Muscle hypertrophy assessments limited to quadriceps biopsies cannot support conclusions about muscle fiber adaptation in other muscle groups such as the deltoid, biceps, or soleus in response to training load.

When muscles are worked hard, the nerve signals that activate them recruit different types of muscle fibers depending on how much force is needed. Muscles with more slow-twitch fibers, like the soleus, respond differently to training than muscles with more fast-twitch fibers, like the deltoid. The way these muscles are built and how they connect to nerves means the same training load doesn't cause the same growth in every muscle. Biopsies from one muscle, like the quadriceps, cannot show what happens in others because each muscle has its own unique way of responding to stress.