Exercise Science is a study that branched off of the popular "P.E." degree back in the 1970's and swayed along the science wave, incorporating a biological foundation. Specifically, it assesses the physiological, anatomical, and biomechanical adaptations brought forth by exercise.
Exercise, within the scope of the study, is quantified to fit specific parameters, and concerned particularly with maximizing human performance.
Exercise can be defined by its separation from standard physical activity through its goal-oriented nature. Physical activity is defined as having a motivation outside of the development of physical fitness. When physical activity enters the realm of upholding a purpose of improving, maintaining, or expressing a particular type of fitness, it is then labeled exercise, and is what the study of exercise science is primarily concerned with.
Furthermore, however, scripting a conclusive definition of physical fitness has proven to be moderately problematic as so many different attributes become factors. The modern elements of physical fitness are considered to be: muscular strength, muscular endurance, cardiorespiratory endurance, joint range of motion/flexibility, body composition. In earlier years, the spectrum of physical fitness included agility in the place of both muscular endurance and body composition, and school testing in Physical Education programs would assess every student through all these characteristic features, reporting results for each distinctly. Modern P.E. programs are absent in the public school systems in virtually every school aside from those in Illinois, and the physical fitness testing has taken the back seat to health education in the classroom, encouraging children to be "healthy" which comes with implications of its own. Health was originally defined as being free of disease. However, as medicine advanced, a healthy individual began to be known as one at low risk for medical need. As such, risk assessment for heart disease, diabetes, etc encompass "health" most thoroughly by today's standards.
But Exercise Science, as a branch of study, has defied the popular progression of the 'health-oriented' focus, and maintains attention and practice on exercise. Though there are some other branches of study that do this as well, the major distinguishing factor from all others that maintain an insight on exercise, is that Exercise Science is science, focusing on anatomy, cellular metabolism, the physics of body mechanics, the physiology of the energy cycles of the body, and so on. It is by no means a macro-analytical perspective.
There you have it...