Final answer:
The hip flexors are the muscles overactive during the overhead squat assessment, causing the low back to arch. The gluteal muscle group is responsible for hip extension rather than posture, and other muscles such as the sartorius and gracilis are involved in the action of crossing the legs. Therefore, correct answer is d.
Step-by-step explanation:
The muscle that would be considered overactive leading to the low back arching during the overhead squat assessment is the hip flexors.
When the hip flexors are overactive, they can create an excessive anterior pelvic tilt, which in turn causes the lower back to arch.
The rectus abdominis and the hamstrings are muscles that would normally counteract this arch by promoting a posterior tilt, and the gluteus maximus is responsible for extension of the hip joint during movements like standing up from a squatting position.
To elaborate further with examples from the provided information, the gluteal muscle group, which includes the gluteus maximus, is responsible for extending the hip joint and attaching the leg to the pelvic girdle.
On the other hand, the sartorius and gracilis muscles are involved in crossing the legs, a movement different from the functions of muscles involved in maintaining posture during the overhead squat assessment.