According to the specificity-of-training principle, if you want to improve endurance, do endurance or aerobic exercise. If you want to improve muscle size and strength, do weight training. Different exercise produces different effects in muscle. Normal weight training, even with minimal rest between sets, doesn’t have much effect on aerobic capacity. It does, however, increase muscle force, glycolytic enzyme activity (the ability of muscles to use glycogen as a fuel source) and muscle creatine content, which also aids energy production. The net effect of weight training, if all goes well, is increased muscular size and strength.
Endurance training produces different effects. When you do aerobics, you increase the number of mitochondria in muscle, because that’s where fat is oxidized during aerobic training. The body also increases capillary density to deliver more oxygen to muscles, since oxygen is required to burn fat as fuel. Myoglobin, the oxygen-containing pigment in muscle, is also upgraded for increased oxygen delivery. Along with those changes, muscle size is often reduced, since endurance training blunts muscle protein synthesis.
About 25 years ago studies began to indicate that doing aerobic exercise concurrently with weight work interfered with muscle gains. Since then other studies have found either the same result or no negative effects of doing both weight training and endurance training. It’s obviously an important issue, since many bodybuilders do aerobics as a means of controlling body composition or losing fat before a contest.
The latest study on the subject examined untrained men assigned to one of three groups: endurance training (ET), resistance training (RT) and concurrent training (CT). The study lasted 12 weeks and featured before and after measurements of body composition, peak oxygen intake and other values affected by aerobic and resistance training.
Weight and lean body mass significantly increased in the RT and CT groups. Bodyfat losses occurred in the CT and ET group but not the RT-alone group. Maximal oxygen intake improved only in the ET group. The gains in muscle size and strength were similar in both the RT and CT groups, indicating that no adverse effects resulted from combining weight training and aerobic exercise.
One surprising effect was the lack of improvement in maximal oxygen intake in the men who were doing both types of exercise’since they were doing aerobic exercise. The researchers suggested that the muscle size and strength those subjects gained may have diluted the gains in mitochondrial density, thereby preventing any significant improvement in oxygen intake.
So the study shows that using a training system that features both weights and aerobics doesn’t adversely affect strength and muscle gains but does hinder improvement in endurance. Doing aerobics alone does improve maximum oxygen intake, so trainees who are concerned about that may want to separate aerobic from weight workouts. Doing weight training alone does increase muscle power more efficiently than, say, doing aerobics right after a weight workout. IM