Back extensions are exercises that require you lift your upper body from a prone position and slightly hyperextend your back. You can use these exercises to strengthen the muscles in your lower back as well as the muscles in your abdomen. The antagonist muscle group used during back extensions is called the rectus abdominis.
Muscle Agonists and Antagonists
Most of the muscles in your body work in coordinated pairs called agonists and antagonists. Agonist muscles have responsibility for moving any given part of your body by contracting, or shortening their length. Antagonist muscles have responsibility for reversing the motion of your agonist muscles and returning your body to its previous position, the Worldwide Education and Awareness for Movement Disorders website explains. Depending on the movement you perform, any given muscle can act as an agonist or antagonist. For instance, when you raise your forearm toward your shoulder, your biceps muscle acts as an agonist and your triceps muscle works as an antagonist. When you lower your forearm, your triceps acts as an agonist and your biceps works as an antagonist.
Back Extension Muscles
The agonist/antagonist pair that allows you to perform a back extension includes the rectus abdominis muscle in your abdomen and the erector spinae in your back. Your rectus abdominis runs from the bottom of your ribcage to your pelvis and is often referred to as your “six-pack.” Your erector spinae runs very close to your spinal column and is actually a coordinated grouping of three muscles: the spinalis, iliocostalis and longissimus, according to the Sports Injury Clinic website. Although the erector spinae is commonly viewed as the antagonist to the rectus abdominis, you actually shorten your erector spinae when you bend your back upward during a back extension making it the agonsit and your rectus abdominis the antagonist.
If your rectus abdominis is weak or lacks the endurance for repetitive activity, you can hyperextend your back to an abnormal degree during a back extension. In turn, abnormal hyperextension can increase your risks for developing a back injury. You can help avoid this possibility by strengthening your abdominal muscles with exercises called crunches. You can strengthen your erector spinae muscles with exercises such as side bends and dorsal bends. You can stretch your erector spinae muscles with back slumps, back arches and lower-back stretches.
Additional muscles in your abdomen that contribute to support and stability include the external and internal obliques, which sit in pairs on either side of your rectus abdominis, and your transversus abdominis, which sits deep in your abdominal wall. In addition to your erector spinae, stability in your back comes from your multifidus muscle, which extends along your spine. Consult your doctor and a certified fitness instructor for more information on how to safely perform back extensions and other exercises for the muscles in your abdomen and lower back.