INTRODUCTION

In the human body, skeletal muscles consist of fast muscle fibers and slow muscle fibers. There is no non-invasive approach for investigating the characteristics of these muscles. The tibialis anterior muscle (TA) and soleus muscle (SOL) of the calf have been widely used in physiological and pathological studies in animals and humans 2. The TA is mainly composed of fast fibers, while the SOL is mainly composed of slow fibers 1. The types of muscle fibers are based on the myosin heavy chain (MHC) isoform compartment and include 1 slow type (MHC I) and multiple fast types (MHC IIa, MHC IIb, MHC IId, and MHC IIx) . Monitoring of skeletal muscle characteristics can help in evaluation of the effects of strength training on skeletal muscles. q-space imaging (qsi) is a quantitative diffusion-weighted imaging (DWI) procedure that makes it possible to detect delicate changes in the microstructure of environments in which free water movement is restricted 2. Previously, we showed that fast muscles and slow muscles can be separated in animal experiments 3 and clinical studies 4 using qsi. The present study aimed to determine whether qsi can distinguish muscle quality by age/sports characteristics.

METHODS

We acquired MRIs in 40 volunteers [control (average 34.5±5.1 y.o.), marathon players (average 24.2±3.1 y.o.), powerlifting players (average 29.8±3.4 y.o.), and teenager (17.2±0.9 y.o.): each n = 10] using a 3-Tesla MRI (MAGNETOM Skyra system, Siemens Healthcare, Erlangen, Germany) and human cadaveric immunohistology on TAs. T2 weighted imaging (T2WI), T2 mapping [ms] and qsi were performed with a field of view of 275 mm × 400 mm and section thickness of 6 mm. b-values had diffusion encoding in six directions. The detailed protocol is described in Figure 1.　The qsi parameter; i.e., a full width at half maximum (FWHM) map, was obtained by averaging the values obtained in the six directions. In addition, the apparent diffusion coefficient (ADC) [10-3 mm2/s] and fractional anisotropy (FA) [arbitrary unit: a.u.] map calculated by diffusion tensor imaging (DTI) were composed using b800 images of qsi. The differences in the values for TA among four groups (controls, marathon players, powerlifting players, and teenagers) were examined using T2 mapping, ADC, FA and FWHM maps. Regarding the human cadaveric immunohistology on TA, by staining MHC type I in red and MHC type IIb in green, we examined the differences in the cell diameters of the slow/fast fibers.

RESULTS

In human cadaveric immunohistology studies on the TA, MHC type IIb cells have a larger cell size than MHC type I cells (Figure 2). It was the same result as in animal experiments, which we performed previously 3. In a study using MRI, T2WI, T2 mapping, ADC, and FA maps could not represent the differences among the four groups (Figures 3 and 4). On the other hand, a FWHM map could visualize the differences in the TA between controls and powerlifting players and the difference in the TA between the control adults and teenagers (p<0.05; Figure 3 and 4).

DISCUSSION

In human cadaveric immunohistology studies on the TA, the fast muscle cells were larger than the slow muscle cells, and the possibility that the cells could be distinguished by observing the cell diameter was shown. In MRI studies, the cell diameter increased in the process of growing up from teenage to adulthood and, among them, the powerlifter had more enlarged fast muscle cells (Figure 5). There is a possibility that changes in cell types due to growth and sports competition characteristics could be captured by qsi. This technique is a promising method that can be used to non-invasively estimate the fiber type ratio in skeletal muscles.

SIGNIFICANCE

This technique is a promising method that can be used to non-invasively estimate the fiber type ratio in skeletal muscles, and it can be further developed as an indicator of muscle characteristics.

Acknowledgements

We are grateful to Masato Suzuki, Yoshifumi Sone for the technical assistance.

References

1: Punkt, K. Acta Histochem. (1998). 2: Callaghan, P. T. Nature (1991). 3: Hata. J. PLoS One (2019). 4: Nakashima. D. ISMRM (2019). 5 Delp, M. Appl. Physiol. (1996)