Author ORCID Identifier

https://orcid.org/0000-0001-7437-2665

Date Available

8-12-2025

Year of Publication

2025

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy (PhD)

College

Education

Department/School/Program

Kinesiology and Health Promotion

Faculty

Dr. Haley C. Bergstrom

Abstract

This investigation examined how movement intent influences neuromuscular excitation strategies during fatiguing resistance exercise, comparing maximal intended concentric efforts versus self-selected volitional efforts during load-equated Smith machine back squat exercise to quantify reserve capacity and characterize underlying neuromuscular mechanisms. Twenty-three resistance-trained participants (12 males, 11 females) completed a repeated-measures design, with replication, across six visits, performing smith machine back squat repetitions-to-failure at 70% 1RM under maximal intent (accelerate rapidly) and volitional intent (self-selected effort) conditions. Maximal voluntary isometric contractions used rapid (instantaneous 10-second effort) and ramp (5-second build-up, 5-second maximal) protocols. Surface electromyography (sEMG) was recorded from vastus lateralis and rectus femoris, with surface mechanomyography (sMMG) from rectus femoris only. Vastus lateralis underwent wavelet-based spectral decomposition to characterize high- and low-frequency contributions, while rectus femoris employed multimodal analysis of conventional signal characteristics. Performance measures included mean propulsive concentric velocity (MPCV), total repetitions, and peak force. Movement intent significantly influenced both force production and neuromuscular activation patterns. During MVIC trials, peak force differed between conditions (F(1,157.08) = 5.97, p = 0.02), suggesting that intent modulates even maximal voluntary efforts. This effect was accompanied by complex neuromuscular adjustments in both muscles examined. Vastus lateralis spectral analysis revealed that the interaction between condition, time, and frequency components was significant (F(1,23326.00) = 8.68, p < 0.01), with mean frequency showing distinct temporal patterns between conditions (F(1,23330.00) = 39.03, p < 0.01). Similarly, rectus femoris demonstrated comprehensive changes across all measured parameters, with significant condition × time interactions for both electrical (sEMG total intensity: F(1,23330.00) = 39.03, p < 0.01; mean frequency: F(1,11403.02) = 41.85, p < 0.01) and mechanical (sMMG total intensity: F(1,11687.01) = 316.02, p < 0.01; mean frequency: F(1,11686.93) = 40.36, p < 0.01) signals. Notably, sex differences emerged specifically for mechanical signal intensity (F(1,22.01) = 26.12, p < 0.01), suggesting differential muscle mechanics between males and females. During repetitions-to-failure trials, performance differences between conditions became more pronounced. MPCV showed a significant condition × repetition interaction (F(1,2508.49) = 158.32, p < 0.01), indicating divergent fatigue trajectories between maximal and volitional efforts. Males consistently achieved higher velocities than females (F(1,22.37) = 29.13, p < 0.01) and completed fewer total repetitions (F(1,22.13) = 8.32, p < 0.01), suggesting a trade-off between power output and endurance. These performance differences were supported by distinct neuromuscular strategies. Vastus lateralis displayed frequency-specific adaptations, with significant three-way interactions between condition, frequency band, and repetition number (F(1,5108.00) = 4.59, p = 0.03), while mean frequency shifted differently across conditions as fatigue progressed (F(1,5111.99) = 14.37, p < 0.01). Sex differences in spectral intensity (F(1,22.00) = 5.08, p = 0.03) further highlighted physiological variations between males and females. Rectus femoris showed complementary patterns, with electrical signal frequency modulated by both condition and fatigue state (F(1,2541.94) = 5.64, p = 0.02), while signal intensity increased with fatigue regardless of intent (sEMG: F(1,2541.99) = 69.09, p < 0.01; sMMG: F(1,2541.72) = 5.14, p = 0.02). The mechanical signals were particularly sensitive to movement intent, with mean frequency showing main effects for condition (F(1,2543.23) = 28.90, p < 0.01), repetition (F(1,2542.85) = 32.39, p < 0.01), and sex (F(1,21.84) = 10.13, p < 0.01), indicating that movement intent fundamentally alters motor unit recruitment and firing patterns throughout the fatigue process. Movement intent produces systematic, frequency-dependent neuromuscular alterations during resistance exercise, though interpretation requires consideration of analytical limitations. MVIC force differences likely reflect methodological constraints rather than physiological differences. Repetitions-to-failure trials revealed that individuals operate substantially below maximal capabilities during volitional efforts, maintaining considerable reserve capacity while reaching similar physiological endpoints. Distinct spectral patterns suggest different energy distribution strategies: maximal efforts employ immediate "front-loaded" excitation, while volitional efforts utilize progressive intensification as fatigue accumulates. The rectus femoris multimodal analysis demonstrated systematic condition-related differences across both sEMG and sMMG measures, though the multifactorial nature of surface myographic signal generation precludes definitive attribution to specific motor unit control mechanisms. While results align with longitudinal evidence supporting maximal intent training for strength adaptations, practical considerations regarding adherence and ecological validity must balance physiological optimization.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2025.341

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