Energy system interaction and contribution during maximal exercise
Introduction
The article explores the interaction and relative contributions of various energy systems during maximal exercise. It emphasizes the critical role of the phosphagen, glycolytic, and oxidative systems in sustaining high-intensity exercise.
Energy System Interaction
During maximal exercise, energy systems interact dynamically to meet the high demands for ATP. The phosphagen system provides immediate energy, the glycolytic system contributes significantly during short bursts, and the oxidative system supports sustained exercise.
The anaerobic systems produce energy at an enormous rate, but the total amount of energy they produce is relatively small. The aerobic system produces energy at a lower rate than the anaerobic systems, but its total capacity for energy production is essentially unlimited.
Earlier analyses set forth the idea that the systems operate discretely, but this is inaccurate. One energy system may predominate over a given time period, but the systems interact throughout the duration of exercise, and their total energy production in high intensity effort equalizes at about 75 seconds of work.
Phosphagen System
The phosphagen system, which includes ATP and phosphocreatine (PCr), provides rapid energy for short, high-intensity efforts. This system is critical for activities like sprinting and weightlifting. Cells can store only small quantities of ATP, so PCr acts as a buffer for near-instant high rate of energy production.
Glycolytic System
The glycolytic system breaks down glucose to produce ATP through anaerobic glycolysis. This system becomes prominent during high-intensity efforts lasting up to a few minutes, such as 400-meter sprints or high-intensity interval training.
The anaerobic systems can produce ATP at a very high rate. Weightlifters produce instantaneous power that is 10-20 times greater than maximum aerobic power. Sprinters can produce power 3-5 times greater than max aerobic power. Experiments have shown that the anaerobic systems are generally capable of about 2.0 – 2.6 times the power output of the aerobic system.
PCr utilization peaks at about 1.3 seconds of max intensity effort and then degrades. Glycolysis peaks at about 5 seconds and then slowly declines after a few second. Glycolysis is probably inhibited by enzymatic activity, because of declining pH.
Oxidative System
The oxidative system generates ATP through aerobic metabolism, using oxygen to oxidize carbohydrates, fats, and occasionally proteins. This system is essential for endurance activities, providing sustained energy over long periods.
The aerobic system contributes energy shortly after the start of high intensity exercise. In fact, aerobic power can reach 90% of VO2max between 30 and 60 seconds of high intensity effort. Even the 30-second Wingate test is around 30% aerobic.
An experiment features 10 rounds of sprints lasting 6 seconds (on the minute for 10 minutes) showed a 64% drop in anaerobic activity, but only a 27% loss in power. The oxidative system filled the majority of the power gap.
The journey to VO2max features a rapid exponential increase in aerobic energy to a sub-maximal level, followed by a slower increase to VO2max. If an athlete is working at intensity above VO2max, the oxidative system may not reach full aerobic contribution
Relative Contribution of Energy Systems
The relative contribution of each energy system varies with the intensity and duration of exercise. Short, intense activities rely heavily on the phosphagen and glycolytic systems, while prolonged, moderate-intensity activities primarily use the oxidative system.
During high intensity activities, the total energy produced is mostly anaerobic early on. But after about 75 seconds, the aerobic system and anaerobic systems have contributed equally.
Likewise, endurance trained athletes generally have higher VO2 maxes than sprinters, but sprinters have been observed to have higher / faster oxygen kinetics, meaning their aerobic system turns on and contributes faster.
Measuring Energy System Contributions
Aerobic energy is easy to measure via respiratory analysis which can determine the precise contribution of fats and carbohydrates consumed aerobically. But anaerobic energy is harder to measure, because it is an intracellular process. Measures of blood lactate can indicate glycolysis, but do not account for PCr activity.
Oxygen debt is an inaccurate measure because it overstates anaerobic contribution. Other factors like temperature increases and hormonal variations can also increase oxygen debt and create inaccurate measurements. Mechanical erg tests can measure work performed but cannot accurately apportion the energy system contribution.
Accurate measurement of max efforts using oxygen deficit is possible if there is quantification of the relationship between oxygen consumption and submaximal efforts. Combining oxygen deficit measurements with needle biopsy measures of metabolites allows more accurate measurements of glycolytic by-products.
Training Implications
While the traditional, system-specific protocols remain valid, athletes whose training features high intensity activities lasting longer than a minute or so should incorporate intensive aerobic conditioning across all muscle fibers.
KEY TERMINOLOGY
Phosphagen System: An energy system that provides immediate ATP through stored ATP and phosphocreatine, crucial for short, high-intensity activities.
Glycolytic System: An anaerobic energy system that breaks down glucose to produce ATP, prominent during high-intensity efforts lasting up to a few minutes.
Oxidative System: An aerobic energy system that generates ATP through the oxidation of carbohydrates, fats, and proteins, essential for endurance activities.
Anaerobic Glycolysis: The breakdown of glucose without oxygen, producing ATP and lactate.
Aerobic Metabolism: The production of ATP using oxygen to oxidize nutrients.
ATP (Adenosine Triphosphate): The primary energy carrier in cells, essential for muscle contractions and various cellular processes.
Phosphocreatine (PCr): A high-energy phosphate compound in muscles, used to rapidly regenerate ATP during high-intensity exercise.
Lactate: A by-product of anaerobic glycolysis, associated with muscle fatigue.
Respiratory Gas Analysis: A method to measure oxygen consumption and carbon dioxide production to assess energy expenditure during exercise.
Muscle Biopsy: A procedure to obtain a small sample of muscle tissue for analysis of energy substrates and enzymes.