If you’re wanting muscle growth, don’t rely only on big compound exercises. Sure, compound multi-joint exercises offer the most bang-for-the-buck in terms of gaining usable strength, but that’s not the main goal of someone wanting to build muscle. It’s just a neat side effect of pursuing your main goal.
Isolation exercises might not be as “functional” in terms of real world applicability, but they put a laser-like focus on the target muscle, making it the recipient of the training stress.
For example, barbell squats are great, but your lower back might give out before your quads. And that’s going to compromise the training stimulus your quads receive. That’s where leg extensions come in. When you take a set of leg extensions to failure, you KNOW that the quads did all the work!
So instead of thinking with an either/or mentality, embrace the fact that neither compound nor isolation exercises are inherently superior… they’re just different.
When I was an undergrad in exercise science, I tracked down the professor most known for his knowledge of lifting. I asked him, “Which is better for size, compound or isolation exercises?”
His answer surprised me. “Neither,” he said. “Each has pros and cons. It’s best to stimulate the muscle in a variety of ways. You’ll likely get more overall hypertrophy doing some of each rather than either compound or isolation exercises exclusively.”
True answers are not as black and white as we’d like for them to be.