Estimation of Critical Torque Using Neuromuscular Electrical Stimulation of the Quadriceps in Humans.

Abstract

Critical torque (CT) is an integrative measure/concept that represents the “critical” or upper boundary of steady-state work that can be performed without leading to exhaustive fatigue. While this concept has been observed across multiple voluntary exercise modalities, it has not been tested using neuromuscular electrical stimulation (NMES). PURPOSE: The purposes of this study were 1) to determine if NMES exercise results in hyperbolic work-duration pattern that plateaus at the end of exercise and 2) determine if NMES exercise performed below CT results in no fatigue. METHODS: Participants (n = 9) were tested. Following familiarization, participants completed 2 identical testing sessions each consisting of four separate 5-minute NMES bouts separated by 20 minutes of rest. The NMES protocol consisted of 3-seconds of stimulation, followed by 2-seconds of rest, for 60 total isometric contractions. Current and pulse duration (200-μs) were held constant among all tests. Initially 100 Hz NMES was used. During the second test a frequency that elicited a torque value below the end test torque during the 100 Hz test (under CT) was used. The third and fourth tests were at 50 Hz and 25 Hz, applied in a random order. The second testing session was performed 2-7 days later. RESULTS: End-test torque (ETT) was calculated as the mean of the last 6 contractions expressed relative to peak torque. Torque values did not differ over the final 6 contractions within each exercise bout—100 Hz (p = 0.49), 50 Hz (p = 0.15), 25 Hz (p = 0.31), and under CT (p = 0.15). Torque declined from 93 ± 18% to 22 ± 7% of peak torque at 100 Hz, from 60 ± 24% to 22 ± 8% at 50 Hz, and from 29 ± 13% to 20 ± 6% at 25 Hz. These ETT values did not differ from each other (p > 0.05). Initial torque and ETT for the under CT bout did not differ (12 ± 8% vs 11 ± 5% of peak torque; p = 0.48), but ETT was lower than the ETT values from the 100, 50, and 25 Hz bouts (p < 0.05). CONCLUSIONS: Intermittent isometric NMES results in a hyperbolic work-duration relationship similar to what is observed during voluntary isometric exercise. NMES exercise above CT declined to a similar torque value regardless of stimulation frequency, while exercise under CT showed no declines in torque. These findings suggest the NMES exercise protocol can be used to determine CT.