Skeletal muscle fiber splitting with weight‐lifting exercise in rats

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Abstract
Adult male albino rats were assigned randomly to control (CON) and weight‐lifting (WL) groups. The WL rats were subjected to a progressive weight‐lifting program against high resistance for 8 weeks. During the last 2 weeks, each WL rat lifted a load equal to 130% of its body weight. The mean weight of the adductor longus muscle was significantly increased in the WL group (p < 0.05). This increased muscle weight was shown to be due to an increase in the number of fibers per unit cross‐sectional area (p < 0.05), and the mean sizes of both fast‐twitch oxidative glycolytic and slow‐twitch oxidative fibers were significantly smaller in the WL rats than in the CON rats (p < 0.05). Light and electron microscopic examination showed that five out of eight WL rats exhibited longitudinally split muscle fibers, while only one CON rat had a few centrally placed nuclei. The splitting process appeared as either a “pinching‐off” of a small segment from the parent fiber or an invagination of the sarcolemma deep into the muscle fiber in a plane parallel to the sarcomeres. There were preliminary indications that this work‐induced fiber‐splitting process may be a physiological adaptation of muscle to the stress of exercise.