Muscles and Antagonistic Pairs
Why Muscles Work in Pairs
Muscles can only pull — they cannot push. Because of this, every joint in the body is controlled by at least two muscles that work in opposite directions. When one muscle contracts and shortens to produce movement, the opposing muscle relaxes and lengthens to allow that movement. This arrangement is called an antagonistic muscle pair. OCR also requires knowledge of a third muscle role — the fixator — which stabilises a nearby joint while movement occurs at another.
Major Muscles of the Body
At the shoulder, the deltoid forms the rounded cap of the shoulder. It is the prime mover for abduction of the arm (lifting it out to the side) and also contributes to flexion and extension. The trapezius is the large, diamond-shaped muscle that runs from the base of the skull down to the middle of the back. It elevates the shoulders, pulls the scapula back and assists head movement. The latissimus dorsi (‘lats’) is the broad muscle of the lower and mid-back, responsible for extension and adduction of the arm at the shoulder — powerful in swimming backstroke and rowing movements. The pectorals (chest muscles) lie across the front of the thorax and act as prime movers in horizontal adduction, such as in the press-up or front crawl pull phase.
At the elbow, the biceps sits on the front of the upper arm. During elbow flexion (the upward phase of a bicep curl), the biceps is the agonist (prime mover), contracting and shortening to pull the forearm up. On the back of the upper arm sits the triceps. During elbow flexion, the triceps acts as the antagonist, relaxing and lengthening to allow movement. The roles reverse completely during elbow extension (the upward phase of a press-up): the triceps becomes the agonist and the biceps becomes the antagonist. The deltoid acts as the fixator at the shoulder throughout the bicep curl, keeping the upper arm stable while the elbow moves.
The abdominals are the core muscles on the front of the trunk. They flex the spine, stabilise the pelvis during running and throwing, and are active in almost every sporting movement as a postural stabiliser and fixator.
At the hip and knee, two large muscle groups dominate. The quadriceps group (four muscles on the front of the thigh) is the prime mover for knee extension — straightening the knee. The hamstrings lie on the back of the thigh and are the prime mover for knee flexion (bending the knee). At the knee, the quadriceps and hamstrings form the classic antagonistic pair: quadriceps agonist during extension, hamstrings agonist during flexion. The gluteals are the large buttock muscles that drive hip extension — critical in sprinting and jumping.
At the ankle, the gastrocnemius runs from behind the knee down to the heel via the Achilles tendon. It is the prime mover for plantar flexion — pointing the toes down, as in sprinting push-off or jumping.
In the upward phase of a bicep curl: biceps = agonist (prime mover, contracts to flex the elbow), triceps = antagonist (relaxes and lengthens to allow the movement), deltoid = fixator (stabilises the shoulder joint so the upper arm stays still). In kicking (knee extension): quadriceps = agonist, hamstrings = antagonist.
The human body contains over 600 named muscles. Muscle fibres are packed with mitochondria — the organelles that produce ATP through aerobic respiration, supplying the energy needed for sustained muscular contraction during sport and exercise.
OCR explicitly requires three muscle roles — agonist, antagonist and fixator — while other boards only require two. Students who name only the agonist and antagonist will miss the fixator marks in OCR questions. A fixator is a muscle that holds a nearby body part still so that the prime mover can work efficiently. It does not produce the movement; it provides the stable base from which the movement occurs. In the bicep curl, the deltoid stabilises the shoulder joint throughout the exercise — without it, the shoulder would move and the elbow flexion would be inefficient. Always include the fixator and name the joint it stabilises.
A very common error in exams is to name a single muscle as ‘the agonist’ without specifying the joint, the direction of movement, or the antagonist and fixator. Marks go to the complete picture: agonist named (1), antagonist named (1), fixator named and its role described (1), action described at the correct joint (1). Always frame answers as: “During elbow flexion, the biceps acts as the agonist (prime mover) while the triceps acts as the antagonist at the elbow joint. The deltoid acts as the fixator at the shoulder, stabilising the upper arm throughout the movement.”