Understanding Muscle Twitch Fibres: Types and Functions

Exercise Benefits, Exercises 9 November 2023

Muscle fibres are the building blocks of our musculoskeletal system, responsible for all our movements, from the smallest twitches to the most powerful. In this blog post, we’ll delve into the different types of muscle twitch fibres, their functions, and how you can train them to optimise your performance.

What Are Muscle Twitch Fibres?

Muscle twitch fibres, also known as muscle fibres or muscle cells, are the individual units within your muscles responsible for generating force and movement. These fibres can be classified into three main types, each with distinct characteristics:

  1. Slow-Twitch (Type I) Muscle Fibres:
    • Slow to contract and fatigue-resistant
    • Well-suited for endurance activities.
    • Contain a rich supply of oxygen and myoglobin (a protein that stores oxygen).
    • Ideal for maintaining posture and prolonged, low-intensity activities like jogging or cycling.
  2. Fast-Twitch (Type II) Muscle Fibres:
    • Rapidly contract but fatigue quickly.
    • Come in two subtypes
      • 2a. Fast-Twitch Type IIA: Somewhat fatigue-resistant, suitable for activities like sprinting and resistance training.
      • 2b. Fast-Twitch Type IIB: Fatigue quickly and are suited for intense, short bursts of power like weightlifting or jumping.
  3. Intermediate (Type IIX) Muscle Fibres:
    • A hybrid between slow and fast-twitch fibers.
    • Offer a balance between endurance and power.
    • Engage in activities that require both

Structure of muscle fibres

  1. Slow-Twitch (Type I) Muscle Fibres:
    • Size: Slow-twitch muscle fibres are relatively small in diameter.
    • Color: They are often referred to as “red fibres” due to their rich blood supply and myoglobin content, which gives them a reddish appearance.
    • Mitochondria: Slow-twitch fibers contain numerous mitochondria, which are essential for aerobic energy production.
    • Capillaries: They are surrounded by an extensive network of capillaries, ensuring a consistent supply of oxygen and nutrients. •
    • Contraction Speed: Contract slowly.
      Fatigue Resistance: Highly fatigue-resistant due to their aerobic energy production and efficient waste removal.
  2. Fast-Twitch (Type II) Muscle Fibres
    • Size: Fast-twitch muscle fibres are larger in diameter compared to slow-twitch fibres.
    • Color: They are often called “white fibres” because they have fewer blood vessels and less myoglobin, giving them a paler appearance.
    • Mitochondria: They have fewer mitochondria than slow-twitch fibers.
    • Capillaries: Fewer capillaries surround them
    • Contraction Speed: Contract rapidly, generating quick, powerful movements.
    • Fatigue: Fatigue more quickly due to reliance on anaerobic energy systems.
  3. Intermediate (Type IIX) Muscle Fibres:
    • Size: Intermediate muscle fibres have a moderate diameter, falling between slow-twitch and fast-twitch fibres.
    • Color: They may appear somewhat reddish due to a modest myoglobin content.
    • Mitochondria: They have a moderate number of mitochondria, allowing for a balance of aerobic and anaerobic energy production.
    • Capillaries: Intermediate capillary density, reflecting their adaptability to mixed activities.
    • Contraction Speed: Contract at a moderate speed, providing a balance between slow and fast-twitch characteristics.
    • Fatigue Resistance: Offer a compromise between slow-twitch endurance and fast-twitch power.

Functions of Muscle Twitch Fibres:

Understanding the types of muscle twitch fibres is crucial because they play distinct roles in various activities:

  1. Slow-twitch fibres excel in low-intensity:
    • Endurance: Slow-twitch fibres are highly fatigue-resistant and are well-suited for sustained, low-intensity activities.
    • Postural Support: They play a crucial role in maintaining posture and stability during everyday activities.
    • Aerobic Activities: Ideal for endurance exercises like long-distance running, cycling, and activities that require prolonged, steady effort.
  2. Fast-twitch fibres are essential for high-intensity, explosive movements:
    • Explosive Power: Fast-twitch fibres contract rapidly and are responsible for generating high levels of force in short bursts.
    • Strength and Speed: Suited for activities that demand explosive strength and speed, such as sprinting, weightlifting, and jumping.
    • Anaerobic Activities: Used in intense, short-duration activities that rely on anaerobic energy systems.
  3. Intermediate fibres bridge the gap between slow and fast-twitch, contributing to mixed activities:
    • Versatility: Intermediate fibres offer a balance between endurance and power.
    • Mixed Activities: Engage in activities that require both strength and endurance, such as martial arts, swimming, or activities with intermittent bursts of power and endurance.
    • Adaptability: They can adapt to the specific demands of various activities, making them versatile in performance.

Training Your Muscle Twitch Fibres:

  1. Slow-Twitch fibre Training:
    • Engage in activities with sustained effort, like jogging or cycling.
    • Perform endurance exercises at a steady pace over an extended period.
  2. Fast-Twitch fibre Training:
    • Focus on high-intensity, explosive workouts such as sprinting, weightlifting, or plyometrics.
    • Use short bursts of maximal effort followed by rest intervals.
  3. Intermediate fibre Training:
    • Combine both endurance and high-intensity training in your routine.
    • Activities like interval training or martial arts can help develop intermediate fibres


In this study, by Morton et al. (20191) the researchers aimed to investigate the effects of resistance exercise on muscle fiber activation and anabolic signaling when lifting both heavier and lighter loads to task failure. They conducted the study on ten recreationally-trained young men and assessed various parameters, including EMG-derived variables, muscle fibre activation, and anabolic signalling.

The key findings of the study are as follows:

  1. The study challenges the common belief that heavier loads (above 60% of maximal strength) are necessary for the activation and stimulation of type II muscle fibres and hypertrophy.
  2. The results showed that muscle fiber activation and anabolic signaling were not dependent on the load (heavier or lighter), repetition duration, or surface EMG amplitude when resistance exercise was performed to task failure.
  3. Both type I and type II muscle fibres were activated when participants lifted both heavier and lighter loads to task failure.
  4. This suggests that the level of muscle fibre activation and potential for hypertrophy is similar, regardless of the load used, as long as the exercise is performed to task failure.

In summary, this study challenges the notion that heavier loads are required for muscle fibre activation and hypertrophy. It suggests that lifting to task failure, whether with heavier or lighter loads, activates both type I and type II muscle fibers, leading to equivalent muscle hypertrophy.


Understanding the different types of muscle fibres and how they function is key to optimising your physical performance. Tailoring your exercise routine to target specific muscle twitch fibres can lead to improved strength, endurance, and overall fitness. So, whether you’re a sprinter, a long-distance runner, or just aiming for a healthier lifestyle, harness the power of muscle twitch fibre to achieve your fitness goals.

  1. Morton RW, Sonne MW, Farias Zuniga A, Mohammad IYZ, Jones A, McGlory C, Keir PJ, Potvin JR, Phillips SM. Muscle fibre activation is unaffected by load and repetition duration when resistance exercise is performed to task failure. J Physiol. 2019 Sep;597(17): 4601-4613. doi: 10.1113/JP278056. Epub 2019 Jul 27. PMID: 31294822. ↩︎

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