Movement and Locomotion

Why muscles work in pairs

A muscle can only produce force in one direction: it pulls when it contracts, but it cannot actively push to return a bone to its original position. To move a joint back and forth, two muscles with opposite actions must work together across that joint, this arrangement is called an antagonistic pair.

When one muscle of the pair contracts and shortens, the other must relax and lengthen so the bone can move freely.

The biceps and triceps: a worked example

1. The biceps and triceps are an antagonistic pair of muscles attached by tendons to the bones of the upper and lower arm, on either side of the elbow joint.
2. To bend the arm (flexion), the biceps contracts and shortens, pulling the forearm upward, while the triceps relaxes and lengthens to allow this movement.
3. To straighten the arm (extension), the triceps contracts and shortens, pulling the forearm downward, while the biceps relaxes and lengthens.
4. Because one muscle in the pair always undoes the movement produced by the other, the elbow joint can be moved in both directions.

Movement vs locomotion

Movement is any change in the position of part of an organism's body, such as bending an arm, blinking an eye, or a leaf turning toward light, it does not necessarily move the whole organism from place to place. Locomotion is a specific type of movement in which the entire organism moves from one location to another, such as walking, swimming or flying.

Locomotion in animals depends on the same basic mechanism as any other movement, a skeleton providing support and antagonistic muscles pulling on it, but is coordinated across many joints and often uses specialised structures such as legs, fins or wings.

How it is examined

Exam questions on this standard often ask you to explain, using the correct terms 'contract' and 'relax', how the biceps and triceps produce a described arm movement, or to state whether a described example (such as a heartbeat or a bird flying) is movement, locomotion, or both. You should be able to justify your answer by referring to whether the whole organism changes location.

Locomotion adapted to different environments

The same underlying mechanism, a skeleton and antagonistic muscles, is adapted differently depending on the medium an animal moves through. A fish has a streamlined body and swings its body and tail fin from side to side using blocks of muscle on alternating sides of its body, reducing water resistance as it swims.

A bird has hollow, lightweight bones and powerful pectoral (chest) muscles that pull its wings downward for flight, with a second set of muscles raising the wings back up for the next stroke. A land animal such as a human or a dog relies on limbs with joints acted on by antagonistic muscle pairs, using the ground for support and grip as it walks or runs.

In each case, locomotion depends on coordinating many joints and muscle pairs together, rather than on a single antagonistic pair as in a simple arm movement.

Common misconceptions

Worked exam-style question

Question. Diagram shows a human arm at the elbow joint, with the biceps and triceps muscles labelled, both attached to the forearm and upper arm bones by tendons. The arm is shown moving from a straight position to a bent position.

(a) Name the type of joint at the elbow. (b) State what happens to the biceps and the triceps as the arm bends.

(c) Explain why both muscles are needed to produce this movement, rather than just one.

Model answer. (a) The elbow is a hinge joint. (b) As the arm bends, the biceps contracts and shortens, pulling the forearm upward, while the triceps relaxes and lengthens.

(c) A muscle can only pull, not push, so a single muscle could bend the arm but could never straighten it again; the biceps and triceps form an antagonistic pair, with one muscle able to reverse the movement produced by the other.

Practice question

A grasshopper hops away when disturbed, moving its whole body from one spot to another using its powerful hind legs. State whether this action is best described as movement, locomotion, or both, and explain your answer.

Exam tip

Key terms

• Antagonistic muscles, a pair of muscles that produce opposite movements at a joint.
• Tendon, a tough band connecting muscle to bone, allowing contraction to pull the bone.
• Exoskeleton, a hard external skeleton, found in animals such as insects, that muscles pull against internally.
• Adaptation, a feature that suits an organism to its way of moving, such as a fish's fins or a bird's wings.

Source:SRC-DSKP-EN

Frequently asked questions

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