Bioenergetics: The Determining Factor in Exercise Duration and Intensity

While we are all familiar with aerobic activity, defined in the early 1970s by Dr. Kenneth Cooper as activity during which the cardiorespiratory system provides enough oxygen for muscular effort, most of us associate anaerobic activity with that very hard effort we do during intervals. The fact is that each non-sequential muscular effort, such as turning your head, entails some measure of energy production in the absence of oxygen, qualifying it as anaerobic. The following discussion of bioenergetics — the study of how the body, i.e., the muscles, receives and uses the energy to do external work — will elucidate how and why the intensity of a muscular effort determines and limits the duration of the exercise.

Short Duration Energy Production within Muscle Cells

There are three primary means by which energy is released for use by muscle cells. Without going into a detailed discussion of biochemistry and cell biology, suffice it to say that the contractile mechanism of the muscle cell requires the presence of adenosine triphosphate (ATP).

When ATP splits in the presence of calcium ions, adenosine diphosphate (ADP) and an inorganic phosphate (Pi) result along with the energy released from the reaction. This energy is used to draw the actin and myosin filaments together yielding the protein, actomyosin, and producing a contraction of those fibers within the motor unit.

As the ADP and Pi reform in the presence of another enzyme that releases the Pi from creatine phosphate (CP), the consequent ATP is used to continue muscle contraction. Additionally, ATP is used to facilitate the relaxation of muscle.1,2,3

These events may be represented as:

ATP + CP  = ADP + C + Pi + Energy

ATP + actin + myosin  = ADP + Pi + actomyosin

All of these actions occur until all the ATP is used up, generally within three seconds. Thus, the immediate source of energy for all muscular effort is ATP-CP. In other words, all muscle contractions lasting fewer than three seconds, whether they are minimal or maximal, are anaerobic. The only difference in the amount of work produced is a function of the number of muscle fibers and groups recruited within that time span.(2,3) Likewise, the duration of muscle action that depletes the available ATP will be determined by the amount of ATP-CP stored and rest time between such efforts; if sufficient, more such actions may be completed. Hence, the energy for short power bursts and weight-lifting regimens is ATP-CP.

Substrate Use in the Production of Long-Term Energy

Everyone knows that one reason we eat is to have the calories to burn by muscle activity. Through a series of reactions in the alimentary system (from the mouth to the large intestines), foodstuffs are broken down into their chemical components for use by the body. These are the carbohydrates, proteins, fats, vitamins and minerals we discuss daily in our roles as educators. Given a choice in the matter, muscles prefer to use carbohydrates (or sugars) as energy sources, i.e., substrates.

However, fats and even proteins may be used. The way scientists know which substrate(s) is/are being used is by determining how much oxygen is being consumed (the difference between O2 inspired and O2 expired) and how much carbon dioxide is being produced (the difference between CO2 inspired and CO2 expired). This ratio is referred to as Respiratory Quotient (RQ):

RQ = volume of CO2 produced/volume of O2 consumed (2,3)

When RQ = 1.0, mostly carbohydrates are being used. Fats are predominant when RQ = 0.71. (Protein is rarely the sole substrate for activity except in certain disease states and during late-stage starvation; RQ = 0.8.) At rest, the RQ is about 0.83. The higher the intensity of exercise, the more dependent muscle is on the readily-available carbohydrates within the muscle and the available sugars in the bloodstream causing RQ to approach or exceed 1.0.

Two other means of energy production are available when muscle contractions are required to continue beyond three seconds. Through complex biochemical reactions, muscles access the stored glycogen within the cells and break it down (glycogenolysis, where -lysis = ‘break down’) into its simplest form, glucose; glucose is broken down further (glycolysis) to yield more ATP to power the above-described muscle contractions. When O2 is available, these reactions occur aerobically yielding 38 ATP per molecule of glucose.

However, when O2 is not being supplied fast enough or in sufficient quantity to enable these reactions to occur aerobically, anaerobic metabolism provides enough ATP (2 ATP/molecule of glucose) to continue muscle action for the next 90-120 seconds.

This non-oxidative glycolysis also yields two by-products for which energy must be expended to remove: CO2 and lactic acid.(1,2,3,4) It is this lactate that so many of us fear or strive for in our own workouts. Likewise, it is lactate we accuse for the “burn” and the pains about which we often hear our clients complain. It should be noted that lactate clearance occurs simultaneously to its production and continues for several minutes after intense exercise.(1,4) Next-day soreness is more likely a result of musculotendinous injury, not lactic acid accumulation.

The concept of the “anaerobic threshold” is a useful one physiologically but is a misnomer.(1) This threshold was noted at approximately 60% of one’s maximal aerobic capacity when CO2 production began to exceed O2 use and one began breathing harder.(2,3,4) One consequence of overproduction of lactate relative to the buffering ability of the body is hyperventilation of CO2 to deacidify the blood. RQ climbs rapidly towards 1.0 and beyond, up to 1.5 in very well-trained athletes.

At the point of deflection from the linear increase in many variables associated with metabolism blood lactate was also accumulating beyond resting values. It was assumed that this was representative of the departure of aerobic metabolism and was dubbed the “anaerobic threshold”.(2,3) Some have argued that while lactate, a very valuable and usable substrate in and of itself, does accumulate because of limited removal processes, anaerobiosis is not necessarily caused by said overproduction; other factors are involved.(1,4)

While this may appear to be a specious rationale favoring lactic acid, there is evidence that lactic acid can be converted for use as a fuel for muscle contraction. As long as respiration continues bringing in oxygen, some lactic acid is able to reenter the aerobic metabolism, the Kreb’s Cycle, after reformation into glucose in the liver.(1,4)

At some point, however, lactate accumulation interferes with the very muscle function it once provided energy for and the muscles, even the respiratory muscles, become fatigued. Exercise is thus terminated; the amount of rest time required before resuming equally intense effort depends on one’s state of conditioning and the nature of the rest period — active vs. inactive.

Finally, aerobic activity, if maintained at an intensity below the threshold where lactate accumulates too rapidly, permits the production of ATP for as long a time period as glycogen in the muscle yields glucose and all other sources of glucose outside the muscle can reach the muscle cells. The two primary sources of exogenous glucose are the blood and the liver. The liver stores glycogen extracted from the gut.

This glycogen is broken down into glucose and released into the bloodstream. (If needed, muscle proteins may be broken down and converted to glucose in the liver via gluconeogenesis.) Blood-borne glucose, in the presence of insulin, is transported across the cell membrane to be used by the muscles.

Thus, exercise intensity determines to a great extent both the type and amount of substrate use by the muscles. The availability of substrates, in addition to the availability of oxygen, determines the duration over which exercise can continue.  High intensity efforts such as power lifts, short sprints or jumps can be done without a breath; in fact, any breaths taken during these brief efforts are more oriented towards CO2 and lactate removal than O2 supply.

The cardiorespiratory system cannot deliver enough oxygen in time. Longer sprints (more than 10 seconds) and weight lifting within reasonable limits of repetitions/set provide time for O2 to be delivered to the muscles but not enough time to clear the lactate from the blood. This condition is self limiting up to 90-120 seconds but can be quite intense during that time.

If continued effort is needed or desired, intensity needs to be reduced to the point whereby lactate production can be managed to avoid further buildup. Assuming not all available carbohydrate stores, eg., glycogen and glucose, are used up in their entirety during the initial accumulation of lactate, exercise can continue for several more hours.

Fortunately for all of us, the body can use fats to yield energy, breaking them down (via lipolysis) into fatty acid and glycerol molecules to be used as we’ve discussed. Likewise, all three energy systems can be trained and conditioned to maximize each system’s ability (within genetically-determined limits) to provide the ATP needed for muscular efforts.

With proper program design, the exercises we prescribe will develop the targeted energy systems and muscle groups such that the desired goals of each client can be approached. Even if it just means turning one’s head.


1. G.A. Brooks, T.D. Fahey, Exercise Physiology: Human Bioenergetics and Its Applications, John Wiley & Sons, 1984

2. H.A. DeVries, Physiology of Exercise for Physical Education and Athletics, Third Edition, Wm. C. Brown Company, 1980

3. D.K. Mathews, E.L. Fox, The Physiological Basis of Physical Education and Athletics, Second Edition, W.B. Saunders Company, 1976

4. M.F. Bergeron, “Lactic acid production and clearance during exercise”, National Strength and Conditioning Association Journal, 13(5):47-50, 1991


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