In a recent study (http://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0149082 ) researchers looked at changes in muscle fiber sizes in response to training with and without endurance exercise.
The resistance training group performed leg presses only, whereas the endurance training group performed cardiovascular exercise and leg presses. Prior to the beginning of the study muscle biopsies to measure muscle fiber types and sizes. There are two primary types of muscle fibers: Type 1 aka slow twitch endurance fibers and Type 2 fast twitch “power” fibers.
After 7 weeks of training both groups increased their one repetition maximum leg press (showing both groups increased in strength). However only the group doing endurance training increased their VO2 Max (cardiovascular exercise capacity) AND increased the size and area of both Type 1 AND Type 2 muscle fibers.
The resistance only group only showed increases in Type II fibers. The mean fiber area (total size) increased by 28% for the Endurance Group where there was no significant increase in the resistance only training group.
This means that the group doing BOTH resistance training and “cardio” gained more muscle mass than the resistance training only group.
When the researchers looked at chemical changes in the muscles they saw an increase in two key anabolic factors in the endurance and resistance training group with elevations in mTOR and Akt which correlated directly with the increase in muscle fiber sizes. This shows that the increased results obtained by adding endurance training to resistance training are the result of increased anabolic signaling not from decrease muscle breakdown.
So the take home message is that to increase muscle mass and strength – you should include some cardiovascular endurance training to your program!