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Titin, antagonistic muscles and the intercostal muscles

Organisms (Form and function) · Muscle and motility · note 3 of 6

Spec B3.3.3, B3.3.8
HL only (what this means)HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels mean
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Titin, antagonistic muscles and the intercostal musclesSpec B3.3.3, B3.3.8

In short

Titin is an immense protein in the sarcomere that acts like a spring: it helps sarcomeres recoil after stretching and prevents overstretching. Antagonistic muscles are needed because muscle tissue can only exert force when it contracts. The internal and external intercostal muscles are an antagonistic pair that move the ribcage in opposite directions.

Titin

  • Titin is the largest known protein. Each molecule runs from the Z line to the centre of the sarcomere, alongside the myosin filament.
  • When a sarcomere is stretched, titin is stretched too and stores potential energy, like a spring.
  • When the stretching force stops, titin recoils, helping the sarcomere return to its resting length.
  • Titin also prevents overstretching, which could pull the actin and myosin filaments apart.

Antagonistic muscles

Muscle tissue can only exert force when it contracts: it pulls but cannot push. A relaxed muscle is lengthened again by a second muscle that pulls the other way. Muscles that produce opposite movements are antagonistic, for example the biceps (flexes the elbow) and triceps (extends it).

Internal and external intercostal muscles

The intercostal muscles lie in two layers between the ribs. Their muscle fibres run in different orientations, at roughly right angles to each other, so they move the ribcage in opposite directions.

Intercostal muscles as an antagonistic pair
MuscleWhen it contractsEffect on the other layer
External intercostal musclesPull the ribcage up and out (inspiration)Stretch the internal intercostals, storing potential energy in titin
Internal intercostal musclesPull the ribcage down and in (forced expiration)Stretch the external intercostals, storing potential energy in titin

This is an example of antagonistic muscle action that moves part of the body internally rather than moving the body from place to place.

Exam tip:

Link to B3.1.5: in ventilation answers, name the external intercostals for inspiration and the internal intercostals (with abdominal muscles) for forced expiration.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

How does a sarcomere contract?

A sarcomere contracts when myosin heads bind to actin filaments, swivel and pull the actin towards the centre of the sarcomere, using energy from ATP. Repeated cycles of binding, pulling and detaching make the filaments slide past each other, so the Z lines move closer together and the sarcomere shortens.

Why do muscles work in antagonistic pairs?

Muscles work in antagonistic pairs because muscle tissue can only exert force when it contracts: it can pull but not push. One muscle moves a body part one way and the other moves it back, stretching the first. For example, the external and internal intercostal muscles move the ribcage in opposite directions.

What is the role of titin in muscle?

Titin is an immense, spring-like protein in each sarcomere. When the sarcomere is stretched, titin stores potential energy and then recoils, helping the sarcomere return to its resting length. It also prevents overstretching, which would pull the actin and myosin filaments apart.

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