Muscle Fibre Adaptations

The aerobic capacity of both fast and slow fibers can be improved significantly by endurance training, however, the slow twitch fibers will always tend to have a greater endurance capacity. On the other hand, fast twitch fibers are, depending on the training techniques used, are more responsive to strength and power. However, slow twitch fibers do respond to resistance training, especially when training methods, such as those used by bodybuilders; involve high volumes of work over a period of years.

As slow twitch muscle fibers adapt to a limited degree to sprint training it is apparent that those born with a high proportion of slow twitch fibers are unlikely to develop into elite sprinters. On the other hand, those born with a high proportion of fast twitch fibers can significantly increase their aerobic capacity although they would still be at a disadvantage compared to those born with a higher proportion of slow twitch fibers.

Fiber type conversions as a result of exercise

The argument as to whether or not the proportion of fast or slow twitch fibers can be altered by training is a confused one. Conventional wisdom once suggested that muscle fibers could not be converted from one type to another. However, recent evidence suggests that in response to endurance training, fast twitch fibers take on some of the functional and structural characteristics of slow twitch fibers. For example, there are a number of studies in which fast twitch muscle fibers have been shown to produce a particular form of the myosin filament normally found in slow twitch fibers. In this regard at least, it must be acknowledged that fiber type conversions do occur. As slow myosin is more economical than fast myosin this adaptation is presumably quite advantageous to the endurance athlete. However, the implication of this information to the speed and power athlete is that endurance training is not only non-specific but potentially deleterious.
There is at present little no evidence that slow twitch fibers can take on the characteristics of fast twitch fibers as a result of training, however, the reader should be aware that there has been very little conclusive research on the effects of speed and power training and fiber characteristics. However, there is evidence that slow to fast conversions do occur in animal muscles which are completely immobilized in casts or by paralysis.

MUSCLE GROWTH - Fiber Hypertrophy and Hyperplasia

Muscle hypertrophy, the growth of muscle tissue as a whole, is brought about largely by the process of muscle fiber hypertrophy, the process by which existing muscle fibers increase in size due to an increase in the content of contractile proteins (actin and myosin) within each fiber.

Hyperplasia is the process by which a tissue grows in size as a result of an increase in the number of fibers or cells. Hyperplasia remains another possible contributing factor to the process of muscle hypertrophy. Whilst there is no conclusive evidence for training induced hyperplasia in adult skeletal muscle, proponents of the hyperplasia theory have proposed that, in response to intense resistance training, existing muscle fibers may split into two smaller fibers. The new fibers are then proposed to hypertrophy until they reach normal (or near normal) size.

Fiber hypertrophy is undeniably the most significant mechanism underlying exercise-induced muscle growth. In contrast, physiologists are uncertain as to whether or not hyperplasia contributes at all to increases in muscle mass during resistance training. Methodological difficulties exist when trying to prove whether or not fiber numbers increase. In human studies small amounts of tissue are extracted from the muscle (a technique known as a biopsy) and one must estimate the total number of muscle fibers based on the average fiber size in the biopsy and the cross sectional area of the whole muscle. As his process involves several assumptions we can not be fully confident of the results of this type of research.

Whilst some researchers have provided evidence suggesting the possibility of training or stretch induced hyperplasia in cats, rats and birds, the validity of the methods used in these studies has sometimes been questioned. Obviously, the existence of hyperplasia in animals does not ensure that it will occur in humans. Finally, it must be said that as long as a strength training program can bring about extreme muscular hypertrophy the mechanism underlying that adaptation is probably of little consequence to the athlete or coach.