The influence of circadian rhythms on peak isokinetic force of quadriceps and hamstring muscles

BACKGROUND: Circadian rhythms, and their potential influence on athletic performance, have been the subject of a number of investigations. OBJECTIVE: This study aimed to determine the influence of circadian rhythms on the peak isokinetic force of the quadriceps and hamstring muscle groups, and the relationship of cortisol level with these measures. METHODS: Twelve male and twelve female participants completed maximal isokinetic flexion and extension movements of their dominant leg at 60 degrees/s using an isokinetic dynamometer. Data were collected at 09.00 (Session A), 14.00 (Session B) and 18.00 (Session C). Salivary cortisol was also obtained immediately prior to each session. Differences in isokinetic strength parameters and salivary cortisol concentrations, between the three sessions, were examined using 2 x 3 mixed ANOVAs. Pearson correlations were used to examine the relationship between isokinetic strength and salivary cortisol. RESULTS: For all of the flexion parameters, but none of their extension counterparts, session C was associated with significantly greater isokinetic strength compared to session A. No significant relationships were observed between measures of isokinetic strength and cortisol. CONCLUSIONS: The findings from this investigation suggest that both male and female athletes performing knee flexion strengthening exercises should conduct their training/competitions in the late afternoon or early evening in order to maximize performance.