In this article some structural and physiological aspects of skeletal muscles are reviewed. One of the recent advances in this topic is the invention of isokinetic exercise systems. With isokinetic exercise systems, subjects work at a fixed speed against maximum and totally accomodating resistance. This feature allows muscle groups to reach their maximum capacity at every point in the range of motion. There is no question that muscle strength is an essential element in physical performance. Depending on the specific type of activity, the requirement for strength is also specific. This review also focuses on some important principles of strength increase and training methods.