Distribution of different fibre types in human skeletal muscles 2. A study of cross‐sections of whole m. vastus lateralis

TLDR

The study aimed to quantify the total number of muscle fibers and assess how the relative proportions of type 1 and type 2 fibers vary within the vastus lateralis, with implications for interpreting fiber‑type data in functional studies. Researchers prepared 15 µm thick whole‑muscle cross‑sections, counted type 1 and type 2 fibers in every 48 mm², and used a computer program to generate contour plots of the bivariate data. Fiber counts varied proximodistally and between individuals, with type 1 fibers comprising 44–57 % (mean 52 %) of the muscle, a depth‑dependent distribution favoring type 2 at the surface and type 1 deeper, and no link between mean fiber area and muscle cross‑sectional area, indicating a non‑random fiber‑type arrangement.

Abstract

In order to determine the total number of fibres and the extent to which the relative occurrence of different fibre types varies within m. vastus lateralis, 15 μm thick cross‐sections of whole muscles were prepared. The total number of type 1 and type 2 fibres was determined in every 48th square millimetre of the section, and the results thus obtained were analysed using a computer program allowing an assessment of bivariate data in the form of contour plots. The total number of fibres varied both in proximal to distal direction in the same muscle and between individuals. No obvious correlation existed between the mean fibre area and the muscle cross‐sectional area. The proportion of type 1 fibres in the whole muscle varied between individuals (from 44% to 57%) with a mean value for all five of 52%. The distribution of different fibre types varied within the muscle, mainly as a function of depth, with a predominance to type 2 fibres at the surface and type 1 fibres in deeper regions of the muscle. Thus, the fibre type distribution in m. vastus lateralis is not random. This must be taken into consideration when data on fibre type composition are compared with functional variables.