Date of Award
Ph.D. in Health and Kinesiology
University of Mississippi
BACKGROUND: The current understanding of fatigue is specific to voluntary exercise (VOL) or neuromuscular electrical stimulation (NMES) separately. Currently, there is no data on the understanding of fatigue in VOL+NMES specific to motor control strategies.
PURPOSE: The main purpose of this research project was to compare the fatigability of two exercise conditions (VOL and VOL+NMES) in the elbow flexors and knee extensors.
METHODS: Thirty-six participants aged 18-35 completed this four-visit randomized, controlled study. Visit one included familiarization to procedures. Visits 2-4 included control, VOL, and VOL+NMES fatiguing exercise with the same pre- and post-measurements. Pre-measures included two isometric maximal voluntary contractions (MVC) and two trapezoids; post-measures were performed in reverse order with only one MVC. The VOL+NMES and VOL visits were identical except, NMES (50-Hertz; at maximal tolerable intensity) was superimposed onto the VOL during the VOL+NMES. The exercise consisted of five sets of 10-second-long isometric muscle contractions at 50% of MVC with two trapezoids immediately following each set. The post-measures were performed at the end of the final set. The control visit involved resting for 5-minutes between the pre- and post-measurements. A minimum of 24-hours was required between non-exercise visits and 48-hours between exercise visits. The elbow flexors (EF) were always completed first, followed by the knee extensors (KE). Several two-way repeated measures analyses of variance (ANOVA) were completed to comparing pre and post maximal voluntary force (MVF, Newtons [N]), and normalized (% pre-MVC) EMG amplitude (EMGa), and EMG median frequency (MDF) for the biceps brachii (BB), vastus lateralis (VL), and vastus medialis (VM). Several two-way repeated measures ANOVAs were completed to compare the pre and post linear slope coefficients (pulses per second [pps]/%MVC) and y-intercepts (pps) of the motor unit (MU) mean firing rate (MFR) vs. recruitment threshold (RT; %MVC) and MU RT vs. derecruitment threshold (DT; %MVC) relationships for the BB, VL, and VM. All statistical analyses were performed with an ⍺ set to 0.05.
RESULTS: A significant interaction was observed (p < .001) for EF MVF. The change in EF MVF was greater in VOL (-42.19 53.76) and VOL+NMES (-45.81 59.18) compared to the control (-3.78 27.01, all p < .001). A significant interaction was observed (p < .001) for KE MVF. The change in KE MVF was greater in VOL (-80.48 65.29) and VOL+NMES (-93.07 78.68) compared to control (-12.52 64.43, all p < .001). A significant interaction was observed (p < .001) for VM EMGa. The change in VM EMGa was greater in VOL+NMES (20.75 34.66%) compared to control (-2.40 25.22%, p = .002). A significant interaction was observed (p < .001) for VM EMG MDF. The change in VM EMG MDF was greater in VOL+NMES (10.62 14.43%) compared to control (1.66 19.14%) p = .009). The slope of the MFR vs. RT relationship was significantly more negative post- (-.704 ± .30), compared to pre-exercise (-.601 ± .28; p = .007) in the BB. The slope was significantly more negative in post- (-.777 ± .68), compared to pre-exercise (-.404 ± .26; p = .047) in the VM. The MU RT vs. DT, the slope was significantly more positive in post- (1.51 ± .016), compared to pre-exercise (1.35 ± .52; p = .044) in the BB.
CONCLUSION: This study suggests that VOL+NMES exercise appears to induce a similar level of fatigue as VOL exercise alone, in the EF and KE muscle groups. This is shown by similar changes in neuromuscular functions and MU firing properties. The MU firing property changes suggest that the addition of NMES increased recruitment of higher-threshold MUs with higher discharge rates, and caused those higher-threshold MUs to derecruit at higher force levels. This study is the first to demonstrate these findings regarding VOL+NMES fatiguing exercise and, thus, provides a foundation for future studies to continue to develop exercise paradigms to examine the fatigability of VOL+NMES.
Miller, William Matthew, "Fatiguing effects of electrical stimulation superimposed onto voluntary contraction" (2022). Electronic Theses and Dissertations. 2446.
Health and Kinesiology