Seated leg extensions, standing squats, and/or leg presses can all be used to strengthen your quadriceps muscles on the front of your thigh. But is one better than the others?
In other words, generally squats are more beneficial, but there are a few special exceptions (e.g. rehab of certain knee surgeries where seated extensions may be more desirable).
The main differences :
– seated extensions put a lot more stress on the knee alone, while squats distribute the stresses btwn all the leg joints (especially important if you have arthritis), and
– squats are a lot more functional, using several leg joints/muscles as well as the core together to perform the kind of motion you actually do in real life (think lifting a big box, sitting down, shooting a free throw…)
And leg presses are somewhere in-between, basically recumbent squats. Good distribution of joint stresses but not a very functional position.