Which major muscles produce the main joint actions in sport, and how do agonist, antagonist and other roles combine to control movement?
Identify the major muscles and the joint actions they produce, and explain the roles of agonist, antagonist, synergist and fixator
A focused answer to the WACE Year 12 Physical Education Studies Unit 3 content on major muscles and joint actions. The main muscles of the upper and lower body, the joint actions they produce such as flexion, extension and rotation, and the roles of agonist, antagonist, synergist and fixator in a coordinated movement.
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What this dot point is asking
WACE expects you to take a named sporting movement, identify the joint action, name the agonist producing it, and assign the antagonist, synergist and fixator roles. The role a muscle plays changes with the movement, so it must be worked out each time.
Major joint actions
Movements are described by their action at a joint. Flexion decreases the angle at a joint and extension increases it. Abduction moves a limb away from the midline and adduction moves it back toward the midline. Rotation turns a bone about its axis, and at the shoulder and hip you also see circumduction. The forearm rotates through pronation and supination, and the ankle uses plantar flexion and dorsiflexion. Naming the correct action is the first step in any analysis.
Major muscles and what they do
In the upper body, the biceps brachii flexes the elbow and the triceps brachii extends it. The deltoid abducts the shoulder, the pectoralis major draws the arm across the body, and the latissimus dorsi extends and adducts the shoulder. The trapezius moves and stabilises the shoulder girdle.
In the trunk, the rectus abdominis flexes the spine and the erector spinae extends it. In the lower body, the gluteus maximus extends the hip, the quadriceps group extends the knee, and the hamstrings flex the knee and extend the hip. The gastrocnemius and soleus produce plantar flexion at the ankle, and the tibialis anterior dorsiflexes it.
The roles within a movement
For any single movement, each involved muscle takes a role.
The agonist, or prime mover, is the muscle that contracts to produce the desired joint action. In knee extension during a kick, the quadriceps is the agonist.
The antagonist is the muscle on the opposite side that relaxes and lengthens to allow the movement. In that same kick the hamstrings are the antagonist.
The synergist assists the agonist, often by helping the action or preventing unwanted movement. The fixator (or stabiliser) contracts to hold a bone or body part steady so the agonist has a firm base to pull against. In a biceps curl the muscles of the shoulder act as fixators to keep the upper arm still.
Working through an example
Consider the upward phase of a squat. The joint action at the knee is extension and at the hip is extension. The agonists are the quadriceps and gluteus maximus, the antagonists are the hamstrings, and the trunk muscles act as fixators to keep the spine stable. This four part breakdown is exactly what an exam analysis rewards.
How this maps to the exam
Expect a movement or image and a command to identify muscles and roles. State the joint action, name the agonist that causes it, give the antagonist that relaxes, and add synergist or fixator roles where asked. Accurate muscle names plus correct role allocation is the marking pattern.