Autonomic nervous alterations associated with daily level of fatigue

Fatigue is a common symptom in both sick and healthy people. We examined autonomic nervous alterations associated with fatigue to clarify the mechanisms underlying fatigue.

The study group consisted of 19 healthy participants who performed a 2-back test for 30 min as a fatigue-inducing mental task session. Before and after the session, they completed the advanced trail making test (ATMT) for 30 min for mental fatigue evaluation, subjective scales to measure fatigue sensation, and underwent electrocardiography to allow assessment of autonomic nerve activities.

After the fatigue-inducing task, the total error counts on the ATMT tended to increase (P = 0.076); the ATMT for total trial counts (P = 0.001), the subjective level of fatigue (P < 0.001), and the % low-frequency power (%LF) (P = 0.035) increased significantly; and the % high-frequency power (%HF) decreased compared with before the fatigue-inducing task although this did not reach the statistical significance (P = 0.170). Although LF measured in absolute units did not change significantly before and after the fatigue-inducing task (P = 0.771), and HF measured in absolute units decreased after the task (P = 0.020). The %LF and LF/HF ratio were positively associated with the daily level of fatigue evaluated using Chalder's fatigue scale. In addition, %HF was negatively associated with the fatigue score.

Increased sympathetic activity and decreased parasympathetic activity may be characteristic features of both acute and daily levels of fatigue. Our findings provide new perspectives on the mechanisms underlying fatigue.