Thursday, February 14, 2008
When an athlete runs on the track, on roads or on firm ground, their legs create propulsive forces which accelerate their centre of mass and drive it forward. The athlete's centre of mass is decelerated during each recovery (early-stance) phase of the gait cycle, only to be accelerated forwards again as propulsive forces are created by the stance leg. As they continue to run, centre of mass is accelerated and decelerated over and over again as it moves steadily forwards.
When the same athlete runs on a treadmill, centre of mass is static (at least in the forwards-backwards plane). There is no forward progress; instead, the running surface 'disappears' behind the athlete. In fact, the treadmill belt moves the athlete's legs and feet under and behind her centre of mass and, to preserve stability, their key task is to move the support leg back in front of the centre of mass in time for the impact with the treadmill belt. The key function of the leg muscles during treadmill running is not to produce propulsive forces but to re-position the legs so as to keep the centre of mass stable.
Because of these major and fundamental differences, some experts have argued that treadmill training is unspecific to 'real running' and should be avoided by athletes who want to improve their running abilities on the ground. The neuromuscular patterns involved in treadmill running are so different from road, track or cross-country running, they argue, that improvements in economy and efficiency are not possible. Some go so far as to contend that treadmill training may actually impair running economy on regular surfaces.
On the other hand, treadmill advocates cite two key advantages of treadmill training: with treadmills, they say, it is possible to perform outstanding 'hill' workouts by setting the treadmill inclination at challenging levels; treadmill athletes can also set training paces with great precision, enhancing their ability to carry out very specific training. They can do ample amounts of work at a goal race pace, for example, without ever having to worry about whether they are straying from the desired velocity.
Critics of treadmill training produce the counter-argument that treadmill running is not really very specific. For example, the weight and forces placed on the treadmill belt with each footstrike cause many belts to slow down briefly with every step. In addition, a discrete pace on a treadmill is not the same, in terms of overall physiological effort, as the same pace on firm ground; for example, six-minute miling pace on a treadmill usually produces lower heart and oxygen consumption rates than same-pace running on the track, cross-country course or football field. The lower cost of treadmill running is definitely attributable to lack of wind resistance; it may also reflect the biomechanical and kinematic differences between treadmill and 'normal' running.
There is little doubt that treadmill training is better for fitness than a completely sedentary lifestyle, and there is no question that non-competitive joggers can improve their overall fitness on a treadmill. But for the reasons given above, it is logical for competitive athletes to be concerned about whether they can actually use treadmill work to boost their performances. Is treadmill running too different from the real thing to have any impact on race times? What does the scientific literature have to say?
Oxygen consumption at higher speeds
One of the first scientific studies to compare treadmill with track running was carried out by physiologist Dr LGCE Pugh in London in 1969. Pugh simply measured the correlation between oxygen-consumption rate and speed in seven athletes running on a track and four running on a treadmill. He found that the relationship was linear during treadmill running, but curvilinear on the track, when oxygen consumption shot up dramatically at higher speeds.
One of Pugh's most interesting findings was that the slope of the line linking oxygen consumption with speed was significantly steeper on the track than on the treadmill: put another way, for each unit increase in running speed there was a correspondingly greater increase in oxygen cost during track running than during treadmill running. Pugh attributed this divergence to the effects of wind resistance - a reasonable assumption since the energy cost of overcoming air resistance increases fairly dramatically as running speed increases. As Pugh pointed out, the energy cost of overcoming air resistance during track running at a speed of 21.5k/hour (about six metres per second, or 67 seconds per 400m) is about 8% of the total energy cost. By contrast, the air-resistance cost doubles to 16% of total expenditure when running speed goes up to 10m/sec (100m in 10 seconds).
Sprint benefits of treadmill training
Pugh's data are extremely interesting, but the real lesson of his research is that it is much easier to run at very high speeds on a treadmill than on track, road or trail. In open-air situations, a significant fraction of energy which could be used for dynamic movement is instead 'wasted' on overcoming the slowing effects of air molecules. This finding that higher top speeds can be attained on a treadmill than a track is interesting and slightly ironic in view of the fact that sprinters almost never train on treadmills, even though they could practise faster-than-track velocities (and presumably improve max running speed) on these devices.
The enhanced energy cost of overground running was also noted by Jack Daniels in his research with young adult male runners. Daniels found that it cost about 10% more to run on an asphalt road than on a treadmill at the same speed.
The first study to evaluate carefully the kinematics of treadmill versus 'overground' running was completed in 1972 at Penn State University in the US, in a study of 16 male members of the university's track and cross-country teams. The runners were evenly divided between distance specialties, with five sprinters, six middle-distance runners and five cross-country runners, all of them required to run at three speeds at three different inclinations on both the treadmill and normal ground.
This groundbreaking study uncovered fundamental differences between treadmill and overground running. For instance, at relatively routine speeds of 3.35 and 4.88m/sec (119 and 82 seconds per 400m respectively), the athletes' stride lengths were absolutely identical on treadmill and firm ground; but when velocity rose to 6.4m/sec (62.5 seconds per 400m), treadmill stride length was about 5% longer than overground strides. The same situation occurred during uphill (10%-grade) running, with strides equal at moderate paces but about 8% longer on the treadmill when velocity rose to 6.4m/sec. During downhill running (10%), stride lengths on the ground and treadmill were equal at all speeds.
Not surprisingly, stride rates were lower on the treadmill during fast running both uphill and on the level: since stride lengths were longer on the treadmill, stride rate would have to slow down in order to keep speed at a constant level. Predictably too, 'support time' (the length in milliseconds of the stance phase of the gait cycle) increased significantly on the treadmill for all speeds during uphill and downhill running - and at the very highest speed during level running. Basically, the runners were trying to create more stability for themselves on the unstable, fast-moving and/or inclined treadmill by keeping their feet on the belt a little longer than usual. In fact, this effect may have been the cause of both the lower stride rates and longer strides: with the stance phase elongated, more propulsive force could have been created, broadening strides but trimming stride rate.
The Penn State study was an important one, because it showed that athletes really do run differently on the treadmill than on the ground. The augmentation of 'support time' discovered on the treadmill is not particularly good news for distance runners, most of whom could benefit more from footstrike shortening (producing the same or more propulsive force in less ground-contact time) than lengthening. In a follow-up study carried out in Australia, the dissimilarities between treadmill and overground running were reinforced, and a new 'wrinkle' was introduced: less experienced, purely recreational joggers react to the treadmill differently from highly-competitive runners.
Comparing male and female joggers
In this investigation, 12 male and 12 female student joggers ran at speeds ranging from 3.33 to 6.2m/sec on both track and treadmill. As with the Penn-State study, disparities in kinematics between treadmill and ground appeared at relatively faster speeds; specifically, above a velocity of about 4.85m/sec (82 seconds per 400m), stride rate on the treadmill for the male runners increased by about 3.4% compared with overground running, while stride length dropped by 3.2%. The female runners showed the same tendency - but with a dramatically increased magnitude; female stride lengths dropped by 10.2% on the mill, while stride rates surged by 10.9% compared with female firm-ground running. As you can see, these changes are exactly opposite to those exhibited by the experienced runners, who augmented stride lengths while reducing stride rates.
Both of these studies are extremely interesting for this reason: research has shown that runners tend to optimise stride length and stride rate in order to produce the most energy-efficient pattern of movement. However, if an athlete who has optimised stride length and rate on firm ground then moves to treadmill training, he or she will be training with sub-efficient economy, practising biomechanical patterns which are less than optimal for firm-ground competition.
In a very interesting subsequent study, Barry Frishberg of the University of Massachusetts studied differences in sprinting on track and treadmill. Frishberg worked with five male varsity athletes who competed in sprint events of 200m or less or ran on the university's 400m relay team. Each athlete carried out a minimum of 10 treadmill training sessions before participating in four filming sessions, two on the treadmill and two on the track. The speeds they achieved over 91.44m (100 yards) on the track were used to set their velocities during the treadmill sprints. Average velocity for the five runners was 8.5m/sec or 11.76 seconds per 100m. If this seems a tad slow, bear in mind that the runners used a standing start, since they could hardly put starting blocks on the treadmills!
Track running leads to greater oxygen debt than treadmill training
Interestingly, the 'oxygen debt' incurred by running 100 metres on the track was 36% greater than in the treadmill condition. Oxygen debt is simply the excess oxygen uptake (above resting oxygen utilisation) that occurs after exercsie is over. It is a somewhat misleading term, because this elevated consumption does not appear to be entirely due to a 'borrowing' from the body's oxygen stores; a better term is excess post-exercise oxygen consumption - EPOC, which is directly related to exercise intensity, in that the higher the intensity the greater the EPOC. Therefore sprinting at 8.5m/sec overground was much more demanding for the Massachusetts athletes than sprinting at the same speed on the treadmill, probably because overground sprinting depleted the leg-muscle cells' energy stores more than treadmill sprinting so that more oxygen was needed to re-establish normal energy supplies within the muscles.
Arguably the most important finding in this study was the sheer magnitude of the difference in cost (36%) between treadmill and track sprinting. As Frishberg pointed out, air resistance alone could not entirely account for this huge disparity, suggesting that biomechanical and kinematic differences must be large enough to contribute to the relative 'cheapness' of treadmill running. Another key result was that, when given the opportunity, the sprinters could exceed their maximal overground running velocities by over 1-2m/sec. A jump of 2m/sec, from 8.5m/sec on the track to 10.5 on the treadmill, would improve 100m performance from 11.76 to 9.5 seconds. So average university runners would be capable of moving along at world-record pace over 100 metres on the treadmill - training which should be beneficial for the improvement of max running speed.
Treadmill 'hills' are a real boon
So what's the bottom line on treadmill training? The ability to regulate a treadmill's inclination is a real boon to athletes plagued by flat terrain. Moving a treadmill's inclination from zero to 6¡ dramatically enhances the power output of key running muscles like the calves, and adjusting the inclination from 6¡ to 12¡ produces a further doubling in power output. Running up real hills has the same effect, but athletes without hills at their disposal can truly benefit from being able to set precisely the length and severity of their treadmill 'hills'.
Even though the oxygen cost of running at specific speeds is lower on the treadmill than on the ground, it is still possible to attain extremely high rates of oxygen consumption during treadmill running, which allows for an increase in maximal running speed. Thus, treadmill workouts can be used quite successfully to improve VO2max. Whether treadmill training can produce the same maximal gain in VO2max as overground training is not known, however. In fact, it is not even certain that a 5% gain in VO2max achieved on the treadmill would necessarily lead to the same gain during overground running.
Since treadmill training intensities can be high, especially since max running velocity is greater on the treadmill, and since athletes can add tremendous heat to their workouts by raising the treadmill inclination, lactate levels during treadmill training can be very high. Thus, treadmill sessions can be used profitably for the improvement of lactate threshold. Again, though, the problem is that such gains may not be as great as those achieved during overground training; nor is there any certainty that threshold improvements will carry over directly and completely to firm-ground running.
As mentioned, athletes can hike maximal running speed instantaneously on the treadmill, often by increasing turnover rate and decreasing the duration of the stance phase of the gait cycle. However, such spikes in speed are accompanied by fundamental differences in running style; the shin of the support leg is less erect at contact and moves through a greater range of motion, with a faster overall angular velocity, on the treadmill, while the thigh of the support leg is more erect during footstrike and moves with a slower overall angular velocity over a more limited range of motion during the stance phase of the gait cycle. (Incidentally, some experts think that the reduced angular motion of the thigh during treadmill running helps to explain why treadmill efforts are energetically cheaper). There is also a reduced forward lean of the trunk during treadmill running. Thus, it is not absolutely certain that improvements in maximal running speed achieved on the treadmill will transfer to the track without 'hitches', since track running will involve significantly different biomechanics.
And these biomechanical considerations explain why it is almost always better for serious athletes to train in real conditions rather than on the treadmill when given the choice. Working out on a treadmill in a controlled environment is better than slipping along on icy roads, plodding through snow drifts, fighting gale-force winds or battling with high heat and humidity, but it is unlikely that treadmill training can improve running economy as effectively as overground work. In fact, the movement patterns of the two types of activity are so different that treadmill training might even retard improvements in overground running economy - an especially troubling thought.