To determine if there is a correlation between the degree of delayed increase in signal intensity (SI) of muscle after exercise on magnetic resonance (MR) images and the amount of ultrastructural (ULS) injury and delayed-onset muscle soreness (DOMS), MR imaging-guided muscle biopsy was performed to obtain tissue from the legs of nine sedentary subjects 48 hours after downhill running on a treadmill. The degree of soreness was subjectively graded. T1-weighted, spin-density, T2-weighted, and short inversion time inversion-recovery images were obtained before and after biopsy, at 48 and 96 hours after exercise, respectively. The delayed SI increase of muscle on images obtained before biopsy was subjectively graded and measured. The degree of ULS injury was determined with electron micrographs. Serum creatine kinase levels were obtained before and up to 144 hours after exercise at 24-hour intervals. The measured SI, SI grades, and DOMS grades were correlated with the degree of ULS injury. Linear regression analysis revealed poor correlation between the DOMS grades and the degree of ULS injury and good correlation between the SI grade and the degree of ULS injury.
Although exercise-induced muscle fiber hyperplasia has been demonstrated through direct fiber counts following nitric-acid digestion of muscle, morphological studies to determine the mechanism of hyperplasia have not been performed previously. In this study, light and electron microscopy were used to evaluate evidence of muscle fiber splitting or de novo formation of new muscle fibers. Since both fiber hypertrophy and hyperplasia may result in alterations in the muscle nuclear populations, myonuclear number and satellite cell frequency were assessed quantitatively to determine their role in regulating muscle fiber size. Ten adult cats performed weight-lifting exercise, and the right (exercised) and left (control) forelimbs were fixed by vascular perfusion. Spaced serial sections were used to evaluate muscle fiber morphology along the length of fibers, and muscle fiber areas were measured. Myonuclei and satellite cells were counted using electron microscopy. Morphological evidence supporting muscle fiber hyperplasia was observed in exercised muscles. These observations included the presence of small fibers which may signify de novo fiber formation. Myonuclear counts indicate that myonuclear density is not a primary regulator of fiber size. Satellite cell frequency was unchanged following exercise. Autoradiographic studies revealed satellite cell activation by uptake of tritiated thymidine in exercised muscles. Satellite cell activation appears to result from increased activity in exercised muscles. These findings confirm previous studies demonstrating muscle fiber hyperplasia following weight-lifting exercise, and suggest that de novo fiber formation is the major mechanism contributing to muscle fiber hyperplasia in this model.
For periods ranging from 26 to 87 weeks, the morphological characteristics of the flexor carpi radialis (FCR) muscle were examined in four cats trained to perform weight-lifting exercise. Four untrained, sex and weight-matched cats served as controls. The right FCR from each cat was surgically isolated, attached to a tension transducer, and set at its optimal length. The forelimb was perfused with 2% glutaraldehyde in 0.1 M cacodylate buffer. Small bundles of fibers were teased from their origin and insertion tendons and embedded in Epon. Spaced serial sections were used to examine the morphological features of the fibers for trained and control animals. Ultrastructural examination revealed muscle fiber degenerative changes, such as pyknotic nuclei, disruption of the sarcolemma, vacuolation, and disorganization of myofilaments. Such changes were observed at a higher frequency in trained muscle than in control muscle. Spaced serial sections of fiber bundles showed that the degree of degeneration varied along the length of the fiber. Fiber area measurements showed that trained muscle had both larger and smaller fibers than control samples. The very small fibers observed in the trained muscle were considered to be regenerating or "new" fibers since they had not undergone degenerative changes. "Satellite-like" cells were observed in trained muscle. Such cells resembled satellite cells but also contained developing myofilaments. Since evidence of degeneration-regeneration was observed in control samples, but at a lower frequency, it was postulated that weight-lifting exercise accelerates muscle fiber turnover in the cat FCR.
Investigations of the structure and function of the flexor carpi radialis muscle (FCR) in the cat have led to the hypothesis that the compartmentalized (nonuniform) distribution of fiber types within the muscle relate to the complex motor skills of the cat. To test this hypothesis a study was undertaken to compare the FCR in four mammalian species of similar body size but with different forelimb motor tasks. The species chosen were: dog, opossum, armadillo, and cat. Comparisons were made among species with regard to general muscle morphology, fiber types and sizes, fiber proportions, and fiber distributions. The FCR of all species was morphologically similar and contained three muscle fiber types (SO, FOG, and FG). The mean area of muscle fibers was largest in opossum, while the FCR fibers of dogs were smallest. The percentage of SO fibers in the dog FCR was greater than in the other species studied. The opossum FCR also contained a high percentage of SO fibers. The armadillo FCR consisted of a high percentage of FG fibers. In the cat FCR the percentages of all three fiber types were similar. For each species, individual fiber proportions were in agreement with the results for fiber percentages. Compartmentalized distribution of fiber types existed in each species with the dog having the most compartmentalized fiber type distribution and the cat the least compartmentalized distribution. Therefore it seems that the compartmentalized organization of the FCR is not related to any specialized motor task, but may be a generalized pattern associated with motor patterns shared among all species studied.
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