When people who already lift weights do either mostly heavy lifts or mostly lighter lifts—but keep the total amount of work the same—both ways make them just as strong after 8 weeks.

Neither heavy nor light lifting made people better at doing more reps in a row—except maybe a small, not certain improvement with lighter lifts in squats.

Whether people lift heavy weights with few reps or lighter weights with more reps—while doing the same total amount of work—both ways make their muscles grow about the same size in the chest and thighs after 8 weeks.

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.

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 strong and lift weights regularly, doing 10–15 hard sets per muscle group each week for 2 months will make you stronger and add a little muscle — whether you train your whole body every day or just once a week.