Do Muscles Compete for Growth?

Resistance training studies typically investigate different training approaches and their impact on growth or strength in the target muscles. This study took a different angle: does training a greater amount of total muscle mass across multiple muscle groups affect outcomes compared with training one area alone? After six weeks, the researchers found no detrimental impact on biceps brachii hypertrophy or strength when quadriceps training was added. It's an interesting finding, but questions remain. Let's take a closer look.

Overview

• What did they test? One hundred five previously untrained young adults (45 men, 60 women) completed six weeks of training. Participants were assigned to one of three groups: low-load training to failure of the biceps brachii alone, the biceps alongside quadriceps training, or a non-exercise control group. Muscle thickness of the biceps and quadriceps, 1RM bicep strength, and isometric quadriceps strength were assessed before and after training.
• What did they find? Performing additional quadriceps training did not impact muscle growth of the biceps when compared with training the biceps alone. Likewise, there were no differences in strength outcomes. 
• What does it mean for you? These findings suggest that additional exercises can be added within a session without blunting muscle growth or strength gains of the muscle trained first. However, given the low training volume, untrained participants, and uncontrolled nutrition, the findings should be interpreted with some caution.

What’s the Problem?

Training studies often look at different styles of training and how effective these may be compared with one another for outcomes such as muscle growth or maximal strength. Some also compare different nutritional or supplemental approaches to determine whether they promote better outcomes. A question less often asked is whether training multiple muscle groups creates competition between them for these outcomes. That is, would training only one muscle group produce better results than training several muscle groups at once?

The authors of the current study appear to have considered this question after noticing a notable difference in biceps brachii muscle growth between some of their previous studies 1 2. The key difference between the studies? One also trained legs, while the other trained arms only. While an interesting observation, there are substantial limitations in comparing results across studies. This leads us to the present study.

Purpose

This study aimed to investigate whether muscle size and strength outcomes are impacted when more total muscle mass is recruited within a training session.

Hypothesis

The researchers hypothesized that additional knee extension training would reduce biceps brachii muscle growth due to competition for resources.