Mark Russell, M.D., Professor of Pediatric Cardiology, Michigan Medicine

Skeletal  muscles  are  designed  to  permit  strong, quick and usually voluntary contractions to allow a wide range of body movements.  Skeletal muscle consists of bundles of muscle fibers, each composed of thousands of individual muscle cells. This muscle is from a zebrafish, but human skeletal muscle is nearly indistinguishable from  this. The contraction force of the muscle is provided by the unique structure of muscle cells, seen here as bands.    When athletes train, there is an increase in the number of individual muscle cells, as well as an increase in the size of muscle fibers such as this one.

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