Submitted on September 5, 2003
Accepted on August 15, 2004
Contractile responses of the rat gastrocnemius and soleus muscles to isotonic resistance exercise
Kris M Norenberg1 and Robert H Fitts1*
1 Department of Biology, Marquette University, Milwaukee, Wisconsin, USA
* To whom correspondence should be addressed. E-mail: robert.fitts{at}marquette.edu.
Male rats were divided into control (C) and weight trained (WT) groups. WT rats performed squat-type exercises twice daily, five days/week, for 14 weeks. They averaged 36 lifts/day with an average weight of 555 g. Muscle to body weight ratio (mg/g) of the soleus (SOL) was not different from C, but increased 11% and 6% in the gastrocnemius (GAST) and plantaris, respectively (P<0.05). The normalized twitch tension of the in situ SOL was elevated by 21%, while single skinned type-I fibers from the SOL showed an increased rate constant of tension redevelopment (Ktr) but no other contractile adaptations to WT. In contrast, the GAST type-I fibers showed an increase (P<0.05) in maximal velocity of shortening (25%), peak power (15%), Ktr (18%), and normalized tension (7%). The Ktr and normalized tension of the GAST type-IIa fibers increased by 24 % (P<0.05) and 12% (P<0.05) respectively, while velocity and power showed a tendency to increase. Fiber size, determined by myosin ATPase histochemistry, was not different for any fiber type from the GAST or SOL. These results indicate that isotonic resistance exercise of the calf targets the GAST (type-I and type IIa fibers) and has little effect on the SOL.
