Electrical stimulation-muscle strength training in people with spinal cord injury

NMES was delivered by a high-voltage constant-current electrical stimulator (400 V, DS7A, Digitimer Ltd., Welwyn Garden City, UK) under the same conditions as the assessment through four self-adhesive stimulation electrodes (Axelgaard, PALS, USA) placed over the rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM). Two 5×10 cm electrodes were placed over RF and one 5×5 electrode was placed on each of the VM and VL approximately at their motor points using a split end cable, to increase the surface area of stimulation. The electrodes were placed to elicit the greatest twitch response with a low stimulation intensity. Each session commenced with a “warm-up” period consisting of paired electrical square-wave stimuli (two 1000 µs square-wave pulses, 5–ms interpulse interval) followed by a maximum of three tetanic trains (tau-t,40mA) delivered to each leg separately every 20 s while the stimulation current was increased from 30 mA in 10-mA increments until a plateau in the maximum peak twitch torque was observed or the maximal current intensity was 99 mA. This plateau was defined as the maximal peak twitch torque (tau-tw,p) and was used as the target torque during the training session. Subsequently, a tetanic train of NMES at 40 mA (tau-t,40mA) was delivered followed by a maximum of three trains of NMES performed at different stimulation current intensities until reaching the closest value to the target torque. After the warm-up period the NMES session commenced with electrically-evoked muscle contractions being elicited at the target torque for 5 sets of 10 repetitions on each leg, with a 1-min rest between sets. To determine the actual training intensity either one of two methods was used. The first method was by evoking the maximal peak twitch torque (tau-tw,p) and setting the current so the tetanic torque was equal to tau-tw,p. However, if tau-tw,p showed a decrease compared to previous sessions, a second method was used whereby the starting current was set to be equal to the highest current used in the previous training session. Within each session, the current was increased by 2 mA per each set of 10 repetitions to maintain a high torque production as fatigue developed; thus, if the second method was chosen, the current selected for set 1 was the same as that used in the final set of the previous session. Using this method, the torque produced in set 1 of training was always higher than that performed in any set of the previous session, so the evoked torque increased incrementally. All training sessions were conducted by the same trained researcher, who was a senior Physiotherapist and were additional to any other rehabilitation exercise. The participants were asked to keep their physical training routine consistent for the duration of the experiment. Training was performed twice a week (with at least one rest day between) for 12 weeks. All assessments were completed at -1 weeks (“Control period”), 0 weeks (0-wk) and 12 weeks (12-wk), except for resting blood samples which were taken at 0-wk and 12-wk only. Post-training assessments were taken 4-6 days after the last training session to allow for recovery of acute, residual effects of intense exercise. The procedures used in this study were similar from the methodology used in a previous study in people with spinal cord injury (SCI).