Which muscle group serves as the prime mover during a curl up on a balance ball?

The rectus abdominis is the prime mover during a curl-up on a balance ball. This muscle group is located in the front of the abdomen and is primarily responsible for trunk flexion, which is the action performed during a curl-up. When performing this exercise, the rectus abdominis contracts to lift the upper body off the ball, engaging in a motion that effectively targets this muscle.

While the transverse abdominis and obliques play important roles in stabilizing the core and assisting with movement, they are not the primary muscles responsible for the main action of curling up. The transverse abdominis helps to stabilize the pelvis and lumbar spine but does not flex the trunk. The obliques assist with rotation and side bending, which are not the main actions in a traditional curl-up. The erector spinae, while involved in maintaining posture and spinal extension, does not actively engage as a prime mover in the flexion motion of the curl-up.

Thus, the rectus abdominis being the primary muscle involved in this specific movement makes it the correct answer.