PART A: The shoulder girdle and shoulder joint collaborate in scapulohumeral movement, or synergy, to facilitate upper-extremity motion. During movements like abduction, flexion, and extension, various muscles act in concert to stabilize and mobilize the scapula and humerus. For instance, during abduction, the deltoid muscle primarily contracts to lift the arm laterally, while the trapezius and serratus anterior stabilize the scapula, ensuring proper alignment. Failure of this synergy could result in shoulder impingement syndrome, where the rotator cuff tendons become compressed between the humerus and acromion process, leading to pain and limited mobility. Additionally, disruption of this coordination could lead to shoulder instability, compromising the joint`s integrity and increasing the risk of dislocation.

PART B: The action of opening a door involves a complex interplay of muscles in the right elbow, forearm, wrist, and hand. When turning a doorknob clockwise, the biceps brachii and brachioradialis muscles contract to flex the elbow, while the pronator teres and pronator quadratus rotate the forearm medially, facilitating knob rotation. Simultaneously, wrist flexors such as the flexor carpi radialis and palmaris longus, along with finger flexors like the flexor digitorum superficialis, grip the knob. In contrast, reversing the motion to turn the knob counterclockwise necessitates activation of the triceps brachii to extend the elbow and the supinator muscle to rotate the forearm laterally. The wrist extensors and finger extensors then engage to maintain grip while pushing the door open.

PART C: Muscles spanning both the hip and knee joints play crucial roles in joint mobility. Full knee extension limits maximal hip flexion actively by engaging the rectus femoris, which crosses both joints. Passively, the tension in the rectus femoris during knee extension restricts hip flexion. Conversely, maintaining excessive hip flexion impedes full knee extension due to passive tension in the hamstrings, which also cross both joints. This limitation is evident in activities like squatting, where excessive hip flexion restricts knee extension. Similarly, maximal hip extension actively engages the hip extensors like the gluteus maximus, limiting knee flexion. Passively, tension in the hip extensors during hip extension restricts knee flexion, as seen when attempting to sit back on one`s heels. Thus, maintaining balanced mobility between the hip and knee joints is essential for functional movement.