Final answer:
During the swing phase of gait, the pelvis rotates slightly to allow for leg movement, despite its overall immobile and weight-supporting role. This controlled pelvic rotation is facilitated by the muscles of the lower body while preserving stability and support for the upper body.
Step-by-step explanation:
True, The student asks about the movement of the pelvis during the swing phase of gait, specifically where it rotates. It's important to clarify that, while the bones of the pelvis are largely immobile and united to form a weight-bearing structure, the pelvis does undergo some motion during the gait cycle. During the swing phase of gait, the pelvis rotates to optimize leg movement. The leg that is off the ground swings forward, and the pelvis tilts down on the same side (ipsilateral hip flexion) while rotating slightly.
The immobility of the pelvic girdle overall is crucial for stability, allowing for the weight of the body to be transferred from the vertebral column, through the pelvic girdle and hip joints, to the lower limb on the ground. The pelvic girdle has less range of motion compared to the pectoral girdle because it is designed to stabilize and support the body.
However, during the swing phase of gait, this stability is briefly compromised to allow for mobility. Muscles of the lower body, including the powerful gluteal group, work to stabilize the pelvic girdle and facilitate the controlled rotation needed during the swing phase to advance the limb for the next step.