Variability of fiber type distributions within human muscles.

The variability of fiber type distributions between different regions of the same human muscle is believed to be small, based on the sampling of between two and four sites. The objective of the present investigation was to determine the variability of slow-twitch (ST) and fast-twitch (FT) fiber distributions using a more extensive sampling technique than those previously employed. The soleus, biceps, triceps, and vastus lateralis muscles were excised from each of four young men who had died suddenly. Between 13 and 17 sites were sampled from each of the muscles; 3 transverse areas were then examined within each sample. Fiber type distributions were determined from photographs of sections stained for myofibrillar adenosine triphosphatase at pH 10.3, 10.0, or 4.3. The numbers of fibers counted in the four muscles ranged between a mean of 13,660 and a mean of 21,601. The variability in fiber type distributions observed between sites and areas within a site were statistically greater (P less than 0.01) than could be expected from muscles whose fiber type distributions are equally distributed throughout the muscle. It was concluded that sampling between 3 and 5 sites in the different muscles was necessary to reduce the between-site standard deviation to 5%.