6.3.5 Slow & Fast Skeletal Muscles

Slow-Twitch Muscle Fibres:

• Specialised for endurance activities (e.g., marathon running).

• Contract slowly and are resistant to fatigue.

• Adapted for aerobic respiration, which is more efficient for prolonged energy supply. Key Adaptations:

• High myoglobin content: Myoglobin stores oxygen, giving these fibres a red colour.

• Rich blood supply: Ensures a continuous supply of oxygen and nutrients.

• Numerous mitochondria: Site of aerobic respiration to produce ATP.

• Low glycogen levels: Reliance on sustained oxygen supply rather than short-term reserves.

Fast-Twitch Muscle Fibres:

• Specialised for short bursts of intense activity (e.g., sprinting or weightlifting).

• Contract quickly and powerfully but fatigue rapidly.

• Adapted for anaerobic respiration, which provides quick energy but is less efficient. Key Adaptations:

• Thicker and more numerous myosin filaments: Allows powerful contractions.

• High glycogen levels: Provides an immediate energy source for anaerobic respiration.

• High concentration of anaerobic enzymes: Enables rapid ATP production in the absence of oxygen.

• Stores of phosphocreatine: Allows quick regeneration of ATP from ADP.

Comparison of Slow- and Fast-Twitch Fibres:

Feature	Slow-Twitch Fibres	Fast-Twitch Fibres
Contraction Speed	Slow	Fast
Resistance to Fatigue	High	Low
Energy Source	Aerobic respiration	Anaerobic respiration
Myoglobin Content	High	Low
Mitochondria	Many	Few
Colour	Red (due to myoglobin)	White