Performing resistance exercises with either isolated or compound movements can increase maximum strength not only in the exercises done but also in other unrelated movements, suggesting that the nervous system adapts in a widespread way.

Whether you train with one joint (like leg extensions) or multiple joints (like squats), you get just as strong at the movement you train — but if you train a simple movement, you get a bit stronger at holding still too, compared to complex movements.

In young, active men, doing compound exercises like squats and deadlifts for 8 weeks leads to bigger gains in strength and aerobic fitness than doing isolated exercises like bicep curls, when the total amount of work is the same.

When you train one muscle at a time (like leg extensions), you get a bit better at holding still with that muscle—more than if you train big compound moves like squats. But either way, you get way stronger at moving than holding.

For exercises that move just one joint—like bicep curls—lifting heavier weights leads to better strength gains than lifting lighter weights, even if both are done until exhaustion.

Lifting lighter weights many times until you can't do more builds muscle strength just as well as lifting heavy weights a few times until you can't do more, for people who already work out.