Speed and Body Weight

I have been asked the question; “How does body weight affect speed?” several times lately. Most of us know that a few extra pounds will affect our running performance, but just how much and in what way?

Extra body weight affects our running performance in several ways. Depending on how much extra weight one has it can affect “running efficiency” by causing one to shorten their stride or alter their upper body’s normal position over the legs in order to handle the weight. Endurance is affected because the muscles tire more quickly the more weight they have to handle. VO2max, sometimes referred to as aerobic capacity, or the measure of the body’s ability to consume oxygen and supply it to the muscles for fuel is affected. While most of us cannot afford to have our VO2max tested, it can be approximated using Dr. Jack Daniel’s VDOT chart. This chart combines running efficiency with VO2max to give an approximation value based on ones time for a given distance. It is not quite the same as VO2max but it can be used as a close approximation. The reason that it differs is that while two people of identical size, sex and weight may have an identical VO2max they may differ as much as 15 minutes in a 10K race because one has a much better running efficiency than the other one.

                                           Below is a part of Dr. Jack Daniel’s VDOT Chart.

Let’s say that an individual is at 185 pounds and a body fat percentage of 23%. This would put him at the outside edge of healthful but by most people’s standard somewhat overweight. An elite male runner would have a body fat percentage of about 4-6% and an elite female runner about 6-10%. Now let’s say that since our 185 pound male is not going to be an elite runner he only wants to get down to 12% body fat. Assuming that his muscle and bone structure remain the same this would take him down to about 162 pounds. Now let’s say that he recently ran his best 10K race, on a flat course, in a time of 1:03:46 putting him at a VDOT of 30. Six months later he is a lean, but not extremely low, weight of 162 pounds. We can predict his new 10K time by using the formula for VO2max of ml/kg/mn i.e. milliliters of oxygen per kilogram of body weight per minute. Using VDOT as VO2max or milliliters of oxygen we multiply 30 by 84.1 (body weight in kg’s) and get a product of 2522.73. If we now divide our answer of 2522.73 by our new body weight in kg’s of 73.63 we find a new VDOT value of 34.26. According to our

chart he should now be able to run a 10K in an approximate time of 57:26 or 6 minutes and 20 seconds faster than his previous time. He lowered his race time by approximately one minute per mile by loosing 23 pounds. In his case, that works out to about 2.66 seconds per mile, per pound of body weight. Of course this is not really that linear but only an average.

The example above only put VO2max into the equation if he also improved his running efficiency and gained more endurance because his muscles didn’t have to work as hard for the same distance his time would probably be reduced at least another half second per mile or more per pound of body weight, and all this without doing speed work. Now add speed work to the equation and the sky is the limit! Well almost!

There are other factors that go into this i.e. if you could loose 23 pounds immediately you could not expect to get all of this result right away as you would still have to get your legs and your nervous system used to going this pace. However, you would reap about 80% of this benefit almost immediately and the rest would come in as little as 4 weeks. I hope this has inspired some of you to loose a little of that extra weight that you really don’t need. By the way when you are loosing weight quickly, and running at the same time, you can expect to feel a little sluggish at times, as it is hard to keep the glycogen in your muscles and loose weight fast. If all you are loosing is a half-pound to a pound a week you should be fine. Any more than that and you will probably find that you are feeling tired a lot of the time and your legs feel like they have lead weights attached to them. Either way it is well worth the sacrifice in the long run to get rid of any excess weight.

Other benefits of loosing those extra pounds:

1. You should be able to increase your weekly mileage with less risk of injury.

2. You will have more energy for other activities.

3. You will look and feel great!

4. You may sweat less.

Happy Trails,

Russ