Final answer:
The pectoral girdle has high mobility for a wide range of upper limb movements, while the pelvic girdle is largely immobile, providing stability and weight-bearing capacity for upright locomotion and support.
Step-by-step explanation:
The pectoral and pelvic girdles of the human body serve different functions and are designed accordingly in terms of flexibility (range of motion) allowed, security, and ability to bear weight. The pectoral girdle, consisting of the clavicle and scapula, offers a high degree of mobility for the upper limbs, which is enabled by the lack of rigid articulations between the girdles themselves and a single, highly mobile joint connecting the clavicle to the axial skeleton. This flexibility permits a wide range of shoulder and arm movements.
In contrast, the pelvic girdle is designed for stability and weight bearing. It forms a robust structure with the sacrum and consists of two hip bones that join anteriorly at the pubic symphysis and posteriorly at the sacrum. The deep sockets and strong ligaments of the pelvic girdle attach securely to the femurs and make it a largely immobile, weight-bearing structure. This immobility ensures that the body's weight can be transferred laterally from the vertebral column through the pelvic girdle, enabling stable locomotion and support.
Therefore, these girdles represent a trade-off between flexibility and security: while the pectoral girdle provides mobility necessary for manipulation and interaction with the environment, the pelvic girdle provides the security and stability necessary for supporting body weight and facilitating movement such as walking.