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.