Stressor-induced increase in muscle fatigability of young men and women is predicted by strength but not voluntary activation

Abstract

This study investigated mechanisms for the stressor-induced changes in muscle fatigability in men and women. Participants performed an isometric-fatiguing contraction at 20% maximal voluntary contraction (MVC) until failure with the elbow flexor muscles. Study one (n = 55; 29 women) involved two experimental sessions: 1) a high-stressor session that required a difficult mental-math task before and during a fatiguing contraction and 2) a control session with no mental math. For some participants (n = 28; 14 women), cortical stimulation was used to examine mechanisms that contributed to muscle fatigability during the high-stressor and control sessions. Study two (n = 23; nine women) determined the influence of a low stressor, i.e., a simple mental-math task, on muscle fatigability. In study one, the time-to-task failure was less for the high-stressor session than control (P < 0.05) for women (19.4%) and men (9.5%): the sex difference response disappeared when covaried for initial strength (MVC). MVC force, voluntary activation, and peak-twitch amplitude decreased similarly for the control and high-stressor sessions (P < 0.05). In study two, the time-to-task failure of men or women was not influenced by the low stressor (P > 0.05). The greater fatigability, when exposed to a high stressor during a low-force task, was not exclusive to women but involved a strength-related mechanism in both weaker men and women that accelerated declines in voluntary activation and slowing of contractile properties.

Sex Differences in Arm Muscle Fatigability With Cognitive Demand in Older Adults

“…We previously showed that young women (18-30 years) and not men had greater fatigability and decreased steadiness during a sustained isometric contraction with the elbow flexor muscles when high CD was imposed but not low CD [16,38]. The current study was designed to identify whether older men and women increased fatigability and decreased steadiness during submaximal isometric fatiguing contractions with the elbow flexor muscles.…”

“…The greater fatigability when high CD was imposed during the isometric sustained contractions in young adults was associated with baseline strength [16] such that weaker individuals were more susceptible to increased fatigability, possibly as a result of altered muscle perfusion [11,12,16]. This was achieved because we were able to match some of the male and female cohort for strength [16].…”

“…For the lower limb, imposing cognitive demand increased the between-and within-subject variability in motor performance among older adults much more than for the young, but fatigability of the ankle dorsiflexor muscles was not increased in either age group [35]. For older adults, however, fatigability of the upper extremity may respond differently to the lower limb as it does in younger persons [16,38]. The upper limb has a greater number of corticospinal connections than lower limb muscles [3], which are more likely to receive interference from the prefrontal cortical areas when a cognitive task is imposed during a motor task [19].…”

“…The experimental setup has been previously described [16,38]. In brief, the elbow flexor muscle forces were assessed with participants seated upright with the nondominant arm abducted slightly and the elbow flexed to 90°.…”

“…For example, when a high-cognitive demand task, which increased stress, was performed while sustaining a contraction with the elbow flexor muscles, young women (18-30 years) had greater reductions in the time-to-task failure than young men [38], suggesting increased fatigability. Initial strength was predictive such that weaker participants (mostly women) exhibited the greatest increases in fatigability when a stressful cognitive demand was imposed [16,38]. The young women also exhibited larger cardiovascular responses (increased blood pressure, heart rate, and rate pressure product) to the heightened cognitive demand at rest and during the fatiguing contraction.…”