Lactate in Fast and Slow Twitch Skeletal Muscle Fibres of Man during Isometric Contraction

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Abstract
Concentrations of lactate in fast and slow twitch fibers, respectively, were determined in quadriceps femoris muscle after sustained contractions at 25, 50 and 75% of maximal voluntary isometric contraction (MVC) until exhaustion and after interrupted exercises at 25% and 50% MVC [in humans]. Maximal lactate concentrations were only found at 50% of MVC performed to exhaustion. Lactate concentration was higher in slow twitch (ST) fibers at 50% MVC compared to in ST fibers at 25% MVC and higher in fast twitch (FT) fibers at 50% MVC compared to in FT fibers at 75% MVC. After short time isometric exercise (i.e., 75% to exhaustion and 50% and 25% performed for the same period of time as 75% MVC) lactate concentration, expressed as lactate ratio (lactate concentration in FT fibers lactate concentration in ST fibers) was positively correlated to percent FT fibers (r [correlation coefficient] = 0.89). Lactate ratio ranged 0.5-2.0, i.e., at onset of isometric exercise, lactate concentration increase was faster in ST fibers in the muscle rich in ST fibers and faster in FT fibers when the muscle was rich in FT fibers.