The circadian rhythm in muscle strength was analysed in 12 males (28 ± 4 years, 79.6 ± 12.3 kg, 1.80 ± 0.05 m) and eight females (28 ± 4 years, 60.3 ± 5.5 kg, 1.61 ± 0.08 m). After two familiarization sessions, participants were tested at six different times of the day (02:00, 06:00, 10:00, 14:00, 18:00 and 22:00 hours), the order of which was randomly assigned over 3–4 days. Rectal temperature (T _rec) was measured over 30 min before each test. Peak isokinetic torques (PT) of knee extensors and flexors were then measured at 1.05 rad s⁻¹ and 3.14 rad s⁻¹ through a 90° range of motion. Maximal isometric voluntary contraction (MVC) of knee extensors and flexors was measured at 60° of knee flexion and the MVC of knee extensors was also assessed with superimposed electrical twitches (50 Hz, 250 V, 200 μs pulse width) in order to control for motivational effects. Three trials were performed in each condition, separated by 3 min recovery, and the highest values were retained for subsequent analyses. A significant circadian rhythm was observed for T _rec in both males and females (acrophase, Φ, 17:29 and 16:40 hours; mesor, Me, 37.0 and 36.8°C; amplitude, A, 0.28 and 0.33°C for males and females, respectively). The mesor of T _rec was higher in males than in females (p < 0.05). Significant circadian rhythms were observed for knee extensor PT at 3.14 rad s⁻¹ in males (Φ, 17:06 hours; Me, 178.2 N m; A, 4.7 N m) and for knee extensor PT at 1.05 rad s⁻¹ in females (Φ, 15:35 hours; Me, 128.7 N m; A, 3.7 N m). In males, the MVC of knee extensors demonstrated a significant circadian rhythm, but only when electrical twitches were superimposed (Φ, 16:17 h; Me, 302.1 N m; A, 13.6 N m). Acrophases of all indices of muscle strength were not statistically different between the two groups and were located in the afternoon (12:47 < Φ < 17:16 hours). The amplitude (percentage of mesor) of extensors MVC (electrically stimulated) was higher in males (6.4%) than in females (4.2%; p < 0.05). Significant circadian rhythms were not consistently observed for all indices of muscle strength whatever the gender. Our group of female subjects tended to show lower circadian amplitudes than the males. In males, maximal voluntary contraction of electrically stimulated muscles followed a circadian curve, which was not significant without the superimposed twitches. These results suggest that motivation could have a masking effect on the circadian rhythm in muscle performance and strengthen the view that peripheral factors are implicated in this rhythm.

The aim of this study was to examine the effect of delayed evening mealtime on sleep quality in young athletes. Twelve rugby players (age 15.8 ± 0.7 years) participated in a crossover within-participant design. Adolescents spent five consecutive days in each of two conditions, separated by a 2-week washout period: routine dinner (3.5 hr before bedtime) and late dinner (LD, 1.5 hr before bedtime). Other mealtimes as well as bedtime and wake-up time were usual and remained the same in both conditions. Their schedules, dietary intakes, and physical activity were controlled and kept constant throughout the study. Sleep was assessed using polysomnography on the first and the last nights in the individual rooms of the boarding school. An increase in total sleep time by 24 min (p = .001, d = 1.24) and sleep efficiency by 4.8% was obtained during LD (p = .001, d = 1.24). Improvement in sleep efficiency was mainly due to a lower wake after sleep onset (−25 min, p = .014, d = −3.20), a decrease of microarousals (−25%, p = .049, d = −0.64), and awakenings ≥90 s (−30%, p < .01, d = −0.97) in LD compared to routine dinner. There were no significant differences in sleep architecture except for a shorter slow-wave sleep (N3) latency (−6.9 min, p = .03, d = −0.778) obtained during LD. In this study, evening dinner 1.5 hr before bedtime leads to better quality and less fragmented sleep compared to evening dinner 3.5 hr before bedtime in young athletes.

Athletes often perceive a performance disadvantage in the morning, in part, because of a recognised deficit in functional muscle force capacity. This diurnal variation in muscle force production has been attributed to higher rectal (T_rec) and muscle (T_m) temperatures in the evening as well as motivational, peripheral, and central factors. A warm-up is an essential component of sporting performance, however moderate hyperthermia reduces sporting gross muscular performance although possibly to a lesser degree in the morning (raising phase) than the peak of the core temperature rhythm (~17:00 h). We investigated whether i) increasing morning T_rec temperatures to evening resting values by an active warm-up leads to quadriceps muscle force production becoming equal to evening values. Or ii) raising T_rec passively in the morning or evening to 38.5°C results in greater quadriceps muscle force production reductions in the evening. Eight active males (mean±SD: age, 25.5 ± 1.9 yrs; body mass, 71.0 ± 6.7 kg; height, 1.79 ± 0.06 m) volunteered and randomly completed five sessions (separated by > 48 h): control morning (M, 07:30 h) and evening (E, 17:30 h) sessions (both with an active 5-min warm-up) and three further trials - an active warm-up 07:30 h trial (M_E, until resting evening temperatures were reached), a morning (M_38.5) and an evening (E_38.5) passive warm-up trial which continued until T_rec values reached 38.5°C (immersed in a water-bath @ ~40°C, 45-50% Relative humidity). During each trial, 5-measures of maximal voluntary contraction (MVC) of the quadriceps on an isometric dynamometer (utilizing the twitch-interpolation technique) were performed with force (peak and mean of the 5-trials) and percentage activation recorded. T_rec, ratings of perceived exertion (RPE) and thermal comfort (TC) were measured. Measurements were made after the participants had reclined for 30-min at the start of the protocol and after the warm-ups/passive heating and prior to the measures for isometric dynamometry. T_rec and T_m (at 3, 2 and 1 cm depths) temperatures were taken at rest, after the passive warm-up, and immediately before the isometric MVC measurements. Data were analysed by general linear models with repeated measures. Isometric force for knee extension showed higher values in the evening than morning (peak Δ83.2 N, mean Δ67.8 N; p < 0.05). T_rec and T_m (at 3 cm depth) values were higher at rest in the evening than the morning (by 0.47 and 0.85°C respectively; p < 0.05) increasing from rest by 0.54 and 2.2°C, 1.78 and 2.2°C, and 1.31 and 1.8°C, in the M_E, M_38.5 and E_38.5 conditions, respectively; ratings of thermal comfort reflecting this (p < 0.05). There was no significant effect of active M_E warm-up and moderate hyperthermia M_38.5 compared to morning control peak (peak or mean) torque (M). E_38.5 reduced "mean" but not "peak" torque in the evening (Δ61.9 N, p = 0.009; p = 0.051). In summary, active warm-up did not improve isometric MVC in the morning and moderate hyperthermia reduced isometric MVC "mean" force only during the peak of the core temperature rhythm (~17:00 h).

Diurnal variations in time-trial performance have been shown in people living normally, where a "standardised protocol" has been employed to reduce bias. We tested the hypothesis that a circadian variation exists for a 10-km running laboratory-based time-trial, where such a standardised approach is used. Twelve recreationally active adult males were recruited. The participants completed three familiarisation time-trials to the best of their ability at a self-selected pace and six 10-km time-trials at 06:00, 10:00, 14:00, 18:00, 22:00 and 02:00 h. Each session was separated by 7-days. Participants were allocated into 6 groups due to finish times (FT); sessions were counterbalanced in order of administration. A cosine fit for resting intra-aural temperature and FT both showed a significant circadian rhythm (p < 0.05) with mesor, amplitude and acrophases of 36.61°C vs 2994 s, 0.34°C vs 149 s; and 17:29 vs 18:44 h:min, respectively. The parallelism of temperature and FT agrees with previously published research. The finding of a 24-h rhythm in 10-km FT (5.0%, d = 0.80; power = 100%) concurs with that of a shorter distance where "standardised protocols" have been employed (4-km, 2.6%, d = 0.34). This finding has implications for scheduling of competition and training. Whether this variation is apparent in other populations, however, is unclear.

Article: Energy availability and macronutrient intake over a 7-day training period in adolescent rugby players - The Journal of Sports Medicine and Physical Fitness 2025 Jan 16 - Minerva Medica - Journals