Physiological hypertrophy of the FHL muscle following 8 weeks of progressive resistance exercise in the rat.

In humans, progressive resistance exercise is recognized for its ability to induce skeletal muscle hypertrophy. In an attempt to develop an animal model which mimics human progressive resistance exercise, Sprague-Dawley rats were trained to climb a 1.1-m vertical (80 degree incline) ladder with weights secured to their tail. The rats were trained once every 3 days for 8 weeks. Each training session consisted of 4-9 (6.02 +/- 0.23) climbs requiring 8-12 dynamic movements per climb. Based on performance, the weight carried during each session was progressively increased. Over the course of 8 weeks, the maximal amount of weight the rats could carry increased 287%, p </= 0.001. The improved training performance was associated with a 23% absolute increase in the weight of the flexor hallucis longus (FHL), with a concomitant 24% increase in both total and myofibrillar protein, p</= 0.001. Peak tetanic tension (Po) of the FHL increased 20%, p </= 0.001, while specific tetanic tension (SPo) was not altered. No change in twitch tension (Pt) was observed, which resulted in a 22% decrease in specific twitch tension (SPt) p </= 0.01. Despite a decrease in resistance to fatigue, p </= 0.05, myosin heavy chain composition, ATP, ADP, creatine, and creatine phosphate concentrations of the FHL were not altered. The results of this study describe an animal model that mimics many of the training parameters and physiological adaptations observed with human progressive resistance exercise.