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⁻¹ and 4.99 ± 0.78 N.kg⁻¹at the peak and at the nadir of the day, respectively. The mean difference (95% CI) was 0.86 ± 0.47 N.kg⁻¹ (0.62; 1.10) for the diurnal variation and 0.30 ± 0.42 N.kg⁻¹ (0.09; 0.52) for the day-to-day variation. The mean maximum isometric arm strength was 1.68 ± 0.33 N.kg⁻¹ at the peak and 1.46 ± 0.19 N.kg⁻¹ at the nadir of the day, respectively. The mean difference (95% CI) was 0.21 ± 0.16 N.kg⁻¹ (0.14; 0.29) for the diurnal variation and 0.06 ± 0.05 N.kg⁻¹ (0.03; 0.08) for the day-to-day variation. The linear mixed-effects model showed little evidence for differences in isometric leg strength between the different times of the day (all <i>p</i>-values &gt;0.983). The present study demonstrated that diurnal variations in leg and arm strength are nearly three times higher than the day-to-day variations, but there was only little evidence for a time-of-day effect on a group level. The diurnal variations observed herein without time-of-day effects are suggestive that individuals achieve their peak performance at different times of the day. Therefore, performance tests should be carried out at the same time of the day to ensure comparability. Furthermore, depending on the difference between the time of competition and the time of peak performance, as well as the individual magnitude in diurnal variation, some athletes can have a clear disadvantage.<b>Abbreviation</b>: 95% CI, 95% confidence interval; SD, standard deviation; ICC, intraclass correlation coefficient.

鉴于力量表现的昼夜效应与竞技体育息息相关，相关研究已得到广泛开展。然而，多数现有研究采用较大的测量间隔，难以追踪单日中力量表现的潜在波动。此外，既往研究仅聚焦于昼夜时段对群体层面力量表现的影响，却忽略了个体最佳表现时段的差异。为此，本研究旨在探究一日内六个不同时段下，受试者腿部、上肢及躯干的等长（isometric）与等速（isokinetic）力量表现的单日节律与日间波动。
在完成熟悉性测试后，19名经过训练的男性受试者（年龄：24.1±2.5岁）使用等速测力仪（isokinetic dynamometer），在一日内六个不同时段（7:00、10:00、13:00、16:00、19:00及21:00）完成等长与等速力量评估。为探究日间波动情况，受试者还在与第七次测试相同的时段完成了第八次测试，并对单日节律与日间波动的差异进行了比较。所有测试间隔至少48小时，首次测试的开始时间采用随机化设置。
受试者单日峰值时段的平均最大等长腿部力量为5.85±0.80 N·kg⁻¹，低谷时段则为4.99±0.78 N·kg⁻¹。单日节律波动的平均差值（95%置信区间，95% CI）为0.86±0.47 N·kg⁻¹（0.62~1.10），日间波动的平均差值则为0.30±0.42 N·kg⁻¹（0.09~0.52）。
受试者单日峰值时段的平均最大等长上肢力量为1.68±0.33 N·kg⁻¹，低谷时段为1.46±0.19 N·kg⁻¹；单日节律波动的平均差值（95% CI）为0.21±0.16 N·kg⁻¹（0.14~0.29），日间波动的平均差值则为0.06±0.05 N·kg⁻¹（0.03~0.08）。
线性混合效应模型分析结果显示，不同时段下的等长腿部力量并无显著差异（所有P值均>0.983）。本研究结果表明，腿部与上肢力量的单日节律波动幅度约为日间波动的三倍，但在群体层面，昼夜时段对力量表现的影响并不显著。本研究未发现群体层面的昼夜时段效应，但却观察到明显的单日节律波动，这提示不同个体的最佳表现时段存在差异。因此，为保证测试结果的可比性，力量表现测试应固定在每日的同一时段进行。此外，结合比赛时段与个体最佳表现时段的差异，以及个体单日节律波动的幅度，部分运动员可能会面临明显的竞技劣势。
**缩写说明**：95% CI（95%置信区间，95% confidence interval）；SD（标准差，standard deviation）；ICC（组内相关系数，intraclass correlation coefficient）。