PART A: The scapulohumeral movement, or synergy, involves coordinated actions of the shoulder girdle and joint to facilitate upper-extremity motion. This complex interplay is essential for various arm movements, including flexion, extension, abduction, adduction, and rotation. Several muscles contribute to these motions, working in harmony to stabilize and mobilize the shoulder complex.

During shoulder flexion, for instance, the deltoid muscle plays a significant role in raising the arm forward, while the serratus anterior stabilizes the scapula against the thoracic wall, facilitating upward rotation. In abduction, the supraspinatus initiates the movement by elevating the arm away from the body, with assistance from the deltoid and rotator cuff muscles. Conversely, adduction involves the pectoralis major, latissimus dorsi, and teres major pulling the arm toward the midline, while the scapular stabilizers maintain proper positioning of the scapula.

Failure of this synergy can lead to dysfunctional movement patterns and increased risk of injury. For instance, if the scapulohumeral rhythm is disrupted, such as decreased upward rotation of the scapula during arm elevation, individuals may experience impingement syndrome or rotator cuff injuries due to altered biomechanics. Additionally, inadequate activation of stabilizing muscles like the rotator cuff or serratus anterior can result in shoulder instability and decreased joint integrity.

PART B: Turning a doorknob clockwise and pushing open a door engages a sequence of motions involving the right elbow, forearm, wrist, and hand muscles. As the hand grasps the knob, the biceps brachii and brachialis contract to flex the elbow joint, bringing the forearm closer to the upper arm. Concurrently, the pronator teres and pronator quadratus muscles rotate the radius over the ulna, allowing the hand to turn the knob clockwise. Simultaneously, wrist flexors, such as the flexor carpi radialis and flexor carpi ulnaris, contribute to wrist flexion, aiding in the twisting motion.

In contrast, to turn the knob counter-clockwise and pull open the door, the antagonist muscles, including the triceps brachii and wrist extensors, oppose the flexor muscles` action. The triceps extend the elbow joint, while the extensor carpi radialis brevis and longus, along with other extensor muscles, facilitate wrist extension. This coordinated effort allows for the reverse motion required to open the door in the opposite direction.

PART C: Maintaining full knee extension restricts the ability to maximally flex the hip due to the principle of passive insufficiency. Muscles like the hamstrings, which cross both the hip and knee joints, become taut when the knee is fully extended, limiting hip flexion. Similarly, excessive hip flexion limits full knee extension by placing the hamstrings in a shortened position, impeding their ability to extend the knee actively or passively.

Conversely, full knee flexion restricts maximal hip extension because muscles like the rectus femoris and other quadriceps muscles, which also span both joints, reach their maximum length when the knee is fully flexed. This limits their ability to actively extend the hip joint. Excessive hip extension, on the other hand, can limit full knee flexion by placing tension on the quadriceps, hindering their ability to flex the knee fully. Thus, maintaining a balance between hip and knee joint positions is crucial for optimizing lower-extremity mobility and function.