At one time, the American College of Sports Medicine recommended using VO2 max--maximal oxygen consumption--to prescribe exercise. "It had been assumed that clients could achieve a given percentage of VO2 max by having them exercise at that same percentage of heart rate reserve (HRR)," according to the book "Exercise Prescription: a Case Study." Now the ACSM believes the use of VO2 reserve (VO2R) is more accurate. Your %HRR equals your %VO2R. Subtract your resting VO2 from your max VO2 to get your VO2R. Unfit individuals benefit most from using the VO2R formula.

Determine your VO2 max. For most people, VO2 max is 10 times higher than their resting VO2 (3.5ml/kg/min). (Generally, unfit individuals measure only five times higher and very fit 20 times higher. For the most accurate VO2 max reading, you must undergo an aerobic submaximal fitness test. "Resting VO2 is constant for everyone," according to the IDEA Health and Fitness Association.

Subtract your resting VO2 from your max VO2. In other words, most people would use the following formula: (3.5 x 10) - 3.5= 31.5ml/kg/min. This is your VO2R.

Put your VO2R into context. "When calculating VO2R, as when calculating heart rate reserve, it is best to determine an intensity range that includes a low end and a high end," according to the IDEA Health and Fitness Association. For example, the aforementioned individual would use the following formula to determine a target exercise range of 70 per cent to 85 per cent: (31.5 --- 0.70) + 3.5 = 25.55ml/kg/min and (31.5 --- 0.85) + 3.5 =30.75ml/kg/min.