The major muscle groups, how muscles work in antagonistic pairs, the roles of agonist, antagonist and fixator, and the types of muscle contraction used in sporting actions.

What this dot point is asking

AQA wants you to locate the major muscle groups, explain how muscles work in antagonistic pairs to create movement, name the agonist and antagonist in a given action, and describe the types of muscle contraction with sporting examples.

The major muscle groups

Antagonistic muscle pairs

The roles in any movement are: the agonist (the working muscle that contracts and shortens, also called the prime mover), the antagonist (the muscle that relaxes), and the fixator (a muscle that stabilises the origin so the agonist can work). Key pairs include biceps and triceps (elbow), quadriceps and hamstrings (knee), and hip flexors and gluteals (hip). In a biceps curl up, the biceps is the agonist and the triceps is the antagonist; lowering the weight reverses the roles.

Types of muscle contraction

• Isotonic contraction: the muscle changes length to create movement. This can be concentric (the muscle shortens, for example the biceps when lifting a dumbbell up) or eccentric (the muscle lengthens under tension, for example the biceps controlling the dumbbell back down).
• Isometric contraction: the muscle stays the same length while under tension and there is no movement, for example holding a plank or a gymnastics balance.

The distinction matters in sport because most movements use a sequence of these contractions. In a squat, the quadriceps work concentrically to stand up (shortening) and eccentrically to lower down under control (lengthening while still tensed to resist gravity). Eccentric contractions are the main cause of delayed onset muscle soreness, because controlling a load as the muscle lengthens creates more micro-damage to the fibres, which is one reason a downhill run leaves the legs sore. Isometric contractions are used wherever a position must be held still against a force, such as a rugby player holding a maul, a gymnast in a crucifix on the rings, or the core muscles stabilising the trunk during almost any lift.

How muscles, bones and joints work together

Muscles never act alone. A muscle attaches to bone by a tendon, and when the agonist contracts it pulls on the bone, which acts as a lever turning about a joint (the fulcrum). The fixator stabilises the origin so the pull is efficient: for example, during a biceps curl the muscles around the shoulder act as fixators to hold the upper arm steady so the biceps can move the forearm. This is why the muscular, skeletal and joint systems are taught together: movement is the product of a muscle pulling, a tendon transmitting the force, and a bone rotating at a joint.