Benitez, B, Dinyer-McNeeley, TK, McCallum, L, Kwak, M, Succi, PJ, and Bergstrom, HC. Load-specific performance fatigability, coactivation, and neuromuscular responses to fatiguing forearm flexion muscle actions in women. J Strength Cond Res 37(4): 769–779, 2023—This study examined the effects of fatiguing, bilateral, dynamic constant external resistance (DCER) forearm flexion on performance fatigability, coactivation, and neuromuscular responses of the biceps brachii (BB) and triceps brachii (TB) at high (80% 1 repetition maximum [1RM]) and low (30% 1RM) relative loads in women. Ten women completed 1RM testing and repetitions to failure (RTF) at 30 and 80% 1RM. Maximal voluntary isometric force was measured before and after RTF. Electromyographic (EMG) and mechanomyographic (MMG) amplitude (AMP) and mean power frequency (MPF) signals were measured from the BB and TB. Performance fatigability was greater (p < 0.05) after RTF at 30% (%∆ = 41.56 ± 18.61%) than 80% (%∆ = 19.65 ± 8.47%) 1RM. There was an increase in the coactivation ratio (less coactivation) between the initial and final repetitions at 30%, which may reflect greater increases in agonist muscle excitation (EMG AMP) relative to the antagonist for RTF at 30% than 80% 1RM. The initial repetitions EMG AMP was greater for 80% than 30% 1RM, but there was no difference between loads for the final repetitions. For both loads, there were increases in EMG MPF and MMG AMP and decreases in MMG MPF that may suggest fatigue-dependent recruitment of higher-threshold motor units. Thus, RTF at 30 and 80% 1RM during DCER forearm flexion may not necessitate additional muscle excitation to the antagonist muscle despite greater fatigability after RTF at 30% 1RM. These specific acute performance and neuromuscular responses may provide insight into the unique mechanism underlying adaptations to training performed at varying relative loads.

Objective. This study examined sex-related differences in fatigability and neuromuscular responses using surface electromyographic (sEMG) and mechanomyographic (sMMG) amplitude (AMP) and frequency (MPF) during fatiguing, maximal, bilateral isometric leg extensions. Approach. Twenty recreationally active males and females with resistance training experience performed continuous, maximal effort, bilateral isometric leg extensions until their force reduced by 50%. Linear mixed effect models analyzed patterns of force, sEMG, and sMMG AMP and MPF responses in the dominant limb. An independent samples t-test compared time-to-task failure (TTF) between sexes. Main Results. There were no significant differences in TTF between males and females. However, males experienced a greater rate of force loss compared to females. Furthermore, sEMG AMP and MPF and sMMG AMP responses followed similar linear trends for both sexes, while sMMG MPF showed non-linear responses with sex-dependent differences. Significance. These data suggest that although TTF was similar, males had a higher rate of force reduction, likely due to greater absolute strength. Furthermore, despite parallel changes in sEMG AMP and MPF, as well as sMMG AMP, the divergent responses observed in sMMG MPF highlight sex-dependent differences in how males and females experience changes in the firing rates of active motor units during sustained maximal contractions.

This study aimed to investigate the test–retest reliability, mean, and individual responses in the measurement of maximal oxygen consumption ($\dot{\mathrm{V}}$O_2max) during a cardiopulmonary exercise test (CPET) and the verification phase during cycle ergometry in women. Nine women (22 ± 2 yrs, 166.0 ± 4.5 cm, 58.6 ± 7.7 kg) completed a CPET, passively rested for 5 min, and then completed a verification phase at 90% of peak power output to determine the highest $\dot{\mathrm{V}}$O₂ from the CPET ($\dot{\mathrm{V}}$O_2CPET) and verification phase ($\dot{\mathrm{V}}$O_{2verification}) on 2 separate days. Analyses included a two-way repeated measures ANOVA, intraclass correlation coefficients (ICC_2,1), standard errors of the measurement (SEM), minimal differences (MD), and coefficients of variation (CoV). There was no test (test 1 versus test 2) × method (CPET vs. verification phase) interaction (p = 0.896) and no main effect for method (p = 0.459). However, test 1 (39.2 mL·kg⁻¹·min⁻¹) was significantly higher than test 2 (38.3 mL·kg⁻¹·min⁻¹) (p = 0.043). The $\dot{\mathrm{V}}$O_2CPET (ICC = 0.984; CoV = 1.98%; SEM = 0.77 mL·kg⁻¹·min⁻¹; MD = 2.14 mL·kg⁻¹·min⁻¹) and $\dot{\mathrm{V}}$O_{2verification} (ICC = 0.964; CoV = 3.30%; SEM = 1.27 mL·kg⁻¹·min⁻¹; MD = 3.52 mL·kg⁻¹·min⁻¹) demonstrated “excellent” reliability. Two subjects demonstrated a test 1 $\dot{\mathrm{V}}$O_2CPET that exceeded the test 2 $\dot{\mathrm{V}}$O_2CPET, and one subject demonstrated a test 1 $\dot{\mathrm{V}}$O_{2verification} that exceeded the test 2 $\dot{\mathrm{V}}$O_{2verification} by more than the respective CPET and verification phase MD. One subject demonstrated a $\dot{\mathrm{V}}$O_2CPET that exceeded the $\dot{\mathrm{V}}$O_{2verification}, and one subject demonstrated a $\dot{\mathrm{V}}$O_{2verification} that exceeded the $\dot{\mathrm{V}}$O_2CPET by more than the MD. These results demonstrate the importance of examining the individual responses in the measurement of the $\dot{\mathrm{V}}$O_2max and suggest that the MD may be a useful threshold to quantify real individual changes in $\dot{\mathrm{V}}$O₂.

Background: Surface mechanomyographic (sMMG) signals have been used to examine sex-specific differences in the mechanical behavior of muscle during fatiguing exercise. However, studies often utilize simple amplitude- and frequency-based analyses, which only reveal the static components of the sMMG signal. Methods: Thus, a wavelet-based analysis was used to examine changes in the spectral intensity of the non-dominant limb’s vastus lateralis during a fatiguing, maximal, unilateral isometric leg extension in recreationally active men (n = 11) and women (n = 10). Relative changes in spectral intensities and instantaneous mean frequency (IMF) were examined using linear mixed-effect models. Time-to-task failure was compared using an independent sample t-test. Results: The neuromuscular responses demonstrated parallel decreases in IMF (p < 0.001). Further, there were parallel, nonlinear, decreases in spectral intensity across wavelets (p < 0.001) and there were no sex differences in time-to-task failure (p = 0.15). Conclusions: These data showed no sex-specific differences in exercise fatigability or muscle mechanics during fatiguing exercise of the leg extensors. However, when collapsed across sex, wavelet-specific changes in spectral intensity over time reveal novel insights into the interplay between low- and high-frequency components during fatigue.