Surface electrical stimulation of skeletal muscle after spinal cord injury

Study design: Survey. Objective: Examine muscle contractile activity during electrical stimulatio n (ES) after spinal cord injury (SCI). Setting: General community of Athens, Georgia, USA. Methods: Eight clinically complete SCI adults (C6 to T12) 4+/-1 (mean+/-SE) years post injury and eight able-bodied adults were studied. Surface ES wa s applied to the left m. quadriceps femoris for three sets of 10, 1 s isome tric actions (50 Hz trains, 400 mu s biphasic pulses, 50 mu s phase delay, 1 s: 1 s duty cycle) with 90 s of rest between sets. Current was set to evo ke isometric torque that was (1) sufficient to elicit knee extension with 2 .3 kg attached to the ankle (low level ES), and (2) intended to equal 30% ( mid level ES) or 60% of maximal voluntary torque of able-bodied adults (hig h level ES, able-bodied only). The absolute and relative cross-sectional ar ea (CSA) of In. quadriceps femoris that was stimulated as reflected by cont rast shift in magnetic resonance images and torque were measured. Results: Six+/-2, 20+/-2 and 38+/-4% of the average CSA of In. quadriceps w as stimulated during low, mid and high level ES, respectively, for able-bod ied. Corresponding values for SCI for low and mid level ES were greater (61 +/-12 and 92+/-7%, P=0.0002). Torque was related to the CSA (cm(2)) of stim ulated muscle (Nm=3.53xstimulated CSA+13, r(2)=0.68, P=0.0010), thus ES of a greater per cent of nz. quadriceps femoris in SCI was attributed to their smaller muscle (24+/-3 vs 73+/-5 cm(2), P=0.0001). The decline in torque r anged from 9+/-1 to 15+/-4% within and over sets for low, mid or high level ES in able-bodied. SCI showed greater (P=0.0001) fatigue (19+/-3 to 47+/-6 %). Conclusion: The territory of muscle activation by surface electrical stimul ation varies among SCI patients. Given sufficient current, a large portion of the muscle of interest can be stimulated. The resulting torque is modest , however, compared to that attainable in able-bodied individuals due to th e small size and limited fatigue resistance of skeletal muscle years after spinal cord injury.