Posted: December 27, 2004

Science of Sport: The Oxygen Debt - One "Loan" That Never Has to Be Re-Paid

You've heard them.

The runner who says that he/she went into "oxygen debt" near the end of a 5K and thus had to slow down.

The coach or expert who states that the ability to incur an "oxygen debt" is so limited that improving aerobic capacity is the only viable way to upgrade endurance performance significantly over time (don't worry - complete sentences are coming).

Do such statements make sense? What is oxygen debt, and how does it actually influence your training and performances?

The term oxygen debt first came into play in the 1920s, after exercise physiologists noticed a "lag" in oxygen uptake at the beginning of exercise. The whitecoats observed that when an athlete completes a warm-up and begins running at a specific speed, his/her rate of oxygen consumption doesn't instantly match up with that velocity but continues to rise for about one to four minutes until a stable ("steady-state") oxygen-uptake rate is attained. If you jumped out of your chair right now and began running at vVO2max (the velocity which causes you to utilize oxygen at the highest-possible rate), for example, you would not instantly hit your topmost measure of oxygen consumption, even though you were running at the speed associated with that kind of oxygen usage. In fact, if you started at vVO2max (100 percent of the speed which elicits VO2max), your average oxygen meterage might approximate 60 percent of VO2max for the first 30 seconds of running, gradually rising from that point on until 100 percent of VO2max was attained. As you can see, there is a definite lag in oxygen uptake; it takes awhile for oxygen usage to "catch up" to the running speed you have chosen.

Observation of this lag led exercise physiologists to coin the term "oxygen deficit", which was defined as the difference between oxygen uptake in the first few minutes of exercise and an equal-in-time period after steady-state oxygen utilization had been reached (1). Interestingly enough, oxygen deficit can be used as a somewhat crude predictor of performance ability. That is, the time to reach steady-state oxygen consumption is shorter in well-trained, high-performing athletes, compared with poorly trained, unfit subjects (2). To put it another way, your oxygen deficit tends to shrink as you carry out high-quality training and get fitter and faster in your races. It is very doubtful, however, that oxygen deficit could be used to predict finishing places in an elite-level marathon race - or even to foretell 10-K finishes in a group of runners who complete the race in around 40 minutes.

After finding out about oxygen deficit, researchers subsequently learned that although oxygen uptake is suppressed at the beginning of exercise (thus creating an "oxygen deficit), it is elevated above normal resting levels after exertion ceases (this is part of what modern-day exercise writers sometimes refer to as the post-exercise "burn"). In other words, just as it takes oxygen an unexpectedly long time to rise from resting to steady-state values at the beginning of a workout, it also takes it a surprisingly long time to fall from steady-state to resting levels after a training session ends.

The great British exercise physiologist A. V. Hill was the first scientist to refer to this gradual drop-off as the oxygen debt, which he simply defined as the excess oxygen uptake above resting levels which occurs after a workout is over. Hill theorized that the oxygen deficit and oxygen debt were tightly linked. That is, he felt that the surplus oxygen consumed after exercise (the oxygen debt) was the body's way of "paying back" the oxygen deficit incurred at the beginning of exertion.

Thus, the oxygen debt is not a phenomenon which you will ever encounter in races; since it occurs when exercise ends, it will not cause you to slow down in your 5Ks. Contrary to popular belief, it is also a highly elastic variable, the magnitude of which depends on both the duration and intensity of the preceding workout. Train for 60 minutes at an intensity of 75 percent of VO2max, for example, and your oxygen debt will be significantly larger, compared with training for 30 minutes at the same intensity. Train for 45 minutes at 85 percent of VO2max, and your oxygen debt will be much greater, compared with working out for 45 minutes at 70 percent of VO2max. Your oxygen debt is like a balloon which can be blown up to great or small proportions; it is not tightly limited.

The oxygen "boom" which occurs after exercise (i. e., the oxygen debt) is related to a variety of factors. First, some oxygen is needed to re-synthesize the high-energy ATP and creatine phosphate which are used up during exercise and also to restore tissue and blood concentrations of oxygen. About 20 percent of the oxygen debt is utilized to convert the lactate generated during exercise back to glucose. In addition, heart rate and breathing rate remain higher than normal after a workout ends, causing oxygen consumption to remain fairly elevated. Body temperature is up, too (compared with before the workout begins), and increases in body temperature tend to accelerate metabolism, leading to a higher draw-down of oxygen than usual. Finally, the key hormones epinephrine and norepinephrine, which tend to spur metabolism and thus oxygen consumption, are often higher than usual at the end of a training session, perhaps putting a little bulge in the debt.

As you might guess (given the factors which contribute to oxygen debt), oxygen debt is not a good predictor of performance. The high-oxygen-debt runner is not necessarily better than the low oxygen-debt harrier, although lofty oxygen debts imply higher-quality training and thus might lead to better performances. A slow-down at the end of a 5K is not caused by oxygen-debt troubles and is more likely to be related to neuromuscular attributes, sub-optimal running economy, a below-par lactate threshold, a troublesome vVO2max, and/or a poor ability to buffer the upswings in muscle acidity which can occur during over-the-top running.

Interestingly enough, oxygen debt is not even tightly linked with oxygen deficit (the "excess" oxygen consumed after a training session shuts down is not necessarily similar to the amount of "missing" oxygen at the beginning of a workout). Your muscles don't become guilt-ridden and anxious during your workouts, hoping to replace exactly the "lost" oxygen associated with your training start-ups. As mentioned, your oxygen debt is a function of what you do during your workout, not just what you did during the first four minutes of training.

So what should you do about your oxygen debt? Simply enjoy the fact that it exists! Oxygen debt helps you burn extra calories, even after you have completely stopped running, and significant calorie-burning can lead to body-composition improvement, a rise in aerobic capacity, enhanced running economy, and better race times. You can also play the oxygen-debt "game" from time to time, reasonably stepping up the intensity and/or lengths of your workouts (without overdoing it and increasing your risk of overtraining) and thus getting a niftier post-training "burn".

I wish all of you a very happy holiday season.

By Owen Anderson, Ph. D.

Copyright © 1998-2004 by Running Research News

References
(1) "Anaerobic Capacity Determined Maximal Accumulated O2 Deficit," Journal of Applied Physiology, Vol. 64, pp. 50-60, 1988
(2) "Oxygen Uptake Kinetics in Trained Athletes Differing in O2 Max," European Journal of Applied Physiology, Vol. 39, pp. 407-415, 1985