The influence of the circadian rhythm on physical performance has received much attention in the literature (24); with evidence of changes in physical performance (e.g., force and power) at varying times of the day (23). Generally, the literature points toward an early morning nadir and a subsequent peak in performance in the late afternoon (5). Within the literature examining the interaction between the time of day and performance/adaptation, there has been some conflict in the findings (17,20). There are data, which suggests that the time of day resistance training occurs does not affect hypertrophy (20) or strength adaptation (19) despite evidence of greater neuromuscular performance in the afternoon, when compared with the morning (17).
Of more importance is perhaps the concept that some sports, e.g., Rugby union sevens, which is set to become an Olympic sport, may require athletes to compete anywhere from 09:00 to 21:00 hours. The literature suggests that the time of day influences body temperature with studies demonstrating small physiological rises in core temperature (Tcore) from AM to PM. It is suggested that body temperature may change by approximately 0.4° C throughout the day (1,14); typically, body temperature reaches its circadian low at approximately 06:00 hours and peaks at 18:00 hours (18). Although typically these changes in Tcore are of nowhere near the magnitude demonstrated in the majority of the literature examining the influence of Tcore on exercise performance (e.g. 8,13), there is some recent evidence in athletes that even small physiological changes in body temperature can indeed influence physical performance (11,23,25). For example, Kilduff et al. (11) demonstrated that differences in Tcore of approximately 0.4° C, induced through the use of a passive heat maintenance garment, can significantly impact upon lower body power output and repeated sprint ability in a group of professional rugby league players.
With this in mind, it is unclear if the small physiological changes in Tcore, resultant from circadian rhythm alone, may influence the performance in elite athletes. Particularly in athletes who are accustomed to training/competing within a wide time frame across the day. This performance information is important to both coach and athlete as strategies, which may minimize the morning low in body temperature could be employed when training/competition takes place early in the day.
Therefore, we examined the influence of the time of day on Tcore and lower body power output in elite rugby union sevens players. We hypothesized that there will be a higher core temperature and a corresponding increased lower body power output, in the afternoon, when compared with the morning.
Experimental Approach to the Problem
This study examined the morning and afternoon Tcore and peak power output (PPO) responses in a group of elite rugby union sevens players. We sought to determine if the natural circadian change in Tcore would influence physical performance in an elite athlete population. Players reported at 10:00 and 17:00 hours for the collection of Tcore and countermovement jump (CMJ) assessment. Countermovement jump data were processed for PPO.
With university ethical approval and informed consent, 16 elite professional rugby union players (age = 21 ± 4 years; body mass = 93 ± 9 kg; height = 182 ± 2.0 cm; and jump height = 40.2 ± 6.4 cm) volunteered to take part in the study. The subjects, all over 18 years of age, recruited were elite rugby union sevens players who routinely train and compete in both the morning (∼10:00 hours) and evening (∼17:00 hours). Subjects were the members of a sevens rugby squad who compete on the institutional review board sevens circuit and have won a number of these events. This study was carried out at the end of a preseason training phase at a training camp. While in camp, players were required to keep a strict sleeping pattern.
Players reported for morning testing at 10:00 hours after consuming their typical training day breakfasts and having refrained from caffeine; moreover, players had refrained from alcohol and strenuous exercise during the previous 24 hours. Upon arrival to the laboratory, players were seated for 15 minutes to measure baseline Tcore (CorTemp Ingestible Core Body Temperature Sensor; HQ Inc., Palmetto, FL, USA). During this time, they were familiarized with the trial procedures. Once in the morning, Tcore measures were collected from the players who performed a standardized warm-up, which was prescribed by the team coach. After the warm-up, players carried out 3 CMJs on a portable force platform (Type 92866AA; Kistler, Ostfildern, Germany). Players remained rested throughout the day and then returned to the testing site at 17:00 hours and repeated the Tcore measurement, warm-up, and CMJ procedure performed in the morning.
Tcore was collected through the ingestion of a temperature sensor (CorTemp Ingestible Core Body Temperature Sensor, HQ Inc.), which transmitted a radio signal to an external sensor (CorTemp Data Recorder, HQ Inc.), which subsequently converted the signal into digital format. Players ingested the sensor 3 hours before the experimental trials (4). The ingestible core temperature device has been demonstrated to be both reliable and valid (4).
The warm-up was standardized across time points and consisted of low intensity running for approximately 200 m followed by a series of dynamic exercises (e.g., speed skips, heel kicks, drop squats, cariocas, and high knee skips) for the main musculature required to perform the CMJ. After this, players performed 5 practice jumps with 1-minute recovery between each jump.
Countermovement Jump Analysis
For the measurement of CMJ PPO, testing was completed on a portable force platform with the participants standing akimbo to isolate the lower limbs (26). After an initial stationary phase of at least 2 seconds in the upright position, for the determination of body mass, participants performed a CMJ, dipping to a self-selected depth and then exploding upwards in an attempt to achieve maximum height. Participants landed back on the force platform and kept their arms akimbo throughout the movement. Players were required to complete 3 maximum jumps with 1.5 minutes rest between efforts. Peak power output was calculated from the CMJ as per West et al. (26). The vertical component of the ground reaction force (GRF) during performance of the CMJ was used in conjunction with the participants' body mass to determine instantaneous velocity and displacement of his centre of gravity (9). Instantaneous power was determined using the following standard relationship:
Power (W) = vertical GRF (N) × Vertical velocity of centre of gravity (m·s−1).
Statistical analysis was performed using SPSS software (version 16; SPSS Inc., Chicago, IL, USA), with significance set at P ≤ 0.05. Data were analyzed using paired samples t-test and Pearson's product moment correlation coefficient. Effect size was calculated using Cohens d. Where significant differences have been identified, 95% confidence intervals (CIs) are presented for an estimation of the population mean difference. Data are presented as mean ± SD.
The Tcore data are presented in Figure 1A. Tcore increased from AM to PM by 1.3 ± 0.3% (95% CI = 0.19–0.31° C; P < 0.0001; Cohens d = 0.98), with a range of 0.10–0.46° C across players. Peak power output data are presented in Figure 1B. Peak power output increased from AM to PM by 5.1 ± 0.7% (95% CI = 120–199 W; P < 0.001; Cohens d = 0.45) with a range of 37–297 W across participants.
The change in Tcore (0.26 ± 0.13° C) and PPO (164 ± 78 W) was significantly related (Figure 2).
The aim of this study was to examine the effect of the time of day on Tcore and lower body power output in elite rugby union sevens players. Here, we demonstrate that Tcore rises from AM to PM, which is in line with existing literature, and that these changes are concomitant with increases in lower body PPO. Furthermore, the magnitude of the change from AM to PM in Tcore and peak power output was strongly associated.
Tcore increased from AM to PM, which supports the existing literature (18). The likely explanation behind this finding is the natural circadian rhythm of Tcore that typically shows a circadian low in the morning (6 AM) before peaking in the early evening (6 PM) (18). The natural circadian change in Tcore is determined by transient changes in heat production and heat loss throughout the day (12). On average, the magnitude of change found in our athletes (range, 0.10–0.46° C) differs to the suggested approximately 0.4° C fluctuation caused by the circadian rhythm (1). This can be explained by the selected time points of measures. Tcore has been demonstrated to start to rise from approximately 06:00 and reaches a peak at approximately 18:00 (18), thus it is likely that our measures/contact time were within the ranges where the greatest fluctuation in Tcore was likely to occur and thus did not represent minimal or maximal Tcore values.
The average PPO increased by approximately 5% from AM to PM, equating to a 164 ± 78 W improvement in CMJ performance; which is in line with prior literature (22). This finding is of particular interest given that these differences in PPO are potentially because of a heightened Tcore, resultant from the circadian rhythm, rather than exercise. In support of this, Kilduff et al. (11) used a passive heating garment to show that a pretest Tcore of 37.5° C resulted in an approximately 5% greater PPO during CMJ when compared with a Tcore of 37.1° C, in a control condition; thus, highlighting the influence of small differences in Tcore within the normal physiological range. Although circadian variation in intracellular inorganic phosphate could also influence the contractile properties of the muscle (15), we believe it is more likely that an elevated muscle temperature (Tmuscle) is responsible for the performance improvements observed (7,16). An elevation in Tmuscle has the potential to increase neural transmission rate in both peripheral and central nerves (10), which increase the speed of muscle contractions and decrease both the time to peak tension and half relaxation time (2,6,11), and decrease the viscous resistance of muscles and joints (3).
Worthy of note, because of the level of athlete being studied, we were limited in our measures/contact time, and therefore, postwarm-up Tcore was not recorded. Thus, the magnitude of the increase in Tcore with the standardized warm-up could not be compared from AM to PM. Prior research has demonstrated that the change in body temperature with a warm-up is indeed encumbered by the circadian low in the morning, and as such, an extended morning warm-up is required and can mitigate the effects of the circadian temperature nadir (21). However, the practical application of an extended warm-up is potentially limited in some sports (e.g., Olympic events) and alternative strategies, such as a passive heating garment, could be used.
The change in Tcore and PPO from AM to PM was significantly related (r = 0.781; P < 0.001). The relationship between change in Tcore and Tmuscle and physical performance has been demonstrated previously (11,16). Kilduff et al. (11) demonstrated that the postwarm-up decline in Tcore and PPO were significantly related in professional rugby league players (r = 0.71). However, to our knowledge, we are the first to demonstrate that a very small physiological change in Tcore, as the result of the circadian rhythm, could be a potential predictor of physical performance in an elite athlete group. This finding is of importance to all sports, which may compete at varying times across the day (e.g., swimming, combat sports).
A limitation of this study was that Tmuscle was not measured, but because of the nature of the data collection, the measurement of Tmuscle was not feasible. The collection of Tmuscle would have allowed for the examination of how the circadian change in Tcore might also influence Tmuscle. In addition, it would have been useful to collect Tcore measures after the warm-up, such that the influence of the time of day on the magnitude of the rise in body temperature, induced by warm-up, could be examined. However, these limitations should not detract from the application of our findings in that we have demonstrated differences in physical performance from AM to PM, which is potentially because of body temperature mediated mechanisms, in an elite group of athletes.
In conclusion, the effects of the time of day on core temperature and lower body power output were examined in an elite group of professional athletes. Our data demonstrate that lower body power output is increased in the evening, when compared with the morning, and this is associated with an increase in core temperature.
The data from this study suggest that athletes could experience reduced performance in the morning, which could be mediated by the natural circadian rhythm of body temperature. There is potential that the morning performance decrement could be offset through the use of appropriate warm-up strategies (21) or through the use of additional tools, such as a passive heating garment (11); however, the application of these strategies needs to be thoroughly investigated in future research to address this study's aforementioned limitations.
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