Glossary: Support and Movement in Humans and Animals

This chapter's terms in context

Chapter 14 works with four terms describing how animals are supported and how their body parts move: exoskeleton, tendon, ligament and antagonistic muscles. An exoskeleton is a hard external skeleton found in animals such as insects and crustaceans, made mainly of chitin in insects, providing support, protection for internal organs, and attachment points for muscles from the inside; because an exoskeleton cannot grow once it has hardened, an animal with an exoskeleton must periodically shed it and grow a new, larger one in a process called moulting, a limitation that vertebrates with an internal skeleton do not share. Comparing an exoskeleton with the internal (endo-)skeleton of vertebrates, in terms of growth, weight, and protection of internal organs, is a frequently tested comparison in this chapter.

A tendon is a tough, non-elastic band of connective tissue that attaches a muscle to a bone, transmitting the force of a muscle's contraction directly to the bone so that the bone (and the body part attached to it) moves. A ligament, despite sounding similar, has a completely different job: it is a band of connective tissue that attaches one bone to another bone at a joint, holding the joint together and limiting how far it can move to prevent dislocation or injury. The single most common exam error in this chapter is confusing tendon and ligament, since both are described as 'connective tissue bands', so the safest way to keep them apart is to remember what each one connects: a tendon always connects muscle to bone, while a ligament always connects bone to bone.

Antagonistic muscles are pairs of muscles that work in opposition to each other across a joint, because a muscle can only actively pull by contracting, it cannot actively push, so movement in two opposite directions at the same joint requires two separate muscles working as a pair. The biceps and triceps at the elbow joint are the standard SPM example: when the biceps contracts, it pulls the forearm up (flexion) while the triceps relaxes, and when the triceps contracts, it pulls the forearm down again (extension) while the biceps relaxes. A frequent error is describing both muscles as contracting together to produce a single movement, when in fact only one of the pair contracts at a time while the other relaxes; if both contracted simultaneously with equal force, the joint would not move at all.

Together, these four terms describe a single working system: the exoskeleton or endoskeleton provides a rigid framework, tendons transmit the pulling force of antagonistic muscle pairs onto that framework, and ligaments hold the framework's joints together so the pulling force produces controlled movement rather than dislocation, a system-level view examiners reward when a question asks for an explanation of movement at a named joint rather than the definition of a single term in isolation.

Paper 2 questions on this chapter often show an unfamiliar joint diagram, such as the knee or the ankle, rather than only the elbow, and ask a candidate to identify the tendon, the ligament and the antagonistic muscle pair from the labels or description given, testing whether the underlying logic has been learned rather than one memorised example. A frequent scoring gap is correctly naming a structure as a tendon or a ligament from a diagram but failing to state its specific function in that particular joint, rather than repeating a generic definition. Practising the same reasoning, what does this structure connect, and what would happen at the joint if it were damaged, across two or three different joints, not only the elbow, prepares a candidate for the unfamiliar-diagram version of this question that examiners frequently set.