Muscle fibres

Not many people are aware that your muscles contain different types of muscle fibre in varying proportions depending upon their principal functions. This has relevance regarding your specific training needs.

Type I - Slow-twitch oxidative

These fibres are the smallest type of fibre and they contract smoothly and slowly with not as much force as type II fibres. They use fat and aerobic beta-oxidation as their energy system. They contain many mitochondria, blood capillaries and myoglobin (a substance that transfers oxygen from the blood to the muscle and gives muscle its characteristic red colour). They are used in endurance type activities and in posture control, sometimes known as ‘core stability’ where the ability to provide ongoing muscular activity without fatigue is essential. Interestingly chronic lower back pain is often thought to result from sedentary life styles where the muscles involved in posture control become weakened placing greater strain on the spine. Core stability exercises can be of great benefit to reduce chronic back pain.

Type IIa - Fast-twitch oxidative glycolytic

These fibres contract faster and more forcefully than type I fibres. They use glycogen/glucose as a fuel in both the anaerobic and aerobic systems as well as the PCr system.

Type IIb - Fast-twitch glycolytic

These fibres are the fastest and most powerful muscle fibres in the body. They use glycogen/glucose only by the anaerobic pathway as well as the PCr system. Because they use the anaerobic systems exclusively for contraction, they have very few mitochondria and low levels of myoglobin and are therefore white in appearance. They are quick to fatigue, however, as compared to type I and type IIa fibres.

Modification of muscle fibres through training

Most muscles contain a mixture of all three fibre types, but postural muscles contain a higher proportion of type I fibres and muscles of the shoulder girdle and arms where intermittent powerful activity is required contain higher proportions of type II fibres. Although the number of each type of muscle fibre within each muscle is determined by our genetic makeup, aerobic endurance training can slowly alter the characteristics of type IIb fibres to become more like type IIa ones, with increases in mitochondria, capillaries and myoglobin. Conversely, intense resistance strength training increases the size and strength of type IIb fibres.

This introduces us to another important concept: The Repetition Maximum (RM). This is the maximum number of repetitions (reps) of an exercise that can be achieved at any given resistance weight. Clearly very heavy weights can only be lifted a few times, whereas light weights can be lifted many times before fatigue sets in. In general, if you wish to develop maximum muscle strength, you do 1 or 2 reps of the heaviest weight you can manage, whereas if you want to develop muscle endurance you do 20 plus reps of a much lighter weight. This latter strategy is better for muscle toning too. RM is also sometimes expressed as percentage intensity. For example, 100% RM is equivalent to a heavy weight that can only be lifted once. The following diagram helps to decide how many reps you should be performing depending upon your fitness goal:

Finally, there is another scientific point that’s helpful to know so as to avoid the dangers of overtraining. Following vigorous exercise, as well as lactate and hydrogen ions that can accumulate to cause muscle soreness, small tears in muscle fibres actually also occur, causing soreness. The body responds to these micro injuries by stimulating repair mechanisms which overcompensate, resulting in increased muscle mass. So, the recovery phase following exercise is crucial. You should never perform resistance training on the same muscle groups on consecutive days, there should be at least 2 days rest in between. You can, however, do resistance training on consecutive days provided you are exercising different muscle groups. It also shows the importance of good nutrition with high protein content to aid in rebuilding micro damage to muscle fibres.

The importance of warm-up exercises and post-workout stretching

Before we go on to provide sample workout routines for each of the identified fitness goals, it is essential to highlight the importance of warm-up and stretching as part of any exercise regime. It used to be thought that as part of the warm-up static stretching exercises were helpful. However, it is better to perform dynamic exercises, which are essentially lower intensity versions of the training exercises that you intend to perform in the main body of your workout session. Warming up properly increases your heart rate and cardiac output to supply blood to your muscles and ligaments. In turn your muscles and ligaments warm up, become more supple and flexible, and the risk of an injury during your training session is reduced. After you have finished your training exercises it is important to do post-workout static stretching exercises as after your muscles have been contracting repeatedly they have a tendency to shorten slightly. Without doing post-workout stretching your range of movement, flexibility and agility can become reduced.