Are workouts twice a day more effective for muscle growth?

Some bodybuilders advocate two-a-days for optimal increases in muscle mass. The legendary bodybuilder and seven-time Mr. Olympia, Arnold Schwarzenegger, often did two-a-days, but can it boost muscle growth as they recommend? The theory behind these types of workouts is that short, intense workouts will lead to higher increases in 24-hour protein synthesis and anabolic hormones such as growth hormone, testosterone, and IGF-1. On the other hand, more frequent exercise can lead to overtraining and loss of muscle strength.

A recent study examined the impact of multiple training sessions on protein synthesis and mTOR in mice. Mice did three workouts in sequence, with either 72 hours, 24 hours, or eight hours of recovery between each. Each workout was five sets of 10 isometric contractions of three seconds in duration, with seven seconds of rest between contractions, and three minutes of rest between sets. So if twice a day workouts are superior to one workout per day, than the workout that was eight hours apart should have had a higher increase in muscle protein synthesis.

Contrary to what one may expect, muscle protein synthesis (MPS) rates are increased when the recovery time between workouts is 24-72 hours, but not when the recovery time is eight hours. Another interesting finding was that the cellular messenger AMPK was reduced when the exercise sessions were eight hours apart, and markers of cellular stress (oxidative stress) was increased. AMPK is a cellular energy sensor of cells and is a downregulator of MPS rates. When AMPK is decreased, it is associated with a decrease in muscle protein synthesis. In sum, based on the findings of this study, training a muscle more frequently does not necessarily lead to more muscle growth. Be sure to give yourself 24-72 hours of muscle recovery between workouts. Training more frequently than 24 hours could lead to stagnation, and possibly even overtraining-induced muscle loss.

Repeated bouts of resistance exercise with short recovery periods activates mTOR signaling, but not protein synthesis, in mouse skeletal muscle. Takegaki, J., Ogasawara, R., Tamura, Y., Takagi, R., Arihara, Y., Tsutaki, A., & Ishii, N. (2017). Physiological Reports, 5(22), e13515.