Preliminary results from the study has shown improved response to stimulation by completely denervated muscles following Functional Electrical Stimulation (FES) for a period of one month, using long pulse durations (biphasic pulse, 120ms, amplitude 160V, 80mA). The stimulator is battery operated (two Li-ion cells), and uses on-demand switched inductor high voltage conversion. The muscle sensor is a load cell based stretch sensor and measures the change in limb girth during muscle contraction. Muscle contraction is isovolumic and shortening in length causes expansion of girth. From the stimulated muscle twitch, the time-to-peak tension (TPT) and half-relaxation time (HRT) were measured as indicators of the underlying fibre property. The force of contraction showed a marked increase after one month of stimulation. The TPT for the leg muscles was 100ms to 180ms. The stimulation threshold for the muscles decreased following a month of such therapeutic stimulation.