New lifters get stronger at leg extensions by doing squats, but experienced lifters need to do leg extensions to get better at them.

We analyzed the available evidence and found that in untrained individuals, compound movements like squats and deadlifts can lead to improvements in isolation movements like leg extensions or bicep curls, likely due to better neuromuscular coordination — the body’s ability to recruit and control muscles more efficiently. This effect appears to temporarily reduce the usual expectation that training must closely match the target movement to produce gains. However, in individuals who are already trained, strength improvements remain tightly tied to the specific movement performed — meaning doing a bench press improves the bench press more than it improves a dumbbell press or push-up. The evidence we’ve reviewed so far includes 82 assertions that support this pattern, with no assertions contradicting it [1]. This suggests that the body’s initial response to resistance training may be more general, especially when someone is new to lifting. As training experience grows, the nervous system and muscles adapt in ways that become more precise to the exact motion practiced. This doesn’t mean compound movements are useless for trained lifters — they still build strength and muscle — but their carryover to other movements becomes more limited. For someone just starting out, adding compound lifts may help improve performance across several exercises, even ones they haven’t directly trained. For someone with more experience, progress in a specific movement usually requires practicing that movement directly. What we’ve found so far points to a shift in how the body responds to training as experience increases — from broad, general improvements early on to highly specific adaptations later. If you’re new to lifting, compound movements may give you a wider boost. If you’ve been training for a while, focusing on the exact movement you want to improve is likely more effective.