Biarticular muscles exhibit preferential hypertrophy when subjected to isolation exercises targeting a single joint action, due to a mechanical disadvantage experienced during multi-joint movements.

Study: Exercise Selection Differentially Influences Lower Body Regional Muscle Development

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.

Study: The effects of hip flexion angle on quadriceps femoris muscle hypertrophy in the leg extension exercise

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.

Study: Effect of adding single-joint exercises to a multi-joint exercise resistance-training program on strength and hypertrophy in untrained subjects.

The study found that doing exercises that move just one joint didn’t help muscles grow more than doing exercises that move two joints at once. So, the idea that muscles crossing two joints need special one-joint exercises to grow isn’t supported.

Study: Influence of Adding Single-Joint Exercise to a Multijoint Resistance Training Program in Untrained Young Women.

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.