Diurnal and day-to-day variations in isometric and isokinetic strength.

Time-of-day effects in strength performance have been extensively investigated due to their relevance in competitive sports. However, most studies use large measurement intervals making it difficult to monitor potential performance changes throughout the day. Furthermore, previous studies have exclusively focused on how the time of day affects strength on a group level and ignored the individual differences in the times of peak performance. Therefore, the main purpose of this study was to investigate the diurnal and day-to-day variations in isometric and isokinetic leg, arm and trunk strength over six different times of the day. Following a familiarization test, 19 trained males (age: 24.1 ± 2.5 years) performed isometric and isokinetic strength assessments at six different times of the day (7:00, 10:00, 13:00, 16:00, 19:00, and 21:00) with an isokinetic dynamometer. An eighth test session was performed at the same time of the day as the seventh test session to investigate the day-to-day variations and the difference between diurnal and day-to-day variations were compared. All tests were separated by at least 48 h. The start time for the first session was randomized. The mean maximum isometric leg strength was 5.85 ± 0.80 N.kg