In order for clients to succeed, they need to exercise at the right intensity. However, a lot of confusion exists regarding terminology surrounding exercise intensity. We all hear terms like “Fat-burning Zone,” “High-intensity,” “Aerobic Training.” This creates a problem in accurately defining an optimal exercise program. The solution is to look at the source of where these terms actually come from.
Traditionally, cardiorespiratory exercise intensities have been defined as a percentage of an individual’s maximum oxygen uptake1 (VO2 MAX), or more correctly the oxygen uptake reserve (VO2 MAX = VO2R – VO2 RESTING). VO2 testing is the accepted gold standard for defining cardiorespiratory fitness.
Why oxygen uptake? The body converts energy for use in different ways depending on the available oxygen supply to the muscle. The term “Aerobic” is used to describe the metabolic pathways that use oxygen. The term “Anaerobic” is used to describe the metabolic pathways that operate without oxygen. At no time, during exercise or rest, does any single system provide the complete supply of energy. The extent to which the body uses a given energy system is dependent primarily on the activity intensity2.
Oxygen availability at the muscle is the key determinant of how the body responds to a given exercise intensity. The definitions for activity intensity relate to the dominant energy system being used for the activity, Aerobic versus Anaerobic. This is why oxygen uptake (VO2) is the basis for defining exercise intensities.
At low to moderate activity levels the body can supply the needed oxygen to support the Aerobic energy system. At higher exercise intensities, where oxygen cannot be supplied to the muscles at a sufficient rate, Anaerobic energy systems supplement the Aerobic system. Higher intensity levels demand a greater percentage of energy to be provided by Anaerobic systems.
One of the key distinctions between moderate and high intensity is the building up of metabolism byproducts within the body – specifically, the build-up of Lactic Acid within the muscle and Lactate within the blood as a result of Anaerobic metabolism. The terms Anaerobic Threshold (AT) and Lactate Threshold (LT) are used to describe the significant increase of these byproducts within the body. These thresholds have a similar physiological basis, but are not synonymous. Knowing either the AT or the LT is important to accurately determine what is to be considered as “moderate” versus “hard” intensity for a given client.
These thresholds occur at approximate percentages of an individual’s VO2 MAX. Studies have shown these thresholds to occur at 50% to 60% of maximal oxygen uptake (VO2 MAX) in untrained subjects and at 70% to 80% in trained subjects3. Consequently, physical activity intensity levels are commonly defined as a percentage of the maximal oxygen uptake.
The below table is from the ACSM Guidelines1. This shows the breakdown of various intensity zones based on the oxygen uptake reserve. (See Figure 1)
Since at “light to moderate” intensity training levels the body has sufficient oxygen supplies for oxidizing fat, these lower intensities are recommended by some as the “Fat-burning Zone.” Knowing the upper limit of the moderate intensity zone allows for a more optimal burning of calories from fat while maximizing the calories burned.
At “hard” intensity training levels, the energy demand is beyond what can be provided by the Aerobic system alone. The body is above its Anaerobic Threshold. This stressing of the Aerobic system provides stimulus for the body to increase its Aerobic capacity. Thus, this is typically called “Aerobic-training Zone.”
Most of the methods, charts, and graphs for determining proper exercise intensity have their origins from studies in VO2 testing. Now that it is practical to do VO2 outside of a university setting, it is possible to use this core science to determine the correct exercise intensity level needed for your clients to succeed.
[1] Whaley, M.H., Brubaker, P.H. ACSM Guidelines for Exercise Testing and Prescription. Seventh Edition. Lippincott William & Wilkins. Philadelphia, PA. 2006.
[2] Baechle TR, Earle RW. Essentials of Strength Training and Conditioning. National Strength and Conditioning Association. Human Kinetics. Champaign, IL, 2000.
[3] Farrel, P.A., Wilmore J.H. Plasma Lactate Accumulation and Distance Running Performance. Med. Sci. Sports 11(4):338-344. 1979.
Scott Kofoed, Ph.D is the President of KORR Medical Technologies. He can be contacted at 801.483.2080, or by email atscott@korr.com, or visit www.korr.com.
