ARCHITECTURAL AND MECHANICAL-PROPERTIES OF THE RAT ADDUCTOR LONGUS - RESPONSE TO WEIGHT-LIFTING TRAINING

Background: The primary objective of this study was to determine the e ffects of an 8 week weight-lifting program on the mechanical, histoche mical, and architectural properties of the rat adductor longus muscle, a predominantly slow adductor muscle. Methods: The weight-lifting pro gram was progressive such that the rats were performing three bouts of ten lifts with 300% body weight load every other day during the last 3 weeks of training, The in situ mechanical properties, fiber type com position, and architectural characteristics of the muscle were determi ned in control and weight-trained rats, Intramuscular electromyographi c recordings were used to verify the recruitment of the adductor longu s during the lifting task. Results: The adductor longus was composed p redominantly of slow fibers (similar to 80% slow oxidative) and had a relatively simple architectural design, i,e., one motor end-plate band near the center of the muscle, virtually no angle of pinnation of the fibers from the line of pull, and a fiber length:muscle length ratio of 0.72, The mean fiber type composition and fiber size, the total fib er number, and the mean physiological cross-sectional area of the addu ctor longus were similar in the two groups of rats. The mean body weig ht of weight-lifting rats was significantly less than control, The wei ght of the adductor longus relative to body weight and its fatigue res istance were higher and the maximum rate of shortening was slower in w eight-lifting than in control rats, No other mechanical property was s ignificantly affected by the training program. Conclusions: The result s indicate that approximately 1 minute of overload every other day by physiological recruitment of motor units can induce remodeling of the adductor longus of growing rats; i,e,, the trained muscles were slower and less fatigable than control, Given that the effects on the archit ectural or force-generating properties of the muscles were small, the marked improvement in the ability to lift heavier loads as the trainin g progressed appears to be more attributable to neurally related than to muscle-related phenomena. (C) 1997 Wiley-Liss, Inc.