Time of day has an important impact on various factors related to muscular strength production. Existence of diurnal (daily) variation in maximum strength with early morning nadirs and peak performance in the late afternoon has been reported consistently (for a review see Drust et al. ). Much less attention has been addressed to time-of-day effects on adaptation to strength training. A study of Souissi et al. (21) showed that repeated training in the morning hours can improve typically poor morning performance to the same or even higher level as their normal daily peak typically observed in the late afternoon. Our previous study confirmed the results of Soussi et al. (21) and further suggested that the degree of the time-of-day-specific adaptation may vary considerably between individuals (20). Nevertheless, both studies found that the absolute magnitudes of the increase in maximum dynamic and isometric strength were similar regardless of whether the training was performed in the morning or afternoon hours. However, exact mechanisms responsible for time-of-day-specific adaptation remain unknown.
In addition to the increase in muscle strength, strength training typically results in significant increases of muscle size (for review see Wernbom et al. ). Biologic factors such as muscle fiber-type distribution, endocrinologic profile, macronutrient intake, age, sex, and many others have been recognized for their importance in hypertrophic adaption to strength training (4,7,9,11,14). Some of these factors are known to vary across a day in human or animal models, such as serum levels of steroid hormones, growth factors (23,24), or genes associated with regulation of protein synthesis and degradation (15). Daily differences in the above-listed factors may provide a different homeostatic environment and result in differential adaptive hypertrophic response if the training stimulus is applied at different times of day. However, this possibility of the time-of-day-specific training-induced hypertrophy has not been scientifically addressed up to now. The purpose of the present study was to examine the effects of time-of-day-specific strength training on muscle hypertrophy and maximal strength of knee extensor muscles in men.
Experimental Approach to the Problem
Selection of the morning and afternoon training hours during the time-of-day-specific training period complies with the occurrence of the diurnal minimums and peaks in maximum voluntary strength (6) as well as with distinct differences in the circulatory levels of some steroid hormones important in skeletal muscle hypertrophy adaptation (e.g., testosterone and cortisol) (23,25) between the selected training clock times. Magnetic resonance imaging (MRI) was chosen for studying hypertrophy induced by time-of-day-specific strength training because of its high accuracy, repeatability, and noninvasive nature (27). In addition, MRI scans allow for monitoring of the changes along the entire length of the selected muscle groups and studying overall and site-specific hypertrophy. However, because of high costs of MRI scanning, a limited number of randomized participants were included for the present report from a larger experimental sample (19,20). Half-squat 1 repetition maximum (1RM) and isometric peak torque during maximum voluntary contraction (MVC) knee extensions performed at training-nonspecific times were selected from the measured strength variables to study the impact of training on maximum strength performance and to study the relationship between changes in muscle strength and in hypertrophy of trained muscle groups. The 1RM represented multijoint, training-specific action and MVC represented isolated single-joint action, well reflecting the changes in quadriceps femoris (QF) maximum strength because of its hypertrophy (1). For the purpose of this report, we only report strength values measured at training-nonspecific times to provide overall time-of-training-independent changes in strength during the training period. Detailed information on the entire experimental sample group, training, and measurement design and methods as well as results on the effects of the time-of-day-training specificity (training specific vs. training-nonspecific clock times of testing) on neuromuscular performance, electromyography, and hormonal concentrations have been described elsewhere (19,20).
Twenty-four male volunteers of 26 subjects randomized for this study into a control group (n = 8) and 2 training groups (morning and afternoon group, n = 9 and n = 7, respectively) successfully accomplished the entire experiment. Initially, they had no history of lower-extremity strength training and did not perform any type of regular physical activity more than once a week during the previous 3 years. This study was approved by the Ethics Committee of the Central Hospital of Central Finland, and an informed consent form was signed by subjects before the investigation.
Before the experiment, the subjects were examined by a physician, and they were considered healthy and had no medical contraindications that would affect the results of this study. Shift workers were excluded. Food records were taken during 3 consecutive days of the last experimental week. Relative macronutrient intake was calculated by a trained nutritionist using the Micro-Nutrica software, 2.5 version (Social Insurance Institution, Turku, Finland) from the food records filled out by the participants. Total body fat percentage and body mass were obtained by a bioelectrical impedance analysis method (BC-418 Segmental Body Composition Analyzer, Tanita, Japan) at baseline (wk 0), mid (wk 10), and end (wk 20). The only statistically significant difference between the morning and 2 other groups was in total body mass (p = 0.049), but there were no significant differences in total body fat percentage and macronutrient intake between the experimental groups (Table 1).
The subjects randomized into the training group first underwent a 10-week preparatory training period (wk 0-wk 10) scheduled between 17:00 and 19:00 hours. Thereafter, the subjects were matched based on their improvements in 1RM half squat after the preparatory training period and randomized either to the morning or afternoon training group. They continued with a 10-week time-of-day-specific training period (wk 11-wk 20) with training times being 07:00 to 09:00 hours for the morning group and 17:00 to 19:00 hours for the afternoon group. Training frequency was increased from 2 sessions/week during the preparatory training (0-10 wk) to 5 session/2 weeks during weeks 11 to 15 and to 3 sessions/week during weeks 16 to 20. Both the preparatory and time-of-day-specific training periods were planned as whole-body periodized programs using a linear method of progression with the main focus on the knee extensors muscles. Half squats (approximately 90° angle of the knee joints), loaded squat jumps, leg presses, and knee extensions were the primary exercises. All training sessions were supervised. Hypertrophic type of training (60-80% of 1RM, 6-12 repetitions per set, 3-5 seconds repetition duration, 2-4 sets) (13) was performed during the time-of-day-specific training only (every second session), and this accounted for 40% of the total training volume in kilograms. High-load (70-100% of 1RM, 1-8 repetitions per set, 2-4 sets) and high-speed protocols (40-60% of 1RM, 5-8 repetitions per set, 2-3 sets, fast or maximum speed during concentric motion) during the time-of-day-specific training accounted for 38% and 22% of the total time-of-day-specific training volume, respectively. The 1RM values for all exercises except for the half squat were obtained at the training-specific times. The control group did not train but was asked to maintain their pre-experimental physical activity level.
Test Protocol and Data Acquisition
The subjects were familiarized with the test procedures a week before initiation of the experiment. Maximum dynamic and isometric voluntary strength were measured on 3 occasions: baseline (wk 0), mid (wk 10), and end (wk 20) starting at a randomly given time of day (between 09:00 and 16:00 hr); however, starting clock times were identical for each individual across the tests. After the bioimpedance measurements and a warm-up consisting of 10 minutes of cycling at a heart rate of 110 to 120 bpm and 3 repetitions of maximum squat jump and countermovement jump, the following tests were performed.
The 1RM half-squat protocol consisted of several submaximal trials of half squats separated by rest periods of 3 minutes. A barbell with weights, inbuilt in a Smith machine (Kraftwerk, Finland) was held on the shoulders. A trial started from the standing position, with the subject squatting slowly down to the 90° knee angle (announced by an audio cue) followed by extension of the knees and hips back to the standing position. The last successful trial performed with the correct technique was taken for further analyses.
Maximum voluntary isometric strength of the knee extensors was tested using unilateral knee extensions of the right leg at a 90° knee angle (180° is knee fully extended). The subjects were secured and strapped to a sitting position on a knee extension device (Leg Ext/Curl Research, Hur Oy, Kokkola, Finland), and the same individual position was ensured in the repeated measures. The subjects were asked to produce maximal force rapidly and maintain it for 3 seconds. Loud verbal encouragement was given by the test personnel. Three trials were performed with a rest period of 1 minute. A trial with the highest peak torque was taken for further analyses.
The entire length of QF was scanned with MRI at week 10 and week 20 (different days as compared with strength tests). Typical morphologic images were acquired in the axial planes by using proton density fast spin echo (FSE) sequence. The axial image slice spacing was 8.5 mm. From the images, the thigh length was measured from the lateral epicondyle of the femur to the lateral tip of the greater trochanter. The cross-sectional areas (CSA) of the rectus femoris and vasti muscle groups were analyzed from the FSE images with freely available Osiris 4.0 software. Because the thigh length varied between subjects, the results were normalized to 15 samples for the vasti muscle group and 12 for the rectus femoris. The reproducibility of drawing the areas was high, with a correlation of 0.99 and mean error of 0.36 ± 0.26% when analyzed by the same person. Muscle volume (cm3) was calculated by multiplying the CSA of each axial slice by the distance between slices and summed across slices. The statistical analyses yielded similar results for both the rectus femoris and vasti muscle groups. Therefore, to simplify the MRI data presentation, the CSA and volume of both the rectus femoris and vasti muscles were merged together (except for the analyses of site-specific hypertrophy) and are presented as QF CSA and volume throughout the article. The site-specific hypertrophy was examined by calculating the percentage difference in respective CSA before and after the time-of-day-specific training separately for the rectus femoris and vasti muscle groups. Percentage differences in CSA were compared among each other within the muscle groups.
Standard descriptive statistics (mean and SD) were calculated. The effects of the preparatory and time-of-day-specific training on strength, muscle volume, body mass, and fat percentage were examined by a one-factor (Training) general linear model with repeated measures (GLM) using absolute values. The Training factor included 3 levels (wk 0, 10, and 20 time points) for 1RM half squat, MVC, and body mass and fat percentage and 2 levels (wk 10 and wk 20 time points) for muscle volume data, respectively. A group effect (Group) was included as a between-subject factor in both GLM models. Site-specific hypertrophy was studied by a one-factor (CSA) GLM with both simple and repeated contrast. When repeated measures GLM revealed significant F-ratios, pair-wise comparisons using the Bonferroni adjustment were used to localize significant differences. In addition, one-way ANOVA was used to test the differences between the groups in relative percentage changes from week 10 to week 20 in muscle volume, CSA, and some anthropometric data. Post hoc tests with Bonferroni adjustment were performed when ANOVA yielded significant F-ratios. Pearson's bivariate correlation coefficients were calculated between some muscle strength and muscle hypetrophy variables for each group separately as well as for groups merged together. All the analyses were performed by means of SPSS 14.0. (SPSS, Inc., Chicago, IL, USA). Statistical significance was set at p < 0.05.
The QF volume was increased significantly (p < 0.001, Training main effect) in both training groups after the time-of-day-specific training period (from 2,180 ± 340 cm3 to 2,237 ± 342 cm3, an increase of 2.7%, and from 2,118 ± 217 cm3 to 2,192 ± 220 cm3, an increase of 3.5%, in the morning and afternoon groups, respectively (Figure 1). The 0.8% difference between the training groups was not significant (p = 0.188). Both training groups increased their QF volume significantly when compared with the control group (from 2,161 ± 191 cm3 to 2,166 ± 193 cm3, an increase of 0.2%) (p < 0.001 in both the Training × Group GLM interaction and post hoc one-way ANOVA).
Similarly to the QF volume, CSA of QF in all 14 slices except the most distal slice was increased significantly in both the morning and afternoon group when compared with the control group but without significant difference between the training groups (Table 2). No significant effect of site-specific hypertrophy was found in the vasti muscle group. In both training groups, the CSA of vastii muscles increased to a similar extent across the entire length. The percentage increase in the distal rectus femoris CSA was somewhat but insignificantly greater than in the proximal part of the muscle in both morning and afternoon training groups (p = 0.101 and 0.141, respectively).
1RM Half Squat
The entire 20-week training period resulted in significant increases in 1RM of similar magnitude in both training groups (p < 0.001, Training main effect). The morning group improved from 125 ± 25 kg to 171 ± 29 kg and the afternoon group from 121 ± 20 kg to 166 ± 19 kg. Most of the improvements occurred during the first 10 weeks of the preparatory training. The second 10-week period of the time-of-day-specific training accounted only for nonsignificant improvements of 7.9 kg (4.5%) and 6.4 kg (3.6%) in the morning and afternoon training groups, respectively. The control group improved its 1RM from baseline to mid (9.4 kg [6.7%], p < 0.01), but this increase was far less pronounced when compared with the morning and afternoon training group (38.6 kg [34.4%] and 38.7 kg [30%], respectively) (p < 0.001, Training × Group interaction).
Isometric peak torque increased significantly in both training groups from the baseline to end tests (p < 0.01, Training main effect). The improvement was significant between all time points in the afternoon group (199.3 ± 52 Nm, 215.1 ± 50 Nm and 229.8 ± 60 Nm at baseline, mid, and end, respectively) but statistically nonsignificant from mid to end in the morning group (205.5 ± 36 Nm, 220.8 ± 39 Nm and 228.6 ± 29 Nm at baseline, mid, and end, respectively). In the relative values, the morning group improved by 7.8% and 4.7% from baseline to mid and from mid to end, respectively. For the afternoon group, the values were 8.9% and 6.3%. Peak toque was not significantly changed in the control group (Figure 2).
Bivariate correlations were performed to study the relationships between the individual increases in muscle strength and muscle size and to study the potential effect of the initial muscle strength and muscle size status (at wk 10) on the 10-week time-of-day-specific adaptation process. None of the correlations yielded significant p values, with r2 values ranging between 0.03 and 0.37 in both positive and negative relationship directions.
The main finding of the present study was that 10 weeks of strength training performed either in the morning or in the afternoon resulted in significant increases in QF muscle size. However, the magnitude of muscular hypertrophy did not statistically differ between the morning and afternoon training groups.
The absolute QF volume after the preparatory training period was well in line with values previously reported for young healthy men by Morse et al. (17) and Ivey et al. (12). The relative increase of 2.7% (the morning group) and 3.5% (the afternoon group) in average QF CSA during the time-of-day-specific training was at the lower end of the range typically reported during strength training, similar to the present study. On the basis of a large number of published scientific studies, Wernbom et al. (28) calculated that the QF CSA increase was on average 8.5%, ranging between 1.1% and 17.3% over an average 10-week strength training period in previously untrained subjects. When taking into account the 10-week preparatory training history of the present subjects in both training groups, we suggest that the relative increase in QF size and the effectiveness of the present time-of-day-specific training could be considered appropriate. Although the preparatory training did not include typical hypertrophy protocols, the high-load and high-speed protocol could induce muscle hypertrophy to some extent (28) and might narrow the adaptation window during the actual time-of-day-specific training (5). It must be kept in mind that the preparatory training was identical in both training groups (timing and training variables). Therefore, the effect of pretraining could be considered similar in both training groups without a significant confounding impact on the studied variables. An interesting finding was the difference in the relative QF volume increase, although statistically insignificant and minor (0.8%) in magnitude, between the training groups in favor of the afternoon group (Figure 1). This tendency was more pronounced (1-1.5%, NS) when considering the mid-section CSA only (CSA 6-CSA 12) (Table 2). The present study does not allow differentiation between true time-of-day effects (e.g., by way of time-of-day-dependent differences in the steroid hormones circulatory levels (25) or in hormonal response to a heavy bout of exercise (2,10,18)) and influence of possible confounding factors. For instance, differences in the timing and amount of protein (amino acids) ingested shortly before and after the morning and afternoon training sessions (11,26) or the interindividual difference in responsiveness to training because of fiber-type distribution (3,22) might be among potential confounding factors of the present study. Moreover, it must be pointed out that the subject number in the present study was rather low. Therefore, the results should be verified with a larger number of subjects and over a longer period of training.
Maximum strength measured in the isometric and dynamic conditions at the training-nonspecific clock times increased similarly in both training groups. Repeated strength training in the morning produces time-of-day-specific adaptations so that it improves typically poor morning performance to the same or even higher level of the daily peak in maximum strength typically observed in the late afternoon (20,21). However, the absolute magnitude of the maximum strength improvements measured either at training-specific times (21) or training-nonspecific times (the present study) appears to be rather similar regardless of the time of day of training. It can be noted that the pattern of the maximum strength development across the entire training period differed between the isometric and dynamic tests in the present study. One repetition maximum in half squat showed a steeper increase during the first 10 weeks and a more pronounced plateau during the time-of-day-specific training (wk 10-wk 20) when compared with the MVC peak torque. This was a typical pattern for 1RM squat in which the performance plateau may start to develop already after 6 to 8 weeks of training in previously untrained men (8). Learning or large neural adaptations (16) could partly explain the high initial improvements in 1RM, as suggested by the significant improvement in the present control group 1RM from week 0 to week 10. In MVC, the role of learning appeared to be minor, probably because of the single-joint nature with fewer muscle groups involved during the execution of the test. The relationship between the relative increase in QF CSA and MVC peak torque was in line with the findings of Aagaard et al. (1), who reported a larger improvement in isometric maximum strength as compared with the improvement in anatomic CSA after 16 weeks of training in previously untrained subjects (increases of 4.5% and 6.3% in MVC vs. 2.7% and 3.5% in the QF volume in the present morning and afternoon groups, respectively).
Strength training in the morning and afternoon hours is similarly effective when aiming for muscle hypertrophy over a shorter period of time (2-3 mo). However, it remains unclear whether time-of-day-specific training performed over longer periods of time (>3 mo) would cause systematic differences in the gain of muscle mass. Maximum strength gains appear also to be similar whether training is performed repeatedly in the morning or afternoon hours. However, there is some evidence that training repeatedly at a particular time of day may be beneficial when daily peak of maximum strength needs to be achieved at that time of day, especially in the morning hours. Strength and power athletes required to compete at a certain time of day (e.g., morning qualifications) may be advised to train repeatedly at that particular time of day for several weeks before the competition.
Gratitude is expressed to the Ministry of Education, Finland, and the Finnish Funding Agency for Technology and Innovation for financially supporting this research.
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