Muscle Fibre Percentage
What changes occur?
I have read a lot of hubs writing about the ability to change a slow twitch muscle fibre (Type 1) to a fast twitch (Type 2a and b) however many of these articles lack scientific evidence. This article aims to clear up some of the current misconceptions and debates.
Skeletal muscles comprise 40% of a humans mass, and come in three forms; skeletal, cardiac or smooth (Karp, 2010), and our understanding of muscles has come a long way since Nobel prize winner Andrew Huxley discovered that muscles contract due to the interaction of actin and myosin. This article is concerned with skeletal, as this muscle type is the only one that adapts to training induced stimuli as cardiac muscle is unique to the heart, and smooth muscle is used to line organs.
Muscle fibre percentage (slow to fast twitch percentage) is largely determined by genetics, although it is possible to change muscle fibre types, largely by increasing the neuromuscular overloading of the muscles. It is important to note that muscle fibre percentages will vary depending upon the individual muscle and muscle groupings (upper/lower body), age, training stimulus and recovery.
Changes in the metabolic properties of muscle fibres are more frequent than changes in fibre morphology, for example with more frequent neuromuscular activity increases in mitochondria and aerobic-oxidative potential occur (Pette, 2002) and with frequent sprint training increases in the glycolytic enzymes of fast twitch muscles can be seen (MacDougall et al., 1998). However frequent training induced stimulation will eventually alter the muscle architecture, although only in the 2b->2a direction, and not in the 1->2a direction.
Type 2b can be thought of as the 'default' gene (Goldspink et al, 1991), and thus with frequent rest or detraining, or a training program consisting of short duration work and lots of rest (such as a sprinter), 2a fibres will 'convert' to 2b. The best example of this change in muscle fibres after an extended period of rest can be seen in patients with spinal cord injuries, for example Burnham et al. (1997).
Conversely, the most frequent change in muscle fibres following a resistance program is changes of 2b to 2a (Campos et al., 2002). This seems odd, as it is well known that 2b present the fastest contractile speed, however it appears that 2b are 'held in reserve' as they are used the least frequently (Staron, 1997) and that 2a is the preferred muscle fibre for weight training. However although the positive change in fibre type following weight training from 2b to 2a is well documented, changes from type 1 to type 2a do not occur (Harber et al., 2004).
For a more intensive review of the literature see Fry's (2004) paper. However feel free to message me or leave a comment and I will get back to you.
Burnham, R., Martin, T., Stein, R., Bell, G., MacLean, I. and Steadward, R. (1997) Skeletal muscle fibre type transformation following spinal cord injury. Spinal Cord, 35(2): 86-91.
Campos, G. E. R., Luecke, T. J., Wendeln, H. K. (2002). Muscular adaptations in response to three different resistance-training regimens: specificity of repetition maximum training zones. European Journal of Applied Physiology, 88: 50-60.
Fry, A. C. (2004). The role of resistance exercise intensity on muscle fibre adaptations. Sports Medicine, 34(10): 663-679.
Goldspink, G., Scutt, A., Martindale, J., Turay, L. and Gerlach, G. F. (1991). Stretch and force generation induce rapid hypertrophy and myosin isoform gene switching in adult skeletal muscle. Biochem Soc Trans, 19(2): 368-373.
Karp, J. R. (2010). A primer on muscles. IDEA Fitness Journal, 7(5): 29-35.
Harber, M. P., Fry, A. C., Rubin, M. R. Skeletal muscle and hormonal adaptations to circuit weight training in untrained men. Scandinavian Journall of Medicine and Science in Sports, 14(3): 176-185.
MacDougall, J. D., et al. (1998). Muscle performance and enzymatic adaptations to sprint interval training. Journal of Applied Physiology, 84(6): 2138-2142.
Pette, D. (2002). The adaptive potential of skeletal muscle fibres. Canaidan Journal of Applied Physiology, 27(4): 423-448.
Staron, R. S. (1997). Human skeletal muscle fibre types: delineation, development, and distribution. Canadian Journal of Applied Physiology, 22(4):307-327.
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