How Does Weight Affect Your Running Speed?
One of things that baffles me all the time is how common the word “weight” appears in conversations among runners, and whether losing any excess baggage may increase running speed. This topic of discussion can be both good and bad. In some ways it promotes a healthier lifestyle; however, in other instances it may lead to obsession over the number on the scale. Generally speaking, lighter equals faster to an extent, and the science behind this phenomenon is discussed below. One the same note, running can’t make you taller, but it can make you lighter. Let’s see more.
Weight and VO2 Max
VO2 max is a term commonly used in running circles because it can predict a person’s performance. This measurement describes the maximum volume of oxygen that an athlete can use during exercise, and is reported in mL per kg of body weight per minute. Theoretically, as weight decreases VO2 max will improve.
A study1 performed in 2000 and reported in Journal of Obesity Related Metabolic Disorders tested this theory. Researchers examined 160 participants, including 31 women both before and after weight loss using a standard treadmill VO2 max test. What the scientists found was that fat-free body mass (i.e. lean body mass including bones, organs, water, etc.) was a better indicative of VO2 max than fat mass. However, participants that had the higher fat mass percentage showed lower sub-maximal aerobic capacity and less time to exhaustion. Therefore, a higher weight will not decrease an athlete’s overall aerobic potential, but will lead to decreased performance in the interim.
Weight and Efficiency
The next question is whether increased body mass decreases an athlete’s efficiency while running. The answer to this query may be obvious. If you have ever tried to run with a backpack full of books or while carrying dumbbells, you likely know the answer. A scientific perspective on this subject can be found in Journal of Experimental Biology2.
Using harnesses and a treadmill, the metabolic cost of running at various body weights and masses was explored. For reference, body weight takes into account the forces from gravity on body mass, or, the amount of physical body that you have. In this study, the metabolic cost of 10 runners was measured in response to changes in weight, body mass, and a combination of the two. The findings indicate that metabolic rate (i.e. the amount of energy expended) was less when weight decreased, and higher when weight increased; although these relationships were not directly proportional. When mass alone was added (the use of a harness compensated for the additional weight), no significant changes in metabolic rate were noted.
Interestingly, these researchers determined that supporting our own body weight comprises approximately 75% of our efficiency measurements. Therefore, while a person’s individual build is not significant, additional weight they are carrying will have an effect on his or her overall efficiency.
Weight and Heat Dissipation
Another factor as to whether body weight matters for running performance is the ability of the runner to dissipate heat. As can be expected, higher adiposity3 provides an insulating layer for internal organs. While this phenomenon is beneficial in cold weather (and may even help keep energy cost down during a winter marathon) inadequate heat dissipation during warmer weather will gravely affect performance.
With less body fat, heat can more efficiently leave the body, leading to less wasted energy and reduced risk of heat exhaustion.
Muscle vs. Fat
A more complicated discussed is that of muscle and fat. Muscle weighs more than fat, and we have already shown that fat (and fat-free mass) affects running performance; therefore, is it logical that muscular gains result in slower running times? The answer to this question is complex.
On one hand, scientists determined that bicep circumference4 was a good performance indicator at the ultramarathon distance, while calf and thigh circumference was not. Additionally, exercise physiologists are well aware that the percentage of slow and fast twitch muscle fibers5 is an important factor for high performance running. So, does an additional pound of muscle slow a runner down in the same way an extra pound of fat might?
Assuming the additional muscle is specifically used in the act of running, the answer is no. Muscle creates power, which is necessary for generating the force of propelling the body forward, and more power will improve metabolic efficiency.
Effect of Hydration
Much of the daily variation from the numbers on the scale are due to hydration, or lack thereof. While some sources suggest that athletes can run 2 seconds faster per mile for every pound lost, it should be noted that these calculations only hold true when excess fat is lost, not muscle or hydration. In fact, even slight reductions in body weight (less than 2%) due to dehydration can hinder running performance significantly6. In one study, the cost of reducing body weight by 1.3% through the use of diuretics was a significantly slowed 5k performance by 91 seconds.
Finding your Race Weight
How can athletes determine whether they are at the proper weight for peak performance? Unfortunately, there is no one-size-fits-all calculation. In general, elite male 800 m athletes were found to have a body mass index7 (BMI) of 20 – 21, while distance runners were primarily in the 19 – 20 range. However, BMI is not always indicative of the whole story, as it does not adequately take into consideration body fat or muscle mass.
Therefore, much of the race weight determination comes from trial and error. Athletes can work towards reducing their weight and quantitatively observing the results, understanding that too low or too high a weight may reduce performance. One way to make the weight determination is to log race performances and workouts alongside weight, and look for patterns. Keep in mind that as athletes grow stronger, optimal weight may rise in correlation to increased muscle mass.
Does Lighter Always Equal Faster?
The topic of weight and running performance can be a dangerous one, given the general Type-A personality of runners. It should be noted that a point of diminishing returns is reached where lower weight does not equate to faster running times. The negative effects of weighing too little include decreased immune function, inadequate storage of glycogen, muscle loss, decreased metabolic efficiency, increased injury risk, and organ malfunction.
When weight is so low that clinical malnutrition is apparent, athletes will not be able to effectively produce energy to fuel their runs, resulting in increased cardiac stress and decreased efficiency. This helps to explain the anorexia paradox, where athletes who experience sudden weight loss generally perform well for a short period of time, typically 3 – 4 months, until experiencing drastic reductions in performance. Once their emergency energy stores are depleted, they risk permanent damage to metabolism, bones, and organs.
1. Goran M., Fields D.A., Hunter G.R., Herd S.L., Weinsier R.L. (2000) Total body fat does not influence maximal aerobic capacity. International Journal of Obesity and Related Metabolic Disorders 24, 841-848. Link
2. Teunissen LP, Grabowski A, Kram R. Effects of independently altering body weight and body mass on the metabolic cost of running. J Exp Biol. 2007;210(Pt 24):4418–4427. Link
3. Savastano DM, Gorbach AM, Eden HS, Brady SM, Reynolds JC, Yanovski JA. Adiposity and human regional body temperature. The American Journal of Clinical Nutrition. 2009;90(5):1124-1131. doi:10.3945/ajcn.2009.27567. Link
4. Knechtle B, Knechtle P, Schulze I, Kohler G. Upper arm circumference is associated with race performance in ultra-endurance runners. Br J Sports Med. 2008;42:295–299. Link
5. Joyner MJ, Coyle EF. Endurance exercise performance: the physiology of champions. The Journal of Physiology. 2008;586(Pt 1):35-44. doi:10.1113/jphysiol.2007.143834. Link
6. Armstrong LE, Costill DL, Fink WJ. Influence of diuretic-induced dehydration on competitive running performance. Med Sci Sports Exerc. 1985;17:456–61. doi: 10.1249/00005768-198508000-00009. Link
7. Sedeaud A, Marc A, Marck A, et al. BMI, a Performance Parameter for Speed Improvement. Bacurau RFP, ed. PLoS ONE. 2014;9(2):e90183. doi:10.1371/journal.pone.0090183. Link