The Effects of Time of Day on Static and Dynamic Postural Control

Instrumented and clinical tests exist to quantify static and dynamic postural control. Physiological and neurological functions in humans are influenced by diurnal patterns that follow a proposed circadian rhythm. There has been little investigation into an influence of the time of day on postural control. PURPOSE: To determine the influence of time of day on static and dynamic postural control. METHODS: Thirty subjects (13 males, 17 females; 21.8±3.74yr.; 173.57±7.90cm; 70.12±13.67kg) reported to the laboratory for six separate testing sessions over a forty-eight hour period during which static and dynamic postural control was assessed at three times (10:00, 15:00, and 20:00) on both days. The order of the static and dynamic tests was randomized. Static postural control was assessed for 30 seconds in a single limb stance on the dominant limb using a forceplate with eyes open and closed conditions. For each condition, three trials were averaged. The order of condition was randomized. Center of pressure velocity in the A/P(COPVX) and M/L (COPV Y) was calculated. Dynamic postural control was assessed with the anterior reach of the Star Excursion Balance Test (SEBT) utilizing the dominant limb. Three reaches were averaged and the reach distances were normalized to leg length (%MAXD). For both COPVX and COPVY, separate Day (one, two) by Time (10:00, 15:00, 20:00) by Eye (EO, EC) by Gender (male, female) repeated measures ANO VA's were performed. For %MAXD, a Day (one, two) by Time (10:00, 15:00, 20:00) by Gender (male, female) repeated measures ANO VA was performed. RESULTS: For COPVX, there was a statistically significant Day by Time interaction (F2,58=3.849; p=.027). On Day 1, COPVX was significantly less at 10:00 (0.120±0.008 m/s) compared to 15:00 (0.125±0.008 m/s). For COPVY, there was a statistically significant Day by Time interaction (F2,58=5.804; p=.005). On Day 1, COPVY was significantly less at 10:00 (0.130±0.007 m/s) compared to 15:00 (0.141±0.008 m/s). For %MAXD, there was a significant Time by Gender interaction (F2,56=3.42, p=.04). Among the females, %MAXD was significantly greater at 10:00 (0.824±0.015) compared to 20:00 (0.811±0.016). A significant main effect for Time (F2,56=3.205, p=.048) was also present for %MAXD, indicating that 10:00 produced the greatest %MAXD (0.821±0.011) compared to 15:00 (0.814±0.012) and 20:00 (0.814±0.012). CONCLUSIONS: There seems to be an influence of the time of day on static and dynamic postural control that suggests postural control may be better in the mornings compared to the afternoons. This may influence serial postural control testing as well as between group comparisons of postural control.