Effects of an anabolic hormone on striated muscle growth and performance.

Chronic administration of an anabolic hormone, nandrolone phenylpropionate, in sedentary female rats for 6 weeks gave a 20% increase in body weight and the same proportional increase in all muscles sampled (heart, diaphragm, soleus, TA, EHP and EDL), such that the muscle/body weight ratio was unchanged. Cardiac muscle was unresponsive to treatment. Acute stimulation of EDL via lateral popliteal nerve gave similar values for contraction time, 1/2 relaxation time and twitch:tetanus ratio in both groups suggesting no slowing of the muscle. Fatigue resistance of EDL was improved with 0.29 +/- 0.029 vs. 0.46 +/- 0.071 of maximum isometric twitch tension being developed after 10 min repetitive stimulation at 4 Hz. This improved endurance was not accompanied by any increase in strength and could not be explained on the basis of cellular hypertrophy, but appears to reflect an increased aerobic capacity of skeletal muscle. The proportion of FOG fibres in EDL increased, 38 +/- 1.1% vs. 46 +/- 1.1%, and this was paralleled in the other skeletal muscles. Specific hypertrophy of FOG and FG fibres could be conclusively demonstrated in soleus and TA, respectively.