According to study published this month, aerobic exercise does not hinder one's attempt to gain muscle mass via resistance training.
Within the physical trainer community there is a misconception that aerobic training reduces muscle mass. The belief is that aerobic activity burns muscle and prevents bulking up. It is true that aerobic activity burns fat; so aerobic activity prevents one from bulking up in that sense. However, a Swedish study published this month found that not only does aerobic exercise not prevent muscle gain, it actually helps build muscle mass (Aerobic exercise does not compromise muscle hypertrophy response to short-term resistance training, 2013. Tommy R Lundberg, et al.).
The study had ten, healthy male participants train one leg with just resistance training and the other with both resistance training and aerobic exercise. The training program lasted 5 weeks. The researchers measured physical characteristics, biochemistry and performance of both legs (specifically the quadriceps) before and after the training program. Perhaps most surprising was the finding that the increase in muscle volume in the leg receiving both aerobic training and resistance training was significantly greater than the leg receiving resistance training alone. In addition, MRI scans showed that the cross sectional area of aerobic and resistance trained legs was significantly greater than legs doing resistance training alone. The changes in muscle volume of the quadriceps muscle with pure resistance training (RE) and both aerobic training and resistance training (AE+RE) can be seen in the figure below.
Although muscle volume (hypertrophy) was increased by additional aerobic training, the maximal velocity and power was compromised by aerobic training. The reason for this may be because even though the aerobic training increases muscle fiber size, it recruits type-II (fast-twitch) muscle fibers to type-I (slow-twitch) muscle fibers. It should be noted that the recruitment was not significant, but there was an effect. The progression of maximal power of each leg over the course of the resistance sessions can be seen in the figure below.
Being physiologists, the researchers also analyzed gene expression of VEGF (a growth factor that increases blood vessel proliferation), myostatin (another growth factor that inhibits muscle growth and differentiation), PGC-1alpha (a transcriptional coactivator involved in energy metabolism), and muscle atrophy factors MuRF-1 and atrogin-1. The graph below shows how each training program affected the respective gene expression.
Apparently, if size is all that matters than aerobic exercise is not of consequence. Maximum power was decreased in aerobically trained legs. This presented a paradox: the larger muscles had less maximal power. Thus, aerobic exercise does not compromise muscle hypertrophy (muscle size), but it does reduce maximum power output and velocity.