AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
The structure, location and general properties of slow and fast skeletal…
Full specification wording
Skeletal muscles are stimulated to contract by nerves and act as: The structure, location and general properties of slow and fast skeletal muscle fibres.
Slow-twitch fibres are built for endurance.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 6 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Slow-twitch fibres are built for endurance. They contract slowly and with less force but resist fatigue, because they respire aerobically: they contain many mitochondria, a dense capillary supply and a lot of myoglobin, which stores oxygen and makes them look red, and they store fat as a respiratory substrate. They are found in postural muscles used continuously, such as the calf muscle. Fast-twitch fibres are built for short bursts of powerful movement, as in the biceps or a sprinter's leg muscles. They contract rapidly and forcefully but fatigue quickly, because they rely on anaerobic glycolysis: they have few mitochondria and little myoglobin, so appear whiter, but they have thicker and more numerous myofibrils, a large store of glycogen and phosphocreatine, and high concentrations of glycolytic enzymes. The lactate they produce lowers pH and causes fatigue.
What earns the marks
- Fast fibres being used during short-term or intense exercise, such as a sprint.
- Slow fibres being used during longer-term or lower-intensity exercise, such as a marathon.
- Slow fibres respiring aerobically, with many mitochondria, many capillaries and much myoglobin.
- Fast fibres respiring anaerobically, with stores of glycogen and phosphocreatine and few mitochondria.
- Linking lactate production in fast fibres to their rapid fatigue.
Common mistakes and how to put them right
Saying slow fibres never fatigue rather than that they fatigue slowly.
Claiming fast fibres contain more mitochondria because they release energy quickly.
Describing myoglobin as transporting oxygen in the blood, when it stores oxygen in the muscle.
Giving a location such as in the legs without naming the activity that justifies the fibre type.
Treating a whole muscle as being made of only one fibre type.
Exam tips
- Pair every structural feature with the property it explains: many mitochondria, aerobic respiration, resistance to fatigue.
- Fast twitch and slow twitch are both accepted, so use whichever you prefer, but stay consistent.
- Where data on an athlete are given, quote the percentages when you justify which fibre type dominates.
Try the lesson
The start of the Skeletal muscles are stimulated to contract by nerves and act as: The structure, location and general properties of slow and fast skeletal muscle fibres. lesson, made from checked teaching notes for this specification point.
Have a go first
Which type of skeletal muscle fibre is adapted for slow, sustained contraction and has a rich blood supply?
Listen
Slow-twitch fibres are built for endurance. They contract slowly and with less force but resist fatigue, because they respire aerobically: they contain many mitochondria, a dense capillary supply and a lot of myoglobin, which stores oxygen and makes them look red, and they store fat as a respiratory substrate. They are found in postural muscles used continuously, such as the calf muscle. Fast-twitch fibres are built for short bursts of powerful movement, as in the biceps or a sprinter's leg muscles. They contract rapidly and forcefully but fatigue quickly, because they rely on anaerobic glycolysis: they have few mitochondria and little myoglobin, so appear whiter, but they have thicker and more numerous myofibrils, a large store of glycogen and phosphocreatine, and high concentrations of glycolytic enzymes. The lactate they produce lowers pH and causes fatigue. A strong answer earns marks for these points. fast fibres being used during short-term or intense exercise, such as a sprint. slow fibres being used during longer-term or lower-intensity exercise, such as a marathon. slow fibres respiring aerobically, with many mitochondria, many capillaries and much myoglobin. fast fibres respiring anaerobically, with stores of glycogen and phosphocreatine and few mitochondria. Exam tip: Pair every structural feature with the property it explains: many mitochondria, aerobic respiration, resistance to fatigue.
Key words
Say what each word means before you turn the card.
slow-twitch aerobic fibres
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fast-twitch anaerobic fibres
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myoglobin and mitochondria
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resistance to fatigue
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See every definition
- slow-twitch aerobic fibres
- Fibres that contract slowly and with less force but resist fatigue, because they respire aerobically with many mitochondria, a dense capillary supply and a lot of myoglobin.
- fast-twitch anaerobic fibres
- Fibres that contract rapidly and forcefully but fatigue quickly, because they rely on anaerobic glycolysis and have few mitochondria and little myoglobin.
- myoglobin and mitochondria
- Myoglobin stores oxygen and makes slow fibres look red, while many mitochondria support aerobic respiration and resistance to fatigue.
- resistance to fatigue
- Slow fibres resist fatigue because they respire aerobically, whereas fast fibres fatigue quickly as the lactate they produce lowers pH.
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