Sex differences in the rate of fatigue development and recovery

Abstract

Background: Many musculoskeletal injuries in the workplace have been attributed to the repetitive loading of muscle and soft tissues. It is not disputed that muscular fatigue is a risk factor for musculoskeletal injury, however the disparity between gender with respect to muscular fatigability and rate of recovery is not well understood. Current health and safety guidelines do not account for sex differences in fatiguability and may be predisposing one gender to greater risk. The purpose of this study was to quantify the sex differences in fatigue development and recovery rate of lower and upper body musculature after repeated bouts of sustained isometric contractions.

Methods: Twenty-seven healthy males (n = 12) and females (n = 15) underwent bilateral localized fatigue of either the knee extensors (male: n = 8; female: n = 8), elbow flexors (male: n = 8; female: n = 10), or both muscle groups. The fatigue protocol consisted of ten 30-second sub-maximal isometric contractions. The changes in maximum voluntary contraction (MVC), electrically evoked twitches, and motor unit activation (MUA) were assessed along with the ability to control the sustained contractions (SLP) during the fatigue protocol using a mixed four-factor repeated measures ANOVA (gender x side x muscle x time) design with significance set at p < 0.05.

Results: There was a significant loss of MVC, MUA, and evoked twitch amplitude from pre- to post-fatigue in both the arms and legs. Males had greater relative loss of isometric force, a higher rate of fatigue development, and were less capable of maintaining the fatiguing contractions in the legs when compared to the females.

Conclusion: The nature of the induced fatigue was a combination of central and peripheral fatigue that did not fully recover over a 45-minute period. The results appear to reflect sex differences that are peripheral, and partially support the muscle mass hypothesis for explaining differences in muscular fatigue.

Figure 1

All force recordings were performed while the participants were secured to custom-built isometric knee extensor (a) and elbow flexor (b) myographs fitted with two independent force transducers. The force transducers were secured to the limbs using canvas straps and allowed for simultaneous recording of the net reaction force acting through the transducer at the site of attachment required to balance the torque generated by the muscular contractions.

Figure 2

a) Truncated force signal trace illustrating the fatigue protocol. (♠) The peak of three 5-second maximum voluntary contractions (MVC) was used to determine the fatiguing contraction intensity, while the interpolated twitch amplitude (IT) along with the associated control twitch (CT) elicited 5-seconds after the contraction were used to calculate the pre-fatigue level of motor unit activation. (♥) Ten 30-second fatiguing contractions with 30-seconds of rest were performed at 50% of MVC with an evoked twitch (ET) elicited 5-seonds after the contraction. The slope of the force plateau (SLP) was used as an indicator of fatigue accumulation. (♦) Recovery was monitored for 45-minutes where an MVC with IT was performed every 15-minutes, with ETs elicited at 5-minute intervals. b) Typical trace for one second of raw EMG data illustrating the 0.25 second overlapping epochs utilized to calculate mean root mean square and mean median frequency values.

Figure 3

Significant time × sex interaction plot for maximum voluntary contraction from pre- to post-fatigue and throughout the 45-minutes of recovery. Time is measured in minutes and relative to the completion of the ten fatiguing contractions. Values are means ± standard deviations. Significant group differences are marked with a^☆.

Figure 4

Significant time × sex interaction plot for percent maximum voluntary contraction throughout the 45-minutes of recovery. Time is measured in minutes and relative to the completion of the ten fatiguing contractions. Values are means ± standard deviations. Significant group differences are marked with a^☆.

Figure 5

Significant time × sex interaction plot for peak evoked twitch amplitude throughout the fatiguing contractions. Values are means ± standard deviations. Significant group differences are marked with a^☆.

Figure 6

Significant time × sex interaction plot for the ability to maintain the required force level during the fatiguing contractions. Values are means ± standard deviations. Significant group differences are marked with a^☆.