Wednesday, July 23, 2008 | Category:
Training
One of the great mysteries of running is why elite African endurance runners have greater fatigue-resistance, compared with runners from the rest of the world.
What do we mean by fatigue-resistance? It is simply the ability to sustain a high-quality pace.
If you tell an elite African runner to run as far as possible at an intensity of 90 percent of maximal aerobic capacity (VO2max), he/she will often be able to race a half-marathon at that level of effort. But, if you provide the same instructions for an elite American or European distance runner, he/she will be able to run for only six or seven miles before slowing down. The elite African has greater fatigue resistance - an enhanced capacity to perform at a high intensity for a sustained period of time without diminishing pace.
Traditionally, we have tried to explain differences in performance between endurance runners by invoking an “aerobic paradigm.” Superior performances were thought to be the result of higher aerobic capacities and therefore faster speeds.
However, the disparity in fatigue-resistance between African and white runners can not be due to differences in VO2max, because research shows that elite white and black runners have similar aerobic capacities. The truth is that runners who share the same VO2max can have great differences in fatigue-resistance – and that endurance runners with higher fatigue-resistance can beat runners with greater max aerobic capacities.
In a study carried out at the University of Cape Town, nine African and eight Caucasian distance runners with similar 10-K race times, VO2max values, and peak treadmill velocities were compared (peak treadmill velocity is simply the maximal speed reached on the treadmill during a VO2max test). Despite these similarities, the African runners possessed superior fatigue resistance: They could run for 21 percent longer at an intensity of 92 percent of peak treadmill velocity, compared with the whites.
Such research suggests that the physiological factors which determine VO2max and fatigue-resistance are quite different. VO2max is easy to figure out: Since the heart is an “oxygen pump,” VO2max depends on cardiac output – and the ability of the muscles to utilize the oxygen delivered by the cardiovascular system. In contrast, exercise scientists have struggled to explain the mechanisms responsible for superior fatigue resistance.
One theory is that differences in fatigue resistance might be explained by glycogen concentrations. Runners with superior fatigue resistance might have a higher capacity to store glycogen in their muscles and liver. If this were the case, they would run low on carbohydrate fuel less quickly during endurance competitions and thus would be able to sustain quality paces for longer periods of time.
Research does show that the appearance of fatigue during distance running often coincides with the development of low liver- and muscle-glycogen levels. Scientific research has been unable to verify this “energy depletion model” of fatigue resistance, however. Note, too, that the energy depletion hypothesis suggests that individuals with different degrees of fatigue resistance would begin their races at similar percentages of VO2max, with the lower-fatigue-resistance athletes gradually falling off the pace as glycogen depletion began to develop. In the real world, runners with greater fatigue-resistance seem to adopt faster paces at very early stages in their races, before glycogen depletion becomes a factor, compared with runners with lower fatigue-resistance.
A competing theory suggests that fatigue-resistance is closely related to an athlete’s ability to dissipate heat while running. A high rate of heat accumulation during running is directly related to fatigue: Race times during the marathon and also during the 3-K steeplechase and 10K worsen as the environmental heat load increases. Runners whose internal temperatures rise slowly during running tend to experience less fatigue, compared with individuals who heat up quickly. Small runners tend to dissipate heat more quickly and experience slower increases in body temperature as they run, compared with larger runners, thanks in part to the larger surface-to-mass ratio in the smaller individuals. Interestingly enough, elite black distance runners tend to be smaller than their elite white competitors. In one study, elite blacks weighed an average of 56 kilograms, compared with elite whites, and the blacks were only 169 centimeters in height, compared with 181 centimeters for the whites. Presumably, this would have allowed the blacks to get rid of heat more easily during prolonged efforts, compared with the whites. In some research, during the shorter events of 1.65 to 3K (when heat dissipation is not such an important factor because of the brevity of the running), the performances of elite blacks and whites have been equivalent.
However, it is unlikely that heat-dissipation capacity can completely account for differences in fatigue resistance. For one thing, black African runners with similar degrees of fatigue resistance can vary tremendously in height and weight. Their wide variations in body size should produce great differences in heat-dissipation ability and thus broad disparities in fatigue resistance, but they don’t.
Furthermore, the heat-dissipation theory suggests that elite African and elite white runners should begin competitions at very similar speeds, with whites falling off the pace as heat dissipation becomes a problem. Instead, elite Africans often compete at higher speeds from the opening seconds of a race, compared with whites, when body temperatures should be equivalent.
A better theory may be that fatigue-resistance is related to the way in which runners’ leg muscles function as “reverse springs” during running. The leg muscles are often referred to as springs, but in reality they function quite differently. When an automobile hits a bump in the road, its springs first compress to soak up the energy of impact and then expand, releasing that energy. When a runner’s foot hits the ground, his/her key leg muscles actually expand (lengthen) at impact instead of compressing and then shorten, the reverse of what happens with a mechanical spring.
This muscular “stretch-shortening” cycle is quite useful to the distance runner. The rubber-band-like “snap-back” of the muscles after they have been lengthened from impact provides much of the propulsive force required to move forward. Once the muscles have been stretched during contact with the ground, the resulting shortening occurs without the need to expend energy (just as no further energy must be added to a rubber band once it is stretched to make it snap back powerfully). In effect, the energy stored in the leg muscles at impact is simply released. This is highly efficient, especially when compared with the alternative, which would require active, energy-consuming muscle contractions to get off the ground and stride forward.
This stretch-shortening cycle is not without its problems and perils, however. For one thing, research suggests that muscles become less willing to be stretched and less enthused about transferring energy in the stretched-to-shortened phase of the cycle during an extended running effort. This breakdown in muscle functioning during running has been called “stretch-shortening muscle fatigue.”
The intolerance of stretch and the slow-down in energy transfer create a situation in which both the braking and push-off components of the stance phase of gait may be elongated, leading to a slow-down in stride rate and thus running speed. The resistance to stretch which develops during a prolonged run could also reduce propulsive force, shortening stride length. These changes can not be explained by variations in oxygen utilization or upswings in body temperature: They are related to the quality of the muscles and their ability to stand up to the stresses of the stretch-shortening process. Runners with “higher-quality” muscles should achieve and sustain optimal stretch-shortening function longer during competitions and thus should have greater fatigue-resistance.
What actually causes the breakdown in stretch-shortening function? Stretch-shortening expert Paavo Komi of Finland notes that the stretch-shortening cycle actually damages muscle cells during prolonged running. Much of the damage probably occurs when muscles are stretched out at impact with the ground, and the muscular mayhem has a significant effect on muscle mechanics, as well as muscle and joint stiffness.
If all this is true, runners with the greatest fatigue resistance would be the ones with the greatest contravention of stretch-shortening muscle injury during running. The key question would then be: What training techniques optimize the limiting of stretch-shortening muscle injury? It would appear that training techniques which accentuate and exaggerate the stretch-shortening properties of muscles, for example very high-speed running and explosive, running-specific drills, would create the greatest advances in stretch-shortening function. On the other hand, high-mileage training might induce the greatest stretch-shortening damage, simply because of the very high volume of stretch-shortening cycles – and the lack of stimulus for improvement of stretch-shortening function (since the cycle is never pushed to its max).
There is also the possibility that increased neural drive in African runners might be responsible for their enhanced ability to sustain a high percentage of VO2max. If this is the case, it would have significant training consequences for non-African runners.
Neural drive is simply the extent to which the nervous system stimulates muscles during activity. High neural drive in runners means that the nervous system is sending lots of nerve impulses to motor units in the leg muscles (motor units are simply collections of muscle fibers which are controlled by a single motor nerve); low neural drive means that the nervous system is letting muscle cells hang like anserine slabs of beef between the bones to which they are connected. If African runners have greater neural drive, it could explain their ability to run for a longer distance at any percentage of VO2max. To put it simply, the nervous systems of African runners would be more willing to stimulate leg muscle cells at a high rate over long distances, compared with the nerve-command systems of white runners. When faced with the task of running as long as possible at an intensity of 90 percent of VO2max, white-runners’ nervous systems might be more apt to say, “Are you kidding me? I don’t want to work that hard for 13 miles. Seven or eight miles should be about right.”
The notion that neural drive is linked with the greater fatigue resistance of African runners is related to the Central-Governor Model (CGM) of fatigue. The CGM says that the nervous system decides, just before a particular effort is commenced, what level of effort can be sustained over the duration of the exertion. This subconscious decision is purportedly based on the nervous-system’s assessment of what intensity can be maintained for the distance at hand without incurring significant muscle damage. A level of effort is chosen which assures a decent running speed – without producing mayhem in the muscles or disturbances to the body’s necessary physiological systems. This chosen level of exertion might be higher in African runners, compared with whites. To put it another way, Africans’ Central Governors might be more tolerant of higher intensities.
If the theory is correct, neural drive should actually decrease when runners become fatigued. If runners slow down while neural drive remains the same, then the fatigue “problem” must reside in muscles, not in the nervous system. A reduction in running speed with the same neural drive would mean that the nervous system was trying just as hard as ever to keep the muscles going, but the poor sinews were simply not up to the task of sustaining pace.
The first part of the testing of the hypothesis – looking at whether drops in neural drive are linked with fatigue – was evaluated in a study carried out by Alf Thorstensson and his colleagues at the Swedish School of Sport and Health Sciences in Stockholm, Sweden, and the Department of Neuroscience at the Karolinska Institute, also in Stockholm. Eight well-trained distance runners with an average VO2max of 69.3 ml.kg-1.min-1 participated in the research.
After a fatiguing, two-hour, 26.8-K run at 75 percent of VO2max (which corresponded with a pace of 13.4 kilometers per hour, or a tempo of 7:13 per mile), the runners’ leg muscles had not lost any of their ability to generate force (this was demonstrated via electrical stimulation of certain leg muscles). Nonetheless, muscular force production had dropped off by about 17 percent, and this was completely explained by an 18-percent reduction in neural drive. In other words, the muscles were not more tired, but the nervous system was! Alf showed that the fatigue associated with running can indeed be caused by reductions in neural drive.
What does this mean for your running? When your calves, hams, and quads begin to “tire” during your marathons or long runs, a significant part of this fatigue is likely to be caused by a reduction in neural drive.
But how do you prevent nervous-system fatigue – and how do you increase neural drive during running, so that you can move along more quickly and set PRs? Research on this subject is in its infancy – no, it is actually in its fertilization stage. But, one activity which has been linked with increased neural drive is strength training. Research suggests that relatively high intensities may be required to upgrade neural drive, employing resistances as great as 80 to 90 percent of the 1RM (one-repetition max). Running-specific strength training with pretty heavy weights may be a great way to boost neural drive (because it forces the nervous system to send mega-waves of impulses to the muscles during running-specific movements).
This is not surprising. What would be shocking would be a finding in which training which employed low levels of neural drive led to a major adaptation in which high levels of neural drive were suddenly utilized during competitions. The actual research has yet to be undertaken, but it seems likely that high-neural-drive running training, i. e., efforts which involve very high-speed running and scalding efforts over hilly terrain, will be the type of work which leads to an increased neural drive in competitive situations - and thus greater fatigue-resistance. Incidentally, this is exactly the kind of training which African runners, including the Kenyans, prefer.
Elite non-African runners who want to compete with the elite Kenyans might want to consider adopting this kind of training, along with intense running-specific strength training, in their efforts to develop Kenyan-like fatigue-resistance. The old, high-volume systems of training, ostensibly created to optimize VO2max, simply won’t get the necessary fatigue-resistance job done.