As described in a recent post, my attempt to recover some of the speed of my youth by engaging in a lifting program to re-build my leg strength was only partially successful. I exceeded my expectations regarding gains in strength, but so far this has not been translated into increased speed. Unfortunately a recurrence of arthritis has confounded my immediate hopes, and at the moment I am more concerned about re-building my aerobic base. However a recent discussion of the merits of gazelles v gliders on the Fetch efficient running thread has prompted me to re-examine the issue of my loss of speed.
The most striking thing about the change in my running style as I have grown older is the fact that my stride length has shortened. Now, whenever I try to increase pace, my cadence automatically increases, often going well above 200 steps per minute even at a modest pace. While many recreational runners might benefit from an increase in cadence, at least up to 180-190 steps/min, I am fairly sure that in my case, the increase in cadence reflects reduced ability to get airborne, leading to a stunted stride. I had attributed this to lack of strength but maybe strength wasn’t the main problem.
The gazelles v striders comparison provides food for thought. Here is a good illustration (though I do not agree with all of the comments by the commentator). The crucial difference between gazelles and gliders is that gliders do not produce as much elevation of the body on each step as gazelles. Because they produce less elevation than gazelles, their stride is shorter and they employ a higher cadence. While I am not sure that even my mother would have ever described me as a gazelle, there is little doubt that I have become a glider as I have aged. This is what I have been trying to correct. However, this video clip provides at least some grounds for questioning the need to overcome my tendency to be a glider. As the video illustrates, Chrissie Wellington, without doubt the greatest female triathlete ever, is a glider. In the video, Chrissie’s gliding is compared with the style of one of the most elegant triathlete gazelles, Mirinda Carfrae.
The costs of gliding
Could it be that gliding is efficient? It is tempting to think that reducing elevation costs must be more efficient, but this would be far too simplistic. When considering efficiency, we need to consider the three major energy costs of running:
1) Overcoming braking. Provided a glider increases cadence to ensure that time on the ground does not increase, the braking cost per step will be the same for both but the cost per mile will be greater for the glider because there are more steps per mile.
2) Limb repositioning costs: these increase with cadence and will be higher for the glider
3) Elevation costs: Although the video commentary incorrectly states there are no elevation costs, in fact elevation of the centre of mass occurs before lift- off as a result of extension of hip, knee and ankle during the late stage of stance. Furthermore due to the higher cadence, the saving in elevation cost per step is partially offset by the greater number of steps per mile. However, the elevation cost per mile will nonetheless be somewhat lower for a glider because elevation cost increases as the square of airborne time, so the saving in elevation cost per step is relatively greater than the extra cost due to more steps per mile.
In estimating the total cost, we need to balance the three variables: braking and limb repositioning costs are greater for the glider, but elevation costs are less. At higher speeds, repositioning costs become the dominant cost and there is no doubt that at high speed (e.g. faster than 7 min/mile) gazelles are more efficient. At intermediate speeds (7min/mile-10 min/mile) braking costs and elevation costs are both quite appreciable and at such paces too, gazelles are almost certainly more efficient. At very low speeds (slower than 10 min/mile) braking cost become relatively small because the leg is never far from vertical and therefore the horizontal ground reaction force that produces braking is always low. At such paces the major goal should be minimising elevation costs. So on balance, I think it is only at very slow paces that gliders might be more efficient than gazelles.
So why is Chrissie Wellington a glider? I do not know, but wonder whether it might be an unconscious attempt to decrease the risk of injury when tired in the late stage of an ironman. With regard to injury, the issue is the relative risk of a larger number of smaller impacts for the glider compared with fewer larger impacts for the gazelle. While the phenomenon of repetitive strain injury demonstrates that repeated small impacts can be damaging, I suspect that size of the impacts plays an even bigger role in damage. Therefore, I am inclined to think that for a tired runner, (either in the late stages of an ultra or an ironman) the risks will be lower for the glider.
Overall, there is little doubt that the gazelle style is better for medium paced and faster running, but there is reason to debate whether or not the glider style might be beneficial for tired, slow runners. I am still eager to become as much like a gazelle as my aging limbs will allow. While I can no longer blame lack of strength, I wonder if maybe lesser ability to capture elastic energy at foot fall is the cause of my gliding. In a future post I will describe my plans for the attempt to recover elasticity. But in the short term, my focus is on re-building my aerobic base, and that is what my next post will address.
Finally, it is noteworthy that on the two occasions when Chrissie Wellington and Mirinda Carfrae went head-to-head in the world ironman championship (at Kona in 2009 and 2011), on each occasion Wellington won the overall event but on both occasions Carfrae ran the faster marathon. In my opinion Wellington is the greatest female triathlete ever but Carfrae is the more efficient, and faster, marathon runner.
Added note (4th May 2013)
In his comments below, Robert had pointed out that I have not provided adequate justification for my claims about the energy costs. My claims are based largely on calculations based on applying Newton’s equations of motion to the simplified model of running described in my posts in January and February 2012. At a pace of 4 m/sec, (which is close to that of Carfrae and Wellington in the world ironman championship in 2011) the calculations demonstrate that the combined cost of elevation and braking is 6% greater for a Glider than for a Gazelle, assuming that that the Glider has an increased cadence sufficient to produce a similar time on stance. Observations suggest that Gliders do increase cadence to maintain a similar time on stance, and furthermore, if a Glider did not have increased cadence they would be even less efficient because the longer time on stance would produce an even greater increase in braking costs (as indicated by my post of February 2012).
The crucial questions is whether or not the simplifications I used in performing these calculations lead to serious errors. There are two respects in which the simplifications might lead to error. First, I assumed a sinusoidal shape for the variation of ground reaction force with time. Variations in the shape of this curve introduce small changes to the results of the calculations, but in the absence of force plate data for each runner, I cannot do a more precise calculation. However, the error due to this simplification is likely to be small.
The other potential source of error is that my calculations are for the total energy expended on elevation and overcoming braking. I have not subtracted the energy saving expected via elastic recoil. If a runner maintains greater tension in the leg muscles, as recommended in the BK running style, it is possible to recover a greater proportion of the required energy via elastic recoil. I cannot exclude the possibility that a Glider might maintain greater tension in the leg muscles, but think that in general this is unlikely as increased tension in the leg muscles results in a greater rate of rise in ground reaction forces and potentially increases the risk of injury. The advocates of the BK style recommend thorough preparation using plyometrics before attempting this. I think that increasing leg muscle tension would not be a good strategy for a tired ultra runner. Thus I doubt that Gliders make greater savings via elastic recoil than Gazelles. I suspect that the opposite might be the case, though this is speculation. However even if equal efficiency of recovery of energy via elastic recoil is assumed, the Glider incurs a 6% greater cost for elevation and braking than the Gazelle.
Also, it should be noted that my calculations do not include limb repositioning costs. These costs are almost certainly higher for the Glider because repositioning cost increases with cadence (as discussed in my post in April 2012) and furthermore, the trajectory of the foot of a Gazelle results in a shorter lever arm of the swinging leg, further increasing efficiency.
On balance, I think it is likely that my calculations provide a fairly realistic estimate of the relative costs of elevation and braking for the two styles.