Muscles and movement — Edexcel International A Level Biology
How muscles, tendons, ligaments and bones produce movement, the structure of a muscle fibre, and the sliding filament theory.
Revision notes
4 short notes, in the order of the specification. Each one in short:
Movement happens when skeletal muscles contract and pull on bones, which act as levers at joints. Tendons attach muscle to bone and ligaments attach bone to bone. Muscles can only pull, so they work in antagonistic pairs, such as the flexor and extensor muscles: one contracts while the other relaxes.
A mammalian skeletal muscle fibre is a long, multinucleate cell with a cell surface membrane called the sarcolemma and cytoplasm called sarcoplasm. It is packed with myofibrils, each made of repeating sarcomeres containing thick myosin and thin actin filaments, and it has many mitochondria, sarcoplasmic reticulum and T-tubules.
Slow twitch muscle fibres contract slowly and resist fatigue because they respire aerobically, with many mitochondria, much myoglobin and a dense capillary supply. Fast twitch fibres contract quickly and powerfully but tire quickly, relying on anaerobic glycolysis with plentiful glycogen and fewer mitochondria.
In the sliding filament theory, an action potential releases calcium ions, which bind troponin and move tropomyosin off the binding sites on actin. Myosin heads attach, pull the actin filaments past the myosin using energy from ATP broken down by ATPase, and the sarcomere shortens as the filaments slide.
Exam questions
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Quick check questions
What is the difference between a tendon and a ligament?
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A tendon joins muscle to bone; a ligament joins bone to bone.
What is an antagonistic muscle pair?
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Two muscles that have opposite effects at a joint: when one contracts the other relaxes.
Which part of a sarcomere stays the same length when a muscle contracts?
Show answer
The A band.
What does calcium bind to in muscle contraction?
Show answer
Troponin.
Why do fast twitch fibres tire quickly?
Show answer
They use anaerobic respiration, so lactate builds up.
Frequently asked questions
How do antagonistic muscles work?
Antagonistic muscles are pairs that have opposite effects at a joint, such as the biceps (flexor) and triceps (extensor). Muscles can only pull. When one contracts and bends or straightens the joint, the other relaxes and is stretched, and then they swap roles to move the joint back.
What is the sliding filament theory?
The sliding filament theory explains muscle contraction. Calcium ions move tropomyosin away from the actin binding sites, so myosin heads bind and pull the actin filaments inwards using energy from ATP. The filaments slide past each other, the sarcomere shortens and the muscle contracts, although the filaments themselves stay the same length.
What are the differences between fast and slow twitch muscle fibres?
Slow twitch fibres contract slowly, resist fatigue, respire aerobically and have many mitochondria, much myoglobin and many capillaries. Fast twitch fibres contract quickly with more force, tire quickly, rely on anaerobic glycolysis and have more glycogen but fewer mitochondria, so they suit sprinting.
What is the role of ATP in muscle contraction?
ATP is used in two ways. It binds to the myosin head to make it detach from actin, and it is hydrolysed by myosin ATPase to provide the energy that returns the head to its original position. ATP is also needed to pump calcium ions back into the sarcoplasmic reticulum.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026