Bioenergetics: How Humans Get The Energy To Move
Human movement, whether involving contractility of the myocardium, the series of contractions involved in the toss of a ball, or muscular activation involved in stepping up a stair, is an energy dependent process. Despite the necessity of energy for sustainable life and function, bioenergetics is often a poorly understood concept. Bioenergetics is a term used to describe the science of energy formation, transfer and use within a biological system.
From an elementary prespective, bioenergetics may be viewed as the processes involved in transferring energy found in a common food source, such as a potato, which can be eaten by a human, undergo digestion and absorption, and pay the liver a visit for conversion into blood glucose, which can supply the muscle with a substrate that allows it to produce usable energy, adenosine tri phosphate (ATP). This, may then enable the muscle to contract to help in manifesting a desired effort or movement. In industrial terms, the transfer of energy is commonly seen in the combustion of fuel which drives an engine to operate.
While the mechanics are considerably different, the overall concept is the same: energy is required for movement. In this chapter, the fundamentals of bioenergetics will be discussed. Readers are encouraged to develop a basic understanding of the following concepts and to be able to employ concepts and terminology described here in conversational terms. If you can talk about it competently, there is a good chance that you understand it and have mastered it.
Thermodynamics and its Application
Thermodynamics refers to the science of energy exchange and/or transfer. This is very similar to bioenergetics, but bioenergetics refers exclusively to the exchange or transfer of energy within biological systems. The first law of thermodynamics states that energy cannot be created or destroyed, but can be changed from one form to another. The implication of this is that while a system and the environment can exchange energy, the total energy within the universe remains constant.
In more practical terms, we consume energy acquired from our environment, convert it to usable energy and perform biologic work with this energy. The energy is changed, but it is not destroyed nor is it magically created. Rather, it is converted to different forms of energy and transferred back to the environment, for example in the production of heat or the work performed in relocating of a stack of bricks. Or, some of it may be retained by the body as manifested by the accumulation of body fat or an increase in muscle mass.
The second law of thermodynamics states that energy transfer always proceeds in the direction of increased entropy. Entropy refers to disorder or randomness. Therefore, this law indicates that the universe’s energy moves toward disorder. Does this law make perfect sense when you consider the hypertrophy of a muscle or the enlargement of a fat deposit? Initially, it may not as such a process results in something that is more organized than its reactants; however, consider that much energy had to be liberated (energy releasing reactions drive energy absorbing reactions as discussed below) and considerable heat released in order to allow for these energy depots to be developed.
Enthalpy refers to the change in energy associated with a reaction. Enthalpy can be measured as the total change in heat caused by a reaction as it moves to equilibrium. The energy harnessed from such a reaction that is available to allow work production (i.e., useful energy) is termed “free energy.” This is the energy that is referred to when considering the ability to perform biologic work.
The Basics of Usable Energy
The rate at which energy can be produced to allow human movement and to power physical performance is dependent upon cellular factors. More specifically, the rate of energy production is governed by the energy systems that are available and how quickly they can resynthesize ATP. Rapid movements that require maximal power output for very short periods of time are virtually entirely dependent upon the ATP-creatine phosphate system (commonly referred to as the phosphagen system or the ATP-CP system).
Intense, sustained movements, such as running a 400 m race (in a young, healthy adult or youth) are dependent upon, not only the phophagen system, but largely upon a slower pathway of ATP production, glycolysis, and to a much smaller extent, the slowest pathway of producing ATP, oxidative phosphorylation. As the duration of maximal effort work is sustained, the absolute power output will decline as there is a shift in the energy system that is most dominant in resynthesizing ATP. In this sense, energy systems may represent a continuum that ranges from rapid energy production to slow energy production. Likewise, this continuum could also be labeled, respectively, as “very limited” to “virtually unlimited.”
It is important to note that at any given time, all energy systems are in operation. While we often like to classify activities as anaerobic or aerobic, each energy system is making a contribution to the total energy production of the organism. For example, if a person is performing maximal power output cycle ergometry, most assuredly, the majority of energy used to supply the actively contracting muscles is being derived from the phosphagen system and anaerobic glycolysis. However, what energy system would the arms be using? How about the heart, or the smooth muscle? Perhaps the only time in which an animal would be in a true anaerobic state would be during the final moment of life.
Heat and Efficiency
Metabolic pathways have been reported to have efficiencies ranging from approximately 31-50% [2,3]. In contrast to mechanical instruments, such as automobile engines, in which inefficiency is a bad thing, some degree of inefficiency is critical to sustain biological systems. If a human were close to 100% efficient in their utilization of substrates for energy production, the organism may suffer hypothermia well before it was able to see the first sunrise. Consequently, humans would not exist as they do today. It is the inefficiency of metabolic pathways that allows the modern human to sustain an average resting body temperature of approximately 98.6° F (37.3°C). Heat that is released during metabolic reactions is dissipated from the body through cooling mechanisms.
Metabolic heat production facilitates the occurrence of other chemical reactions by inducing an increase in enzyme activity. The temperature related increase in enzyme activity is known as the Q10 effect and it refers to a doubling of the enzyme activity for every 10°C increase in temperature. While such an increase in brain temperature would be deadly, muscle temperature may increase by 10°C during sustained, intense exercise [2]. Thus, as work intensity increases and more metabolic heat is produced, enzymes that facilitate energy production via metabolic pathways will be incrementally more active.
Chemical reactions that produce heat are known as exothermic reactions. Conversely, chemical reactions in which heat is absorbed are known as endothermic reactions. An exergonic reaction allows for the spontaneous release of energy. It should be noted, however, that exergonic reactions do involve a small energy input often referred to as the “cost of activation” or the “activation energy.” Thus, “spontaneous” may be a misnomer. Enzymes reduce the activation energy of a given reaction. If the energy given off in an exergonic reaction is heat, the reaction is termed “exothermic.” Examples of exergonic reactions include the energy liberating combustion of an ATP molecule and substrate oxidation, which is also an energy liberating process. By nature, these are also catabolic reactions.
In contrast to energy liberating reactions, a reaction in which the input of free energy is required for the reaction to proceed is termed an endergonic reaction. Endergonic reactions involve absorption of energy from the surrounding environment. Examples of endergonic reactions include building a glycogen molecule from glucose units, formation of a triglyceride molecule from glycerol and free fatty acids. These may be considered anabolic reactions in which a larger, higher energy molecule is being produced. Such reactions are coupled with exergonic reactions. That is, the exergonic reaction supplies the energy necessary to drive the endergonic reaction.
To piece these concepts together, the glycolytic pathway is noted to have an efficiency of approximately 31% under controlled circumstances [2]. What this means is that the usable energy produced through this pathway represents less than one third of the total energy that is liberated in the combustion of one mole of glucose. The balance of the liberated energy goes to heat production. The usable energy that is produced allows for biologic work. However, as implied above, biological work is not highly efficient. Humans can certainly improve their mechanical efficiency through repetition and training, but much energy is still lost in the performance of such work.
One way to think about this is to consider the actual energy required to pedal a cycle ergometer at 100 watts. Using standard conversion factors (1 kcal = 70 w), 100 watts is equivalent to approximately 1.43 kcal/min. Thus, if one were to cycle at this power output for 30 minutes, only 42.9 kcal would be expended! Fortunately, humans are not terribly efficient when it comes to mechanical work. This lack of efficiency requires much more energy to be expended to achieve a given power output. Based on metabolic equations for cycle ergometry [1], the energy that must be put in to produce a cycling work rate of 100 watts is approximately 7.22 kcal/min. Thus, the energy input (i.e., the actual energy being expended to perform the work) is 216.6 kcal over the 30 minute period. This sounds far better than 43 kcal, yet there are still tremendous problems in motivating people to exercise. Of course, consider the motivational problems if humans happened to be closer to70 or 80% efficient in the performance of mechanical work! In this example, the efficiency is 19.8% ( [work output/energy input] x 100). Again, much of the energy that is going into performing the work is lost as heat, rather than being realized as work output.
Summary
Bioenergetics is the science of energy transfer in living systems. The understanding of energy transfer and utilization is critical to developing a working knowledge and vocabulary for an exercise physiologist. Energy is fundamental to life and is clearly of utmost importance when considering how to optimize training or performance. Consequently, it is necessary for exercise physiologists to embrace the complexities of energy transfer, energy availability, energy production and utilization. In this chapter, the fundamentals of energy transfer, and thermodynamics have been described and concepts of efficiency discussed. In the following chapter, a more detailed analysis and discussion of the pathways that sustain energy production will be presented.
References
1. American College of Sports Medicine. (2000). ACSM’s Guidelines for Exercise Testing and Prescription, 5th ed. Media, PA: Williams & Wilkins.
2. Brooks, G.A., Fahey, T.D., White, T..P., and Baldwin, K.M. (2000). Exercise Physiology: Human Bioenergetics and Its Applications, 3rd ed. Mountain View, CA: Mayfield Publishing Company.
3. McArdle, W.D., Katch, F.I., and Katch, V.L. (2001). Exercise Physiology: Energy, Nutrition, and Performance, 5th ed. Baltimore, MD: Lippincott, Williams & Wilkins.