|Research Sheds New Light on the Exercise "Afterburn"
Len Kravitz, Ph.D.
The increase of energy expenditure following exercise is referred to as excess post-exercise oxygen consumption, or EPOC (See Figure 1). This recovery process oftentimes referred to as the exercise 'afterburn', is largely a bodily restoration process of several physiological variables that were elevated during exercise. Much confusion exists as to how long EPOC actually lasts, with some studies suggesting no more than 35 minutes and other studies reporting 24 hours (Borsheim & Bahr 2003). Borsheim and Bahr note these conflicting results can be attributable to the various differences in exercise intensity, duration, as well as the particular mode of exercise (cardiovascular, resistance training, circuit training). Adding to these divergent findings is the fact that the scientific methodologies and technologies vary quite a bit between investigations in how EPOC is measured (Knab et al., 2011). However, for weight management goals of clients, the contribution of EPOC is helpful, and thus of particular interest to personal trainers. Some recent studies add some new findings on EPOC, which personal trainers can apply directly with their client program designs.
Background Knowledge on EPOC
The understanding of an elevated metabolism after exercise was first noted in the 1920's, however a ground breaking investigation by Drs. Glen Gasser and George Brooks in 1984 resolved many of the conflicting ideas and explained several previously unknown mechanisms of EPOC. Gasser and Brooks comprehensively clarify that chemical and physical changes (see Figure 2) that occur in the body's contracting cells during exercise (elevating oxygen metabolism) persist during recovery, accounting for a meaningful amount of EPOC. The authors further explain that this elevated oxygen recovery additionally involves the recovery energy needs of the muscle cells to restore themselves to pre-exercise levels. Contemporary studies on EPOC describe the training load of exercise as a 'homeostatic stress' or 'disturbance' to physiological and metabolic processes, which exists during and continues after exercise (Mann et al., 2014).
What Effect Does Exercise Intensity Have on EPOC?
Perhaps one of the most influential factors that contribute to EPOC is exercise intensity (Borsheim & Bahr, 2003). In a present-day study, Mann et al. (2014) investigated the rate of recovery from 20 minutes of running after bouts of 60%, 70% and 80% of their VO2max in 38 male and female runners between the ages of 20 and 40. Substantiating previous research, the 80% of VO2max bout had the greatest magnitude in EPOC from the 15 minutes of measured EPOC.
In a very unique study, Knab et al. (2011) completed an EPOC investigation in a metabolic chamber. Metabolic chambers are small room-like enclosed spaces where subjects can live independently for up to 24 hours (and longer). Scientists can very precisely measure how metabolic rate is affected by exercise, food, sleep and physical movement in these enclosed chambers. Knab and colleagues investigated the effects of exercising at 70% of VO2max for 45 minutes on a cycle ergometer in a metabolic chamber with 10 healthy male subjects (25 yrs.). The authors chose 45 minutes of exercise, because this corresponds to the middle of the range (30-60 minutes) suggested by the physical activity guidelines for Americans. Subjects were told to avoid exercise for a few days before their trials and were given a specific food list to attain a diet that represented 55% carbohydrate, 30% fat, and 15% protein. In order to control all variables affecting energy metabolism, subjects completed two trails in the metabolic chamber for 24 hours on non-consecutive days (i.e., Mon/Wed or Tue/Thur). One trial in the metabolic chamber served as the control, where for 24 hours (starting at 8:00 am) the subjects stayed in a rested state with meals (55% carbohydrate, 30% fat, and 15% protein) provided to them during the day. This established true resting energy expenditure for 24 hours. In the second trial, subjects reported to the metabolic chamber at the exact same time, but did a 45-minute cycle ergometer workout at 11 am. The scientists provided extra snacks and food in the meals (for the 24 hours in the chamber) to match the number of calories expended by each subject for the 45-minute cycle bout.
The results showed that the 45-minute workout had an average energy cost of 519 kilocalories. Following exercise, the resting energy expenditure (i.e., EPOC) was significantly elevated for 14 hours post-exercise, which represented an increase of 190 kilocalories as compared to the resting day trial. This study suggests that vigorous intensity exercise sustained at a moderate duration (45 minutes) has a very meaningful EPOC (190 kilocalories).
What Affect Does High Intensity Resistance Training vs. Traditional Resistance Training Have on EPOC?
In a pioneering investigation, Paoli et al (2012) investigated the effect of high intensity resistance training (HIRT) vs. traditional resistance training (TRD) on EPOC with 17 resistance-trained males (28 yrs.) who had 4-6 yrs. of resistance training experienced). All subjects completed the HIRT trial and the TRD trial, which were separated by 7 days. Order for the HIRT and TRD was randomly determined for each subject. Subjects all had a similar standardized breakfast (20% fat, 55% CHO, 25% protein), followed by one of the workouts with similar meals during (provided by researchers) during the day. Subjects maintained their daily life activities throughout the day and then reported back to the laboratory 22 hours later to measure EPOC 22.
The TRD intervention consisted 4 sets of 8 exercises; bench press, latissimus dorsi pull down, military press, bicep curls, triceps extensions, leg press and leg curls, and sit-ups. Subjects performed 8 to 12 repetitions to failure (which was 70% to 75% of their 1-RM) with one minute of rest between sets for single-joint exercises and two minutes of rest for multiple-joint exercises. The TRD training session lasted approximately 62 minutes (including a 10-minute warm up on a treadmill).
The HIRT protocol did bench press, leg press and latissimus dorsi, performing exercises in a specific training 'sequence' for each exercise. The 'sequence' consisted of lifting a weight to failure which was loaded at a weight comparable to the subject's 6-RM (meaning subjects can do 6 repetitions but not 7 or more); this was followed by 20 seconds rest and then the subjects lifted the same weight again to failure (~2 to 3 repetitions); followed by 20 seconds rest and then the subjects lifted the same weight again to failure for a third time (~2 to 3 repetitions). The subjects then rested 2.5 minutes and repeated the entire sequence a second time. With the leg press the subjects rested 2.5 minutes and repeated the 'sequence' a third time. The training session lasted approximately 32 minutes (including a 10-minute warm up on a treadmill).
With both protocols, exercises were performed with a 1-second concentric followed by a 2-second eccentric phase. During the three days before each training session and during the day after, participants were provided a standard diet that provided approximately 20% fat, 55% CHO and 25% protein.
The results showed that at 22 hours, the TRD protocol energy expenditure was 5% (99 kilocalories) greater than the resting values. At 22 hours the HIRT protocol was 23% greater (452 kilocalories). Notably, since the authors measured gas analysis, the additional kilocalorie expenditure (above rest) was determined to be primarily fat fuel.
These recent studies demonstrate that high intensity exercise and/or a combination of higher exercise intensity with duration in aerobic exercise impacts EPOC significantly. In resistance training, higher intensity exercise with shorter rest periods influence EPOC meaningfully. The magnitude and duration of EPOC appears to be quite sustained, with well-controlled studies showing elevations in resting energy metabolism for up to 24 hours. Since these newer studies had male volunteers, the question comes up whether there might be an EPOC difference in genders. Borsheim and Bahr, in their 2003 review article summarize the evidence indicating that EPOC in terms of absolute magnitude is higher in men versus women, but this difference disappears when EPOC is corrected for body mass. Furthermore, the authors continue that there are no differences between genders when EPOC is expressed as a percentage of total energy expended.
Borsheim, E & Bahr, R. (2003). Effect of exercise intensity, duration and mode on post-exercise oxygen consumption, Sports Medicine, 33 (14): 1037-1060.
Gaesser, G.A. & Brooks, G.A. (1984). Metabolic bases of excess post-exercise oxygen consumption: a review. Medicine and Science in Sports and Exercise, 16(1), 29-43.
Knab, A.M., Shanely, R.A., Corbin, & K.D et al. (2011). A 45-minute vigorous exercise bout increases metabolic rate for 14 hours. Medicine & Science in Sports & Exercise, 43(9), 1643-1648
Mann, T.N., Webster, C., Lamberts, R.P., & Lambert, M.I. (2014). Effect of exercise intensity on post-exercise oxygen consumption and heart rate recovery, European Journal of Applied Physiology, 114:1809-1820.
Paoli, A., Moro, T., Marcolin, G. et al. (2012) High-intensity interval resistance training (HIRT) influences resting energy expenditure and respiratory ratio in non-dieting individuals. Journal of Translational Medicine, 10:237 (Open access article)