If college guys who've lifted weights before do a specific 12-week workout plan—3 times a week, 8 exercises, 3 sets each—they'll get 25% stronger in their arms, based on how much they can lift in one go on a curl machine.

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.

After doing strength training for 12 weeks, older women—whether they started strong or weak—gained about the same amount of strength in their arms and the same amount of muscle, no matter how strong they were at the start.

If young guys lift weights all over their body 3 times a week for 9 weeks, they'll get stronger — no matter if they switch up their exercises or stick to the same ones.

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 already trained, doing light weights with short or long breaks between sets gives you about the same strength boost in your chest and legs after 8 weeks.