Spatial fiber type distribution in normal human muscle

Abstract

The variability of fiber type distribution in nine limb muscles was examined with histochemical and tensiomyographical (TMG) methods in two groups of 15 men aged between 17 and 40 years. The aim of this study was to determine the extent to which the relative occurrence of different fiber types and subtypes varies within human limb muscles in function to depth and to predict fiber type proportions with a non-invasive TMG method.

The distribution of different fiber types varied within the muscles, as a function of depth, with a predominance of type 2b fibers at the surface and type 1 fibers in deeper regions of the muscle. For all the analyzed muscles the contraction times measured at stimulus intensity 10% of supramaximal stimulus (10% MS) were significantly ($p<0.05$) shorter than the contraction times measured at 50% of supramaximal stimulus intensity (50% MS). The Pearson's correlation coefficient between percentage of type 1 muscle fibers measured at the surface of the muscle and contraction time at 10% MS, obtained by TMG was statistically significant ($r=0.76,P<0.01$). Also the Pearson's correlation coefficient between percentage of type 1 muscle fibers measured in the deep region of the muscle and contraction time at 50% MS obtained by TMG was also statistically significant ($r=0.90,P<0.001$).

These findings suggest that the contraction time obtained by TMG may be useful for non-invasive examining of muscle fiber types spatial distribution in humans.

Introduction

The relationship between the histochemical and contractile properties of human muscles has been established with non-invasive tensiomyography (TMG) (Dahmane et al., 2001) and with invasive measuring methods (Buchthal and Schmalbruch, 1970; Parker et al., 1984; Sica and Mc Comas, 1971; Stein et al., 1972). It has been demonstrated that there is a significant correlation between the contraction time of muscle response measured by TMG and the percentage of slow twitch muscle fibers obtained by the histochemical method. These fibers are distributed in a mosaic pattern, and the percent distribution of each fiber type can vary significantly between different regions of a muscle. Many studies have compared the fiber type arrangement in superficial and deep sites of human muscles. A difference has been reported by certain authors (Edström and Kugelberg, 1968; Elder et al. 1982; Johnson et al., 1973; Lexell et al. 1983; Nygaard, 1981; Polgar et al., 1973), but this has not been confirmed by others (Edgerton et al., 1975). The reasons as to why these authors failed to clearly show differences in the regional distribution of fiber types are multiple: the investigated groups were not homogenous concerning age, sex and health condition, the analyzed samples were too small or too few. Our study will provide information based on more data, suitably controlled (both analyzed groups were homogenous concerning age, sex, and health condition). Previous studies showed differences in regional distribution of slow or fast fiber types, but did not precisely distinguish among the different types of fast twitch fibers. The principal objective of the present study was to determine the extent to which the relative occurrence of different fiber types and subtypes varies within human limb muscles and to predict fiber type proportions with a non-invasive measurement technique (TMG).

Since TMG uses surface neuromuscular electrical stimulation (ES) for muscle activation, it can also be used for studying the influence on the motor unit (MU) recruitment order. As a result, this study also compares the recruitment order of different muscle fiber types during various stimulus intensities of twitch type (1 ms stimulus duration) ES.