Bioenergetics is a complex branch of biochemistry that focuses on how cells transform energy, often by producing, storing, or consuming adenosine triphosphate (ATP), or put more simply, the study of how cells transform energy from one form to another. For personal trainers, this helps us understand how muscles receive and use the energy to do external work that we assign to our clients. This discussion will also elucidate how and why the intensity of a muscular effort determines the duration of the exercise.

Dr. Kenneth Cooper, known in the fitness industry as “The Father of Aerobics”, defined aerobic activity in 1970 as that during which the cardiorespiratory system provides sufficient oxygen for muscular effort. Any non-sequential muscular effort, such as turning one’s head, entails some measure of energy production in the absence of oxygen, qualifying it as anaerobic.

Several bioenergetic pathways can be utilized to accomplish energy production:

  • Phosphagen (Creatine-ATP)
  • Glycolysis
  • Gluconeogenesis
  • Ketosis
  • Lipolysis (Aerobic)
  • Oxidative (Aerobic)

We will discuss the most common pathways during exercise below.

Phosphagen Bioenergetic Pathway: ATP Production

To release energy, the contractile mechanism of the muscle cell requires 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 draws the actin and myosin filaments together to yield the protein actomyosin, while also producing a contraction of those fibers within the motor unit.

ADP and Pi reform in the presence of another enzyme, creatine phosphate (CP), thereby releasing the Pi from creatine phosphate (CP) to generate ATP for muscular contraction.

We can think of these reactions as mathematical equations:

ATP + CP = ADP + C + Pi + Energy

ATP + actin + myosin = ADP + Pi + actomyosin

Such reactions continue until all the ATP gets utilized, generally within three seconds. Thus, the immediate source of energy for all muscular effort is ATP-CP.  The difference in the amount of work produced represents a function of the number of muscle fibers and groups recruited within that period.

Likewise, the duration of muscle action that depletes the available ATP relies on the quantity of ATP-CP stored as well as rest time between efforts; if sufficient, more such actions may be completed. Hence, the energy for short power bursts and weight-lifting regimens hails directly from ATP-CP. Glycolysis is the process by which the body produces ATP from available carbohydrates after the stores of cellular ATP have been depleted.


As stated above, the anaerobic process of glycolysis can break down the body’s glycogen stores to render glucose, yielding additional ATP to power muscle contractions. In the presence of oxygen, this aerobic reaction can yield 38 ATP per molecule of glucose.

However, in the absence of sufficient oxygen, anaerobic metabolism will only provide enough ATP (2 ATP/molecule of glucose) to continue muscle action for an additional 90-120 seconds. This non-oxidative glycolysis also yields two by-products for which energy must be expended to remove: CO2 and lactic acid.

Lactic acid has long been the scapegoat for post-exercise muscle soreness. However, lactate clearance occurs simultaneously with its production and continues for several minutes after intense exercise, rendering next-day soreness likely due to musculotendinous micro-injury, not lactic acid accumulation.

Oxidative System: Aerobic Capacity and Anaerobic Threshold

We often reference the concept of the “anaerobic threshold”, defined as approximately 60% of one’s maximal aerobic capacity when CO2 production starts to exceed O2 (evidenced by the need to breathe harder). Hyperventilation, a consequence of the overproduction of lactate relative to the buffering ability of the body, reflects its attempt to deacidify the blood. RQ climbs rapidly towards 1.0 and beyond, up to 1.5 in very well-trained athletes.

Evidence supports lactic acid conversion into fuel for muscle contractions. As long as respiration continues bringing in oxygen, some lactic acid can re-enter aerobic metabolism–the Krebs or citric acid cycle–after reformation into glucose in the liver.

At some point, however, lactate accumulation interferes with the very muscle function for which it was previously providing energy, resulting in muscle fatigue. Once the exerciser is forced into muscle failure, 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.

When working out at an intensity below the threshold, the production of ATP will last as long as glycogen in the muscle yields glucose. Our blood supply and our liver reign as the two primary sources of exogenous glucose. Blood-borne glucose, in the presence of insulin, moves across the cell membrane for use by the muscles.

Nutritional Production of Long-Term Energy

While many individuals live to eat, food ultimately provides the calories we burn during muscle activity. Digestion further breaks down the food into usable forms of energy: sugars, amino acids, fatty acids, vitamins, and minerals. The human body’s preferred source of energy, glucose, comes from the breakdown of carbohydrates.

In the absence of ample available carbs, the body can resort to utilizing fats and proteins to produce energy. The term lipolysis refers to the body’s use of fats to yield energy, breaking them down into fatty acid and glycerol molecules.

Scientists have devised a method for determining which nutrient the body prefers by measuring the amount of oxygen and carbon dioxide consumed. The term Respiratory Quotient (RQ) refers to the ratio of these two quantities:

RQ = volume of CO2 produced/volume of Oconsumed

Carbohydrate usage renders an RQ = 1.0, while proteins equate to .8 and lipolysis results in an RQ of 0.71. (RQ can be lower with certain disease states like diabetes and/or late-stage starvation). At rest and when consuming a mixed diet, the RQ levels out at about 0.83. The higher the intensity of exercise, the more our muscles will depend on a readily-available carbohydrate source within the muscle.

The Impact of Intensity

To a great extent, exercise intensity determines 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 lean more oriented toward CO2 and lactate removal than O2 supply.

Sometimes the cardiorespiratory system cannot deliver sufficient oxygen for an athlete’s needs. Longer sprints 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. As mentioned above this window lasts anywhere from 90-120 seconds.

If the athlete remains determined to continue, they may successfully do so by reducing the intensity of the exercise, to the point where the body can manage lactate production to avoid further buildup. Assuming not all available carbohydrate stores (glycogen and glucose) get depleted during the initial accumulation of lactate, exercise can continue for several more hours.

Take-Home Message

Bioenergetics are not exactly simple to grasp when taken in-depth. However, understanding the basic processes outlined above will help you to recognize your clients’ fitness and thresholds. Through careful program design, we can train and condition these three energy systems to maximize each one’s ability (within genetically determined limits). Savvy trainers can choose exercises for their clients to match their goals while taking advantage of the human body’s bioenergetics systems.


  2. G.A. Brooks, T.D. Fahey, Exercise Physiology: Human Bioenergetics and Its Applications, John Wiley & Sons, 1984
  3. H.A. DeVries, Physiology of Exercise for Physical Education and Athletics, Third Edition, Wm. C. Brown Company, 1980
  4. D.K. Mathews, E.L. Fox, The Physiological Basis of Physical Education and Athletics, Second Edition, W.B. Saunders Company, 1976
  5. M.F. Bergeron, “Lactic acid production and clearance during exercise”, National Strength and Conditioning Association Journal, 13(

Cathleen Kronemer

Cathleen Kronemer is an NFPT CEC writer and a member of the NFPT Certification Council Board. Cathleen is an AFAA-Certified Group Exercise Instructor, NSCA-Certified Personal Trainer, ACE-Certified Health Coach, former competitive bodybuilder and freelance writer. She is employed at the Jewish Community Center in St. Louis, MO. Cathleen has been involved in the fitness industry for over three decades. Feel free to contact her at She welcomes your feedback and your comments!