The reverse crunch has been modified to increase the degree of difficulty by performing it on an incline board. When the hip is forced into flexion, the pelvis must follow into backward tilt, which will then cause the spine to flex.
The sacrum is the last vertebral area that is in contact with the pelvic bone on either side of the pelvis. The abdominal muscles are contracting in spinal flexion in this exercise as a consequence of the pelvis moving into backward tilt. As this muscles contract concentrically they neutralise any unwanted spinal rotation, lateral flexion or lateral tilt of the pelvis. The prime movers of the reverse crunch are the rectus abdominis, external obliques and the internal oblique. In a reverse crunch the spinal joint is flexed by the concentric contraction of the rectus abdominis, external obliques and the internal obliques. The same muscles that concentrically contracted to lift the body are those that are eccentrically contracting to lower the body. In the down phase of the exercise the body is lowered slowly with gravity. Joint Action Contraction Muscle Group Spinal Joint Flexion Concentric Spinal Joint Flexors Down Phase