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 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 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 a young guy who's never lifted weights before and you do pec deck exercises, your upper and lower chest muscles will grow the same amount—no one part gets bigger than the other.

After nine weeks of lifting weights—whether heavy or light—men who work out casually don’t get bigger pectoral muscles, even though other muscles in their body do grow. This might mean some muscles just don’t respond the same way, or maybe they’re not doing the exercises right.

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.