Functional overload of rat fast-twitch skeletal muscle during development.

The intent of this study was observe the effects of different training programs on selected biochemical properties in lantaris muscle of young rats. Littermates (10 days old) were assigned to endurance (E), sprint (S), and control (C) groups. Animals in each group were partitioned into either 21- or 51-day groups and 10-day controls. Myofibrillar ATPase activities at 21 and 51 days (0.720 +/- 0.028 and 0.719 +/- 0.012 mumol Pi.mg-1.min-1) were higher than the 10-day activity (0.191 +/- 0.022 mumol Pi.mg-1.min) for C animals (P less than or equal to 0.05). At 51 days, E training resulted in a lower ATPase activity than both the 21-day E and 51-day C animals (P less than or equal to 0.05). Sarcoplasmic reticulum protein yield and Ca2+ binding and uptake increased from 10 to 51 days for C animals (P less than or equal to 0.05). The Ca2+-binding ability for 51-day E animals (26.1 +/- 3.4 nmol Ca2+/mg) was greater than for the 51-day C animals (20.0 +/- 1.69 nmol Ca2+/mg) (P less than or equal to 0.05). The plantaris muscle of S animals were not altered at 21 and 51 days compared to C animals were not altered at 21 and 51 days compared to C animals (all variables). The results suggest that exercise training alters the biochemical characteristics of skeletal muscle during growth.