If untrained guys do leg exercises to failure using medium-heavy weights, going slow or going a bit faster on each rep gives about the same strength gains.

If new guys do leg exercises with slow reps (6 seconds) instead of faster ones (2 seconds), they might get stronger in a specific knee position—even if they do the same number of sets and their muscles grow the same amount.

To get stronger, it matters more how heavy the weight is than whether you push to your absolute limit—lifting heavier weights makes you stronger, no matter if you stop short of failure.

If untrained guys do leg extensions slowly (6 seconds per rep) vs. quickly (2 seconds), they’ll push harder at the end of the movement with the slow version, but harder at the start with the fast one — so speed changes where you’re strongest during the lift.

If new guys do leg exercises to failure using medium weights, slowing down the movement doesn’t make their muscles grow more than doing faster reps — both ways work about the same.

If you lift heavy weights with either short or long breaks between sets—while doing the same total work—you’ll get about the same stronger in your leg muscles after 10 weeks.