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The 1% Incline Treadmill Myth

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As is usual in winter, the interest on these forums concerning running on a treadmill is at a peak. There have been several threads recently on this subject on the Training and Marathon forums....perhaps on other forums, as well. In most of those threads, someone usually mentions the “guideline” of using a 1-2% incline when running on a treadmill to compensate for the lack of air resistance that one experiences when running outdoors. In fact, it happened again as recently as yesterday in a thread titled “OK to do LR on treadmill?” on the Training Forum.


When I see mention of the “equalizing” incline factor, I often post a caution against blindly following the concept, as I did a few days ago in another thread on the Training Forum titled “Treadmill vs. Jogging On Ground”. (See http://forums.runnersworld.com/message.jspa?messageID=7649859&tstart=50) In that same thread, there was a lengthy “debate” concerning the value of the incline adjustment, which I didn’t read until yesterday. On one side of the debate was arepper, who postulated that: there is no merit to using the incline; running at zero incline on a treadmill is equal to running on a flat surface outdoors; and a study that ExPhysRunner referenced to demonstrate that need for a 1-2% incline is flawed. On the other side of the debate were ExPhysRunner, jwd1113 and dcx693 who argued that arepper’s position had no scientific basis and theirs did. ExPhysRunner challenged arepper and anyone else to offer other studies that might shed more light on the subject. No one did. The debate finally ended when, I think, nothing was being resolved and all participants tired of it.


So, which side of the debate was correct? In my opinion, both were....at least in what each side was trying to say. ExPhysRunner, jwd1113 and dcx693 were absolutely correct in that there is an air resistance, and that “some” effort is needed to overcome it, when running outdoors that is absent when running on a treadmill. That has long been a scientifically proven fact. OTOH, arepper was correct in that the use of an incline to compensate for the lack of air resistance and, thus, make treadmill running equivalent to outdoors is a greatly overblown theory, in most cases is not necessary at all, and in many instances can make the treadmill more difficult than outdoor running at the same pace. Let me attempt to explain that apparent dichotomy.


I am going to reference two of the most technically comprehensive books on running physiology that have ever been published:


1) “Better Training for Long Distance Runners” by Dr. David Martin, physiologist, and Peter Coe, engineer and father and coach of Sebastion Coe, who was perhaps the greatest middle-distance runner of all time.


2) “Lore of Running” by Dr. Tim Noakes, physician, physiologist, and marathoner.


Martin and Coe  In their book, they discuss at length a treadmill stress test that they have used to measure the cardiopulmonary fitness of runners. They described four general constraints they considered in designing their treadmill test protocols. They described the fourth constraint as follows (the bold emphasis is mine to highlight relevance to our subject):


“The fourth testing constraint is that the environmental data collection conditions should be kept as constant as possible to optimize detection of changes in athletes’ fitness from one test session to the next. We kept relative humidity at 35% to ensure effective evaporative cooling. We also maintain our laboratory room at a relatively cool 17 degrees C (63 degrees F) during testing. It is also appropriate to keep the treadmill running conditions as similar to over-ground running as possible. At least through velocities as fast as 6 min/mile (268 m/min), submaximal O2 as measured with treadmill running is insignificantly different than that measured with track running (McMiken and Daniels 1976). Biomechanical differences in running stride between the moving treadmill belt and over-ground running are minimal.


Although over-ground running creates air resistance, such resistance brings an added aerobic demand only at velocities considerably faster than those routinely used in our evaluations. According to the studies of Pugh (1970), the effect of air resistance starts to increase O2 consumption measurably only at faster paces. As an example, at a pace of 4:35 min/mile (13 mi/hr; 350 m/min), the additional aerobic demand is 5.7 ml/kg/min. Indeed, this added energy demand to a front-runner in a fast-paced race is used to advantage as a tactical maneuver by runners who remain in that runner’s wind shadow.”


The treadmill stress tests conducted by Martin and Coe lasted for 26 minutes with no rest breaks until the final 3 minutes of recovery. The first 14 minutes were run at paces of 7:30 min/mile for the first 2 minutes, 6:40 for 3 minutes, 6:00 for 3 minutes, 5:30 for 3 minutes and 5:00 for 3 minutes....all at zero percent incline. The next 9 minutes were run at 6:00 min/mile and progressive inclines of 4%, 6%, 8% and 10% (2 minutes each), with a final one minute at 11%. The test ended with 3 minutes of recovery.


A couple of things are interesting to note:


1) They viewed running at zero percent incline as being “insignificantly different than that measured with track running” and did not see a need to compensate for lack of air resistance by introducing a treadmill incline until the late stages of the test when inclines were used to max out VO2, which was one of the objectives of the test.


2) They also addressed the other major consideration that most often comes up when discussing treadmill running here on these forums....biomechanical differences with over-ground running. And they didn’t consider such differences to be significant.


Noakes  He doesn’t discuss treadmill running, per se, in his book. However, he does discuss a similar subject relative to the effect of air resistance and that Martin and Coe made reference to....drafting in a race. The principle is the same, but reversed. In the case of drafting, the object is to avoid air resistance to conserve energy, as opposed to running on a treadmill where the “myth” is to use incline to simulate air resistance to increase energy used to be the same as running outdoors. Both concepts are predicated on the effect that air resistance has on the runner. The following is what Noakes has to say about air resistance and drafting (again, the bold emphasis is mine):


“One of the first scientists to study the influence of wind speed on running performance was the great British physiologist Dr. Griffiths Pugh, whose work on effects of altitude on athletic performance is among the classic contributions on that topic.  Pugh performed four different studies designed to measure how wind speed and the gradient of the running surface influence the oxygen cost of running (1970).  His studies showed that the extra cost of running into a facing wind increased as the square of the wind speed.  Thus the oxygen cost of running into a 66-km/hr head wind increases by 30 ml/kg/min.  Similarly, running up an 8% incline increases the oxygen cost of running by about 20 ml/kg/min.


“Pugh also showed that at the speeds at which middle-distance track events are run (6 m/s or about 67 seconds per 400m), about 8% of the runner’s energy is used in overcoming air resistance.  But by running directly behind a leading runner (or drafting) at a distance of about 1 m, the athlete can save 80% of that energy.  In a middle-distance race this would be equivalent to a savings of about 4 seconds per lap.  However, Pugh considers it unlikely that in practice the following athletes would ever be able to run as close to the lead runner to benefit to this extent.  By running slightly to the side of the lead runner, the following runner would probably benefit by about 1 second per lap.


“Another researcher to study the benefits of drafting was Californian Chester R. Kyle (1979).  His calculations suggest that at world-record mile pace, a runner running 2 m behind the lead runner would save about 1.66 seconds per lap, which generally confirms Pugh’s estimations.


“These findings explain why track athletes find pacers to be such essential ingredients in aiming for world records.  In addition, these findings explain why world records in the sprints are set at altitude.  During sprinting, the energy cost of overcoming air resistance rises to between 13 and 16% of the total cost of running.  Thus, the sprinter benefits greatly by running at an altitude where air resistance is considerably reduced.  It is interesting that when a runner is racing on a circular track, an optimum strategy is to accelerate into the wind and to decelerate when the wind is from behind, the opposite of what one would expect.


“The Briton Dr. Mervyn Davies (1980/81) extended Pugh’s findings.  Davies used essentially the same techniques as Pugh but included observations on the effects of running downhill and of following winds of different speeds.


Davies found that when a runner was measured on a treadmill, facing winds of up to 18 km/hr had no effect on the oxygen cost of running.  But the same conditions on the road will have a very marked effect.  On the treadmill, the athlete does not move forward and thus does not expend energy overcoming wind resistance.  However, an athlete who runs on the road into a wind of 18 km/hr faces an actual wind speed equal to that of his or her running speed plus that of the prevailing wind.


“The practical relevance of this is that on a calm day, anyone running slower than 18 km/hr (about a 2:21 marathon pace) will not benefit by drafting in the wake of other runners.  However, runners  stand to gain significantly by drafting at faster speeds or when running into winds that, when added to their running speeds, would make the actual wind speed greater than 18 km/hr.”


Everything that Noakes says about Pugh’s and Kyle’s studies with air resistance involves either strong head winds or very fast running paces. It was Davies who extended Pugh’s studies to calm wind and following wind conditions.


I infer from Davies and Noakes’ conclusions that, if 18 km/hr (11.16 mi/hr or 5:22 min/mile) is the threshold for drafting to become beneficial in calm air, then, similarly, 5:22 pace is the threshold at which the lack of air resistance becomes a factor when running on a treadmill.


Actually, the study that ExPhysRunner referenced also found that there was no difference between oxygen consumption on a treadmill at 0% incline and outdoor running at slower paces, although the paces at which there was no difference (9:11 and 8:03 min/mile) were considerably slower than those determined by Davies (5:22 min/mile) and referenced by Martin and Coe (“at least to 6:00 min/mile”). Perhaps these variations resulted from differences in test protocols. Or, perhaps it relates to one of the reasons arepper felt that the study that ExPhysRunner referenced was flawed....a very small sample base of nine runners. And, apparently, all of them were highly trained runners since the test conditions extended to a pace of 5:22 min/mile. In any event, the sample certainly wasn’t representative of a broad cross section of runners. Of course, that might also be the case in Pugh’s and Davies’ studies.


In my opinion, the bottom line to all of this is that ExPhysRunner and his supporters are right in that there is air resistance imposed on all runners at all paces when running over-ground. However, arepper is also right in that for most runners it’s a “So what?” issue. The impact of energy consumption expended to overcome calm air resistance for most runners is insignificant, immeasurable, and not worth attempting to specifically compensate for with a predetermined incline adjustment when on a treadmill. In fact, all of the above data, including that in the study referenced by ExPhysRunner, indicates that cranking in an incline “adjustment” just because someone says you should can make treadmill more difficult than running over-ground for everyone running slower than 8:00, 6:00 or 5:22 min/mile....take your pick of whose data you think is most credible.


And there is a final consideration, which was the basis of my previous post on this subject, that this is really a very minor variable among several that determine the differences between treadmill and over-ground running. Other more significant variables include outdoor climate, treadmill calibration, and outdoor terrain. Heck, in Noakes book, he even mentions the drag caused by short hair, loose fitting clothing, or long hair as having as much or more energy cost....4%, 4.2% and 6.3%, respectively....as air resistance for the pace that most runners run. A long haired runner can just get a haircut and gain more than 2%. In comparison, calm air resistance costs the 4:30 min/mile runner, the middle-distance track runner referenced in Pugh’s study above, 8%. And it is a very small fraction of that for us mere mortals.


My recommendation is to ignore any advice that says that it is necessary to always use a 1-2% incline adjustment to compensate for the lack of wind resistance. Instead, use the combination of speed control and incline adjustment that best makes a treadmill run “feel like” it is giving you the training benefit that you desire.