Muscle Fatigue and Motor Output Variability with Acute Stress in Healthy Young Adults and Veterans with Posttraumatic Stress Disorder

MUSCLE FATIGUE AND MOTOR OUTPUT VARIABILITY WITH ACUTE STRESS IN HEALTHY YOUNG ADULTS AND VETERANS WITH POSTTRAMATIC STRESS DISORDER Manda L Keller, DPT Marquette University, 2011 Acute stress can alter motor performance differently for men and women. The first aim of this dissertation addresses possible causes for the sex difference in the motor response of a low-intensity fatiguing contraction of the elbow flexor muscles to an acute stressor (difficult mental math) in young, healthy adults. Muscle fatigue increased for men and women when exposed to the stressor, but impairment was more prominent for the women. This work showed that fatigue in the central nervous system, specifically in cortical motor and premotor areas, as well as relaxation rates of the muscle (quantified with cortical stimulation) were not responsible for the stress-induced motor fatigue. The greater fatigue with stress was associated with the strength of the individual such that weaker individuals (mainly women) fatigued more quickly when exposed to the stressor than stronger individuals (mainly men). Thus, the mechanism of stress-induced fatigability in weaker subjects may be due to differences in blood perfusion to the muscle. Furthermore, women were less steady than men for very low-intensity contractions in the presence and absence of stress. Steadiness was impaired at failure of the fatiguing contraction but persisted more for women than men, up to 20 minutes recovery. A second aim was to determine muscle fatigability and steadiness in veterans with posttraumatic stress disorder (PTSD) in the presence and absence of the cognitive stressor. Male veterans with PTSD and male civilian controls performed a low-intensity contraction till task failure with the handgrip muscles. Veterans with PTSD fatigued more quickly and were less steady than the control subjects. When exposed to a cognitive stressor, neither the veterans with PTSD nor control subjects had greater fatigability or reduced steadiness. Veterans with PTSD however, were more fatigable compared with the control subjects in both stressful and non-stressful conditions, suggesting that the chronic stress condition (vs. acute stress) has a greater influence on motor performance for the hand muscles. Understanding how motor control is altered in the presence of acute stress and in clinical populations can lead to more tailored treatment interventions for optimized rehabilitation programs for specific motor impairments and for clinical populations.