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...

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.

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.

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 heavy weights for just a few reps makes you stronger in exercises that work one muscle at a time, more than lifting lighter weights for many reps, especially if you already work out regularly.

Beginners get the biggest strength boosts from lifting weights, but even they get way stronger at moving weights than at holding still — the gap between those two types of strength stays the same no matter how fit you are.