As a strength athlete, you can do various exercises bilaterally (left and right simultaneously) or unilaterally (left and right separately). What is better, if muscle growth is your main training goal?
BILATERAL DEFCICIT
Many strength athletes report that they feel the muscle group they are training work harder when they train it unilaterally. For example, with the single-arm machine row or the single-arm preacher curl. This could be related to what is scientifically called the bilateral deficit.
The bilateral deficit is characterized by a reduction in the force produced by a single limb during maximal bilateral contractions. This has led proponents of unilateral training to argue that the nervous system may recruit muscle fibers or motor units better during unilateral training. And this could lead to greater muscle growth, making unilateral training more effective than bilateral training.
RESEARCH
But what does scientific research say about this? A recent meta-analysis by Kassiano et al compared strength gains and muscle growth of unilateral versus bilateral training.
In this analysis, there are only two studies that focus on hypertrophy. The outcome? Bilateral and unilateral are equally effective.
There are several studies available on strength gains. The results suggest that the principle of specificity applies: you gain strength in bilateral results by training bilaterally and you get stronger in single arm or leg movements by training unilaterally.
More studies are needed to determine for sure whether bilateral and unilateral are indeed equally effective for muscle growth.
ADVICE
But if that is the case, why then do unilateral training (which takes more time)? A good reason could be if you have too little weight available, for example in a hotel gym on holiday. Another reason is simply preference.