All things have to be taken in context, and understanding the target audience. Some compound movements like Squats and Deadlifts have a great anabolic effect, which puts the body in a place where it is more likely to put on muscle. Particularly beginners are best served focusing on compounds for both the strength and size considerations. However, that is not the end of the story, nor can it be.
Goals of Your Training
Your goals affect the type of training. They are paramount, as they help you determine what kind of a program and what exercises are necessary to achieve those goals. It also affects what type of results you want.
- If your goal is to have large masses of muscle, you train for size
- If your goal is to perform XYZ, you train for the technique, strength, and mobility required to do XYZ
Muscles can grow in a couple ways, both are hypertrophy, but the nature of the hypertrophy is different:
- Sarcoplasmic hypertrophy is an increase in the energy support systems, and is the type of muscle growth sought out by bodybuilders. Muscles are large and spongy.
- Myofibrilar hypertrophy is an increase in the protein pairs that perform contraction, and is the type of muscle growth sought out by strength athletes (power lifting, olympic lifting, strong man). Muscles are dense and hard--i.e. not as large.
Most athletes do need a combination of both of these, but depending on your goals one may take priority over the other.
What causes growth?
This is a very valid question, and affects whether compounds or isolations are necessary for your goal at hand. There are some fundamental things that must be balanced in your training that work for everyone:
- Training volume--more volume will increase muscle, but tax your recovery. Varying set/rep ranges also achieve different proportional results (myofibrilar vs. sarcoplasmic)
- Nutrition--you need raw materials like protein and energy to build muscles
- Fatigue--you can use fatigue as a training variable (i.e. reducing rest times or training to failure)
- Rest--all growth occurs during recovery
Essentially, there is a balance of training stress and recovery that has to happen. The stress puts the muscles in a state where they will have priority when the body repairs itself. The recovery is where the actual growth occurs.
- Compound lifts stress more of your body at once. However, this requires that you take your training and rest as complete days on or off. This approach seems to work well for strength sports.
- Isolation lifts focus the stress on one part of your body. This allows you to really stress that part of your body one day, and cycle to other parts of your body while the stressed one recovers.
This is not entirely an either/or proposition. These are simply training options. Some people do well with mixing approaches. For example you could use squats, deadlifts, and bench press to elicit its general anabolic effects; but then focus on smaller body parts in isolation.
It would be a mistake to completely ignore either compounds or isolation work. Particularly since some isolation work can address weaknesses we may have in part of the kinetic chain of muscles. For example, if you have a strength imbalance from the left and right side, you need to exercise them independently to make the weaker side stronger.