there are many biological factors that are important for muscular hypertrophy like hormone levels, age, sex, muscle fiber type, diet, among others. These factors have been recognized as extremely important for the hypertrophic adaption to strength training. Many of these factors, however, are known to vary throughout the day. Can the daily differences in the above factors like hormone levels, affect the adaptive response to strength training? What time of day is the best time to train?

A study published in the Journal of Strength and Conditioning Research set out to examine the effects of time-of-day-specific strength training on muscle hypertrophy maximal strength in men. The training group underwent a 10 week preparatory training regimen. Afterwards, the subjects were randomized to either a morning training group or an afternoon training group. The groups trained for another 10 weeks with training times between 07:00 and 09:00 hours and 17:00 and 19:00 hours in the morning group and the afternoon group respectively. Cross-sectional areas and volume of the quadriceps femoris were obtained by magnetic resonance imaging at weeks 0, 10, and 20. Maximum voluntary isometric strength during unilateral knee extensions and the half squat one repetition maximum were tested at weeks 0, 10, and 20.

The entire 20-week training period resulted in significant increases in maximum voluntary contraction and 1RM in both training groups. In this study, the magnitude of muscular hypertrophy and strength did not statistically differ between the morning or afternoon group. However, this study was of short duration and like most research concerning physical improvement through exercise, there needs to be more subjects over longer periods of time.
(J Strength Cond Res 23(9):2451-2457)

Exercise induced endogenous hormone levels have been studied extensively. Researchers have examined how the different components of training including sets, repetitions, load and rest intervals affect serum levels of hormones such as testosterone, growth hormone and cortisol. Many studies have demonstrated there is an acute increase in serum levels of anabolic hormones after intense resistance exercise.

To be more specific, high intensity exercise coupled with short rest intervals that is performed with large muscle groups are associated with large rises in these hormones when compared to other training methods. Conversely, training small muscle groups like the biceps has been shown to have no effect on serum hormone levels. Because of the findings in many studies, training programs have been constructed to maximize the post-exercise rise in these hormones based on the assertion that exercise-induced increases in hormones like testosterone and GH will enhance muscle size and strength. But, considering the fact that these increases in hormone levels are very small and of short duration, will they produce muscular gains.

A study from the Kinesiology Dept. of McMaster University in Canada found that exercise induced hormone levels had no effect on muscle size or strength after 15 weeks of resistance training.

There is evidence that a minimal basal level of testosterone is required to support strength and hypertrophy gains, which are otherwise attenuated. Therefore, the hormone-sensitive processes that underpin muscle anabolism at hypo- and supra-physiological hormone levels are not being activated appreciably by exercise-induced increases in hormone availability or at least do not result in any measurable enhancement of strength or hypertrophy.

(J Appl Physiol 108(1); 2010)

“It has been well documented for more than eight decades within the world of exercise physiology that high-intensity muscular contraction is the most important requirement for the stimulation of rapid increases in muscular size and strength, whereas the duration of the exercise is not important in this regard. High-intensity muscular contraction prevents even the possibility of a large number of such contractions within a given unit of time.

Intensity and duration, in otherwords, exist in an inverse ratio to one another; you can either train hard or train long, but you can’t do both.”

High Intensity Training: The Mike Mentzer way.