Explain how the skeletal and muscular systems interact through joints, lever systems and muscle contraction to produce movement

What this dot point is asking

WACE wants you to describe the working musculoskeletal system, not just label a diagram. You need to name major muscles and the joint actions they produce, classify muscle roles in a movement, identify the type of contraction, and analyse the lever class at a joint. Examiners reward answers that link a named muscle to a named joint action in a real sporting movement.

Joints and joint actions

Most movement happens at synovial joints, which have a joint capsule, synovial fluid and articular cartilage that reduce friction. Learn the joint types and their possible actions: the ball-and-socket shoulder and hip allow flexion, extension, abduction, adduction, rotation and circumduction; the hinge knee and elbow allow mainly flexion and extension; the pivot joint at the neck and forearm allows rotation. Use the correct movement terms: flexion decreases the joint angle, extension increases it, abduction moves a limb away from the midline, adduction toward it, and plantarflexion and dorsiflexion describe the ankle.

Muscle roles in a movement

For any movement, classify each muscle by its role. The agonist (prime mover) is the muscle that contracts to produce the movement. The antagonist is the opposing muscle that relaxes and lengthens to allow it. Synergists assist the agonist or stabilise the joint, and fixators hold a body part still so the agonist has a firm base to pull from. In an elbow flexion such as a biceps curl, the biceps brachii is the agonist and the triceps brachii is the antagonist; in elbow extension the roles reverse. This reciprocal arrangement is why muscles work in antagonistic pairs.

Types of muscle contraction

Name the contraction type by what the muscle length does. In a concentric contraction the muscle shortens while developing tension, for example the quadriceps during the upward drive of a jump. In an eccentric contraction the muscle lengthens under tension, controlling a load, for example the quadriceps absorbing impact on landing. In an isometric contraction the muscle develops tension without changing length, for example the core muscles holding a plank or a rugby player holding a scrum bind. Isotonic is the umbrella term for contractions that produce movement (concentric and eccentric), while isometric produces no joint movement.

Lever systems

A lever has a fulcrum (the joint), an effort (the muscle force), a resistance (the load or body weight) and two lever arms. The three classes differ by the order of these components.

First-class levers have the fulcrum between the effort and the resistance, like a see-saw. The nodding of the head at the neck is the classic example. Second-class levers have the resistance between the fulcrum and the effort; these favour force and are rare in the body, with rising onto the toes (plantarflexion at the ball of the foot) the main example. Third-class levers have the effort between the fulcrum and the resistance; these are the most common in the body, including elbow flexion, and they favour speed and range of motion at the cost of mechanical advantage.

How this maps to the exam

Functional anatomy questions are usually a stimulus photograph of an athlete with parts labelled. You will be asked to name the joint action, the agonist, the contraction type and often the lever class for that instant. Practise full movements in phases (preparation, execution, follow-through), because the agonist and contraction type change between phases.