Abstract
Acute cognitive stress affects neuromuscular fatigue, but the underlying mechanisms are not well understood. The aim of this study was to identify the influence of stress on central fatigue and associated functional changes in the brain. Thirty participants, balanced by sex, performed intermittent elbow flexion contractions at 30% of their maximum strength till voluntary fatigue. Endurance time, strength loss, and voluntary activation loss rate were collected. Functional hemodynamic changes in the participants’ prefrontal and motor areas of the brain were also collected and used to calculate functional integration and segregation in the PFC and M1. Results show that stress had a facilitative effect on fatigability and the fatigue related decline in the functional integration and segregation of prefrontal cortex were not observed under stress. These findings demonstrate how the imposition of dual task demands in the workplace delays the perception of neuromuscular fatigue, thereby increasing the risk of injury.
Fatigue and fatigability are often affected by task specific factors like concurrent cognitive demand or stress (Mehta & Parasuraman, 2014) making context and task specific studies of fatigue necessary. One such context specific factor that has been shown to heavily influence fatigue outcomes is stress, a physiological response of the body when an individual perceives their environmental demands beyond their capacity, and therefore, feels a threat to their well-being as a result (Calvo & Gutierrez-Garcia, 2016). Acute cognitive stress applied concurrently with low intensity contractions of upper body muscles reduces the time to fatigue significantly, especially for women (Shortz & Mehta, 2017; Yoon et al., 2009). Overall central fatigue was similar between the stress and control conditions, and the effect of stress on fatigability is dependent on neuromuscular strength (Keller-Ross et al., 2014). One possible mechanism of fatigue under stress may be due to the modulation of the prefrontal cortex (PFC) activity which regulates exercise performance by influencing the activity of the motor cortex (M1; Tyagi & Mehta, 2023). This study investigates the central mechanisms of fatigue under stress and how stress affects central fatigue and Function of the PFC and M1 under fatigue. We hypothesized that participants would have a lower endurance time in the stress condition as compared to control condition and higher central fatigue.
Thirty right-handed men and women, free from cardiovascular diseases, chronic conditions, and musculoskeletal injuries, and between the ages of 18 and 55 were recruited from a college community. Participants performed intermittent submaximal elbow flexion contractions at 30% of their maximum elbow flexion strength with (stress condition) and without (control condition) a challenging mental math task where they were asked to serially subtract 13 from a random 4-digit number. Participants’ endurance times, strength loss, and initial strength were measured. Functional integration (measured via global efficiency) and segregation (clustering coefficient) within the PFC and M1 were modeled using a graph theoretical approach via functional near-infrared spectroscopy (fNIRS) during the fatiguing contractions. Global Additionally, transcranial magnetic stimulation (TMS) was used to measure voluntary activation of the M1 before and after the fatiguing task to capture changes in neural drive. A condition (stress, control) X sex (male, female) repeated measures ANOVA was performed on strength loss, endurance time, and VA loss rate of the participants. A condition (stress, control) X phase (early, late) X sex (male, female) repeated measures ANOVA was performed on, global efficiency, and clustering coefficient, with significance criteria set at α = 0.05
There was a significant effect of sex on initial strength (p < 0.001), where initial strength of males was significantly higher than that of females. There was a significant effect of condition on the endurance time (p = 0.034), which was higher in the stress than the control condition. There was a significant effect of condition (p = 0.026), sex (p = 0.019), and their interaction (p = 0.031) on the VA loss rate of the participants. Post hoc analysis revealed that the VA loss rate in males was significantly higher than females in the control session (pBonferroni = 0.008) but reduced significantly (pBonferroni = 0.029) to be comparable to females in the stress session. There was also a significant effect of phase (p = 0.038) and its interaction with condition (p = 0.017) on the functional integration of the PFC which was significantly higher in the early phase as compared to the late phase (pBonferroni = 0.011) for the control session only. Similarly, a significant interaction effect of phase and condition was also observed for clustering coefficient of the PFC (p = 0.016) where it was significantly higher in the early phase as compared to the late phase (pBonferroni = 0.040) for the control condition. There was no effect of condition on the global efficiency and clustering coefficient of the M1.
These findings show that stress had a facilitative effect on fatigability where both males and females exhibited greater endurance under stress as compared to the control condition. In the control condition, males exhibited a significantly higher rate of central fatigue than females as expected (Hunter, 2009). However, the stress condition had a facilitative effect on fatigability in males through central mechanisms. Moreover, the fatigue related decline in global efficiency and clustering coefficient observed in the control condition was not present under stress which may have been because of the mental demand imposed by the concurrent mental math task. However, since the PFC is also primarily responsible for regulating muscle fatigue (Jiang et al., 2012), the added cognitive burden of the mental math tasks could have been responsible for delaying the perception of fatigue among the participants.