Purpose: Shoulder resistance exercise (SRE) is commonly used to improve the function of upper extremity. To provide more sensory inputs, SRE is usually combined with head movement. However, previous studies reported that passive head extension and flexion position caused specific changes of both otoliths and neck muscle afferents. These changes affect in heart rate, blood pressure, heart rate variability and muscle sympathetic nerve activity. The purpose of this study was to evaluate the effects of head position on the cardiac autonomic activities during SRE. Methods: Heart rate(HR)and mean artery pressure(MAP)were continually measured using electrocardiography and blood pressure monitor. Power spectrum was used to analyze the fluctuations of heart rate variability(HRV)in 13healthy male subjects (mean age 21.3±1.5yr).SRE protocols were 30% of 1 RM and 30repetitions per minute for four minutes. Subjects were assessed for HRV during the following two experimental conditions in randomized order. The two trails were(1) head in midline with eyes closed(HM),(2)head in hyperextension 30°with eyes closed(HE) to activate otoliths and neck muscle afferents. All the variables were tested with Wilcoxon signed ranks test. Results: There is no significant influence of head position on the HR and MAP in two trails. All the HRV variables showed significant difference between HM and HE in recovery stage (p＜0.05) but not in exercise stage. The values of normalized low-frequency power were 21.25±4.98, 46.50±2.37% respectively during HM and HE. The values of normalized high-frequency were 46.91±1.98, 52.82±2.15% respectively and the ratio of low-frequency to high-frequency were 1.17±0.078, 0.923±0.089 respectively during HM and HE(p＜0.05). Conclusion: Our data indicate that the HE results in an increased vagal activity during recovery stage, including increases in HFnu and decreases in LF/HF. The results suggest that the vestibular apparatus during head hyperextension mediates the reduction in sympathetic activities which contrast to in skeletal muscles sympathetic afferents increase and consquently, neither MAP nor HR was affected significantly by head hyperextension during exercise stage.
