Fast Twitch Fibres

Slow-twitch fibres

Slow-twitch fibres are also known as type I fibres. These myofibers contract more slowly than fast-twitch fibres and generate less powerful contractions. Type I fibres are red due to large amounts of myoglobin and the large numbers of mitochondria. Myoglobin is an oxygen-binding protein with a higher affinity for oxygen than haemoglobin (Figure 2). Therefore, myoglobin in the myofibres assists in unloading and delivering oxygen from the blood haemoglobin to the muscle fibre. These features allow type I muscle fibres to generate large amounts of ATP through the aerobic respiration cycle and avoid lactic acid build-up. As a result, type I muscle fibres are resistant to fatigue and can produce repeated, low-intensity contractions for an extended period, making them best suited for endurance type capabilities.

Fig. 1 - Microscopic cross-sectional image of a skeletal muscle

In a nutshell, the slow-twitch fibre adaptations are:

1. High myoglobin content
2. A rich supply of blood vessels for delivering oxygen and glucose
3. High numbers of mitochondria

The muscles containing type I fibres are often postural muscles such as those in the neck and spine due to their endurance needs. In addition, athletes such as marathon runners have a higher number of type I fibres. Both genetics and training are important determinants of muscle fibre compositions.

Equilibrium dissociation curve

Fig. 2 - Oxygen dissociation curve - oxygen dissociation when carried by haemoglobin and myoglobin

The affinity level of a molecule refers to how well it can interact and bind with another and is reported by the equilibrium dissociation constant (K_D).

Myoglobin and haemoglobin bind to oxygen. "pO₂" refers to the partial pressure of oxygen, and “saturation” refers to how saturated myoglobin and haemoglobin are with oxygen. As the partial pressure of oxygen gas increases, the oxygen saturation also increases until haemoglobin/myoglobin are saturated. Myoglobin has a higher affinity for oxygen and will become saturated with oxygen at lower pressures.

Fast-twitch fibres

Fast-twitch fibres are also known as type II fibres. They are capable of contracting rapidly and more strongly than the slow-twitch fibres. However, type II muscle fibres are not suited for prolonged contraction and are only adapted for short bursts of powerful contractions. Type II fibres are further subdivided into two types:

Type IIa

Type IIa fibres are also sometimes referred to as fast oxidative fibres and are a hybrid of type I and II fibres. These fibres possess large numbers of mitochondria and myoglobin; as a result, they are red. They generate and break down ATP rapidly by employing aerobic and anaerobic metabolism. Type IIa muscle fibres can produce fast, intense muscle contractions with relative amounts of fatigue resistance. However, they are more prone to fatigue than type I fibres.

Type IIa fibres are recruited for activities that require high intensity, such as sprinting and weightlifting.

Type IIb

Type IIb fibres are also commonly known as fast glycolytic fibres. They contain a small amount of myoglobin and, as a result, are white. Fast glycolytic fibres contain very few mitochondria and mainly generate ATP via anaerobic respiration (glycolysis alone) at a slow rate. However, they can use ATP rapidly. This results in short and rapid bursts of power followed by fatigue. Type IIb fibres are the largest of the three fibre types.

Type IIb fibres can be turned into type IIa by long-duration training, allowing them to be more resistant to fatigue.

Similar to the type IIa fibres, type IIb fibres are suited for speed, strength, and power type activities, but they are less resistant to fatigue. Activities such as heavyweight training with 1-3 reps, powerlifting, and 100 m sprints predominantly require type IIb muscle fibres.

A comprehensive comparison between different muscle fibre types is in Table 1.

Table 1. Comparison of different skeletal muscle fibres.

Determinants of the muscle fibre composition

Our muscles are not composed of only one type of muscle fibre. Instead, they are a combination of fast-twitch and slow-twitch muscle fibres. The composition of the fibre types in muscles depends on one’s genetics, physical activity level, and age.

Genetics

The genes we inherit from our parents determine many things including our muscle composition.

Physical activity level

Individuals with no athletic occupation tend to have a 50:50 ratio of fast and slow-twitch fibres. The composition of the top-performing and professional athletes, however, differs. Power athletes such as weightlifters and sprinters tend to have a higher ratio of type II fibres, while endurance athletes such as marathon and long-distance runners have more type I fibres.

Age

As we age, we start to lose our lean muscle mass. This is accompanied by a decline in our fast-twitch fibre content. The loss of lean muscle mass can contribute to age-related pathologies such as metabolic dysfunction and increased risk of falls. Resistance training can prevent this loss.