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7.1.1Topic 7 · Run for your Life

Movement of the Skeleton — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 9 parts Lesson 3 of 25 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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7.1.1 Movement of the Skeleton — Edexcel A Level Biology A (SNAB) 9BI0 notes: Explain how antagonistic muscle pairs, tendons, ligaments and joints work together to move the skeleton.Movement at a joint needs three kinds of tissue working together.

  • Skeletal muscle
  • antagonistic pairs
  • hinge joint
  • ball-and-socket joints
  • Exam-style question 1
  • Exam-style question 2

Part 1 of 7Skeletal muscle, bones and joints

Movement at a joint needs three kinds of tissue working together. Skeletal muscle generates force, bones act as rigid levers and joints allow the levers to pivot. Skeletal muscle is also called striated or voluntary muscle because it shows a banded appearance under the microscope and is under conscious control through the somatic nervous system.

Muscles are attached to bones by tendons: tough, inelastic cords of collagen that transmit the force of contraction to the bone without stretching.

Ligaments
Bones are linked to other bones at a joint by ligaments: strong, slightly elastic bands of collagen and elastic fibres that hold the joint together and limit how far it can move.
Cartilage covers the ends of the bones and, with synovial fluid, reduces friction.

Part 2 of 7Antagonistic muscle pairs

A muscle can only pull by contracting; it cannot push. Muscles therefore work in antagonistic pairs. When one muscle (the agonist or prime mover) contracts, its partner (the antagonist) relaxes, and vice versa. Each pair is arranged so that the two muscles pull the bone in opposite directions.

The standard example is the elbow joint:

Flexion
(bending the arm): the biceps contracts and the triceps relaxes.
  • The biceps is attached to the radius, so the forearm is pulled up towards the shoulder.
Extension
(straightening the arm): the triceps contracts and the biceps relaxes.
  • The triceps is attached to the ulna, so the forearm is pulled down and out.

Other pairs include the quadriceps and hamstrings at the knee, and the tibialis anterior and gastrocnemius at the ankle. A contracting muscle shortens and thickens; a relaxing muscle is simply stretched back out by its partner, which is why a single muscle cannot do both jobs.

Part 3 of 7The elbow as a lever

The elbow is a hinge joint: movement is possible in one plane only.

Fulcrum
The humerus is held still while the forearm moves around the joint, which acts as the fulcrum.
The biceps tendon attaches very close to the fulcrum, so the muscle only has to shorten a small distance to move the hand through a large distance.
This gives speed and range of movement but means the muscle must exert a force larger than the weight being lifted.

Synovial joints are enclosed in a capsule that secretes synovial fluid. The fluid lubricates and nourishes the cartilage. Hinge joints contrast with ball-and-socket joints (hip and shoulder), which allow movement in all planes, and with fixed joints (skull sutures), which allow none.

Part 4 of 7Comparing the connective tissues

FeatureTendonLigament
ConnectsMuscle to boneBone to bone
ElasticityVery little, so force is transmitted efficientlySome elasticity, allowing limited movement
Main roleTransmits force to move the skeletonStabilises the joint and prevents dislocation
Main componentParallel collagen fibresCollagen with elastic fibres

Part 5 of 7Linking to wider biology

Each skeletal muscle contains many muscle fibres whose contraction depends on the sliding of actin and myosin filaments (see the next lessons).

Linking to wider biology
The nervous system coordinates antagonistic pairs: motor neurones stimulate the agonist while inhibitory interneurones in the spinal cord reduce stimulation of the antagonist.
The ATP that powers contraction comes from respiration,
  • so sustained movement depends on the cardiovascular and respiratory systems supplying oxygen and glucose.

Part 6 of 7Injury and disorders of the skeletal system

Because tendons and ligaments are made mainly of collagen with a poor blood supply, they heal slowly when damaged.

Injury and disorders of the skeletal system
Sprain
A sprain is a stretched or torn ligament; a strain is damage to a muscle or tendon.
A dislocation occurs when the ends of the bones are forced out of their normal position, often tearing ligaments.
Arthritis involves breakdown of the cartilage and inflammation of the synovial membrane,
  • so the bones rub together and the joint becomes stiff and painful.

In an exam, link structure to function: the tendon's collagen fibres are packed in parallel and cross-linked, which gives great tensile strength; the synovial fluid reduces friction between cartilage surfaces; and the joint capsule keeps the fluid in place. Remember that the amount of movement at a joint depends on the shape of the bone ends and on how tightly the ligaments hold them.

Part 7 of 7Exam-style questions

  1. Explain how the biceps and triceps allow the forearm to be raised and then lowered. [4]4 marks

  2. State two differences between a tendon and a ligament. [2]2 marks

  3. A student says that the triceps 'pushes' the arm straight. Evaluate this statement. [2]2 marks

Key terms

TermMeaning
Antagonistic pairTwo muscles that pull a bone in opposite directions; one contracts while the other relaxes
TendonInelastic collagen cord attaching muscle to bone
LigamentSlightly elastic band joining bone to bone at a joint
FlexionBending a joint, decreasing the angle between bones
ExtensionStraightening a joint, increasing the angle between bones
Hinge jointJoint allowing movement in one plane, such as the elbow
Synovial fluidLubricating fluid in a joint capsule that reduces friction
AgonistThe muscle that contracts to produce a movement

Avoid these mistakes

1
Saying muscles push bones
  • muscles only pull when they contract;
    • the antagonist pulls the opposite way.
2
Swapping tendons and ligamentstendons are muscle-to-bone, ligaments are bone-to-bone.
3
Describing the elbow as a ball-and-socket joint: it is a hinge joint.
4
Thinking the relaxed muscle of a pair is doing work: it is passive and is stretched by its contracting partner.

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