The muscles that are worked when you do back extension is dependent on whether you use an exercise machine, what type of machine you use and whether the back extension is done in a seated position or while standing. The upright, seated back extension machine available at most gyms works the erector spinae muscles and the multifidus muscles. The standing back extension machine works the erector spinae muscles in addition to a large muscle group of the legs called biceps femoris.
Erector spinae muscles
The erector spinae muscles are a group of paired muscles on both sides of the spine made up of three smaller muscles called iliocostalis, longissimus, and spinalis. These muscles connect to the hips, mid to lower spine, and ribs on one end and to the skull, upper spine and ribs on the other. These muscles allow you to extend the back, bend side to side, and they maintain the proper alignment of the spine. Weakness of these muscle can lead to poor posture and back pain. These muscles are worked when using the upright, sitting back extension and the standing back extension machines.
The multifidus muscles are small muscles that extend the entire length of the spine that stabilize and maintain proper posture. They allow for extension of the spine, side to side bending, and rotation of the spine. These muscles are exercised when using the upright, sitting back extension machine.
The biceps femoris muscles are large muscles that run along the back of your legs as a part of the hamstrings. Although they are leg muscles, they assist in extension of the lower back because they attach to the lower part of the hip bones. These muscles are worked when using the standing back extension exercise machine.
Developing strength of your back extensors is beneficial because it promotes good posture and support of your spine. Thus, you are less likely to develop back pain and the complications that arise from having poor posture. Research studies show that strength of back extensors protects against vertebral fractures associated with osteoporosis.