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A recent study by a group at Ball State University in Indiana claims that significant muscle hypertrophy can be achieved with aerobic exercise. This piqued my interest because it has been well established that aerobic exercise is not an effective means of inducing muscle growth. Not only that, there is some evidence that muscle fibers may actually decrease in size to facilitate aerobic metabolism with long-term training. So I thought this study deserved a closer look.
The first thing to figure out is what exactly do they mean by aerobic exercise? They had untrained young (~20 years) and older-aged (~74 years) subjects work out three to four times per week on a stationary bike for 12 weeks. Exercise intensity was set according to each subject’s “heart rate reserve” (HRR). Your HRR is the difference between your resting heart rate and your predicted maximum heart rate. So the bike was set up so each subject pedaled for 20 minutes at 60% of their HRR initially and worked up to a maximum of 45 minutes at 80% HRR by the seventh week and then this intensity was maintained for the remaining five weeks. To give you a better idea of what 80% HRR means, it’s about 50% of your max heart rate.
At the end of 12 weeks, several different measurements of muscle size and function were taken. Of interest to us of course is the effect on muscle growth. Quadriceps muscle volume increased ~5% in younger and ~6% in older subjects. That’s nothing to sneeze at so I kept reading. Upon closer examination, it was only type-I (slow-twitch) fibers that actually grew. Type-II (fasttwitch) fibers actually shrunk in the younger subjects although this did not reach statistical significance by 12 weeks.
After reading the study closely, I think the implied claim made by the title of the study, aerobic training induces skeletal muscle hypertrophy, probably requires an asterisk with the disclaimer, “only in untrained young and elderly men.” If you train your legs with weights regularly, jumping on a stationary bike and pedaling at 50% your max heart rate is not going to do jack to build mass in your quads. This is a perfect example of the Hypertrophy-Specifc Training principle, which states, “the effectiveness of any given load is determined by the condition of the tissue at the time the load is applied.” There is a load-stress and metabolic-stress threshold in muscle tissue that must be crossed for significant hypertrophy to occur. The process of adapting to that stress pushes this threshold up over time, rendering previous loads ineffective at inducing growth. If it weren’t so, we could all just skip the squat rack and jump on the bike until we had quads Tom Platz would be proud of. In the real word, it just ain’t so. To the authors’ credit, they do come clean in the final conclusion by clarifying that aerobic exercise should be considered a viable option for “combating the decline of aerobic capacity and loss of muscle mass that occurs with the normal aging process.” Until then, you and I will have to stick with good old weight training.