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1 Orthopaedics, University of California, Irvine, CA, USA; Physiology and Biophysics, University of California, Irvine, CA, USA; Otolaryngology, University of California, Irvine, CA, USA
2 Orthopaedics, University of California, Irvine, CA, USA
3 Otolaryngology, University of California, Irvine, CA, USA
4 Physiology and Biophysics, University of California, Irvine, CA, USA
* To whom correspondence should be addressed. E-mail: vjcaiozz{at}uci.edu.
Previous studies have reported the existence of skeletal muscle fibers that co-express multiple myosin heavy chain isoforms. These surveys have usually been limited to studying the polymorphic profiles of skeletal muscle fibers from a limited number of muscles (i.e., usually less than 4). Additionally, few studies have considered the functional implications of polymorphism. Hence, the primary objective of this study was to survey a relatively large number of rat skeletal muscle/muscle regions and muscle fibers (n 
5000) to test the hypothesis that polymorphic fibers represent a larger fraction of the total pool of fibers than do so-called monomorphic fibers, which express only one myosin heavy chain isoform. Additionally, we used Hill's statistical model of the force-velocity relationship to differentiate the functional consequences of single fiber myosin heavy chain isoform distributions found in these muscles. The results demonstrate that most muscles and regions of rodent skeletal muscles contain large proportions of polymorphic fibers, with the exception of muscles such as the slow soleus muscle and white regions of fast muscles. Several muscles were also found to have polymorphic profiles that are not consistent with the I
IIAIIXIIB scheme of muscle plasticity. For instance, it was found that the diaphragm muscle normally contains I/IIX fibers. Functionally, the high degree of polymorphism may: i) represent a strategy for producing a spectrum of contractile properties that far exceeds that simply defined by the presence of four myosin heavy chain isoforms; and ii) result in relatively small differences in function as defined by the force-velocity relationship.
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