Change in the LF/HF ratio of heart rate variability. AUC: area under the curve. MAX 1~MAX 3: the trapezius muscles contract with maximum effort for 1 min. Relax 1~Relax 3: the trapezius muscles relax for 2 min . Values are presented as mean, SE. †, different from the LF/HF ratio first rest (p < 0.01). *, **, different from the LF/HF ratio of the control group (p < 0.05, p < 0.01).