• 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