Physiology is the scientific study that explains the physical and chemical factors responsible for the development and maintenance of life, while the physiology of sports tells us about the ultimate physical capabilities during effort (HALL, 2016). Numerous interconnected and interdependent physiological processes occur inside the human body at any moment, whether the body is in a state of rest or in a state of physical activity. This dependence is manifested through the intricate communication between cells, tissues, organs and organic systems that work perfectly. During any exercise, the activity of most systems increases significantly. The aim of this paper is to try to explain how the body adapts to increased physiological demands due to recreational sports and fitness, more precisely how the muscular system adapts to the additional demands of physical activity. In order to understand how muscles adapt to certain physiological requirements, it is first necessary to understand how movement occurs, which is also the basic element of any physical activity. Next, there is the question of who initiates and controls the action of the muscles and what the muscles need in order for muscle contraction and the movement itself to occur. When a movement occurs and in order for all further movements to take place smoothly, various chemical reactions take place inside the muscles, which are collectively called metabolism. The dominance of a particular energy system will largely determine the result of adaptive changes within the muscles. In order to understand the adjustments that occur within the muscles, we must also understand the methodology of physical activity, which is the main guideline in developing muscle abilities. Ultimately, what we are interested in is the answer to the following question; what are all the chronic adjustments that occur within the muscles to systematically engage in any of the activities? Engaging in physical activities also has some unwanted consequences such as muscle injuries, so it is important to understand the causes of injuries and how injuries can be classified according to their severity. The approach to muscle injuries will greatly affect the further engagement in activities. And finally, it is necessary to know what adjustments are achieved by our strongest muscle in the body, the heart.