Table 1.

Definition of nomenclatures and abbreviations used in the text

Nomenclature and abbreviation	Definition
DC component	0th order of the transformed series in FFT. It represents the summation of signals from all input pixels.
FFT	Fast Fourier transformation
(Global) T-tubules	The entire tubule system, including both TE and LE.
Global T-tubule regularity	The magnitude of the major frequency normalized to that of the DC component, calculated from skeletalized global T-tubule system of a cell. TTpower, previously defined as an index for T-tubule regularity in Wei et al. (15), is a measure of the magnitude of the major frequency without normalization.
PIT	Pixel intensity threshold, a value used for thresholding the input image based on the intensity of pixels, to detect ROI.
ROI	Region of interest, the intracellular region enclosed by the peripheral membrane.
TE density	The total number of pixels of skeletalized TE divided by the total Number of pixels of intracellular area (ROI).
LE density	The total number of pixels of skeletalized LE divided by the total number of pixels of intracellular area (ROI).
TE spacing	The average horizontal spatial interval between TE, derived from the spatial frequency.
TTint	Index of T-tubule integrity, which takes into account both regularity and density. For analysis of global T-tubules, TTint = global T-tubule density × global T-tubule regularity; for analysis of TE only, TTint = TE density × TE regularity.