Author ORCID Identifier
Year of Publication
Doctor of Philosophy (PhD)
Kinesiology and Health Promotion
Dr. Haley C. Bergstrom
The purposes of this study were to: 1) examine sex-related differences in the performance of repetitions to failure below (CL-10%, CL-20%) and above (50%, 60%, 70%, and 80% 1RM) the critical load (CL) to make inferences regarding the mechanisms responsible for variability in the performance of repetitions to failure at varying loads; 2) examine the time course of neuromuscular and muscle oxygen saturation responses during the performance of repetitions to failure below and above the CL; 3) to make inferences regarding motor unit activation strategies used to maintain force during the completion of repetitions below and above the CL; 4) determine if there are differences in performance fatigability during repetitions to failure performed below and above the CL; and 5) to use performance fatigability, neuromuscular, and muscle oxygen saturation responses observed below and above the CL to examine fatigue on an individual basis and determine if the performance of repetitions to failure below the CL result in a lower degree of fatigue-induced changes compared to above the CL. Eleven women (22.1 ± 2.2 yr; 167.8 ± 5.9 cm; 68.4 ± 6.0 kg) and 10 men (23.8 ± 4.2 yr; 179.9 ± 4.6 cm; 81.8 ± 11.7 kg) completed a 1RM leg extension test, repetitions to failure at 50%, 60%, 70%, and 80% 1RM to determine the CL (defined as the slope of the line of the load lifted [kg x repetitions completed] vs. the repetitions completed), and repetitions to failure 10% below (CL-10%) and 20% below (CL-20%) the CL. Immediately before and after each failure set, the subjects performed a maximal voluntary contraction (MVC) for the assessment of performance fatigability. During all visits, electromyographic (EMG) and mechanomyographic (MMG) responses were measured from the vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF) and muscle oxygen saturation (%SmO2) was measured from the VL of the dominant limb. Statistical analyses consisted of four-, three-, and two-way mixed model ANOVAs, with appropriate follow-up three-, two-, and one-way repeated measures or mixed model ANOVAs, and paired and independent samples t-tests. Polynomial regression and Student Newman-Keuls tests were used to determine the time course of changes among the neuromuscular parameters and %SmO2. An alpha level of p ≤ 0.05 was considered statistically significant. The men demonstrated greater absolute (kg) strength compared to the women at all loads (p ≤ 0.05), but there was no difference in the relative (%1RM) load corresponding to CL-10% or CL-20% between the sexes. The women completed more repetitions to failure than the men at 50%, 60%, and 80% 1RM (p ≤ 0.044), but not at 70% 1RM, CL-10%, or CL-20% (p ≥ 0.085). There were no differences between the sexes in total volume accumulation or time under tension (p ≥ 0.058). The women demonstrated less performance fatigability than the men 70% 1RM (p = 0.039), but no difference in performance fatigability for any other load (p ≥ 0.097). For both the men and the women, performance fatigability was greater after performing repetitions to failure below the CL compared to above the CL (p ≤ 0.039). For the women, muscle activation during the initial 10% of repetitions completed for the VL, VM, and RF was less than muscle activation during the final 10% of repetitions completed (p ≤ 0.005). In addition, muscle activation for the VM and RF during the initial 10% of repetitions completed below the CL was less than above the CL (p ≤ 0.002), and for the RF, muscle activation during the final 10% of repetitions completed above the CL was greater than below the CL (p ≤ 0.047). For the men, muscle activation during the initial 10% of repetitions completed was less than the final 10% of repetitions completed (p < 0.001) and muscle activation during the final 10% of repetitions completed above the CL was greater than below the CL (p ≤ 0.006). For %SmO2, all loads demonstrated quadratic or cubic decreases from baseline that were less than the initial repetition from 20-100% of total repetitions completed. From 40-100% of total repetitions completed, the %SmO2 was greater during repetitions completed below the CL compared to above the CL. In addition, the women demonstrated greater %SmO2 than the men from 40-100% of total repetitions completed. There were similar increases in EMG amplitude (AMP) and MMG AMP and decreases in MMG mean power frequency (MPF) for the men and the women for all 3 muscles, indicating both sexes relied on either the Onion Skin Scheme or Muscular Wisdom motor unit activation strategy for continued force development. For EMG MPF, the men and women demonstrated similar responses above the CL. Below the CL, the women demonstrated an earlier onset of fatigue-induced decreases but were able to maintain repetitions to failure longer after the onset compared to the men, indicating women may be less influenced by increases in metabolite accumulation that decrease the EMG MPF signal. There were no sex-specific responses in performance fatigability around the CL, but there were sex-specific responses in the number of repetitions completed at all loads that suggested the women were more fatigue-resistant at loads above and below the CL compared to the men. These findings allude to variability in the mechanisms that cause fatigue at lower versus higher loads. Thus, the CL may be a tool to investigate the “lowest” load that is required for adaptations in muscular strength and neuromuscular adaptations to occur.
Digital Object Identifier (DOI)
Dinyer, Taylor Kayln, "Examination of Sex- and Intensity-Specific Fatigue During Bilateral Leg Extension Exercise" (2021). Theses and Dissertations--Kinesiology and Health Promotion. 83.