If you're already trained in lifting weights, whether you use free weights or machines, you'll get about the same strength gains after 8 weeks—as long as you're lifting the same amount, at the same intensity, and moving through the same range of motion.

If you're already used to lifting weights, using free weights or machines won't make a noticeable difference in how much stronger you get or how fast you can push the weight after 8 weeks of training.

Whether you use machines that change resistance as you lift or keep the same weight all the way through, after 20 weeks of training, healthy men gain about the same strength and muscle size either way.

If two groups of young men lift weights with the same total effort but one rests longer with heavier weights and the other rests briefly with lighter weights, they both get just as strong after 8 weeks—even though you’d think the heavy weights would win.

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.

If you're already fit and lift weights, whether you do partial reps with the muscle stretched or full reps, you'll gain about the same tiny amount of strength after 12 weeks.