Hyperplasia can be defined as:
"the growth of an organ due to an increase in the number of cells."
The breast tissue undergoes hyperplasia in a lactating mother (1). The tonsils grow by hyperplasia to enhance the immune response in a child with a throat infection (2). As it relates to skeletal muscle, hyperplasia defines muscle growth due to an increase in the number of muscle fibers.
In contrast, hypertrophy defines an increase in the size of existing cells or fibers rather than an increased number of cells.
Human skeletal muscle undergoes hypertrophy (i.e. it gets bigger) following a resistance training program. But is this whole-muscle hypertrophy the result of fiber hypertrophy or fibre hyperplasia. In other words, do muscles get bigger due to an increase in existing fibre size or an increase in the number of fibres?
It's a subject that had stirred much debate amongst researchers. While fibre hypertrophy is well accepted and documented, very few studies have measured fibre hyperplasia in humans. Studies on animals have shown conflicting results.
Studies on cats have found that hyperplasia occurs in response to heavy resistance training - cats were trained to move a heavy weight with their paw in order to get food (3,4). In contrast, other studies on chickens, rats and mice have found that overload resulted in muscle hypertrophy only and no change in the number of muscle fibres (5,6,7). The differences in results between the cat and other animal studies may be a result of the overload used. The cats were exposed to high resistances and low repetitions as opposed to more endurance-type activity used in the other studies.
A further study performed on birds reported an increase in the number of muscle fibres in the wing in response to chronic stretching by attaching a weight to it (8). Follow up studies using a similar model have both confirmed and contradicted these results (9).
Research into fibre hyperplasia in human muscle in scarce. Nygaard and Neilsen (10) reported that fibre size remained unchanged in the shoulder muscles of swimmers despite the whole muscle belly becoming larger. It was argued that such muscle development must have resulted from hyperplasia.
Larsson and Tesch (11) compared bodybuilders to active but untrained controls. They discovered that despite the significantly greater muscle cross-sectional area in bodybuilders, individual fiber size was not significantly different to the controls'. However, Schantz and co-workers found a significant difference in individual muscle fiber size between bodybuilders and male and female physical education students (12).
Proponents of hyperplasia suggest that it can take place through two mechanisms...
1) By the splitting up of pre-existing fibers
2) By the activation of satellite cells surrounding muscle fibers, which have the potential to mature into muscle fibers themselves.
One longitudinal study followed a group of recreational resistance trained subject. After 12 weeks of more intense resistance training, the number of muscle fibers in the biceps brachii in some of the subjects increased significantly. The fact that not all subjects responded in the same way suggests that if hyperplasia is possible in humans it may only occur in a few individuals under certain conditions.