This paper was prepared in partial fulfillment for Doctoral Degree in Physical 
Therapy at Texas Woman's University in Houston, TX. Resistance training is 
frequently used in rehabilitation to improve musculoskeletal function. The 
increased ability of skeletal muscle to generate force following resistance 
training results from two important changes: 1) the adaptation of the muscle 
fiber, and 2) the extent to which the motor unit can activate the muscle (neural 
adaption). The purpose of this article is to provide a review of research 
investigating the effects of resistance training on muscle fibers and on nervous 
system input. Muscle fiber adaptations caused by resistance training include 
increased cross-sectional area of the muscle (hypertrophy, hyperplasia, or 
both), selective hypertrophy of fast twitch fibers, decreased or maintained 
mitochondrial number and capillary density of muscle, and possible changes in 
energy sources. Changes in nervous system input resulting from resistance 
training include recruitment of an increased number and firing rate of motor 
units, increased reflex potentiation, and improved synchronization. An 
understanding of the adaptations occurring in muscle in response to resistance 
training provides a fundamental basis for which appropriate clinical exercise 
training programs can be developed for the rehabilitation of patients. J Orthop 
Sports Phys Ther 1990;12(6):248-255.