Affiliations: [a] Division of Sport, Exercise and Nutritional Sciences,
University of Central Lancashire, Lancashire, UK | [b] School of Psychology, University of Central
Lancashire, Lancashire, UK
Note: [] Corresponding author: Jonathan Sinclair, Division of Sport,
Exercise and Nutritional Sciences, School of Sport Tourism and Outdoors,
University of Central Lancashire, Preston, Lancashire, PR1 2HE, UK. E-mail:
Jksinclair@uclan.ac.uk
Abstract: 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°/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 × 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.
Keywords: Circadian rhythms, isokinetic force, time of day
