This is the first paper to look at what happens to muscles that are not exercised during resistance training. These authors had 21 participants perform resistance training for 10 weeks in which they performed specific exercises to target their biceps, hamstring, and quads. No other muscles were exercised. At the start and end of the study, the authors performed MRI imaging to measure three-dimensional volume of 28 muscles in the body, including all of the muscles that were targeted and a range of non-exercised muscles. They also monitored self-selected protein intake and had all subjects consume 0.3 g/kg of whey protein after each workout. This amount is the equivalent of 150 pound person consuming 20 g of whey protein post workout.

Because a study like this has never been done before, the authors were unsure of the outcomes. Most people would have theorized the nonexercised muscles might have benefited from the growth hormone released from the exercised muscles and increased in size slightly. Surprisingly, this was not the case. While the muscles that were exercised increased in volume anywhere from 2.2 to 17.7%, the muscles that were not exercised decreased in volume, especially in people with low protein intake. This study has profound effects for managing injuries and improving athletic performance, as some of the nonexercised muscles underwent significant atrophy: adductor longus decreased in volume by up to 9% (average 5.6%) while soleus decreased in volume by as much as 5.5% (average 3.7%). Adductor magnus also decreased in volume in the nonexercised group, but only by an average of 1.5%, possibly because the vertical fibers of adductor magnus were slightly recruited during the hamstring curl exercises.

Interestingly, the muscle atrophy in the nonexercised muscles primarily occurred in people with low protein intake. This paper has significant implications not just for the management of athletic performance, but also for injury prevention. Weakness of the adductor longus is strongly correlated with the development of groin injuries, and weakness of the soleus correlates with impaired running performance and a range of injuries, most notably knee and Achilles injuries. Weakening of nonexercised muscles can also significantly impair athletic performance, and this can all be avoided by prescribing strengthening routines that target all of the major muscle groups, particularly the calves. The authors claim that when early hunter gatherers had limited access to dietary protein, they lost the evolutionary advantage associated with muscle hypertrophy and strength. This study also explains why long-distance runners who often only exercise their lower extremities have such poor muscle development in their upper extremities, and why heavy weightlifters who focus on their chest and arms often have extremely thin legs.

While the authors noted the difference in muscle atrophy depending upon whether they had high or low protein intake, I felt one of the biggest surprises of this study is what the authors considered low protein intake. Although not addressed in the abstract, there wasn’t that much difference between the high protein group and low protein group as both groups consumed .3 g per kilogram of whey protein after each workout. After adding in dietary protein, the high-protein group consumed 1.4 g per kilogram of body mass each day, while the low protein group consumed 1.2 g per kilogram. That’s the equivalent of 150 pound person in the high-protein group consuming 95 g of protein per day, while a 150 pound person in the low protein group would be consuming 81 g of protein each day. That small difference in protein consumption was responsible for producing significant atrophy in the low protein group. Based on these numbers, it’s safe to say that you should multiply your weight in pounds by .7 to determine your minimum protein consumption while resistance training.

This paper makes a strong case for both exercising as many muscles as possible when performing strengthening exercises, especially by changing your routine over time. It also makes it clear that the process of generating muscle mass is not simple, and you need to have high quality protein in your diet in order to not consume nonexercised muscles. As mentioned, this is particularly important with the calf muscles because these muscle atrophy at a faster rate than other muscles as we age, so they need to be exercised regularly as we get older.