Table 1.

Classification schemes

Muscle/Fiber-Type Classifications	Example Measurement	Tissue/Fiber Specific	Representative Reference	Recommended Use	Continuous vs. Discrete	Subjective vs. Objective	Method Complexity
Muscle color	Red/white visual	Tissue	(1)	General descriptor: coloring based on metabolic features, not myosin	Continuous (more red or less red)	Subjective	Low
Myosin ATPase	Myosin enzyme activity measure: visualization of product formed	Fiber	(2)	Myosin typing individual fibers	As an enzyme activity assay, staining intensities are continuous, but it is used discretely	Subjective	Mediump H sensitive; requires extensive optimization by species and tissue
Myosin heavy chain antibody	Detects specific myosin heavy chain proteins	Fiber	(3)	Current best practice for classifying fibers by myosin type	Discrete	Objective	Low
SDH activity: succinate dehydrogenase activity	Mitochondrial enzyme activity assay	Fiber	(4)	Used to delineate variations in oxidative metabolism; localized to mitochondria	Continuous	Subjective	Medium Attention to timing required
NADH-TR activity: reduced nicotinamide adenine dinucleotide-tetrazolium reductase	Mitochondrial enzyme activity assay	Fiber	(5)	Used as a marker of oxidative metabolism; the sarcoplasmic reticulum as well as mitochondria contribute to activity	Continuous	Subjective	Medium Attention to timing required
Muscle fiber size	Use laminin or dystrophin antibodies to outline muscle membrane	Fiber	(6)	Morphological property	Continuous	Objective	High (see Trap 2)
Contraction velocity: maximum velocity of unloaded shortening	Mechanical measurement of speed of shortening, correlated to twitch contractions time during isometric contraction	Muscle	(7)	Functional property	Continuous variable but can bin by speeds	Objective	High Requires specialized equipment; sensitive to conditions such as temperature, isometric versus concentric
Fatiguability	Mechanical measure: rates of force loss during repeated contractions	Muscle	(8)	Functional property	Continuous	Subjective	High Highly dependent on choice of contraction protocol; sensitive to conditions such as temperature, in vivo versus in vitro, electrical stimulation versus voluntary contractions
Specific force	Mechanical measure: maximum isometric force normalized for physiological cross section	Muscle or fiber	(9)	Functional property	Continuous	Objective	High (see Trap 3)