Myoglobin, a protein found in muscle tissue, plays a crucial role, especially in slow-twitch muscle fibres. It is responsible for the storage and transport of oxygen within muscle cells. Myoglobin has a high affinity for oxygen, even higher than that of haemoglobin found in red blood cells. This characteristic allows it to serve as an oxygen reserve in muscle tissue. In slow-twitch fibres, which are adapted for prolonged, aerobic activities, the presence of high levels of myoglobin is vital. It ensures a steady supply of oxygen to the mitochondria, where it is used in the aerobic generation of ATP. The abundance of myoglobin in these fibres is what gives them their characteristic red colour and enables them to sustain prolonged activity without fatigue. Thus, myoglobin is essential for the efficient functioning of slow-twitch fibres during endurance activities, as it enhances their capacity to utilise oxygen for energy production.

Muscle fibre composition has significant implications on susceptibility to muscle fatigue. Slow-twitch fibres, with their high oxidative capacity and efficient use of oxygen, are more resistant to fatigue. They are capable of sustained, low-intensity activity over extended periods without losing their functional capacity. This resistance to fatigue is due to their efficient energy production through aerobic pathways, extensive capillary networks, and high myoglobin content. In contrast, fast-twitch fibres, particularly Type IIb fibres, fatigue more rapidly. They rely on anaerobic glycolysis for energy production, which is less efficient and leads to the accumulation of lactate and other metabolic byproducts. These byproducts contribute to the decline in muscle performance and the onset of fatigue. Therefore, individuals with a higher proportion of fast-twitch fibres may experience quicker onset of fatigue during high-intensity activities, while those with more slow-twitch fibres can sustain activity for longer before tiring.

The difference in energy storage between slow-twitch and fast-twitch muscle fibres significantly affects their function. Slow-twitch fibres are primarily adapted for long-duration, low-intensity activities and rely on aerobic metabolism. They store energy in the form of fats, which provides a more sustained and long-term energy source. This allows slow-twitch fibres to function efficiently for prolonged periods without depleting their energy reserves rapidly. In contrast, fast-twitch fibres are designed for short, high-intensity activities. They store energy predominantly as glycogen, which can be rapidly converted to glucose for immediate energy through anaerobic metabolism. This quick access to energy is crucial for activities requiring sudden bursts of strength or speed. However, glycogen stores are limited and deplete quickly, which aligns with the fast-twitch fibres' propensity for rapid fatigue. The distinct energy storage mechanisms in these fibre types underpin their specialised functions in muscle activity and endurance.

Genetic factors play a significant role in determining the distribution of muscle fibre types in individuals. The ratio of slow-twitch to fast-twitch fibres in a person is largely influenced by their genetic makeup. This genetic predisposition is crucial because it often dictates an individual's natural aptitude for certain types of physical activities or sports. For instance, a person with a higher proportion of slow-twitch fibres might naturally excel in endurance sports such as long-distance running, whereas someone with a predominance of fast-twitch fibres may find they have a natural advantage in sprinting or strength-based sports. The influence of genetics on muscle fibre composition is a key area of interest in sports genetics, as it can provide insights into personalised training and performance optimisation. While training can modify the characteristics of muscle fibres to a certain extent, the baseline distribution set by genetics usually remains a significant determinant of an individual's muscle fibre composition.

Muscle fibres can exhibit a degree of plasticity, meaning they can change their characteristics with appropriate training, though a complete conversion from one fibre type to another is relatively rare. Training can induce changes in the metabolic and physical properties of the muscle fibres. For example, endurance training can lead to biochemical and structural changes in fast-twitch fibres, making them more oxidative and increasing their fatigue resistance. This process involves changes in the expression of specific genes that control the characteristics of muscle fibres. The fibres may start to develop more mitochondria and increase their capillary density, becoming more similar to slow-twitch fibres in their function. Similarly, strength and power training can enhance the glycolytic capacity of slow-twitch fibres, though they typically do not transform into fast-twitch fibres. This adaptability of muscle fibres allows for a certain level of flexibility in how they respond to different types of physical training, ultimately contributing to improved performance in various physical activities.