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.
Scientific Claim
Muscles that cross two joints (biarticular muscles) exhibit preferential hypertrophy with isolation exercises that target a single joint action, due to mechanical disadvantage during multi-joint movements.
Original Statement
“Regarding hypertrophy, rectus femoris growth across all three regions were greater for the leg extensions. Vastus lateralis growth at the proximal and middle regions were not significantly different, but growth at the distal region was greater for the back squat group. If you're a frequent viewer, you'll likely be aware that these findings generally align with the other research we have on the quadriceps. That is compound exercises that involve simultaneous knee and hip extension such as squats and leg presses seem to be highly effective for growing the vastus heads of the quads but fail to optimize development of the rectus femoris. This is likely explained by the fact that the rectus femoris crosses over two joints.”
Context Details
Domain
exercise
Population
human
Subject
Biarticular muscles
Action
exhibit
Target
preferential hypertrophy with isolation exercises due to mechanical disadvantage during multi-joint movements
Intervention Details
Evidence from Studies
Supporting (0)
Contradicting (6)
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.
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.
Drop-Set Training Elicits Differential Increases in Non-Uniform Hypertrophy of the Quadriceps in Leg Extension Exercise
The study found that doing leg extensions alone made one part of a two-joint muscle grow more, but it didn’t compare that to exercises that use two joints at once, so we can’t say if the muscle grows better because it’s at a disadvantage in big movements.
Comparison of Muscle Hypertrophy and Strength Adaptations Induced by Back Squat and Leg Extension Resistance Exercises.
The study found that doing leg extensions (isolating the knee) made the rectus femoris muscle grow more than doing squats (which use multiple joints), which matches the claim—but it didn’t prove why this happens, so we can’t say it’s because of mechanical disadvantage.
Effect of adding single-joint exercises to a multi-joint exercise resistance-training program on strength and hypertrophy in untrained subjects.
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.
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.