The effect of circadian rhythmicity on lower extremity strength and balance in female athletes

The aim of this study was to investigate the effect of circadian rhythmicity (CR) on lower extremity strength and balance performance in 22 female athletes (19-24 y). Participants underwent isokinetic knee strength, Single Leg Hop Tests (SLHT), Y Balance Test (YBT), and isokinetic static/dynamic balance tests. All measurements were randomized and performed at 9:00, 14:00, and 19:00 to assess CR variation. In total, 3 angular strength, 5 jump, and 2 balance tests were analyzed. When comparing results across time zones, body temperature, isokinetic 60°/sec extension (Ext) strength, and several bipedal static balance parameters Forward-Backward Standard Deviation (SBFD), Average Forward-Backward Speed (AFBS), and Ellipse Area (EA)) showed statistical significance (p>0.05). No significant differences were found in other isokinetic strength tests, all SLHTs, Y balance, or dynamic balance tests (p>0.05). Significant CR effects on lower extremity strength were observed only in the 60 sec extension, with overall strength peaking at 14:00. Strength asymmetry remained similar across time points. These findings suggest that time-sensitive performance parameters like strength and static balance are influenced by CR, while balance and coordination parameters appear more stable. Accordingly, scheduling strength-based training during midday hours may optimize performance by leveraging CR-related variations.