Also known as oxygen debt, EPOC is the amount of oxygen required to restore your body to its normal, resting level of metabolic function (called homeostasis). It also explains how your body can continue to burn calories long after you’ve finished your workout.
Exercise that places a greater demand on the anaerobic energy pathways during the workout can increase the need for oxygen after the workout, thereby enhancing the EPOC effect. Differences to consider Between Male and Female Athletes.
Strength training with compound, multijoint weightlifting exercises or doing a weightlifting circuit that alternates between upper- and lower-body movements places a greater demand on the involved muscles for ATP from the anaerobic pathways. Increased need for anaerobic ATP also creates a greater demand on the aerobic system to replenish that ATP during the rest intervals and the post- exercise recovery process. Heavy training loads or shorter recovery intervals increase the demand on the anaerobic energy pathways during exercise, which yields a greater EPOC effect during the post-exercise recovery period.
The body is most efficient at producing ATP through aerobic metabolism; however, at higher intensities when energy is needed immediately, the anaerobic pathways can provide the necessary ATP much more quickly. This is why we can only sustain high-intensity activity for a brief period of time—we simply run out of energy. HIIT works because during high-intensity exercise ATP is produced by the anaerobic pathways; once that ATP exhausted, it is necessary to allow ATP to be replenished. The rest interval or active-recovery period during an anaerobic workout allows aerobic metabolism to produce and replace ATP in the involved muscles. The oxygen deficit is the difference between the volume of O2 consumed during exercise and the amount that would be consumed if energy demands were met through only the aerobic energy pathway.
Higher intensities require ATP from anaerobic pathways. If the ATP required to exercise at a particular intensity was not obtained aerobically, it must come from the anaerobic pathways. During EPOC, the body uses oxygen to restore muscle glycogen and rebuild muscle proteins damaged during exercise. Even after a HIIT workout is over, the body will continue to use the aerobic energy pathway to replace the ATP consumed during the workout, thus enhancing the EPOC effect.
In an extensive review of the research literature on EPOC, Bersheim and Bahr (2003) concluded that “studies in which similar estimated energy cost or similar exercising VO2 have been used to equate continuous aerobic exercise and intermittent resistance exercise, have indicated that resistance exercise produces a greater EPOC response.” For example, one study found that when aerobic cycling (40 minutes at 80 percent Max HR), circuit weight training (4 sets/8 exercises/15 reps at 50 percent 1-RM) and heavy resistance exercise (3 sets/8 exercises at 80-90 percent 1-RM to exhaustion) were compared, heavy resistance exercise produced the biggest EPOC.
THE EPOC EFFECT FROM A HIIT OR HIGH- INTENSITY STRENGTH-TRAINING WORKOUT CAN ADD 6 TO 15 PERCENT OF THE TOTAL ENERGY COST OF THE EXERCISE SESSION.
High-intensity workouts require more energy from the anaerobic pathways and can generate a greater EPOC effect, leading to extended post-exercise energy expenditure. Heavy weight training and HIIT workouts appear to be superior to steady-state running or lower- intensity circuit training in creating EPOC (LaForgia, Withers and Gore, 2006).
Admittedly there is some debate about the significance of the EPOC effect for the average exercise participant because the high-intensity exercise required for EPOC can be extremely challenging. However, if you want results and are up for the challenge, increasing the intensity of your workouts by using heavier weights, shorter rest intervals or high-intensity cardio intervals may be worth the effort. While HIIT or heavy resistance training is effective and beneficial, remember to allow at least 48 hours of recovery time between high-intensity exercise sessions and try to limit yourself to no more than three strenuous workouts per week.
Bersheim, E. and Bahr, R. (2003). Effect of exercise intensity, duration and mode on post-exercise oxygen consumption. Sports Medicine, 33, 14, 1037-1060
2. Post-workout Nutrition
Recent research in the field of nutrient timing suggests that when nutrition is consumed relative to exercise may be more important than what is consumed. After exercise the body needs to replenish energy with carbohydrates and repair tissue with protein. Having a post- workout snack or drink with a proper ratio of carbohydrates to protein can help meet both needs. The carbohydrates will refuel energy needs as well as increase insulin levels, which helps to promote the post-exercise utilization of protein for muscle repair. Proper nutrition is especially important after high-intensity exercise, which can promote the release of the muscle-building hormones: testosterone (T), human growth hormone (GH) and insulin-like growth factor-1 (IGF-1). Refueling your body with the recommended nutrition within the recommended time frame will help your body to effectively use GH, T and IGF-1 to repair and build new muscle tissue. Research indicates that having a snack or drink with a 3–4:1 carbohydrate-to- protein ratio within 30 to 45 minutes post-exercise can help you recover from the day’s activity and get ready for tomorrow’s workout.
Normal glycogen restoration occurs within between 24 and 36 hours whereas a more aggressive approach can regenerate glycogen stores within 20 hours. Aggressive strategies therefore become an important consideration for those participating in more frequent, vigorous training programs and less important for those participating in moderate-to- vigorous bouts of cardio every 24 – 48 hours who need nothing more than their regular diet.
If contemplating an aggressive strategy, the following factors will influence the rate of glycogen re-synthesis:
• Timing of carbohydrate ingestion following exercise.
• Type of carbohydrate ingested.
• Quantity of carbohydrate ingested.
• Inclusion of protein (to accelerate glycogen re-synthesis).
Glycogen re-synthesis rates (glycogen synthetase activity) are most active in first hour following exercise, becoming progressively less active with each passing hour. In fact, rates are approximately 100 % greater in the first hour than two hours post-exercise. If one waits two hours after exercise to begin eating carbohydrates, muscle glycogen stores at the 4-hour mark (post-exercise) will be 45 % lower than levels where eating began within the hour following exercise.
Glycogen re-synthesis rates are approximately 50 % slower from fructose than from glucose for several reasons.
Glucose undergoes a more rapid, active absorptive process versus fructose, which is absorbed via a slower, more passive process. Glucose is also absorbed higher up (sooner) in the GI tract and fructose generally requires conversion to glucose within liver cells before it becomes biologically available to the muscle cells. As glucose and fructose are absorbed via different mechanisms and at different location, consideration of multiple-source carbohydrate foods may potentially increase muscle delivery. However, the issue of glycemic load and the body’s insulin response may require some consideration.
Not only do high-glycemic load foods trigger greater insulin responses and the classic post-prandial blood sugar crash, but high levels of circulating insulin also inhibit the action of Hormone Sensitive Lipase (HSL), the enzyme responsible for mobilizing fats from storage for use as energy.
Traditionally, the post-exercise window (EPOC) is a period where the body typically favors strong utilization of fats as a fuel.
Volumes of research have investigated the effects of feeding different quantities of carbohydrates on glycogen re-synthesis rates and while positive results are associated with 0.7 – 1.5 g carbohydrates / kg BW / hour for the first 4 hours, the consensus appears to be around 1.0 – 1.2 g / kg BW / hour. This amounts to significant quantities of carbohydrates (e.g., 175 lb (79.5 kg) runner would consume 80 – 95 g / hour or 320 – 380 kcal / hour for the first 4 hours. While the first hour is most critical, total calories needed should always be considered when planning post- exercise dietary strategies. The inclusion of some protein (≤ 20 g) can accelerate glucose uptake for first 40 – 60 minutes after exercise, but only if carbohydrate intakes are < 1.0 g / kg BW.
Recent research demonstrates that quality protein consumed before exercise (containing 6 g of essential amino acids = approximately 15 – 18 g of whey isolate) appears to create the largest increase in protein synthesis rates in recovery (~ 150 – 200% increase in rates over resting levels versus ~ 150 % increase in rates when feeding after exercise). While 15 – 18 g of quality whey isolate appears to optimize the post-exercise anabolic effect, many questions exist over the ideal quantity of protein that should be consumed within 30 – 45 minutes after exercise.
Various studies have investigated this effect by comparing quantities as small as 5 g to over 60 g and it appears that 20 g of quality protein consumed within 30 – 45 minutes is generally considered optimal (averages around 0.23 – 0.27g / Kg BW for average male and female). A whey isolate is suggested given how quickly it’s assimilated into the body and given the fact that it is a rich source of Leucine, an amino acid associated with protein synthesis. Casein on the other hand, while cheaper, and contains less Leucine than whey isolate, and takes considerably longer to empty from the stomach. However, it is a protein of choice to take a few hours after your workout to sustain continued protein synthesis, long after the whey isolates have been utilized.
For individuals with goals to replenish both glycogen stores and build protein, a 4:1 mixture of carbohydrates-to- protein is indicated from research (approximately 1.0 g / Kg BW of carbohydrates; 0.25 g / Kg BW for protein).
Accelerade™ is a commercially-available product that meets this recommendation, whereas the new line of recovery products from Gatorade™ only deliver a 1-to- 1 or 2-to- 1 ratio. Regardless of your goal however, hydration must always be your first consideration before adopting strategies for glycogen and protein synthesis – always factor total caloric quantity and protein intake for the day.
Biomechanical and physiological differences between men and women should be recognised to get the best out of every athlete.
A good example of this is the impact of nutrition and training on immunity. Although it’s hugely beneficial for health, one of the few downsides of exercise is that immediately after training (especially long or hard sessions), the immune system becomes temporarily depressed, increasing the risk of coughs, colds, sore throats and other upper respiratory tract infections (URTIs).
Why it differs for women
Scientists now think that the increase in the body’s core temperature produced by exercise temporarily interferes with the normal functioning of the body's immune cells, increasing the risk of infection and illness. But this has particular implications for women; due to hormonal changes, the body’s core temperature fluctuates during the menstrual cycle, which begs the question: are exercising women more at risk of URTIs than men at certain times of the month?
To try and answer this, researchers have studied the immune responses in women who undertook cycling trials at different times in their menstrual cycle*: the follicular phase (typically around 5-14 days after the start of menstruation) and the luteal phase (typically around 14-28 days after menstruation).
The women completed four challenging cycling trials at two different points in the menstrual cycle:
• Follicular phase while consuming a carbohydrate drink
• Luteal phase while consuming a carbohydrate drink
• Follicular phase while consuming a carbohydrate-free (placebo) drink
• Luteal phase while consuming a carbohydrate-free (placebo) drink
In all four trials, the ambient temperature was maintained at a very warm 30°C – the added heat stress making any core temperature and immune changes more pronounced.
It turned out that when the women were in the luteal phase of their cycle and didn't consume carbohydrate, they experienced a large and significant rise in immune cells called leucocytes, indicating compromised immunity and increased risk of an URTI. However, when the women had consumed the carbohydrate drink while performing the luteal phase trial, the rise in leucocytes was not nearly as great – i.e. consuming carbohydrate during exercise had helped to reduce the amount of immune stress resulting from exercise.
What does this mean if you're a female athlete in hard training?
Firstly, you should consume carbohydrate during your hard training sessions to reduce the dip in post-exercise immunity, especially in the second half of the menstrual cycle when you may be particularly vulnerable. More generally, you should consider backing off the training workload during times of stress at work or at home (stress hormone release lowers immunity), and ensure you consume a high-quality diet, paying particular attention to zinc, vitamin D and omega-3 intake, all of which are key immune nutrients.
Is this approach really worth the effort I hear you ask? Well, to quote Aristotle again, "The aim of the wise is not to secure pleasure, but to avoid pain!