Muscles, joints and muscle fibre types

Muscles and the skeleton

• Tendons join muscle to bone; they are strong and inelastic, so the pull of the muscle is transmitted to the bone.
• Ligaments join bone to bone at joints; they are slightly elastic and hold the joint together.
• Muscles can only pull (contract); they cannot push. So they work in antagonistic pairs: when one contracts the other relaxes and is stretched.
• A flexor bends a joint (biceps at the elbow); an extensor straightens it (triceps).

At a synovial joint, cartilage covers the bone ends and synovial fluid lubricates the joint.

Structure of skeletal muscle

A muscle is made of bundles of muscle fibres. Each fibre is a long, multinucleate cell surrounded by the sarcolemma (cell membrane), with T-tubules that carry impulses inside, a specialised endoplasmic reticulum called the sarcoplasmic reticulum (stores Ca2+), many mitochondria, and many myofibrils.

Myofibrils are divided into repeating sarcomeres, bounded by Z lines:

• thin filaments of actin attached to the Z lines
• thick filaments of myosin in the centre, attached at the M line
• the A band (dark) is the length of the myosin; the I band (light) has only actin; the H zone has only myosin

Slow and fast twitch fibres

Skeletal muscles contain a mixture of two main fibre types. The proportion varies between muscles and between people, and training can change it.

Worked example

A myofibril 20 µm long contains sarcomeres each 2.5 µm long. How many sarcomeres are there along it?

1. Number = 20 / 2.5
2. = 8

Answer: 8 sarcomeres.

Check yourself

1. A myofibril 45 µm long is made of sarcomeres 2.25 µm long. How many sarcomeres does it contain?
   Show answer

   20. 45 / 2.25 = 20.

2. What does a tendon connect?
   • Bone to bone
   • Muscle to bone
   • Muscle to muscle
   • Nerve to muscle
   Show answer

   Muscle to bone. Tendons transmit the pull of muscle to bone.

3. Why do skeletal muscles work in antagonistic pairs?
   • Muscles push and pull equally
   • One muscle supplies ATP to the other
   • So the pair always contract at the same moment
   • Muscles can only pull, so a second muscle is needed to move the bone back
   Show answer

   Muscles can only pull, so a second muscle is needed to move the bone back. Contraction only shortens a muscle.

4. Which muscle is an extensor of the elbow?
   • Triceps
   • Biceps
   • Diaphragm
   • Intercostal muscle
   Show answer

   Triceps. The triceps straightens the arm.

5. What is the function of the sarcoplasmic reticulum?
   • To produce ATP
   • To make actin
   • To store and release calcium ions
   • To carry impulses from the brain
   Show answer

   To store and release calcium ions. Ca2+ release triggers contraction.

6. Which feature is typical of slow twitch fibres?
   • Many mitochondria and lots of myoglobin
   • Few capillaries
   • Rapid fatigue
   • Mainly anaerobic respiration
   Show answer

   Many mitochondria and lots of myoglobin. They are built for sustained aerobic activity.

7. Which part of the sarcomere contains only thin actin filaments?
   • The A band
   • The H zone
   • The I band
   • The M line
   Show answer

   The I band. The I band is the light region with actin only.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.