The concept of excess post-exercise oxygen consumption (EPOC) has been described as a benefit of high-intensity exercise and a major player in weight management and weight loss–topics front and center for every certified personal trainer.
But what exactly is going on and how important is it to weight management? Why is respiration more labored sometimes more than others? Can a 60-minute workout continue to burn more calories for the rest of the day?
The answers lie in understanding the demand for oxygen by the muscles and how much oxygen reserve is currently available in the body.
Oxygen Availability
There is a supply and demand oxygen economy that takes place with intense exercise and many variables dictate how that oxygen is supplied to working muscles, especially when all the available oxygen is used up and the muscles need more. If the balance of oxygen debt exceeds the bank of reserves, the muscles will suffer. Only through increased respiration and accelerated heart rate can the oxygen debt get repaid. The body requires energy to do this!
EPOC refers to the amount of oxygen our body consumes following a bout of exercise that elicits an excess of the pre-exercise oxygen consumption at the baseline level. Essentially, the body uses more oxygen after exercise than before exercise, enabling it to expend additional calories during recovery from exercise than prior to engaging in it.
The purpose of EPOC is to restore the body to its resting state and create physiological adaptations which will help the body handle the same amount of exercise-related stress more easily in the future.
The Science and Implications Behind EPOC
After a bout of exercise, the body strives to restore itself to homeostasis. This recovery process requires energy, which explains the increase in calories expended post-exercise compared to pre-exercise. Reynolds & Kravitz (2001) state that the following occurs during EPOC:
- replenishment of energy resources
- re-oxygenation of blood and restoration of circulatory hormones
- decrease in body temperature
- return to normal breathing rate and heart rate
High-intensity and/or longer-duration cardiorespiratory and resistance exercise seems to elicit the greatest EPOC response. As a general rule of thumb, the higher the intensity (and the more the exercise disrupts homeostasis), the greater the magnitude and duration of EPOC following exercise. Though longer exercise sessions have been shown to elicit a greater EPOC response compared to shorter sessions, exercise intensity is suggested to be the main contributor to EPOC.
EPOC is the greatest immediately post-exercise. Some studies have found that EPOC lasts up to 24 hours, while others have found its duration to be much shorter, less than an hour in some cases. The large range of EPOC durations across studies has been attributed to differences in exercise intensity and duration, as well as variations in study methodologies.
Despite claims from professionals in the fitness industry, some areas of research suggest that the EPOC effect is fairly small, contributing only slightly to weight loss as compared to the energy cost during actual exercise. In fact, some studies have shown that when tracking energy expenditure for several hours following completion of a training session, HIIT and continuous cardio burn about the same amount of post-exercise calories. Different studies report varying, yet small, caloric expenditures that can be attributed to EPOC, ranging from 51 (Haltom et al. 1999) to 127 (Burleson et al. 1998) kilocalories.
Given this modest boost in expenditure, any weight loss benefits would be seen slowly and steadily over time, with all other factors being stable.
The Role of Oxygen in the Phases of Muscle Contractions
In attempting to understand the complex process of EPOC and how it relates to lifting, it helps to know exactly what muscles experience during a typical workout:
Stage 1: At first exertion, sufficient ATP is present to fuel the muscle cells for only a few seconds. Once ATP becomes depleted, creatine phosphate takes over, appearing within the muscle tissue in very limited supplies. Depletion of this substrate typically occurs after the muscle has sustained a contraction for about 25-30 seconds, giving the muscle energy for a total of about 35 seconds. Longer-term energy demands require oxygen.
Stage 2: Metabolic oxidation kicks in; this describes the process whereby pyruvate, the energy source derived from the muscle’s glycogen stores (which converts to lactic acid during rest pauses), combines with oxygen to produce additional ATP. Energy production via metabolic oxidation will only occur in the presence of sufficient oxygen levels. Since muscle tissue does not store oxygen, it must be taken up from outside the cell. This buys the heart and lungs some time to catch up and supply more oxygen.
Stage 3: A small drop in blood oxygen levels triggers a complex cascade of responses, eliciting more labored breathing and an elevated heart rate. The freshly oxygenated blood shuttles energy into the muscles within the first few minutes of exercise and ignites a process known as aerobic metabolism. Glycogen remains the major fuel source here; but now the body can utilize oxygen to generate water and carbon dioxide (CO2). This allows the body to maintain exercise without creating additional lactic acid.
If at the point of creatine phosphate depletion there has been no movement of oxygen into the muscles, an oxygen debt quickly develops and must be repaid by the body.
Aerobic Training and EPOC
During aerobic activity, initial labored heart rate/breathing will ultimately lead to comfortably sustaining continued exercise; the point at which the oxygen debt is repaid during aerobic activity is commonly referred to as experiencing one’s “second wind”. Breathing resumes a more regular pace, the vascular tissue dilates and the heart’s stroke volume is optimal as sufficient oxygen reaches the working muscles. As intensity of aerobic activity increases, the oxygen demand elevates and the cardiorespiratory system responds by once again becoming more labored.
As heart rate increases, fat slowly gets released from storage. Within 10 to 15 minutes, the release of fat reaches its peak; it circulates in the blood and gets taken up by the muscles. Inside a muscle cell, both glycogen and fat are used as the fuel mix to sustain the energy required for continuous muscle contractions.
Resistance Training and EPOC
When resistance training, increased recovery time between sets is recommended to counter the labored cardiorespiratory activity.
Under optimal training conditions, the muscle cell experiences intermittent periods of momentary relaxation. During these pauses (like the pause between eccentric and concentric phases), the cell walls become permeable and allow for the inward movement of nutrients (including oxygen) and the outward movement of waste (carbon dioxide).
If a maximum contraction, as in high weight and very low reps, is sustained, oxygen cannot be introduced; the contraction will fail upon depletion of internal creatine phosphate. This explains in part why maximal muscular contractions cannot be sustained for longer than 25-30 seconds. As long as intermittent bouts of relaxation are combined with forceful muscular contractions, a set can be sustained for a longer period. A longer set will increase time under tension and allow for higher volume.
Likewise, if the contractions are sub-maximal, as in lower weight and high reps, some degree of permeability exists in the cell wall, allowing for intermittent oxygen transport into the cell and prolonging the contractile period. To test this, you can perform a maximally sustained contraction and measure the time required to reach failure. Then, after sufficient recovery, try maintaining a sub-maximal contraction of this same movement, measuring the contractile period and observing its longer duration. If a maximum contraction is executed with intermittent periods of relaxation, even longer contractile periods should be achievable.
Metabolic Boost During Recovery
Once exercise ceases, the process of recovery immediately begins. Some individuals continue to sweat, often for hours after completing a training session. The body utilizes sweat production to remain cool as muscles, liver, heart and immune system begin the complex and energy-demanding process of recovering. This persistent increase in metabolism explains how EPOC comes into play.
Many recovery processes make use of this extra energy. Lactic acid, a chemical by-product, must be cleared and converted into a useful energy source. The pathway to turn lactic acid back to glucose (then glycogen), an important function of the liver, demands energy.
At the same time, muscles need to repair and adapt. This process of protein breakdown and synthesis also requires energy. Nerves need to make fresh neurotransmitters, and hormones used during exercise need to be freshly synthesized. We can easily begin to recognize this collective process as extracting a high energetic cost to the body.
Commitment to Recovery Efforts
Like the workout itself, good recovery technique can improve performance and boost the benefits of training. Vital recovery begins with proper hydration and nutrition. Adequate fluid consumption facilitates the removal of lactic acid and allows glycogen stores to be regenerated. High-quality carbohydrates and proteins should feature prominently as part of any prudent recovery meal. This in turn should be followed up with adequate, good-quality sleep. A restful night helps muscles recover for another day of fun, high-energy, EPOC-laden output!
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