The document discusses bioenergetics and the three biological energy systems that replenish ATP in human skeletal muscle during exercise: (1) the phosphagen system provides ATP primarily for short-term, high-intensity activities; (2) glycolysis breaks down carbohydrates to resynthesize ATP; (3) the oxidative system uses primarily carbohydrates and fats as substrates to provide ATP at rest and during low-intensity activities. The extent to which each system contributes depends on the intensity and duration of muscular activity, with the phosphagen system supplying ATP for short, high-intensity activities and the oxidative system for long, low-intensity activities.

1. chapter
Bioenergetics
2
of Exercise And
Training
Bioenergetics
of Exercise
and Training
Joel T. Cramer, PhD; CSCS,*D; NSCA-CPT,*D; FNSCA

2. Chapter Objectives
• Understand the terminology of bioenergetics and
metabolism related to exercise and training.
• Discuss the central role of ATP in muscular activity.
• Explain the basic energy systems present in human
skeletal muscle.
• Recognize the substrates used by each energy
system.
• Develop training programs that demonstrate an
understanding of bioenergetics and metabolism.

3. Section Outline
• Essential Terminology

4. Key Terms
• bioenergetics: The flow of energy in a biological
system; the conversion of macronutrients into
biologically usable forms of energy.
• catabolism: The breakdown of large molecules into
smaller molecules, associated with the release of
energy.
• anabolism: The synthesis of larger molecules from
smaller molecules; can be accomplished using the
energy released from catabolic reactions.
(continued)

5. Key Terms (continued)
• exergonic reactions: Energy-releasing reactions that
are generally catabolic.
• endergonic reactions: Require energy and include
anabolic processes and the contraction of muscle.
• metabolism: The total of all the catabolic or exergonic
and anabolic or endergonic reactions in a biological
system.
• adenosine triphosphate (ATP): Allows the transfer of
energy from exergonic to endergonic reactions.

6. Chemical Structure
of an ATP Molecule
• Figure 2.1 (next slide)
– (a) The chemical structure of an ATP molecule
including adenosine (adenine + ribose), triphosphate
group, and locations of the high-energy chemical
bonds.
– (b) The hydrolysis of ATP breaks the terminal
phosphate bond, releases energy, and leaves ADP,
an inorganic phosphate (Pi), and a hydrogen ion (H+).
– (c) The hydrolysis of ADP breaks the terminal
phosphate bond, releases energy, and leaves AMP,
Pi, and H+.

7. Figure 2.1

8. Section Outline
• Biological Energy Systems
– Phosphagen System
• ATP Stores
• Control of the Phosphagen System
– Glycolysis
•
•
•
•
•
Glycolysis and the Formation of Lactate
Glycolysis Leading to the Krebs Cycle
Energy Yield of Glycolysis
Control of Glycolysis
Lactate Threshold and Onset of Blood Lactate
(continued)

9. Section Outline (continued)
• Biological Energy Systems
– The Oxidative (Aerobic) System
•
•
•
•
Glucose and Glycogen Oxidation
Fat Oxidation
Protein Oxidation
Control of the Oxidative (Aerobic) System
– Energy Production and Capacity

10. Biological Energy Systems
• Three basic energy systems exist in muscle
cells to replenish ATP:
– The phosphagen system
– Glycolysis
– The oxidative system

11. Key Point
• Energy stored in the chemical bonds of
adenosine triphosphate (ATP) is used to
power muscular activity. The replenish-ment
of ATP in human skeletal muscle is
accomplished by three basic energy
systems: (1) phosphagen, (2) glycolytic,
and (3) oxidative.

12. Biological Energy Systems
• Phosphagen System
– Provides ATP primarily for short-term, high-intensity
activities (e.g., resistance training and sprinting) and
is active at the start of all exercise regardless of
intensity

13. Biological Energy Systems
• Phosphagen System
– ATP Stores
• The body does not store enough ATP for exercise.
• Some ATP is needed for basic cellular function.
• The phosphagen system uses the creatine kinase
reaction to maintain the concentration of ATP.
• The phosphagen system replenishes ATP rapidly.
– Control of the Phosphagen System
• Law of mass action: The concentrations of reactants or
products (or both) in solution will drive the direction of the
reactions.

14. Biological Energy Systems
• Glycolysis
– The breakdown of carbohydrates—either glycogen
stored in the muscle or glucose delivered in the
blood—to resynthesize ATP

15. Glycolysis
• Figure 2.2 (next slide)
– ADP = adenosine diphosphate
– ATP = adenosine triphosphate
– NAD+, NADH = nicotinamide adenine dinucleotide

16. Figure 2.2

17. Biological Energy Systems
• Glycolysis
– The end result of glycolysis (pyruvate) may proceed
in one of two directions:
1) Pyruvate can be converted to lactate.
• ATP resynthesis occurs at a faster rate but is limited in
duration.
• This process is sometimes called anaerobic glycolysis (or
fast glycolysis).
(continued)

18. Biological Energy Systems
• Glycolysis
– The end result of glycolysis (pyruvate) may proceed
in one of two directions (continued):
2) Pyruvate can be shuttled into the mitochondria.
• When pyruvate is shuttled into the mitochondria to undergo
the Krebs cycle, the ATP resynthesis rate is slower, but it
can occur for a longer duration if the exercise intensity is
low enough.
• This process is often referred to as aerobic glycolysis (or
slow glycolysis).

19. Biological Energy Systems
• Glycolysis
– Glycolysis and the Formation of Lactate
• The formation of lactate from pyruvate is catalyzed by the
enzyme lactate dehydrogenase.
• The end result is not lactic acid.
• Lactate is not the cause of fatigue.
• Glucose + 2Pi + 2ADP → 2Lactate + 2ATP + H2O

20. Cori Cycle
• Figure 2.3 (next slide)
– Lactate can be transported in the blood to the liver,
where it is converted to glucose.
– This process is referred to as the Cori cycle.

21. Figure 2.3

22. Biological Energy Systems
• Glycolysis
– Glycolysis Leading to the Krebs Cycle
• Pyruvate that enters the mitochondria is converted to
acetyl-CoA.
• Acetyl-CoA can then enter the Krebs cycle.
• The NADH molecules enter the electron transport system,
where they can also be used to resynthesize ATP.
• Glucose + 2Pi + 2ADP + 2NAD+ → 2Pyruvate + 2ATP +
2NADH + 2H2O

23. Biological Energy Systems
• Glycolysis
– Energy Yield of Glycolysis
• Glycolysis from one molecule of blood glucose yields a net
of two ATP molecules.
• Glycolysis from muscle glycogen yields a net of three ATP
molecules.

24. Biological Energy Systems
• Glycolysis
– Control of Glycolysis
• Stimulated by high concentrations of ADP, Pi, and ammonia
and by a slight decrease in pH and AMP
• Inhibited by markedly lower pH, ATP, CP, citrate, and free
fatty acids
• Also affected by hexokinase, phosphofructokinase, and
pyruvate kinase
– Lactate Threshold and Onset of Blood Lactate
• Lactate threshold (LT) represents an increasing reliance on
anaerobic mechanisms.
• LT is often used as a marker of the anaerobic threshold.

25. Key Term
• lactate threshold (LT): The exercise intensity
or relative intensity at which blood lactate
begins an abrupt increase above the baseline
concentration.

26. Lactate Threshold (LT) and OBLA
• Figure 2.4 (next slide)
– Lactate threshold (LT) and onset of blood lactate
accumulation (OBLA)

27. Figure 2.4

28. Biological Energy Systems
• Glycolysis
– Lactate Threshold and Onset of Blood Lactate
• LT begins at 50% to 60% of maximal oxygen uptake
in untrained individuals.
• It begins at 70% to 80% in trained athletes.
• OBLA is a second increase in the rate of lactate
accumulation.
• It occurs at higher relative intensities of exercise.
• It occurs when the concentration of blood lactate reaches
4 mmol/L.

29. Biological Energy Systems
• The Oxidative (Aerobic) System
– Primary source of ATP at rest and during lowintensity activities
– Uses primarily carbohydrates and fats as substrates

30. Biological Energy Systems
• The Oxidative (Aerobic) System
– Glucose and Glycogen Oxidation
• Metabolism of blood glucose and muscle glycogen begins
with glycolysis and leads to the Krebs cycle. (Recall: If
oxygen is present in sufficient quantities, the end product
of glycolysis, pyruvate, is not converted to lactate but is
transported to the mitochondria, where it is taken up and
enters the Krebs cycle.)
• NADH and FADH2 molecules transport hydrogen atoms to
the electron transport chain, where ATP is produced from
ADP.

31. Krebs Cycle
• Figure 2.5 (next slide)
– CoA = coenzyme A
– FAD2+, FADH, FADH2 = flavin adenine dinucleotide
– GDP = guanine diphosphate
– GTP = guanine triphosphate
– NAD+, NADH = nicotinamide adenine dinucleotide

32. Figure 2.5

33. Electron Transport Chain
• Figure 2.6 (next slide)
– CoQ = coenzyme Q
– Cyt = cytochrome

34. Figure 2.6

35. Table 2.1

36. Biological Energy Systems
• The Oxidative (Aerobic) System
– Fat Oxidation
• Triglycerides stored in fat cells can be broken down by
hormone-sensitive lipase. This releases free fatty acids
from the fat cells into the blood, where they can circulate
and enter muscle fibers.
• Some free fatty acids come from intramuscular sources.
• Free fatty acids enter the mitochondria, are broken down,
and form acetyl-CoA and hydrogen protons.
– The acetyl-CoA enters the Krebs cycle.
– The hydrogen atoms are carried by NADH and FADH2 to the
electron transport chain.

37. Table 2.2

38. Biological Energy Systems
• The Oxidative (Aerobic) System
– Protein Oxidation
• Protein is not a significant source of energy for most activities.
• Protein is broken down into amino acids, and the amino acids are
converted into glucose, pyruvate, or various Krebs cycle intermediates to produce ATP.
– Control of the Oxidative (Aerobic) System
• Isocitrate dehydrogenase is stimulated by ADP and inhibited by
ATP.
• The rate of the Krebs cycle is reduced if NAD+ and FAD2+ are not
available in sufficient quantities to accept hydrogen.
• The ETC is stimulated by ADP and inhibited by ATP.

39. Metabolism of Fat,
Carbohydrate, and Protein
• Figure 2.7 (next slide)
– The metabolism of fat and that of carbohydrate and
protein share some common pathways. Note that all
are reduced to acetyl-CoA and enter the Krebs
cycle.

40. Figure 2.7

41. Biological Energy Systems
• Energy Production and Capacity
– In general, there is an inverse relationship between
a given energy system’s maximum rate of ATP
production (i.e., ATP produced per unit of time) and
the total amount of ATP it is capable of producing
over a long period.
– As a result, the phosphagen energy system primarily
supplies ATP for high-intensity activities of short
duration, the glycolytic system for moderate- to highintensity activities of short to medium duration, and
the oxidative system for low-intensity activities of
long duration.

42. Table 2.3

43. Table 2.4

44. Key Point
• The extent to which each of the three energy
systems contributes to ATP production
depends primarily on the intensity of
muscular activity and secondarily on the
duration. At no time, during either exercise
or rest, does any single energy system
provide the complete supply of energy.

45. Section Outline
• Substrate Depletion and Repletion
– Phosphagens
– Glycogen

46. Substrate Depletion and Repletion
• Phosphagens
– Creatine phosphate can decrease markedly
(50-70%) during the first stage (5-30 seconds) of
high-intensity exercise and can be almost eliminated
as a result of very intense exercise to exhaustion.
– Postexercise phosphagen repletion can occur in a
relatively short period; complete resynthesis of ATP
appears to occur within 3 to 5 minutes, and
complete creatine phosphate resynthesis can occur
within 8 minutes.

47. Substrate Depletion and Repletion
• Glycogen
– The rate of glycogen depletion is related to exercise
intensity.
• At relative intensities of exercise above 60% of maximal
oxygen uptake, muscle glycogen becomes an increasingly
important energy substrate; the entire glycogen content of
some muscle cells can become depleted during exercise.

48. Substrate Depletion and Repletion
• Glycogen
– Repletion of muscle glycogen during recovery is
related to postexercise carbohydrate ingestion.
• Repletion appears to be optimal if 0.7 to 3.0 g of
carbohydrate per kg of body weight is ingested every
2 hours following exercise.

49. Section Outline
• Bioenergetic Limiting Factors in Exercise
Performance

50. Table 2.5

51. Section Outline
• Oxygen Uptake and the Aerobic and
Anaerobic Contributions to Exercise

52. Low-Intensity, Steady-State
Exercise Metabolism
• Figure 2.8 (next slide)
.
– 75% of maximal oxygen uptake (VO2max)
– EPOC = excess postexercise oxygen consumption
.
– VO2 = oxygen uptake

53. Figure 2.8

54. Key Term
• excess postexercise oxygen consumption
(EPOC): Oxygen uptake above resting values
used to restore the body to the preexercise
condition; also called postexercise oxygen
uptake, oxygen debt, or recovery O2.

55. High-Intensity, Non-Steady-State
Exercise Metabolism
• Figure 2.9 (next slide)
– 80% of maximum power output
.
– The required VO2 here is the oxygen uptake that
would be required to sustain the exercise if such an
uptake were possible to attain. Because it is not
possible, the oxygen deficit lasts for the duration of
the exercise.
– EPOC = excess postexercise oxygen consumption
.
– VO2max = maximal oxygen uptake

56. Figure 2.9

57. Table 2.6

58. Section Outline
• Metabolic Specificity of Training
– Interval Training
– Combination Training

59. Metabolic Specificity of Training
• The use of appropriate exercise intensities
and rest intervals allows for the “selection”
of specific energy systems during training
and results in more efficient and productive
regimens for specific athletic events with
various metabolic demands.

60. Metabolic Specificity of Training
• Interval Training
– Interval training is a method that emphasizes
bioenergetic adaptations for a more efficient energy
transfer within the metabolic pathways by using
predetermined intervals of exercise and rest periods.
• Much more training can be accomplished at higher
intensities
• Difficult to establish definitive guidelines for choosing
specific work-to-rest ratios

61. Table 2.7

62. Metabolic Specificity of Training
• Combination Training
– Combination training adds aerobic endurance
training to the training of anaerobic athletes in order
to enhance recovery (because recovery relies
primarily on aerobic mechanisms).
• May reduce anaerobic performance capabilities, particularly
high-strength, high-power performance
• Can reduce the gain in muscle girth, maximum strength,
and speed- and power-related performance
• May be counterproductive in most strength and power
sports