Daily rhythmicity in hand grip strength

Muscle strength is known to fluctuate across the day, typically peaking in the late afternoon. However, studies examining diurnal variation in hand-grip strength have produced inconsistent findings, likely due to methodological limitations such as short or sparsely sampled time series. Clarifying whether hand-grip strength exhibits a robust daily rhythm is essential for understanding the circadian modulation of motor performance. Our sample included 22 participants (11 men and 11 women) ranging in age from 21 to 90 years and residing in both Europe and the United States. Each participant conducted measurements at 12 time points scheduled every two hours across a 24-hour cycle and distributed over two or three days to avoid unintended sequential effects. We also measured sublingual temperature as a supplemental variable. Using ANOVAs and cosinor rhythmometry, we found that hand-grip strength and sublingual temperature exhibited robust daily rhythmicity with average peaks at 15:30 (grip strength) and 16:18 (temperature). We conclude that hand-grip strength exhibits robust daily rhythmicity in healthy individuals. Negative results in previous studies were likely due to inadequately short time series and insensitive data analysis procedures.