Changes in morphological and functional characteristics of male rat EDL muscle during growth.

Morphological and functional changes as well as changes in fibre-type composition were investigated in the left extensor digitorum longus (EDL) muscles of male Wistar rats of approximately 40, 60, 120 and 700 days old. A number of morphological changes occurred in the EDL muscle during growth. While from 40 to 120 days muscle mass and cross-sectional area (CSA) increased by 247 and 192%, changes in muscle and fibre lengths were much smaller (44 and 17%, respectively). Besides morphological changes tetanic force was also found to increase (approximately 307%) up to 120 days. Because this increase in force was greater than the increase in CSA, specific force increased by approximately 29% between 40 and 60 days. Thereafter, specific force stayed rather constant. From 40 until 60 days changes were also found in the force-frequency and force-velocity curve, which indicate a slowing of the muscles (until 60 days). Changes in fibre-type composition of the EDL muscle were found to occur later during growth between 60 and 120 days. In this period an increase in the relative total area of Type IIBd fibres and a decrease in the relative total area of Type IIBm fibres (corresponding to the Type 2X and IIB fibres, respectively), were found; this was apparently due to a conversion of many Type IIBm into Type IIBd fibres and not to a difference in cross-sectional growth between these fibres.(ABSTRACT TRUNCATED AT 250 WORDS)