The performance of a muscle can be measured by the contractile tension it can generate and the duration for which it can sustain it. The former one is usually referred to as strength, the latter one as endurance. These two variables are largely influenced by the types of muscle fibres that make up the muscle. These can be fast, slow, or intermediate
Fast fibres or fast twitch fibres can be of two types. Type IIb fibres or fast glycolytic (or just fast) generate energy by using ATP stored in the muscle. Type IIa or fast oxidative glycolytic (or intermediary) fibres use oxygen to convert stored glycogen to ATP.
Type IIb fibres respond to stimulus in 0.01 seconds, hence their name. They have a large diameter and can store a lot of glycogen. They also have few mitochondria and are densely packed with myofibrils. Because of the high myofibril number, they can generate a lot of tension. Since they rely primarily on anaerobic metabolism, they fatigue quickly. They appear pale or whitish because they have a small number of capillaries per unit area.
Type IIb fibres will only be activated if the demand on the muscle exceeds the capacity of slow twitch fibres. They can reach peak contraction very quickly and generate a higher amount of force compared to slow twitch. Phasic, or muscles responsible for moving us, contain mostly fast twitch fibres. Phasic muscles such as the deltoid, rectus abdominis, and the glutes fatigue easily and are prone to inhibition and weakness. This can contribute to postural abnormalities and pain.
The number of fast fibres recruited for a specific activity can be increased through strength and power training. These fibres usually contribute most significantly to the size and definition of a particular muscle.
Fast-twitch fibres are more suited to explosive activities. In strength and power-based sports, like powerlifting or rugby, having more of these can be an advantage.
Some training modalities predominantly develop fast twitch fibres.
Slow fibres also go by the names slow twitch, slow oxidative and Type I. Compared to fast fibres, they are smaller and take about 3 times longer to contract in response to a stimulus.
Slow twitch fibres have numerous mitochondria and an extensive capillary network. Due to their high content of a red pigment called myoglobin and more blood flowing through them, slow fibres appear red to the naked eye. Myoglobin, which is similar to the oxygen-carrying protein haemoglobin in the blood, makes oxygen available when necessary. The mitochondria in the fibres can then use oxygen to create ATP (adenosine triphosphate). ATP then fuels muscle contraction. Hence, slow twitch fibres are considered aerobic.
Slow twitch fibres can sustain force for extended periods because they, in a sense, provide their own energy. The downside is that they cannot produce large amounts of force like the fast ones can. Slow fibres are activated easily and are the first to be recruited when a muscle contracts. The fast fibres will only be activated if the demand exceeds the capacity of the slow ones.
Postural muscles, which maintain posture against gravity contain mostly slow fibres. This helps postural muscles like the scalenes, levator scapulae, the lumbar erectors, and the psoas sustain a degree of contraction over extended periods. The downside is that these muscles are also prone to hyperactivity and tightness.
Steady, endurance type activities can increase mitochondrial density and increase the body’s ability to use oxygen for creating energy. Hence, activities improving aerobic efficiency are well-suited to developing slow fibres. Examples of effective ways of training slow twitch fibres include:
Fast twitch fibres can be converted through endurance training into intermediate, Type IIa or fast oxidative glycolytic fibres. As the name implies, intermediary fibres share some of the properties of fast and some of slow fibres.
They may appear white or reddish depending on the concentration of myoglobin and capillarisation. Intermediate fibres have more endurance, are somewhat slower to contract and have a smaller diameter than fast fibres.
The distribution and proportions between fibre types depend on a number of factors. Ancestry and genetic factors have significant influence. Training and habitual activities can also alter the balance.
Muscles that have to move rapidly, like the eye muscles or the small muscles of the hand, have mostly fast fibres. Most postural muscles, e.g. the calves and core muscles, have mostly slow fibres.
Working knowledge of muscle fibre types and their distribution in different muscle groups can be very helpful. It can provide guidance as to which muscle groups may be short and tight and which may need activation and strengthening. This, in turn, would inform how to best serve the client through a combination of soft tissue techniques, stretching and exercise.