If you're new to lifting, doing just one set of pec deck exercises three times a week can make you just as strong as doing three sets — after 12 weeks, both groups get about 45% stronger.

If you're a young guy who hasn't lifted weights before and you do pec deck exercises for 12 weeks, you'll probably get about 43–46% stronger, no matter if you do just one set or three sets each time.

If you're a young guy who hasn't trained before and you do pec deck exercises for 12 weeks, your chest muscles will get thicker—about 17–18% thicker in the upper part and 21% thicker in the lower part—no matter if you do just one set or three sets each workout.

If untrained young guys do pec deck workouts three times a week for three months, following a specific plan, their chest muscles get noticeably thicker—by about 17 to 21%—in both the upper and lower parts of the chest.

If you're new to working out, doing just one set of chest machine exercises three times a week gives about the same muscle growth as doing three sets — both lead to around a 20% bigger chest muscle after 12 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.