Variability in the water, sodium, potassium, and chloride content of human skeletal muscle

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Abstract
Muscle from 47 individuals were analysed for water, sodium, chloride, potassium, and fat. In 35 patients, samples were taken from three sites in one muscle, and in five from several muscles. The average composition of different muscles differed, even in the same individual and the composition of samples varied considerably. Variability is the larger the more diverse the material. Coefficients of variation of muscle constituents are given for material from a biopsy, a muscle, a subject, or a given muscle in any subject. Some of these are large. Variability in the composition of samples is shown to be largely orderly. Water is closely correlated with tissue content of sodium plus potassium; sodium and chloride contents of samples show a positive correlation; potassium and sodium contents are strongly negatively correlated. Residual stochastic variability is small. Thus, when account is taken of the sodium content of a biopsy, the coefficient of variation for a potassium estimate falls from 12% to about 6%. Some practical implications are deduced.