Lifting weights with one joint (like leg extensions) transfers better to holding a static position than complex lifts like squats, because it’s easier for your body to use the same muscle pattern in both cases.

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.

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.

Doing extra arm isolation exercises like bicep curls and triceps extensions doesn't help you get stronger or build more arm muscle than just doing big compound moves like bench presses and pull-downs — at least not for beginners after 10 weeks.

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.

Doing extra arm exercises along with big compound lifts like squats and presses makes arms a little bigger, but doesn’t make you stronger than doing just the big lifts alone.