Variation of the length of skeletal muscle fibers and sarcomere lengths were investigated in extensor digitorum longus (EDL) and soleus (SOL) muscles in mice (ICR) during postnatal growth in the range from 2 to 12 weeks after the birth. Lengths of fiber were measured under the dissecting microscope. For measuring sarcomere lengths, laser diffraction technique was used. For measuring lattice spacing, X-ray diffraction technique was used. Following results were obtained;
　(1) Muscle fiber length increased rapidly up to 8 and 5 weeks in EDL and SOL, respectively.
　(2) Relative fiber length at 12 weeks against 2 weeks were 1.30 in EDL, and 1.32 in SOL.
　(3) Sarcomere lengths at both ends of the fiber showed no significant change during growth.
　(4) On the other hand, sarcomere lengths at mid point showed maximum values at 8 and 5 weeks in EDL and SOL respectively. These observations suggest that the sarcomere are stretched especially at mid point when lengths of muscle fibers increase rapidly. It would be possible to consider that this change of the sarcomere length during postnatal growth affect the contractile properties of muscles.

DESCENTE SPORTS SCIENCE Vol.5/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE