VO2 max is typically always expressed as a value between 15 and 80, with higher being preferable. It is expressed as milliliters of oxygen used per kilogram of body weight per minute of activity (mL/kg/min). Because the measurement is based on body weight, it is similar across individuals. However, men may have higher VO2 max values than women of the same age because they have more muscle mass and thus more oxygen-using capacity. This test can only be done by a certified athletic trainer or physician's assistant. The watch is attached to your chest using straps and wires that connect to a machine called a treadmill. You walk on the treadmill while breathing in air through a tube placed in your mouth. The flow of air is measured by the machine, which calculates your maximum volume of oxygen you can breathe in during each minute of exercise. This is called your VO2 max. You must wear all clothing used during the test, including undergarments. Hanging items on the treadmill belt are not permitted. The technician monitoring the test will inform you if any accessories cannot be worn during the test. They will specify what needs to be removed or replaced. The test begins with a five-minute warm-up period at an easy pace. This allows your body to reach normal temperature after time spent sitting still. After the warm-up, you will start walking on the treadmill.