Some muscles grow better with exercises that move just one joint, because they work across two joints and can’t fully contract during multi-joint moves.

The study found that some two-joint muscles grew better with exercises that only move one joint, but it didn’t prove this was because those muscles are at a mechanical disadvantage during multi-joint moves — so the reason given in the claim isn’t backed up.

This study found that doing leg extensions with your hip bent less makes the rectus femoris muscle grow more, but it didn’t compare single-joint exercises to multi-joint ones, so it doesn’t prove whether biarticular muscles grow better in isolation due to being at a mechanical disadvantage.

Adding arm curls and extensions to push-ups and pull-ups didn’t make arms grow any bigger or stronger than doing just the push-ups and pull-ups alone.

Adding arm isolation exercises didn’t make arms grow much more than just doing compound lifts like push-ups and rows—so the idea that biarticular muscles need special isolation work to grow isn’t supported.