Squats transfer to leg extensions at 16% — the reverse is 9%
In short
In a volume-matched 10-week study, both a squat group and a leg extension group increased their one-rep max and muscle thickness, but the gains were larger in the squat group. Maximum isometric torque improved 14% in the squat group against 10% in the leg extension group, and changes in pennation angle, fascicle length and optimal fascicle length appeared only in the squat group. Transfer was asymmetric too: squat training raised leg extension one-rep max by about 16%, while leg extension training raised squat one-rep max by about 9%.
"Do the compound lifts first" circulates as common sense, but data that matches volume between the two is scarce. This study combined ultrasound, EMG and dynamometry to compare two programmes of equal training volume over ten weeks.
Twenty-four participants were split into a squat group and a leg extension group — one multi-joint exercise, one single-joint. Before and after, the researchers quantified knee extensor muscle architecture and the torque–fascicle length relationship of the vastus lateralis.
What happened to muscle and strength?
Both groups significantly increased their one-rep max (P<0.001). Muscle thickness rose in both (P=0.008 for leg extension, P<0.001 for squat), but the increase was greater in the squat group (P=0.031).
Maximum isometric torque improved significantly in both groups, with the squat group gaining 14% against 10% for leg extension (P=0.041).
The architectural changes separated more sharply. Pennation angle, fascicle length and the optimal vastus lateralis fascicle length increased in the squat group only. The two groups did not just grow by different amounts — the muscle reorganised differently.
How much transferred?
This is the most usable result. Both groups improved on the exercise they had not trained, but the amounts were asymmetric.
- Squat-only group — leg extension one-rep max rose about 16%
- Leg-extension-only group — squat one-rep max rose about 9%
The squat group's transfer was significantly larger (P=0.037). The researchers attributed it to the high knee torque required to maintain a given external load. At the same volume, the squat places a greater demand on the knee extensors, and the neuromuscular adaptation that follows is broader.
This was 24 participants, and the comparison is against the specific leg extension setup used here. It does not mean single-joint work is useless — the leg extension group also significantly increased one-rep max, muscle thickness and maximum torque. It remains a valid option for anyone whose knees or injury history rule squatting out.
How this relates to your strength score
A relative strength score is the sum of squat, bench press and deadlift one-rep maxes, which turns the transfer measured here into a scoring question. If your time is limited, spending it on the multi-joint lift is the better bet for the total.
The numbers make it plain. Train the squat and the untrained single-joint movement follows by 16%; train the single-joint movement and the squat follows by 9%. Accessory work cannot substitute for the main lift. Use accessories to shore up a weak range, and let the score be built by handling the Big 3 themselves heavy. Which lift deserves your time first is covered in your total is set by your weakest lift.
Frequently asked questions
Are compound or isolation exercises more effective?
In a volume-matched 10-week study, the squat group beat the leg extension group on muscle thickness gain, maximum isometric torque (+14% versus +10%) and transfer to the untrained movement. The isolation group still increased one-rep max and thickness significantly.
Does squatting make your other leg exercises stronger?
Yes. The squat-only group improved their untrained leg extension one-rep max by about 16%. Transfer in the other direction was about 9%, roughly half as much.
Is isolation work unnecessary?
No. In this study the leg extension group significantly increased one-rep max, muscle thickness and maximum torque. It remains a valid option for anyone who has to avoid squatting because of knee pain or injury.
Why did the squat produce larger adaptations?
The researchers pointed to the high knee torque required to maintain a given external load. At equal volume, the squat places a greater demand on the knee extensors, producing broader neuromuscular adaptation.
Did muscle architecture differ between the groups?
Yes. Increases in pennation angle, fascicle length and optimal vastus lateralis fascicle length appeared only in the squat group, meaning the muscle reorganised differently rather than simply growing by a different amount.
Source: PubMed