Cardio' refers to the heart and 'Respiratory' refers to the lungs. In other words, the referent is the circulatory system for oxygen that is taken into the blood through the lungs. Cardio-respiratory fitness measure helps to your body’s circulatory and respiratory system how well it is able to transport oxygen to your muscles during prolonged exercise and also of how well your muscles are able to absorb and use the oxygen, once it has been delivered, to generate adenosine triphosphate (ATP) energy via cellular respiration (cellular respiration is a chemical process in your body's cells that converts the energy stored in the food you eat into the ATP form of energy that is recruited for use by your muscles). Essentially, your cardio respiratory fitness level is a measure of the strength of your aerobic energy system. If you haven't already read the exercise energy systems (EES) article you can do so to get a better understanding of what ATP is, what cellular respiration and what the aerobic energy system.The purpose of the study was to investigate the effect of cardio respiratory training and core strength training on selected physical and physiological variables among college football players.