6.3.5 Slow & Fast Skeletal Muscles

• 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

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