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. Understanding these pairs — and always naming them in the context of a specific joint and a specific direction of movement — is essential for exam answers.
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 rotator cuffs are a group of four muscles that wrap around the shoulder joint, stabilising the ball-and-socket joint and assisting rotation — critical in throwing sports such as cricket bowling and javelin. The pectorals (chest muscles) lie across the front of the thorax and act as prime movers in horizontal adduction, such as in the push-up or front crawl pull phase. The latissimus dorsi (‘lats’) is the large fan-shaped muscle of the back, responsible for extension and adduction of the arm at the shoulder, powerful in swimming and rowing movements.
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 concentrically to shorten and 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.
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. The hip flexors are the antagonists to the gluteals, acting as agonists during hip flexion (lifting the knee forwards in running).
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. The tibialis anterior on the front of the lower leg is the prime mover for dorsiflexion (pulling the toes upward), and the two muscles form an antagonistic pair at the ankle.
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.
In a bicep curl upward phase: biceps = agonist (concentric contraction), triceps = antagonist. In kicking (knee extension): quadriceps = agonist, hamstrings = antagonist. In a sprinting push-off (plantar flexion): gastrocnemius = agonist, tibialis anterior = antagonist.
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. A full answer names the agonist, names the antagonist and describes the action at the correct joint. Writing ‘the biceps is the agonist’ alone will not earn full marks. 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.”
Types of Muscle Contraction
Muscles do not always contract in the same way. The spec identifies three types of contraction: isotonic concentric, isotonic eccentric and isometric.
Isotonic concentric contraction occurs when a muscle shortens while it contracts, producing movement. The biceps during the upward phase of a bicep curl contracts concentrically — it shortens, and the elbow flexes. This is the most obvious form of contraction in sport: the quadriceps contracts concentrically to extend the knee when kicking a ball.
Isotonic eccentric contraction occurs when a muscle produces tension while it lengthens. This happens when the muscle is acting as a brake. During the downward phase of a bicep curl, the biceps contracts eccentrically — it is still producing tension to control the lowering of the weight, but it is getting longer as the elbow extends. Eccentric contractions are responsible for DOMS (delayed onset muscle soreness) because the lengthening under load causes tiny tears in muscle fibres.
Isometric contraction occurs when a muscle is active but the joint does not move. A gymnast holding a crucifix position on the rings keeps the arms outstretched perfectly still — the deltoids and pectorals are working hard isometrically. A rugby player locked in a scrum uses isometric contractions to maintain position against an opposing force.
Concentric = muscle shortens (produces movement). Eccentric = muscle lengthens under tension (controls or brakes movement). Isometric = muscle produces tension but no joint movement occurs. All three can happen in the same exercise: a squat involves concentric quadriceps on the way up, eccentric on the way down, and isometric if you pause at the bottom.
Tendons are strong, inelastic connective tissues that attach muscles to bones. Without tendons, a muscle contraction would not move the skeleton. The Achilles tendon, for example, attaches the gastrocnemius and soleus muscles to the calcaneus (heel bone). When the calf muscles contract concentrically, the Achilles tendon transmits the force to the heel, producing plantar flexion for sprinting. Tendons are denser and less elastic than muscles, which is why they take longer to strengthen with training. Overloading tendons without adequate rest is a common cause of injury in growing athletes.