1. Muscle - What is it? - How do we make it grow?

2. What is the purpose of muscle?
The body contains over 400 voluntary skeletal muscles, which constitute 40-50% of the total body weight. Skeletal muscle performs three major functions:
1. force production for locomotion and breathing
2. force production for postural support
3. heat production during cold stress

3. What makes up a muscle?
Individual muscle fibers are composed of hundreds of threadlike protein filaments called myofibrils. Myofibrils contain two major types of contractile protein:
1. Actin – part of thin filaments
2. Myosin – part of thick filaments

4. Skeletal Muscle Tissue

5. Microstructure of Muscle

6. Hypertrophy
Defined: Increase in muscle size (with no change in the number of muscle fibers).
Hypertrophy of existing muscle fibers.

7. Hyperplasia Defined: Increase in the number of cells in a tissue.
Generation of new muscle fibers.

8. Resistance Training & Hypertrophy/Hyperplasia
Studies have proven hypertrophy occurs with prolonged weight training.
Some studies report that elite bodybuilders have more fibers per motor unit than the average person, raising the point that hyperplasia might occur with long-term training (1,2).

9. Muscle Fibers
Slow-twitch/Type I fibers: large capacity for aerobic metabolism and a high resistance to fatigue.
Intermediate Fibers/Type IIa: viewed as a mixture of both type I and type IIx. These fibers are extremely adaptable.
Fast-twitch/Type IIx: limited capacity for aerobic metabolism, less resistant to fatigue, and large anaerobic capacity. Force production is 10-20% greater than force produced by slow, type I fibers (3).

10. Typical Muscle Fiber Composition in Elite Athletes (3).
Sport % slow fibers % fast fibers
(type I) (type II)
Distance runners
Track sprinters
Non-athletes

11. References:
1. Atha, J Strengthening muscle. In Exercise and Sport Science Reviews, vol. 9, ed. D. I. Miller, Philadelphia: The Franklin Institute Press.
2. Larsson, L., and P.A. Tesch Motor unit fibre density in extremely hypertrophied skeletal muscles in man: Electrophysiologial signs of muscle fibre hyperplasia. European Journal of Applied Physiology 55:
3. Bottinelli, R., M. Canepari, C. Reggiani, and G. Stienen Myofibrillar ATPase activity during isometric contractions and isomyosin compostion in rat single skinned muscle fibers. Journal of Physiology (London). 481: