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.
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.
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).
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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