The Muscle That Is The Prime Mover Of Back Extension

Two muscles function as agonists, or prime movers in back extensions -- your spinal erectors and your hamstrings. Both muscles work together to help raise your torso whenever you start out leaning forward. Other muscles such as your hips and abdominals contribute to the exercise, but they provide stability instead of power. Consult a health-care practitioner before beginning any strength-training program.

Extensions

Back extensions require a special bench that you can rest your hips on. Hook your feet firmly under the ankle supports and ensure that you can lower yourself forward freely. Cross your arms on your chest and, without rounding your back, lean forward as far as you can. Then arch up until your torso goes above parallel to the ground. If you can perform more than 15 to 20 repetitions, you might wish to increase the difficulty of this exercise by holding weight to your chest. Never place your hands behind your head.

Spinal Erectors

Your spinal erectors are columns of muscle than run up either side of your spine to the middle of your back. During normal activity, these muscles maintain the posture of your lower back, keeping you from slumping forward. These muscles work heavily to pull your torso erect when you have leaned forward. This occurs during exercises such as the good morning, the deadlift and the back extension. These muscles also keep you upright when squatting, so work them heavily, but carefully, to support your training and performance goals.

Hamstrings

Your hamstrings consist of three muscles: semitendinosus, the semimembranosus and the biceps femoris. Your biceps femoris flexes your knee, and does not get recruited during back extensions. Your semitendinosus and semimembranosus work with your spinal erectors to help your torso extend -- or straighten up. These muscles are also prime movers during the back extension. These muscles also work heavily during the squat and deadlift, and in those exercises work in conjunction with your spinal erectors.

Supporting Muscles

Your hips, mainly the gluteus maximus, work with your spinal erectors and hamstrings to help you straighten during the lift, but they are not a prime mover during the exercise. Fatigue in this area can sometimes limit exercise performance, according to a 2004 study in the "Scandinavian Journal of Medicine and Science in Sports." Your abdominals and obliques -- the other muscles of your core -- contract to help you maintain a stable torso when performing a back extension. As no movement takes place, these muscles are not prime movers.