Rates of performance loss and neuromuscular activity in men and women during cycling: evidence for a common metabolic basis of muscle fatigue

Abstract

The durations that muscular force and power outputs can be sustained until failure fall predictably on an exponential decline between an individual's 3-s burst maximum to the maximum performance they can sustain aerobically. The exponential time constants describing these rates of performance loss are similar across individuals, suggesting that a common metabolically based mechanism governs muscle fatigue; however, these conclusions come from studies mainly on men. To test whether the same physiological understanding can be applied to women, we compared the performance-duration relationships and neuromuscular activity between seven men [23.3 ± 1.9 (SD) yr] and seven women (21.7 ± 1.8 yr) from multiple exhaustive bouts of cycle ergometry. Each subject performed trials to obtain the peak 3-s power output (P_max), the mechanical power at the aerobic maximum (P_aer), and 11-14 constant-load bouts eliciting failure between 3 and 300 s. Collectively, men and women performed 180 exhaustive bouts spanning an ~6-fold range of power outputs (118-1116 W) and an ~35-fold range of trial durations (8-283 s). Men generated 66% greater P_max (956 ± 109 W vs. 632 ± 74 W) and 68% greater P_aer (310 ± 47 W vs. 212 ± 15 W) than women. However, the metabolically based time constants describing the time course of performance loss were similar between men (0.020 ± 0.003/s) and women (0.021 ± 0.003/s). Additionally, the fatigue-induced increases in neuromuscular activity did not differ between the sexes when compared relative to the pedal forces at P_aer These data suggest that muscle fatigue during short-duration dynamic exercise has a common metabolically based mechanism determined by the extent that ATP is resynthesized by anaerobic metabolism.

New & noteworthy: Although men and women differed considerably in their absolute cycling performances, there was no sex difference in the metabolically based exponential time constant that described the performance-duration relationship. Similarly, the fatigue-induced increases in neuromuscular activity were not different between the sexes when compared from a metabolic perspective. These data suggest that men and women have similar rate-limiting mechanisms for short-duration dynamic exercise that are determined by the extent the exercise is supported by anaerobic metabolism.

Keywords: critical power; metabolism; performance-duration relationship; sex differences; skeletal muscle.

Fig. 1.

Representative EMG data from the vastus lateralis of a male subject during a constant-load fatiguing trial that elicited failure in 216 s. The amplitude of the EMG was greater at the end compared with the beginning of the trial (A) and increased continuously throughout the bout until the point of failure (B). Trials began with 5–10 s of unloaded pedal revolutions to obtain a cadence of 80 rpm before the constant load was applied to the ergometer’s flywheel (B). The electromagnetic triggers mounted at the base of the pedals ensured that the amplitude of the contraction-by-contraction EMG was computed over the same portion of the pedal revolution.

Fig. 2.

Mean power profiles for men (A) and women (B) during the all-out 3-min test to estimate critical power. The power output at the end of the all-out 3-min test fell to similar relative intensities below the highest power output that could be sustained for 5 min and elicited the V̇o_2max (P_aer; demarked by horizontal black lines) in both sexes. Note the similar profiles for the men and women despite the markedly greater power outputs achieved by the men. Gray solid lines and dashed horizontal black lines represent ±1 SD.

Fig. 3.

Individual performance-duration relationships for mechanical power (A) and pedal force (C) in relation to the times to failure from the male and female subjects with the most rapid (● and ◆) and slowest (○ and ◇) rates of muscle performance loss (k). Measured values of P_max, P_aer, F_max, and F_aer were used to fit the individual curves in accordance with Eqs. 2 and 3. When the absolute performance-duration relationships were expressed as a percentage of the nonsustainable performance range (e^-kt), the time course of the relative loss in power (B) and force (D) for the four subjects were accurately described by a single exponential time constant.

Fig. 4.

Study means for the absolute levels of mechanical power (A) and pedal force (C) produced by the men and women in relation to the durations these performances could be sustained until failure. While the mean absolute cycling performances were markedly greater in the men compared with the women (A and C), the time constants (k) describing the relationship between the relative power (B) and force (D) and the times to failure were the same for both sexes (P > 0.05). For example, the mechanical power (B) and pedal forces (D) at 50% of the nonsustainable performance range were maintained for similar durations before failure in men and women (P > 0.05). Shaded gray regions represent ±1 SD.

Fig. 5.

The time course of the relative loss in power (A) and pedal force (B) from the 180 exhaustive trials (men = 90 and women = 90) performed by all 14 subjects were accurately described by a single exponential time constant. The time constants for the power- and force-duration relationships were 0.0207/s (R² = 0.96) and 0.0201/s (R² = 0.96), respectively.

Fig. 6.

Contraction-by-contraction, rectified, and averaged EMG (AEMG) from the vastus medialis of the right leg during two constant-load trials for a representative male (A) and female (B) subject. In the constant-load trials that were sustainable and supported primarily by aerobic metabolism (gray outlined symbols), there was no increase in the amplitude of the AEMG; however, in the constant-load trials that elicited failure in <300 s, the amplitude of the AEMG progressively increased until the point of failure (black outlined symbols). The sustainable trials depicted for both individuals were performed at the same power output as the critical power determined by the all-out 3-min test.

Fig. 7.

Rates of change in EMG activity (ΔAEMG/ΔTime) from the vastus medialis (A and B) and lateralis (C and D) of both legs for a representative male and female subject in all trials >20 s. In the constant-load trials performed with pedal forces below the maximum forces sustained primarily by aerobic metabolism (force < F_aer), EMG activity from all four quadriceps muscles remained essentially constant for both subjects, i.e., ΔAEMG/ΔTime ≈ 0. In contrast, the trials with force outputs that exceeded F_aer, and thus required a continued reliance on anaerobic metabolism, AEMG increased in all four muscles for the male and female subject, i.e., ΔAEMG/ΔTime > 0. Linear regression lines for both subjects indicate that more rapid rates of increased EMG activity occurred in trials where the force output exceeded F_aer by a greater extent.

Fig. 8.

Rates of change in EMG activity (ΔAEMG/ΔTime) from the vastus medialis (A) and lateralis (B) of both legs for all subjects and all trials plotted against the pedal force expressed as a percentage of F_aer. In constant-load trials performed with pedal forces below F_aer, EMG activity from all four quadriceps muscles remained essentially constant; however, in the constant-load trials with force outputs that exceeded F_aer, and thus required a continued reliance on anaerobic metabolism, rates of AEMG increased similarly in both muscle groups for the men and the women (P > 0.05 for both muscle groups).