When untrained guys do leg extensions with a medium weight until they can't do any more, moving faster (2 seconds per rep) makes their quads work harder for most of the movement — but moving slower (6 seconds per rep) makes them work harder right at the end. So, how fast you lift changes which part of the move your muscles feel most.

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.

After doing hard leg pushes with 5-minute breaks, people’s muscles stay stronger and more activated afterward than after 2-minute breaks, meaning they don’t get as tired.

If you're a guy who lifts weights, your front thigh muscle works way harder during leg extensions than during leg presses — probably because the movement challenges how that muscle crosses two joints at once.

When guys who’ve never lifted weights before start training, the more their thigh muscles ‘fire up’ during max efforts, the stronger they tend to get—both in holding a heavy position and in lifting one heavy weight.