Short-term exercise performances have been shown to follow the human circadian rhythm. Indeed, it has been well documented that, in a temperate climate, maximal anaerobic muscle power exercise is generally increased by the end of the afternoon (11) at the peak of the temperature curve (10,14,15,33). Some studies have suggested that the simultaneous increases in body temperature and maximal anaerobic performance are causally related and that the circadian rhythm of body temperature acts as a passive warm-up (6,7,28,32).
Few studies have been reported concerning the influence of time of day on exercise training. For aerobic performance, Hill et al. (19) compared morning and evening training and reported a higher post-training ventilatory anaerobic threshold for participants at the time of day at which they had trained. They thus concluded a “circadian specificity” of training. Torii et al. (36) showed that afternoon training sessions had a greater positive effect on O2 max compared with morning or evening training sessions. More precisely, Hill et al. (20) demonstrated that greater improvements in aerobic endurance performances occurred at the time of day at which high-intensity training was regularly performed. This was also recently emphasized by Edwards et al. (12) for aerobic cycling performance, but not for swimming performance (4).
For anaerobic muscle power performance, there are very few studies to our knowledge that have evaluated its circadian and temporal specificity after training. Only Oschütz (30) and Souissi et al. (35) evaluated the influence of training at different times of day on sprint, jump performances, and peak anaerobic power and peak knee extension torque, respectively. The first study indicated greater improvements in the groups that trained in the evening, and the second demonstrated increased post-training performances corresponding to the time of day at which the group had trained.
The variation in training benefit noted at different times of day is probably caused by the diurnal increase in performance. Because the habitual time of training is currently necessarily multiple during a single day, whereas the time and place of a competition may vary, it is not clear whether the previous results would be reproduced in a specific natural environment. In this first report, the purpose of this study was to verify whether short-term muscle power performance is dependent on the time of training and provide some recommendations on how to adapt training hours in the month preceding a competition.
Experimental Approach to the Problem
To test our hypothesis, subjects performed maximal brief squat jumps (SJ) and countermovement jumps (CMJ) and short-term cycle sprints exercise tests before and after 5 weeks of training. They were randomly assigned to either a Morning-Trained Group (MTG, 7:00-9:00 hr) or an Evening-Trained Group (ETG, 17:00-19:00 hr). They performed the evaluation tests in both the morning and evening.
Sixteen physical education students (12 males and 4 females) gave written consent to participate in this study after receiving a thorough explanation of the protocol. The study was approved by the appropriate local ethics committee, and the study protocol complied with the Helsinki declaration for human experimentation. There were 2 experimental groups randomly assigned to either the MTG or ETG. The mean age, height, and body mass of the subjects of the MTG (7 males and 1 female) were 19.7 (±1.5) years, 1.76 (±8.7) m, and 71.3 (±6.3) kg, respectively. The mean age, height, and body mass of the subjects of the ETG (5 males and 3 females) were 20.1 (±1.3) years, 1.75 (±9.3) m, and 71.6 (±14.4) kg, respectively. All the subjects were moderately active (physical activity, MTG: 10 ± 2 hr/wk; ETG: 10.5 ± 2.5 hr/wk). Moreover, all the subjects were classified as “neither type” from their responses to the self-assessment questionnaire of Horne and Östberg (21), which determines morningness-eveningness.
The study was conducted in Guadeloupe, French West Indies, which has a warm and moderately humid natural environment (the mean environmental temperature and humidity were 27.9 ± 0.5°C and 65.4 ± 12% outside). All the subjects had been acclimatized to this environment for years.
Subjects were tested at 2 moments, a week before the 5-week training period and a week after. The order of the morning and evening test sessions was randomly assigned for all subjects; however, the tests in both sessions were performed in the same order. The tests consisted of cycling sprints (derived from the force-velocity test) to evaluate the power output and jumps to evaluate jump height (CMJ and SJ). The MTG participants trained only in the morning and performed tests in both the morning and evening. The ETG participants trained only in the evening and were also tested in both the morning and evening. The morning and evening tests were scheduled at the same time of day as the morning and evening training sessions. All the tests were performed in the same laboratory. The laboratory conditions were recorded with an electronic thermometer-hygrometer (Novo 16755, Novo, France, precision 0.1°C) and were 28.1°C (±0.6°C) and 62.5% (±9%) for temperature and humidity, respectively, throughout the sessions. The subjects were instructed to avoid any kind of strenuous activity for 24 hours before each test, to sleep normally, and to wear the same sportswear and shoes for every test. Compliance with these instructions was checked before each test.
The participants trained 3 times a week for 5 weeks with their assigned group. The training sessions were conducted on Mondays, Tuesdays, and Thursdays and totaled 3 hours a week. The sessions were conducted outdoors in a warm and moderately humid natural environment and focused of sprints, jumps, and standard track and field exercises (Table 1).
The cycling test consisted of a maximal sprint lasting approximately 7 seconds against a friction resistive load set at 60 g.kg−1 body mass applied on the periphery of the flywheel (Monark 824E, Stockholm, Sweden). The subjects were instructed to accelerate as fast as possible while remaining in the seated position and were strongly and similarly encouraged throughout the test sessions. The cycle was equipped with toe-clips to prevent the subject's feet from slipping. All sprints were accompanied by a count down and were performed from the same foot-start position. The total force developed by the subject was calculated from both the force developed against the friction load (constant) and the force developed against inertia to accelerate the flywheel (calculated following the method of Martin et al.) (26). The velocity was recorded every 8° of pedal revolution by a photoelectric cell and a disk with alternating blind and clear portions. The power output was calculated by multiplying the velocity by the total force per pedal revolution. The data were collected by an acquisition card (DAQ-Pad 6020E, National Instruments, TX, USA) and analyzed by software developed in our laboratory with a LabVIEW interface (LabVIEW, National Instruments, TX, USA). Maximal power (Pmax) was calculated from the pedal revolution with the highest power development. Two different jump tests were performed: a CMJ and an SJ. The CMJ test consisted of leg flexion from the standing position immediately followed by a maximal jump with the hands on the hips. The SJ test consisted of a maximal jump from a flexed position with the hands on the hips. These jump tests were monitored with the Takei Kiki Kogyo vertical jump meter (T.K.K. 5106), which recorded the jump height. The subjects performed 2 trials of each jump, and the best was kept.
Statistical tests were processed using Statistica software (Statsoft, Maisons-Alfort, France). Each variable was tested for normality using the skewness and Kurtosis tests. Once the assumption of normality was confirmed, parametric tests were performed. The effects of group, time of day, and training were verified by a three-way analysis of variance with repeated measures (ANOVA 2 * 2R * 2R, group * time-of-day * training). The reproducibility of measurements was studied by regression analysis between the first and second jumps and at different times before and after training (test-retest, intraclass correlations). Data are displayed as mean (±SD), and the statistical significance was set at p ≤ 0.05.
After 5 weeks of training, there were no significant interaction effects of group, time of day, or training for any of the parameters. In contrast, there was a significant training effect for all the parameters.
The Pmax was significantly increased (F = 5.83, df = 14, p = 0.02) for both groups from 803 ± 175 to 840 ± 173 W in the morning and from 805 ± 181 to 858 ± 163 W in the evening (Table 2). However, the ETG showed no improvement in the morning tests, although the statistical analysis did not reveal any interaction.
The jumps tests also presented a training effect (SJ: F = 9.62, df = 14, p = 0.007, Figure 1 and 2; CMJ: F = 8.89, df = 14, p = 0.009, Figures 3 and 4), but this was not associated with any interaction.
Both the MTG and ETG groups showed improvement in the jumps, regardless of the time the tests were performed. Expressed in percentage, for the SJ test, MTG increased its performance by 5.35% in the morning and 2.97% in the evening, whereas the ETG improved by 0.68% in the morning and 4.4% in the evening. For the CMJ, MTG increased its performance by 4.4% in the morning and 5.57% in the evening, whereas the ETG improved by 3.46% in the morning and 4.72% in the evening.
The major result of our study was that exercise training displayed no temporal specificity. Short-term exercise training was not influenced by the daytime time of day. On the other hand, 5 weeks of training resulted in improved performances.
The subjects were randomly assigned to 1 of 2 training groups required to exercise either only in the morning, MTG (7:00-9:00 hr), or only in the evening, ETG (17:00-19:00 hr). All the subjects had been acclimatized to the warm and moderately humid environment for years.
We chose not to include a control group because subjects often defect from studies that require several weeks of training sessions, mainly because of the high demands on motivation and physical effort, and this is particularly so for control groups. In fact, 24 subjects were originally recruited for the study, and only those subjects who completed the 5 weeks of training were included in the analysis (8 for each group). None had been involved in strength training in the 4 months before the study.
In circadian studies, test reliability has to be sufficient to detect variations of small magnitude. For the MTG CMJ, the reproducibility was r = 0.886 (p < 0.003) and r = 0.934 (p < 0.000) for morning versus evening before and after training, respectively. The reproducibility between the first and second CMJ was 0.936 < r < 0.966 (p < 0.001). For the MTG SJ, the reproducibility was r = 0.562 (p < 0.146) and r = 0.865 (p < 0.005) for morning versus evening before and after training, respectively. The reproducibility between the first and second SJ was 0.928 < r < 0.977 (p < 0.001). For the ETG CMJ, the reproducibility was r = 0.701 (p < 0.05) and r = 0.745 (p < 0.03) for morning versus evening before and after training, respectively. The reproducibility between the first and second CMJ was 0.717 < r < 0.957 (p < 0.05). For the ETG SJ, the reproducibility was r = 0.289 (p < 0.487) and r = 0.246 (p < 0.556) for morning versus evening before and after training, respectively. The reproducibility between the first and second SJ was 0.829 < r < 0.940 (p < 0.01). More variation was therefore observed for ETG than MTG, particularly for the SJ, but the ANOVA analyses did not reveal any interaction. The test-retest between the first and second jumps showed good to very good correlations.
Moreover, it is well known that the improvement in anaerobic performance is very specific and reversible (2,8), and some studies have reported significant increases in short-term performance and power as a result of 5 or more weeks of training in both trained (23, for example) and untrained subjects (2). The test times were chosen with regard to the literature, particularly those studies conducted in warm and humid environments (32), and the sample size of this study is frequently observed in training research.
The subjects made up a mixed sex population, but the study aim was not to compare sex. They were recruited on a voluntary basis to have as many subjects as possible, considering the possible abandonment because of this kind of short-longitudinal study. We assumed that this factor had very few effects on the results. The circadian variations in anaerobic performance do not appear to differ between men and women (17). Moreover, no significant differences in maximal anaerobic performance (force-velocity and jumps tests) were demonstrated during the various different menstrual cycle phases (16), and no interaction effects between time of day and menstrual cycle phase were observed on muscle strength indicators (isokinetic peak torque of knee extensors and flexors, maximum voluntary isometric contraction of knee extensors and flexors, and electrically stimulated isometric contraction of the knee extensors) (5).
The tests were chosen for their reliability and are currently used in exercise laboratories. The cycle sprint test is derived historically from the force-velocity test to evaluate maximal anaerobic power (force × velocity). It is known that the relationship between the braking resistive force (F) and the maximal velocity of the subject (V) is linear and can be expressed as: V = b - aF (37). The coefficient of this relation is calculated from the linear regression linking F and V for each test and the intercepts with the velocity axis (V0) and the force axis (F0). Power corresponded to the product of F and V and followed a parabolic evolution with a maximal value for (0.5 V0) * (0.5 F0). This is therefore a very good index of maximal strength in movement. The SJ and the CMJ principally activate the series elastic component, with an active part (crossbridges of fiber types) and a passive part (tendons) according to the model of Shorten (34), which contributes greatly to performance and expresses the force and Pmax that can be reached during a very brief exercise. The parallel elastic component (sarcolemma, titin, connective tissue) is involved only in passive conditions.
In contrast with the data obtained in temperate climate, our results in a tropical climate failed to show either time-of-day effects on performance or the temporal specificity of maximal muscle power training. In a short report by Oschütz (30), the influence of 4 weeks of speed power training at different times of day was evaluated in terms of sprint and jump performances. Three groups trained at different times (group 1: 08:00; group 2: 14:00; group 3: 18:00 hr). The results indicated an increase in performance for all groups but greater results for group 3, which trained in the evening, compared with group 2. However, the groups trained only at the time assigned to them at the beginning of the program and not at other times of day. More recently, Souissi et al. (35) studied the effect of 6 weeks of training twice a week on the peak anaerobic power obtained from a Wingate anaerobic test and the peak knee extension torque from isokinetic measurements. Two groups trained at different times (group 1: 07:00-08:00 hr; group 2: 17:00-18:00 hr). The peak anaerobic power showed a significant group × training × time-of-day interaction. Before training, both groups showed increases in performance over the course of the day, whereas, after training, the evening training group showed a greater improvement in the evening than in the morning tests, and the morning training group showed no differences between the morning and evening tests. The authors demonstrated, as expected, a significant training effect for peak torque and peak anaerobic power, associated with greater improvements for the groups at the time they trained. They concluded that training at a specific hour increased the peak torque and the peak anaerobic power specifically at this hour, and thus that strength training displays temporal specificity. Although Racinais et al. (31) demonstrated that an active warm-up did not blunt the time-of-day effect in a temperate climate, this finding can be explained in part by the fact that the specific training time in the morning group may have blunted the variation in circadian performance because of the increase in muscle temperature. In this case, training would have acted above all as an active and specific warm-up, particularly in the morning, with the result being that the performances remained unchanged at the 2 test times. It was, however, surprising that their evening training group did not demonstrate more greatly improved performance compared with the morning group because they certainly trained near their peak core temperature. However, the high SD for the peak anaerobic power values in the morning training group for the evening tests (doubled values compared with the evening training group) may also have played a role in the interaction and statistical results.
In our study, the lack of difference between morning and evening training could be explained in part by the moderately warm and humid environmental conditions, in which the natural light remains similar from 6:00 to 18:00 hours. Previous studies conducted in our laboratory in a moderately warm environment failed to show any daytime variations in anaerobic performance (31,32). Moreover, this particular tropical environment changes little over the entire year, with few variations in temperature. The passive warm-up effect of this environment has been suggested to blunt the passive warm-up effect of time of day (32). This may thus lead to specific physiologic adaptations to exercise (3) and certainly influences the circadian regulation of some neurohormonal metabolisms. It might have acted as a stabilizer, and the results of the good intraclass correlations for the CMJ as well as the good to very good test-retest correlations for all jumps support this point. Indeed, previous studies conducted in the same environment showed a stability in performance throughout the day, and the training benefit thus appears as strong at any time of day.
This is an important observation because, up to now, such stability has only been shown for short-term acute but not chronic exercise. Moreover, it is particularly interesting when improved maximal muscle power performance is sought because training should be carried out at the time of day when performance is highest and maximal (30).
On the other hand, the training intensities were similar in the morning and evening because the performances were stable throughout the day, as were the environmental conditions, and thus the 2 training groups made identical progress. Passive warm-up has been suggested to be an important factor in explaining the lack of time-of-day effect on acute exercise in this environment (31), and it may be the same for chronic exercise. Moreover, chronic (22) and acute (18) moderately hot environments have been shown to enhance jump performance and the anaerobic power developed on a cyle ergometer (13). This specific environment thus certainly acts as an additional thermic load that is favorable to improve the enzyme kinetics and the development of greater muscle adaptations. Another explanation could be linked to epinephrine or arousal activities, which might certainly have played an important role, allowing subjects to ramp up even if the time of day was not to their liking.
However, it is important to note that the subjects had trained and were evaluated only in their naturally warm and moderately humid environment. A comparative study with the same subjects in both tropical and temperate conditions would be necessary to understand the true importance of the environmental conditions for testing and living.
In line with the litterature, the 2 groups showed improved performances after 5 weeks of training, 3 times per week, in exercise that requires high speed and intense muscle contractions. It is well known, for example, that explosive or high-intensity interval training can improve jump or cycle sprint performance, and numerous studies have reported that training 3 hours per week or 5 weeks of training were sufficient to improve maximal anaerobic performance (1,9,24,27,29). Moreover, the exercises performed during the 5 weeks were similar to those performed during the tests and, as emphasized by MacDougall amd Sale (25), the more similar the training contents are to the tests, the more efficient the training will be.
The percentage of benefit was approximately 5-6% for the 2 exercises. This was quite similar to the results of Souissi et al. (35), who showed an improvement in peak anaerobic power of approximately 8% after 6 weeks of training.
Therefore, brief and short-term muscle power performance in a tropical climate does not appear to be dependent on regular training at a specific time of day. A comparative study should be conducted in a warm and neutral environment to resolve the questions raised by these results, and the results should also be confirmed for other types of anaerobic and strength-building exercise. The central or peripheral origins of this lack of circadian variation should also be explored in further research.
In summary, our data indicated that in a moderately warm and humid tropical environment, maximal muscle power training sessions can be performed at any time of day with the same benefit. This is an important point because it implies that training sessions can be conducted throughout the day without taking into consideration the time of day of the competitive event, particularly in the month preceding a competition.
This article is dedicated to the memory of Professors Guy Falgairette (1954-2005) and Mario Bedu (1948-2006).
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