FAST-CONTRACTING OR SLOW- CONTRACTING FIBRES
Each muscle in our body is composed of a combination of fast and slow muscle fibres, with other fibres falling between these two extremes. These muscle fibres exist on a continuum ranging from slow to fast.
They can be categorised into two main types: slow-twitch (Type I) and fasttwitch (Type II) and vary in properties from each other as follows: 1. Contraction Speed: Slow-twitch (Type I) fibres contract slowly, suited for endurance activities, while fasttwitch (Type II) fibres contract rapidly, ideal for quick, powerful movements 2. Metabolism: Slow-twitch fibres primarily use aerobic respiration, making them fatigueresistant, while fast-twitch fibres rely on anaerobic respiration, leading to quicker fatigue 3. Myosin ATPase Activity: Fast-twitch fibres, specifically Type IIx, have high myosin ATPase activity, which allows them to generate more force. However, the heavy reliance on anaerobic metabolism also makes them highly fatigable. 4. Force Production: Fasttwitch fibres provide bigger and more powerful forces for shorter durations, while slow-twitch fibres produce lower power contractions over longer periods 5.Fatigue Resistance: Slow-twitch muscle fibres have a higher resistance to fatigue as they primarily rely on aerobic respiration, while fast-twitch fibres fatigue more quickly, especially those primarily using anaerobic glycolysis 6. Capillary Supply and Myoglobin Content: Slow-twitch fibres have a rich capillary supply, numerous mitochondria, and a high concentration of myoglobin, making them red in colour, while fast-twitch fibres generally contain fewer blood vessels, mitochondria, and myoglobin, resulting in a paler colour. 7. Twitch Speed: Slow-twitch fibres possess slower twitch speeds, while fast-twitch fibres have higher twitch speeds. 8. Endurance vs Power: Slow-twitch fibres are designed for endurance activities that require long-term, repeated contractions, like maintaining posture or running a long distance. Fast-twitch fibres are ideal for quick contractions of short duration 9. Fibre Type Transitions: Muscle fibres can transition from slow to fast and vice versa with exercise training, impacting their characteristics and utility for specific activities 10. Colour: Slow-twitch fibres are often referred to as red fibres due to their high myoglobin content, while fast-twitch fibres are paler in colour due to lower myoglobin content.7.7
Source:
Rana Tanmoy (ed.). Principles of Veterinary Animal Physiology. CRC Press,2026. — 290 p.. 2026
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