There has been much debate about the exact role of overtraining and resistance exercise. Some, such as Mike Mentzer, thought that just about all bodybuilders were overtraining—and yet there were many scientists who overwhelmingly concluded that individuals could never reach a true overtraining state. Researchers were interested in a controlled study standardizing the diet and training with excessive training load and insufficient recovery time between bouts on muscle markers of anabolism and catabolism. Male rats were randomly assigned to either a trained or a sedentary group. 

The trained group was subjected to a 12-week resistance-training program with excessive training load and insufficient recovery between bouts that was designed to induce leg-muscle atrophy. Briefly, rats underwent consecutive training sessions (five days/week) that consisted of jumps (repetitions). The overtraining protocol consisted of a progressive number of sets (2–4) and repetitions (5–12) with a 30-second rest between each set and carrying an overload of 50% body weight (a loaded vest strapped on the animal’s chest). 

After the 12-week experiment, the muscle fibers were collected to analyze the markers of muscle catabolism and anabolism, such as MAFbx, MyoD, myogenin, and IGF-I protein expression. The overtraining program resulted in a signifcant reduction in muscle size in the legs (-17%,) compared with the control group. Reciprocally, there was a significant (~20%) increase in MAFbx protein expression (i.e., marker of muscle catabolism), while the markers of muscle anabolism such as MyoD (-27%), myogenin (-29%), and IGF-I (-43%) protein levels decreased significantly. The data indicates that muscle atrophy induced by excessive training load and insuficient recovery was associated with upregulation of the MAFbx catabolic protein and downregulation of the MyoD, myogenin, and IGF-I anabolic proteins. These findings suggest overtraining is a very real phenomenon for athletes who continuously increase their volume in the gym; to prevent overtraining, one should look into a blocked periodization-type program where volume is variable.