The results of the study, which were published in the Canadian Journal of Applied Physiology, showed that four hours after exercise, the rate of muscle protein synthesis was 50 percent greater in the exercised arm compared to the non-exercised arm. Twenty-four hours after exercise, the rate of muscle protein synthesis was 110 percent in the exercised arm. By 36 hours after exercise, the rate of muscle protein synthesis was only 14 percent greater than the control group.
According to the researchers, the most effective training frequency would be such that the next training session for the same body part would not occur until the protein synthesis rate for that body part returned to its pre-training rate. For a small muscle group like the biceps, this looks to be no earlier than 36 hours. Larger muscle groups, such as the quadriceps, may take even longer to recover.
Training a body part before its synthesis rate returns to the pre-exercise rate may actually be detrimental to the muscle and lead to overtraining. If you want to grow, you need to take time to let your body recover.