Final answer:
An arched low back during butt kicks could indicate tightness in the hamstring group, which are responsible for moving the back of the lower legs up and toward the buttocks, and moving the thigh down and back. Stretching and strengthening the hamstrings may alleviate this tightness.
Step-by-step explanation:
When a client is performing butt kicks and the personal trainer notices the client arching their low back at the height of the jump, it could indicate tightness in the hamstring group of muscles. The hamstrings, located in the posterior compartment of the thigh, are responsible for moving the back of the lower legs up and toward the buttocks, as well as moving the thigh down and back. Tight hamstrings can lead to an overcompensation by the lower back muscles during activities like butt kicks, leading to an arched lower back as the body tries to achieve the motion mostly through the lower back muscles instead of the hamstrings.
The gluteal group of muscles plays a significant role in raising the knee at the hip, assisting lateral rotation of the thigh, and helping maintain posture. If the hamstrings are tight, the function of the gluteal group could be compromised, and the individual may arch their low back to complete the movement. Therefore, stretching and strengthening exercises targeting the hamstring muscles might be recommended to alleviate this tightness and improve overall leg movement.