Psychological and physiological parameters associated with human performance vary according to the time-of-day. A circadian rhythm in muscle force has been also described for maximal dynamic contractions (11) and isometric contractions (10,18). In most studies, the acrophase (time of the maximal level of the rhythm) of the muscle capacity to develop maximal force has been found in the evening compared with the morning. The diurnal variations in muscle performance can be influenced by several factors, such as core temperature, sleep deprivation, warm-up duration, and hormones' concentration such as cortisol and catecholamine (4,10,29). Moreover, several studies on physical training have also suggested an effect of training time on performance (5,25,28). These studies advanced that (a) adaptation to strength training is greater at the time-of-day at which the training was conducted, (b) training in the morning can improve typically poor morning performances to the same or even higher level as their normal daily peak typically observed in the evening, and (c) morning training could decrease the amplitude of the performance diurnal variations. Thus, the choice of morning or evening training time (temporal specificity of training) seems to be crucial to improve performance especially when training exercise is carried out at a specific time-of-day.
However, the findings cited above concerned only conventional resistance training while an original (self-resistance) training consisting of maximal isometric voluntary co-contractions (MIVCCs) of elbow antagonistic muscle pairs without any external load resulted in significant improvements of strength in recent studies (8,16,17). The question of whether self-resistance training adaptations may depend on the time-of-day at which training was scheduled has not been studied. Therefore, the temporal specificity of self-resistance training on muscular adaptation is unknown. Furthermore, the answer to this question may potentially provide information and may help those who will adopt this type of training in establishing the best time-of-day of training to enhance muscular strength and power. We hypothesized that the higher enhancement in muscle strength induced by self-resistance training could be obtained in the morning and could depend on the training time-of-day.
The aim of this study was (a) to investigate the impact of a 6-week time-of-day–specific MIVCC training of elbow flexor and extensor muscles on the adaptations and diurnal variations of maximal voluntary force (MVF) and maximal rate of force development (MRFD) and (b) to study whether the temporal specificity of MIVCC training scheduled in the morning or evening may change the shape of the muscle strength diurnal variation.
Experimental Approach to the Problem
The temporal specificity of MIVCC training on muscular adaptation is unknown. Therefore, by comparing 2 resistance training programs, one scheduled in the morning and the other in the evening, we sought to investigate the importance of time-of-day MIVCC training on the improvements and the diurnal variation of maximal and explosive muscle strength. To achieve this purpose, participants performed MVF and MRFD during isometric elbow flexion and extension before and after a 6-week MIVCC training. They were randomly assigned to either a morning training group (MTG, trained only between 07:00 and 08:00 hours) or an evening training group (ETG, trained only between 17:00 and 18:00 hours). They performed the evaluation tests both in the morning (between 07:00 and 08:00 hours) and the evening (between 17:00 and 18:00 hours).
Twenty right-handed physical education students participated to this study. They were randomly assigned to either the MTG (n = 10; 23.6 ± 2.6 years; age range between 20 to 33 years for MTG and between 20 to 27 years for ETG; 72.3 ± 8.7 kg; 175.3 ± 0.09 m) or the ETG (n = 10; 23.7 ± 4.8 years; 68.3 ± 8.2 kg; 179.1 ± 0.08 m). Each participant received thorough explanations about the protocol and provided written informed consent before any data collection. The experimental protocol was approved by the Institutional Review Board of the University and carried out according to the guidelines of the Declaration of Helsinki. The participants had no history of upper- or lower-extremity strength training during the 6 months before the study. All the participants had the same standard isocaloric meal at 12:30 (±15 minutes). They were categorized either as “moderately morning type” (n = 6) or as “neither type” (n = 14) on the basis of their answers to the self-assessment questionnaire (14), which assesses morningness-eveningness ability to overcome drowsiness, and flexibility of sleeping habits. Before the study, the participants were familiarized with the experimentation by performing all types of task involved in the study at least twice for each task. All measurements were conducted on the right arm.
The pretraining session (T0) and the post-training session (T1) after 6 weeks of MIVCC training were performed for each training group. During each session, all participants were tested at 2 times of the day, 07:00 and 17:00 hours. The time interval between 2 consecutive test sessions was at least 24 hours. The morning and evening test sessions were completed in a counterbalanced design. Subjects' oral temperature was recorded using a digital clinical thermometer (Omron, Paris, France; accuracy, ±0.05° C) in the beginning of each test session. All test sessions took place at a similar ambient temperature and relative humidity (20 ± 2° C and 37 ± 7%, respectively) of the laboratory. T0 and T1 consisted of the same set of strength measurements (MVF, MRFD) collected through unilateral isometric voluntary contraction of elbow flexor and extensor muscles at 90°elbow angle. T1 was scheduled at least 48 hours after the last training session.
Two MIVCC training programs were performed, one scheduled in the morning (between 07:00 and 08:00 hours) for the MTG and the other scheduled in the evening (between 17:00 and 18:00 hours) for the ETG. The groups were matched according to their age, body mass, height, and their MVF performed at T0 (24). Each subject of each group participated in a 6-week MIVCC training program of elbow antagonistic muscle pairs of the right arm, 3 times per week (18 training sessions) with a 1-day recovery between sessions (2-day recovery during the weekend) (17). Each training session consisted of 48 MIVCCs (6 sets of 8 repetitions) interspersed with 30-second recovery between repetitions and 2-minute recovery between sets. The participants were instructed and encouraged to perform and sustain each MIVCC for 5 seconds in the same position as during the test session (elbow at 90°).
Measurements and Data Processing
Maximal voluntary force during elbow flexion (MVFF) and extension (MVFE) was measured when the participants were fastened on a special chair by means of straps and belts at the shoulders and waist levels as described in a previous study (27). The participants exerted their force by pulling on an inelastic strap located around their wrist and placed in series with the force transducer (2 kN) sampled at 1,000 Hz through a data acquisition card and PC. The right elbow angle was positioned at 90° with a manual goniometer (8). After warm-up and a 3-minute period, three MVFF and MVFE with 2-minute recovery between trials were performed in a randomized order. Maximal voluntary force was recorded during 5 seconds. Flexion and extension exercises were performed in the same order in T0-T1 sessions for the same participant.
The maximal rate of force development of elbow flexion (MRFDF) and extension (MRFDE) were measured during the first second of the maximal voluntary contraction. The participants were instructed to provide the most explosive force, by concentrating on the fastest contraction. Thereafter, they were encouraged to extend their effort and to reach their MVF before relaxing (23). The participants were not given any feedback during tasks. Trials with an initial countermovement were discarded, and extra trials were performed.
Data were processed using the software package of a Vicon system (Motion Systems, Oxford, United Kingdom). Maximal voluntary force was assessed as the highest force level recorded during each contraction, which was equal to the maximal value of a 20-millisecond moving average. During each test session, the mean value of the 2 maximal trials was used for further analysis. Maximal rate of force development was equal to the highest RFD during 20-millisecond sampling window (23). Maximal rate of force development was expressed in Newton per second and in relative units, i.e., as a percentage of MVF per second (MRFD% = [MRFD/MVF] × 100) (23).
Data were presented as mean ± SDs. Normality of the data was confirmed using a Shapiro-Wilk test. The effects of group, time-of-day, and training on MVF and MRFD were tested by using a 3-way analysis of variance (ANOVA) with repeated measures (2 groups [MTG-ETG] × 2 training [T0-T1] × 2 time-of-day [morning-evening]) for each exercise (flexion-extension) separately. To verify the specificity hypothesis that morning test session would be especially affected by time-of-day specific training, a 2-way ANOVA with repeated measures (2 groups [MTG-ETG] × 2 time-of-day [morning-evening]) was performed on the amplitude of diurnal variation and the relative increase of MVF and MRFD. To study whether MVF and MRFD improvement depends on the specific hour of training or not, we combined the results corresponding to the same schedules for training and testing (S) (morning-morning and evening-evening, n = 20) and different schedules for training and testing (D) (morning-evening and evening-morning, n = 20), then the 2-way ANOVA with repeated measures was performed (training [T0-T1] × time-of-day of scheduled training [S-D]). When repeated measures revealed significant F-ratios, pairwise comparisons using the Bonferroni adjustment post hoc test were carried out to localize significant differences. To assess the practical significance of our findings, the effect size was calculated using partial eta-squared (
). The statistical significance level was set at p ≤ 0.05. All data were processed using STATISTICA software 7.1 (Statsoft, Maisons-Alfort, France).
Before and after training, the body temperature depended on time-of-day (F(1.18) = 96.9; p < 0.001;
= 0.84). There was no group (F(1.18) = 0.20; p = 0.6) or training (F(1.18) = 0.1; p = 0.05) effect. The interaction between (group [MTG-ETG] × training [T0-T1] × time-of-day [morning-evening]) was not significant. At T0, the Bonferroni test demonstrated that temperature increased significantly from morning (MTG: 36.24 ± 0.14; ETG: 36.16 ± 0.15) to evening measures (MTG: 36.67 ± 0.07; ETG: 36.73 ± 0.07) (p < 0.001). The same results were observed at T1, with a difference in body temperature observed between the morning (MTG: 36.07 ± 0.13; ETG: 36.04 ± 0.08) and evening (MTG: 36.76 ± 0.05; ETG: 36.74 ± 0.06) (p < 0.001).
Training Effects on Maximal Voluntary Force of Elbow Flexion
The three-way ANOVA (group × training × time-of-day) performed on MVFF showed a significant main effect of time-of-day (F(1,18) = 22.91; p < 0.001;
= 0.56), training (F(1,18) = 137.14; p < 0.001;
= 0.88), and group × training × time-of-day interaction (F(1,18) = 27.79; p < 0.001;
= 0.60). In contrast, there was no group effect (F(1,18) = 0.6; p = 0.4).
The Bonferroni post hoc analysis revealed that there was no difference between groups (MTG-ETG) at T0. In both training groups, MVFF was higher in the evening than in the morning (p < 0.001; Figure 1A). At T0, the amplitude of diurnal variation was 5.9% for the MTG and 6.5% for the ETG. After training, MVFF increased (p < 0.001) in the MTG and ETG at the 2 time points (Figure 1A). However, in the morning test, MVFF gain after training was greater in the MTG compared with the ETG (p = 0.03). Moreover, the relative increase was larger in the morning than in the evening for the MTG (12.0 ± 3.9% vs. 7.0 ± 3.1%; p = 0.02), whereas the gain was lower in the morning than in the evening for the ETG (5.0 ± 4.4% vs. 9.8 ± 5.2%; p = 0.02). As a consequence, intraday variation in MVFF disappeared after training in the MTG (p = 1) but persisted in the ETG (p < 0.001).
Training Effects on Maximal Voluntary Force of Elbow Extension
The same findings were observed for MVFE with significant effects of time-of-day (F(1,18) = 33.53; p < 0.001;
= 0.65), training (F(1,18) = 154.57; p < 0.001;
= 0.89), and group × training × time-of-day interaction (F(1,18) = 67.19; p < 0.001;
= 0.78). The group effect was not significant (F(1,18) = 0.53; p = 0.4). At T0, the Bonferroni post hoc analysis showed that there was no difference between the MTG and ETG, but MVFE was higher in the evening (p < 0.001). At T1, MVFE increased in both training groups in the morning and the evening (p < 0.001; Figure 1B). However, the relative increase of MVFE was greater at the specific time of training for each group: strength gains were higher in the morning than in the evening for the MTG (17.6 ± 3.9% vs. 9.0 ± 4.3%; p < 0.001), whereas the gain was lower in the morning than in the evening for the ETG (6.6 ± 3.8% vs. 13.4 ± 5.3%; p < 0.001). In addition, in the morning test, the relative increase after training was significantly higher in the MTG when compared with that in the ETG (p = 0.002). After training, the relative variation between the evening and morning decreased for the MTG (p = 0.002) but increased for the ETG (p < 0.001). As a consequence, the intraday variation in MVFE persisted at T1 in the ETG (p < 0.001) but disappeared in the MTG (p = 1).
Training Effect on Maximal Rate of Force Development of Elbow Flexion
The 3-way ANOVA with repeated measures (group × training × time-of-day) showed a main effect of time-of-day (F(1,18) = 23.2; p < 0.001;
= 0.56) and group × training × time-of-day interaction (F(1,18) = 4.88; p = 0.04;
= 0.21) on MRFDF. However, there was no effect of group and training.
The post hoc test revealed that before training there was a diurnal variation in MRFDF for both training groups (p < 0.01) (Figure 2A). The MRFDF improved between the morning and evening (p < 0.01) for the MTG and ETG. After training, this diurnal variation was blunted in the MTG (p = 1) and persisted in the ETG (p < 0.001). When we consider the effect of training, despite a trend associated with the training time-of-day, both training groups did not significantly improve their MRFDF neither in the morning (p = 0.06 vs. p = 1) nor evening (p = 1 vs. p = 0.058) for the MTG and ETG, respectively. Concerning the relative increase of MRFDF, there was no effect of group and time-of-day. Despite the trend, there was a nonsignificant effect of group × time-of-day interaction (p = 0.054).
However, when MRFDF was related to MVFF (MRFDF%), training was the only significant main effect (F(1,18) = 24.08; p < 0.001;
= 0.57) on MRFDF%. There was an effect of group × training × time-of-day interaction (F(1,18) = 6.22; p = 0.02;
= 0.25). At T1, MRFDF% decreased when compared with T0 values in the morning and evening (p < 0.001) for the MTG. However, for the ETG, MRFDF% decreased only in the evening (p < 0.001) (Table 1), with the group and time-of-day being not significant. At T0, the post hoc analysis showed that MRFDF% did not change from the morning to the evening for both training groups.
Training Effect on Maximal Rate of Force Development of Elbow Extension
Statistical analyses showed that time-of-day (F(1,18) = 28.59; p < 0.001;
= 0.61) and group × time-of-day interaction (F(1,18) = 5.81; p = 0.02;
= 0.24) were the only significant effect in MRFDE. At T0, the post hoc test showed diurnal variations of MRFDE for both training groups, that is, MRFDE improved between the morning and evening (p = 0.02 for the MTG and p < 0.001 for ETG; Figure 2B). At T1, this diurnal variation was blunted with the MTG (p = 1) and persisted with the ETG (p < 0.001). Maximal rate of force development of elbow extension in the morning and the evening did not improve in the MTG and ETG. Moreover, there was no effect of group, time-of-day, and group × time-of-day interaction on relative increases of MRFDE.
When MRFDE was related to MVFE (MRFDE%), training (F(1,18) = 25.32; p < 0.001;
= 0.58) and group × training × time-of-day interaction (F(1,18) = 29.05; p < 0.001;
= 0.61) were the only significant effects and as observed for MRFDF, MRFDE% decreased when compared with T0 (p < 0.001), especially in the MTG (Table 1).
Results of Combined Data
In this section, we combined data of 20 participants together to study whether MVF and MRFD improvement depends on a specific hour at which training was scheduled or not. Thus, the data corresponded to the same schedules for training and testing (S) (morning-morning and evening-evening, n = 20) and different schedules for training and testing (D) (morning-evening and evening-morning, n = 20) (Figure 3). This comparison confirmed the results of the 2-way ANOVA performed on relative gain between T0 and T1. The results revealed a main effect of MIVCC training on MVFF (F(1,19) = 197.03; p < 0.001;
= 0.91) and MVFE (F(1,19) = 140.81; p < 0.001;
= 0.88), as well as an interaction between training × time-of-day of scheduled training for MVFF (F(1,19) = 13.60; p = 0.001;
= 0.41) and MVFE (F(1,19) = 70.48; p < 0.001;
= 0.78). Otherwise, there was no difference between S and D data before training (T0, black columns) for both MVFF and MVFE. The MVFF and MVFE increased between T0 and T1 for S and D (p < 0.001) (Figure 3A). Moreover, when training and testing schedules were the same (S), the MIVCC training effects on MVFF and MVFE were higher (p < 0.001) compared with D.
Regarding MRFD, there was only an interaction effect of training × time-of-day of scheduled training on MRFDF (F(1,19) = 5.15; p = 0.03;
= 0.21). The Bonferroni test showed that MRFDF (p < 0.001) and MRFDE (p = 0.03) increased significantly between T0 and T1 only when training and testing schedules were the same (S). Furthermore, at T1 there was a difference between S and D for MRFDF (p < 0.001) and MRFDE (p = 0.001) (Figure 3B).
When related to MVF, the results showed an effect of training (F(1,19) = 23.78; p < 0.001;
= 0.55 and F(1,19) = 24.85; p < 0.001;
= 0.56) and training × time-of-day of scheduled training interaction (F(1,19) = 6.57; p = 0.01;
= 0.25 and F(1,19) = 30.64; p < 0.001;
= 0.61) for MRFDF% and MRFDE%, respectively. Moreover, there were a decrease of MRFDF% and MRFDE% for S and D (p < 0.001) Figure 3C.
The major findings of this study were that (a) MIVCC training improved muscle strength at the 2 time points independently of the time-of-day training and (b) the muscle strength gain depended on the temporal specificity of MIVCC training in agreement with previous studies using conventional resistance training (6,26,28). Greater improvements in muscle isometric strength were observed when force was measured at the same time as training sessions for both training groups (ETG and MTG). Moreover, after MIVCC training performed in the morning hours, typically low morning performances can reach the same levels as the daily peaks usually observed in the evening. As a consequence, the typical usual pattern of MVF and MRFD was blunted after MIVCC training in the morning group but not in the evening group.
The effect of the circadian or diurnal variations has been well demonstrated for various indices of short-term maximal performances (5,9). These short-duration performances present a common characteristic to be better in the afternoon (16:00 and 20:00 hours) than in the morning (between 06:00 and 10:00 hours). In this study, before training, there were significant diurnal variations in MVF and MRFD for flexor and extensor muscles. The amplitude of the variations in this study (about 6% for MVFF and 8% for MVFE for both training groups) is in agreement with that in previous studies on MVF during elbow flexion (3% for Freivalds et al. (9) and 12.06% for Gauthier et al. (10)).
The mechanisms responsible for such diurnal fluctuations in maximal short-term performance are poorly understood. Some studies suggested that the higher evening muscular performances such as MVF and MRFD may be linked to variation of muscle contractile properties rather than variation in central nervous command (13,18). The effect of intraday variation on muscle contractile properties could be attributed to (a) the intracellular variation in the muscle, for example, the circadian rhythm in inorganic phosphate concentration and intracellular Ca2+ controlled processes (13,18), (b) the products of the neurohormonal system such as cortisol (4) and catecholamine (29), and (c) the circadian rhythm in central temperature (2,11), which could exert a passive warm-up effect (2) enhancing metabolic reactions, increasing the extensibility of connective tissue, reducing muscle viscosity, and increasing the conduction velocity of action potentials through calcium release by the sarcoplasmic reticulum (18,21). Oral temperature increased with an amplitude of ∼0.6° C between the morning and evening in agreement with previous studies (7,13). However, variations in core temperature less than ≤1° C seem to be insufficient to entirely explain the changes in muscle strength performance (18). Other factors such as motivation could explain the diurnal variation in short-term muscle performance (22). Furthermore, diurnal variation in muscle strength has been also linked to changes in both the architecture (i.e., the arrangement of muscle fibers relative to the axis of force generation) of muscle fibers and patella tendon stiffness (20) over the course of the day (i.e., higher in the morning).
In agreement with previous findings (8,16,17), both training groups showed improved MVF of the elbow flexor and extensor muscles after 6 weeks of MIVCC training performed 3 times per week. These findings showed that short-term training consisting of simultaneous maximal voluntary co-contractions of antagonistic muscle pairs could provide similar results to those obtained with traditional isometric training programs (15,19). Furthermore, the results of combined data performed in 20 participants confirmed that MIVCC training could enhance MVF in the morning and evening independently of the training time, but the highest strength gains were observed at the time-of-day at which training was scheduled for both training groups.
Moreover, after training, the MRFD of the elbow flexor and extensor muscles showed a slight but not significant increase for both training groups. These results confirmed the assumption that explosive strength increases depend on training modality (1,12,30,31). In contrast, when sample size was increased with combined data, the slight increase of MRFD becomes significant only if training and test session were scheduled at the same time. Consequently, these results suggested that isometric training (i.e., MIVCC) could enhance rapid force production even without emphasis on explosive contractions (1). When normalized to MVF, MRFDF% and MRFDE% decreased after training in the evening and morning for the MTG, whereas these same parameters decreased only in the evening for the ETG probably because the increases in MVF in the morning after training were smaller in this group (Figure 1).
Six weeks of MIVCC training at a specific time-of-day modifies the diurnal variations of MVF and MRFD performances in the MTG. Indeed, the amplitude of diurnal variations in MVF and MRFD of flexor and extensor muscles decreased and became nonsignificant in the MTG but persisted in the ETG. These results were in agreement with those of some previous studies that reported significant temporal specificity after conventional strength training designed to promote muscle strength (6,25,26,28). Bird and Tarpenning (3) suggested that plasma concentrations of testosterone and cortisol were higher after evening strength training than after morning strength training. Sedliak et al. (25) observed that the MTG showed a significant decrease in the serum cortisol concentrations, after 10 weeks of strength training, in the morning, whereas the ETG showed persisting higher morning serum cortisol concentrations after training. However, Sedliak et al. (24) failed to show any specific adaptation in the electromyogram during unilateral isometric knee extension peak torque after strength training at a specific time-of-day. They concluded that peripheral rather than neural adaptations are the main source of temporal specificity in strength training. More recently, Sedliak et al. (26) observed that the magnitude of muscular hypertrophy (quadriceps femoris cross-sectional areas and volume) did not differ between morning and evening training.
Maximal isometric voluntary co-contraction training enhances muscle strength whatever the time-of-day at which training was scheduled. Muscle strength adaptations to MIVCC are greater at the time-of-day when training is performed. Maximal isometric voluntary co-contraction training at the same time-of-day has the potential to alter the normal diurnal variation of maximal and explosive muscle strength. Indeed, MIVCC performed in the morning can improve typically low morning performances to the same (or even higher) level as their normal daily peaks typically observed in the evening.
The MIVCC training could be adopted to improve muscle strength independently of time-of-day at which training was scheduled and without alteration of explosive force. In contrast, to optimize the muscle strength our results suggested that morning training may be accompanied by the greatest muscle strength gain and blunted maximal and explosive muscle strength variation observed between the morning and evening. Otherwise, it is interesting in further research to add a control group and to explore the possible central or peripheral origin, or both, of the diurnal variation in co-contraction training effects.
The authors thank all the participants in this study for their maximal effort and cooperation.
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