Muscle Fatigability and Control of Force in Men with Posttraumatic Stress Disorder

Abstract

Acute stress can increase fatigability and decrease steadiness of sustained low-force contractions that are required for functional tasks in upper limb muscles. Whether motor performance is more impaired in people with a chronic stress disorder is not known. This study compared the fatigability and steadiness (force fluctuations) of handgrip muscles in veterans with posttraumatic stress disorder (PTSD) and civilian controls in the presence and absence of varying levels of cognitive demand. Eighteen veterans with PTSD and 21 healthy controls (33 ± 9 yr) attended three randomized experimental sessions to perform an isometric fatiguing contraction (20% of maximal strength) with the handgrip muscles. Two sessions involved performing a cognitive task during the fatiguing contraction: 1) difficult mental math task (stressor) and 2) a simple mental math task (mental attentiveness). A third session involved a fatiguing contraction with no mental task (control). Stress elevated heart rate, blood pressure, and levels of anxiety in veterans with PTSD (P < 0.05) but blunted cortisol levels (P < 0.05). Time to failure was briefer (7.2 ± 2.5 vs 9.3 ± 5.2 min, P = 0.03), and force fluctuations increased at a greater rate for veterans with PTSD than for controls (P < 0.05). Cognitive stress did not influence time to failure or force fluctuations for either group (P > 0.05). Veterans with PTSD demonstrated greater fatigability and loss of steadiness (greater force fluctuations) of the handgrip muscles compared with healthy controls. Male veterans with PTSD demonstrated altered neuromuscular function of arm muscles that potentially affects functional tasks during daily, ergonomic, and military activities.