If you're already fit, you might get stronger faster using gym machines like the chest press than doing classic lifts like squats or bench press — at least over 9 weeks — maybe because you're already used to the free weights.

If you're already strong and lift weights, training each muscle group either once or five times a week for eight weeks will give you about the same strength gains on the bench press, squat, and row.

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.

Lifting heavier weights for fewer reps makes your legs much stronger than lifting lighter weights for more reps, but both types of training help your upper body a little bit too, and this might work differently for different muscles.

If you're a guy who already lifts weights, doing 4 workouts a week for 8 weeks will make you way stronger—adding about 25 pounds to your squat and 10 pounds to your bench, whether you stick to the same routine or mix it up.

If you're a guy who's been lifting weights for a while, doing either 2 or 4 workouts per week for each muscle can boost your strength by about 8–11% in just 8 weeks—as long as you do the same total amount of work overall.