Now it’s time to choose the muscles you’ll be working on. Some trainers prefer to build groups of muscles at once, while others like to isolate. It’s totally up to you to decide which muscle or groups of muscles to blast in your workout.  If you need ideas, here are the muscles targeted by most weight lifters: Latissimus dorsi (back) and pectorals (chest)  Deltoids (upper shoulder) and trapezius (upper back) Posterior deltoids (rear of the shoulder) and anterior deltoids (front of the shoulder) Erector spinae (lower back) and rectus abdominis (abdomen) Right external obliques (right side of abdomen) and left external obliques (left side of abdomen) Hamstrings (back of the thigh) and quadriceps (front of the thigh) Gastrocnemius (calf) and tibialis anterior (near the shin) Triceps (back of upper arm) and biceps (front of upper arm)