AQA A Level Biology

Revision Notes

6.3.5 Slow & Fast Skeletal Muscles

Slow & Fast Skeletal Muscle Fibres

  • There are two types of muscle fibres found in muscles
    • Fast fibres
    • Slow fibres
  • Human muscles are made up of both types of muscle fibres
  • Some muscles have higher proportions of a particular fibre type due to their different properties

Fast muscle fibres

  • Fast muscle fibres contract rapidly
    • The myosin heads bind and unbind from the actin-binding sites five times faster than slow muscle fibres
    • Their rapid contraction-relaxation cycle means they need large amounts of calcium ions present to stimulate contraction
  • They rely on anaerobic respiration for ATP supply
  • They are suited to short bursts of high-intensity activity as they fatigue quickly due to the lactate produced from anaerobic respiration
  • These muscle fibres are often found in high proportions in the limbs of animals that flee a predator or hunt prey at high speeds
    • For example the wings of a robin and legs of a cheetah
  • There are high proportions of fast muscle fibres in human eyelids
    • They contract in short bursts and do not need to sustain the rapid movement
  • Fast muscle fibres have fewer capillaries
    • Blood containing glucose and oxygen flow through the capillaries
    • This means they have quite a slow supply of oxygen and glucose for aerobic respiration
  • Low amounts of myoglobin are present in fast muscle fibres
    • Myoglobin is a red pigment molecule that is similar to haemoglobin
    • Myoglobin functions as a store of oxygen in muscles and increases the rate of oxygen absorption from the capillaries
  • Due to this fast muscle fibres appear paler in colour than slow muscle fibres

Slow muscle fibres

  • Slow muscle fibres contract more slowly and are suited to sustained activities like walking and perching
  • They rely on aerobic respiration for ATP
  • They fatigue less quickly due to less lactate production, making them ideal for endurance
  • These muscle fibres are often found in high proportions in the limbs of animals that migrate or stalk prey over long distances
    • For example the wings of geese and legs of wolves
  • Human back muscles have a high proportion of slow muscle fibres
    • These muscles have to contract for long periods of time in order to keep the skeleton erect when standing or sitting
  • Slow muscle fibres have a denser network of capillaries
    • Blood containing glucose and oxygen flow through the capillaries
    • This means they have a short diffusion distance and a good supply of oxygen and glucose for aerobic respiration
  • High amounts of myoglobin, haemoglobin and mitochondria are present in slow muscle fibres
    • This increases the rate of oxygen supply, oxygen absorption and aerobic respiration
  • Due to the high amounts of red pigment, slow muscle fibres appear a dark red

Fast and Slow Muscle Fibres

Fast and Slow Muscle Fibres, downloadable AS & A Level Biology revision notes

The effect of training on muscle fibres

  • Most humans tend to have an equal amount of slow and fast fibres in their arm and leg muscles
  • However, some people (commonly trained athletes and sportspeople) tend to have a higher proportion of one muscle fibre type in these muscles
  • The higher proportion of the certain fibre type enhances their performance in their specific sport or event
  • Athletes that train for short-burst, high-intensity activities (sprinting, weightlifting etc) tend to have higher proportions of fast muscle fibres and lower proportions of slow muscle fibres in their arms and legs
  • Athletes that train for endurance activities (marathons, long-distance cycling etc) tend to have higher proportions of slow muscle fibres and lower proportions of fast muscle fibres in their arms and legs
  • Some individuals have muscles that are more suited to particular sports, but training can massively increase their success
  • Scientists have stated that training can have several effects on an individual’s muscles:
    • It can influence which fibre types develop
    • It can increase the number of capillaries and mitochondria present in muscles

Exam Tip

If an exam question asks how an individual’s muscles get bigger as a result of exercise, remember that it is the size of the fibres and not the number of fibres that increase! An increase in the length and number of contractile units within each fibre causes this increase in size.

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.
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