TIME-OF-DAY DEPENDENCE OF ISOKINETIC LEG STRENGTH AND ASSOCIATED INTERDAY VARIABILITY

The purpose of this study was to assess the interday variability and t ime-of-day effects on selected isokinetic leg strength indices. Nine a dult collegiate sportsmen (mean(s.e.) age 19.6(0.5) years; mean(s.e.) height 1.81(0.02) m; mean(s.e.) body mass 76.5(3.1) kg) completed a se ries of nine test sessions, organized so that each subject was tested three times within a day (08.00-09.00 hours; 13.00-14.00 hours; 18.00- 19.30 hours), on three occasions, each separated by a minimum of 7 day s. Gravity-corrected indices of extension peak torque (EPT), flexion p eak torque (FPT), and the peak torque ratio (PTR), at contraction velo cities of 1.05 rad s(-1) and 3.14 rad s(-1), were calculated for each subject using an isokinetic dynamometer.dynamometer. Two-way repeated measures analysis of variance of coefficient of variation (V%) scores revealed no significant differences in performance variability across within-subject factors of time-of-day and performance index (P > 0.05) . Overall mean(s.e.) V% for scores across experimental conditions were 3.97(0.72)% at 1.05 rad s(-1) and 5.98(1.23)% at 3.14 rad s(-1), sugg esting that similar levels of measurement error occur between 08.00-19 .30 hours. One-way repeated measures analysis of variance of absolute strength indices (EPT, FPT and PTR) revealed that significantly higher scores were achieved during session 3 (18.00-19.30 hours), with mean( s.e.) values of 249.1(40.0) N m, 149.0(32.3) N m, 59.5(5.0)% at 1.05 r ad s(-1) and 172.1(38.7) N m, 121.3(27.7) N m, 71.1(6.2)% at 3.14 rad s(-1), respectively (P < 0.05). This finding appears to be consistent with current knowledge about time-of-day effects on the assessment of muscular strength. Thus for stable and maximal values to be obtained d uring isokinetic leg testing, the use of multiple-trial protocols is r ecommended, with testing occurring as close to 18.00-19.30 hours as po ssible. In addition, the observed significant time-of-day effect sugge sts that appropriate comparison of maximal isokinetic leg strength can only be achieved based on data obtained within 30 min of the same tim e of day.