Differential response of the dog quadriceps muscle to external skeletal fixation of the knee.

The change in muscle fiber size and fiber percentage was studied in three heads of the dog quadriceps following 10 weeks of immobilization using an external skeletal fixator. Muscle biopsy morphometric analysis revealed that muscle fiber atrophy was greatest for the vastus medialis and least for the rectus femoris. The atrophic response for type 1 fibers was, in order from most to least atrophied: VM greater than VL greater than RF, whereas for type 2 fibers the corresponding order was VM = VL greater than RF. An increase in connective tissue was also observed for all muscles. These results are discussed in terms of skeletal muscle architecture, initial fiber-type distribution, and level of use. As a result, predictions are made as to the muscles that are most vulnerable to disuse atrophy, namely the postural muscles that contain a relatively large proportion of slow muscle fibers and cross a single joint. Conversely, those that are least susceptible to atrophy are those that are not used as postural muscles, that cross multiple joints, and that are predominantly composed of fast muscle fibers.

Long-term effects of spinal cord transection on fast and slow rat skeletal muscle. II. Morphometric properties.

Morphometric properties of rat soleus and extensor digitorum longus muscles were studied 1 year following complete thoracic spinal cord transection (spinal cord level T9). Both muscles demonstrated almost complete type 1 to type 2 muscle fiber type conversion after 1 year. Muscle fiber atrophy was observed in both muscles. Type 2 fiber atrophy occurred to about the same extent in both muscles. Atrophy was most severe for the soleus type 1 fibers (50% decrease in size). Calculations based on the fiber type and size changes observed indicate that the percentage of the muscle cross-sectional area occupied by each fiber type was almost the same for both muscles 1 year after transection. Discriminant analysis of the data indicated that the percentage of type 2 fibers present in the muscle was the best discriminator between the various groups. These morphometric data provided a basis for understanding the contractile results presented in the previous study as well as insights into the mechanism of transformation in skeletal muscle. Furthermore, inherent differences between type 1 and type 2 fibers were demonstrated between predominantly slow and predominantly fast muscles. Thus, after almost one-half a lifetime of transection, rat muscles are almost completely transformed to fast muscle, and, regardless of initial conditions, have nearly identical properties.