Histochemical and contractile properties of soleus muscle trained during development.

The effects of endurance training on the histochemical and contractile properties of developing rat soleus was studied. Five 18 day old Long Evans rats were trained twice a day for 28 days (E group); workloads were increased to 30 m/min (15% grade) for 40 min/bout. Contraction time (Tc), twitch tension (Pt), half relaxation time (t1/2R), tetanic tension (Po), maximal rate of rise in tension (t50% Po) and fatigue index (FI, % decline in force after 15 s tetanic stimulation) were measured and compared to a control (C) group. The contralateral soleus was excised, rapidly frozen and histochemical (myosin ATPase pH 9.4 and 4.3) and histological sections prepared for analysis of fibre type, number and area. Similar fibre types (mean = 70 and 71% ST), fibre numbers (mean = 2115 and 2242) and areas of ST (mean = 2000 and 2320 micron 2) and FT fibres (mean = 1476 and 1628 micron 2) were found in C and E groups respectively. Po (p less than 0.05) was greater and t50% Po (p less than 0.01) was significantly faster in E. Pathological changes were found in about 1% of fibres in each trained muscle, but not in controls. Histochemical differentiation of fibre types was unaffected and contractile properties only minimally by training during early development.