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The Physics of Faster Running

Part 2 - Body Weight

It's no surprise to serious runners that body weight plays a role in running performance.  Experience teaches that the more weight you are carrying the more effort it takes to run and that losing excess body fat can contribute to improved performance.  However, is it possible that body weight, specifically muscle mass,  plays a much greater role in running performance than has been previously recognized?  We learned in part 1 of this series that faster running is achieved by applying more force against the ground.  What role does body weight and/or muscle mass have play in the generation of force?  Part 2 of this series will examine the role body weight and muscle mass play in the physics of running faster.

Different Runners, Different Sizes

A casual observation of elite runners who specialize at different distances, from the 100 meter sprint to the marathon, reveals a striking difference in size and muscle mass.  Sprinters tend to be larger and more heavily muscled while long distance runners tend to be small, slender, and lightly muscled.  There is no reason to believe the relationship between muscle mass and racing distance is happenstance.  After all, it doesn't make sense that elite sprinters would carry additional muscle and body mass if it were detrimental to their performance.  Nor would elite distance runners generally carry less body and muscle mass than is necessary to run their best performance. 

If, as observation suggests and logic supports, muscle mass directly influences running performance the question naturally rises as to how it does so.  What is it about running different distances that requires different amounts of body and muscle mass?  Two exercise physiologists set out to answer this question.  They "obtained the mean values for body mass, stature, and racing speed for the world's fastest 45 male and female specialists, respectively, over...14 years (1992 - 2003) at each of eight standard track racing distances from 100 to 10,000m" (1).  By examining the body mass, stature, and racing speeds of these elite runners they hoped to determine if and how body and muscle mass influence racing performance.

Ground Support Forces

Physiologists have long known that running at increasingly faster paces places increasingly higher force demands on the body.  These force demands are known as ground support forces and they are a measure of the amount of force required to both support and propel the body while running.  Running at a slow jog requires producing a force of around 1.5 times bodyweight.  As running speed increases, ground support forces increase also, reaching as high as 2.5 times bodyweight while running all out on level ground and even more when running downhill.  Clearly, the required ground support forces are considerable. 

Based on the increase in required ground support force, the researchers hypothesized that "the greater body masses of faster specialists are directly related to the greater ground support forces required to attain faster running speeds."  In other words, they suggested that the variations in body and muscle mass are directly related to the level of ground support forces required for the runners to run at the paces maintained during their respective specialized racing distances.  They hypothesized that the greater ground support forces required during faster running means that greater body and muscle mass are necessary and that the lesser ground support forces required during slower running means that less body and muscle mass are necessary.  If this is true, then it would explain why sprinters are more heavily muscled and distance runners are more slender.

Relationship Between Mass and Racing Performance

The researchers analyzed the body mass, height, and performance of the world's fastest 45 male and female runners at each of eight standard racing distances from 100 - 10,000m in relation to the required ground support forces for each of the racing distances. Their research revealed that speed-specific ground support forces did indeed explain the differences in body and muscle mass, accounting for 97% of the variation in performance.

"Our idea that speed-specific ground support force requirements might explain the body mass variation present among highly adapted runners of different specializations was well supported by the elite human runners in our sample."

What this means is that there is a specific amount of body and muscle mass required to perform optimally and that the faster the average race pace the more body and muscle mass that are required to perform optimally.  Not only did the researchers confirm their hypothesis about the link between performance at varying distances and muscle mass, they also discovered that there is a single constant that describes the relationship between performance and body mass.

"Regardless of specialization or sex, we found a single constant accurately links the ideal body mass for running performance to the ground support force required of the performer.

 

The existence of a single structure-function relationship that links the mechanical requirements of running to the optimum body masses for performance has several basic implications.  Most immediately, we can conclude that the fundamental determinants of body size optima for performance is the support force that runners apply to the ground at their racing speeds.  Because different runners use the same biological tissues to satisfy this physical requirement, we find that running performance has a common structural basis."

As you see, running performance has a common structural basis.  Or said another way, running performance is influenced by body and muscle mass.  Too much or too little results in sub-optimal performance.  Note that it is not just a matter of too much or too little body mass, but also of muscle mass.  In part 1 of this series we learned that in order to run faster, more force has to be applied to the ground by the working muscles and that your top running speed is reached when the fastest force generating capacity of your muscles balances against the available ground contact time.  Adding to that knowledge, we now see that the amount of muscle mass you carry has a direct impact on performance.  At increasingly faster speeds, more muscle fiber is required to generate the increasing higher amounts of ground support force required for faster running.  Let's see how this applies in real life situations.

Explaining Other Research

In the country of South Africa, black athletes dominate in the long distance running events while white athletes dominate the shorter middle-distance events.  In 1993 a team of South African researchers attempted to provide a physiological explanation for the differences in performance at longer distances between the white and black athletes (2).  They recruited the fasted 9 white and 11 black South African middle-to-long distance runners.  The white runners were faster at distances less than 5k, while the black runners were faster at distances of 5k and further.  The athletes were subjected to a battery of tests in order to try and determine the physiological basis for the difference in performance, but the researchers were unable to explain the differences in performance between the two groups based on standard physiological measures. 

"All we can conclude is that the ability of the elite black runners in this study to both train more intensively and sustain a higher %VO2max during competition was not due to any measurable differences in muscle fiber type composition, VO2max, or running economy but may have been related to lower blood lactate concentration at any running speed."

What could explain the difference in performance between the two groups?  One of the things the researchers noted was that the white runners were physically larger and more muscled than the black runners.  "...the main anthropometric differences between these runners were that the black distance runners were significantly shorter and lighter than the white middle-distance track athletes and had a considerably smaller muscle mass and lean thigh volume."  Additionally, even when corrected for height, the white athletes are heavier than the black runners.  Table 1 presents the height, weight, and height/weight ratio for the white and black runners.

Table 1:  Anthropometric measurements in elite white and black South African runners

Even though the researchers were unable to explain the difference in performance between the two groups, our knowledge of the structural basis for running performance allows us to propose a resolution to the issue.  The faster performance at shorter distances of the white athletes could be explained on the basis of their greater body and muscle mass.  According to the structural basis of running the additional muscle mass of the white runners allowed them to generate the necessary force to perform better at the shorter distances.  However, their greater muscle mass was a disadvantage as the racing distance increased.  The lesser muscle mass of the black runners was a disadvantage at the shorter, faster distances that require greater ground support forces, but an advantage at increasing distances.  This can be seen in the increasingly larger differences in performance as the racing distance increased or decreased from 5k.  At increasingly shorter distances, the white runners were increasingly faster than the black runners, running 3.5% faster at the 1.65k distance.  This situation was reversed at distances of 5k and further.  At 5k the white runners were just 1.4% slower than the black runners (13:55 vs 13:43).  At 10k the white runners were 3.8% slower than the black runners (29:38 vs 28:33) and 7.5% slower at the half marathon distance (67:19 vs 62:39).  Table 2 presents the differences in performance between the two groups.

Table 2:  Running times over various distances of elite white and black South African runners

The difference in performance between the two groups is completely in line with the structural basis of running performance.  Shorter distances are raced at faster paces and, therefore, require increased ground support forces.  The greater body and muscle mass of the white runners provided a structural advantage at these short, fast distances.  Longer distances are raced at slower paces and, therefore, require less ground support forces.  The lesser body and muscle mass of the black runners provided a structural advantage at these longer, slower distances.

Training Implications

What are the training implications of what we have learned in this 2 part series.  In order to maximize your running performance I suggest the following:

First, train to increase the strength and rate of force production of your muscles.  Increasing just the strength of your muscles will help, but not as much as improving both strength and rate of force production.  Conduct both strength training exercise and explosive type training (such as bounding, plyometrics, Olympic lifting, etc.) for your leg muscles.

Second, focus on increasing your strength and rate of force production while keeping weight gain to a minimum.  Gaining excess mass will counteract the effect of increased strength and power - carrying too much muscle mass is detrimental to training.  Focus on exercises that result in increased strength, power, and contraction speed, but not weight gain.

Third, if you aren't already there, get your percent bodyfat down to low, but healthy levels.  For males 8-10% bodyfat levels seem to be close to optimal for distance runners and for females 15 - 20% seems to be the level maintained by elite female distance runners.

References:

1.  Weyand P, Davis J., Running performance has a structural basis, J Exp Bio, 2005, 208, 2625-2631

2.  Coetzer P, Noakes T, Sanders B, Lambert M, Bosch A, Wiggins T, Dennis S., Superior fatigue

resistance of elite black South African distance runners, J Appl Physiol, 1993, 75(4), 1822-1827