Antagonistic muscles

In the syllabus and the exam

Antagonistic muscles appear in the topic of support and movement, most often illustrated using the biceps and triceps acting on the elbow joint or, less commonly, muscles working across the knee. The key idea tested is that skeletal muscle can only pull, never push, on a bone; movement in two opposite directions therefore requires two muscles arranged as a pair, one on each side of a joint, so that one muscle's contraction is balanced by the other's relaxation. In Paper 2 structured questions, candidates are usually shown a diagram of a joint and asked to identify which muscle is contracting and which is relaxing for a stated movement, or to explain why a single muscle cannot both bend and straighten a joint on its own. A common mistake is stating that both muscles contract together, or describing the relaxed muscle as also pulling, when in fact a relaxed muscle simply lengthens under the pull of its partner. Candidates are also expected to link this pair to the tendons that attach each muscle to bone, since without a tendon anchoring both ends of the muscle, contraction would not transmit force to the skeleton at all. Confusing which muscle is called the flexor and which is the extensor for a given movement is another frequent source of lost marks. Examiners sometimes extend this idea to other joints, such as the muscles moving the lower leg at the knee, so candidates should be ready to apply the same reasoning about paired contraction and relaxation to a joint they have not seen labelled before, rather than memorising the biceps and triceps example alone. This principle is not limited to vertebrates: candidates should recognise that any movable joint controlled by muscle, whether in a human limb or another animal's limb, generally relies on at least one antagonistic pair rather than a single muscle acting alone.

Example

When labelling a diagram of the arm, a candidate writes: 'The biceps and triceps are antagonistic muscles: when the biceps contracts to bend the arm, the triceps relaxes, and when the triceps contracts to straighten the arm, the biceps relaxes.'

Related terms

TendonLigamentReflex action

Source:SRC-DSKP-EN