Abstract
The effectiveness of neuromuscular electrical stimulation (NMES) for rehabilitation is proportional to the evoked torque. The progressive increase in torque (extra torque) that may develop in response to low intensity wide-pulse high-frequency (WPHF) NMES holds great promise for rehabilitation as it overcomes the main limitation of NMES, namely discomfort. WPHF NMES extra torque is thought to result from reflexively recruited motor units at the spinal level. However, whether WPHF NMES evoked force can be modulated is unknown. Therefore, we examined the effect of two interventions known to change the state of spinal circuitry in opposite ways on evoked torque and motor unit recruitment by WPHF NMES. The interventions were high-frequency transcutaneous electrical nerve stimulation (TENS) and anodal transcutaneous spinal direct current stimulation (tsDCS). We show that TENS performed before a bout of WPHF NMES results in lower evoked torque (median change in torque time-integral: − 56%) indicating that WPHF NMES-evoked torque might be modulated. In contrast, the anodal tsDCS protocol used had no effect on any measured parameter. Our results demonstrate that WPHF NMES extra torque can be modulated and although the TENS intervention blunted extra torque production, the finding that central contribution to WPHF NMES-evoked torques can be modulated opens new avenues for designing interventions to enhance WPHF NMES.
Highlights
The effectiveness of neuromuscular electrical stimulation (NMES) for rehabilitation is proportional to the evoked torque
The aim of the present study was to investigate the influence of high-frequency transcutaneous electrical nerve stimulation (TENS) and transcutaneous spinal direct current stimulation (DCS) on the mechanical and EMG responses to wide-pulse, high-frequency (WPHF) NMES
Our first hypothesis was that high-frequency TENS would reduce WPHF NMES-evoked torque due to reduced reflexive recruitment of motor units, and our results support this hypothesis (Figs. 3B, 4C, 5B)
Summary
The effectiveness of neuromuscular electrical stimulation (NMES) for rehabilitation is proportional to the evoked torque. Using a low constant stimulation intensity to evoke an initial torque of 5–10% maximal voluntary contraction (MVC) torque, WPHF NMES-evoked torques have been reported to reach up to 80% MVC9 This progressive increase in WPHF NMES-evoked torque in addition to what would be expected from activation of motor axons through peripheral pathways alone has been termed “extra torque”[7,8] and occurs in ~ 50% of individuals[10]. This highlights the potential for WPHF NMES to evoke torque levels effective for rehabilitation and training, while overcoming the main limitation of conventional NMES, namely d iscomfort[11]. Since WPHF NMES-evoked torque involves, at least partially, motor units recruited reflexively in the spinal cord, the use of TENS or anodal tsDCS during, or before, WPHF NMES may alter WPHF NMES-evoked torque
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