Friday, March 23, 2012

Weight vs. Running Speed: A Case Study

Last week, I tore apart the “research” behind the notion that losing weight will help you run faster. It was fun. I even got some thoughtful comments from Amby Burfoot, the author of the original Runner’s World article. In his comment, he asked if I thought that losing weight made runners slower.

Something to ponder. Since the study cited in the Runner’s World article wasn’t worth the bandwidth I used to download it, and wasn’t based on a sampling of runners’ weights and race times, I couldn’t turn to that work. What I could do, however, is use myself as a case study, since I have a record of my race times, and a consistent record of my weight.

Since some people get all freaked out about mention of specific weights, I’ll just note a couple of things before I dive into the data and analysis:
  • I keep a regular log of my weight because I have kidney issues and consistent weigh-ins alert me to the onset of excessive water retention so that I can get into the doctor before the issue gets out of control.
  • All of the listed weights put me in the government-designated “healthy” BMI range of 19-25.
  • I use “X,” my lowest weight as a runner, as a baseline weight and refer to other weights as X+10, etc.
  • “X” is a little on the low side for my height and build. At about 5 pounds lower than that, I have trouble sleeping because my hipbones are uncomfortably digging into my mattress.
  • However, “X” is probably still more than almost all of you weigh. I’m relatively tall, have bones denser than lead, and lats that most guys are jealous of.
Now, on to the data.

Below is a table with race dates, race distances, race times, and my weight. For comparison purposes, I converted each race time to a 5k equivalent time using the trusty McMillan running calculator.

Without any in-depth analysis, you can see that my slowest race time was when I weighed the least, and my fastest race time was when I weighed the most. And the differences in weight and time are not small. It’s a 20 pound weight difference and a 4 minute time difference in a 5k, and this comparison is quite fair because both races were 5ks.

So heavier=faster? No. First, making that kind of conclusion based on two data points would be asinine. Second, many other factors are important – training volume, training quality, and race experience, among others.

Still, it does make me wonder about the lighter=faster hypothesis, so I made a scatter plot of my race times vs. my weight.

While the scatter plot doesn’t immediately show a clear relationship between my weight and times, I did a “best fit line” evaluation – in other words, an estimate of the linear relationship between my race times and my weight. That relationship does, in fact, indicate that I race faster when heavier, and that I actually drop about 8 seconds in a 5k for every pound I GAIN.

Of course, one can find a weak linear relationship between any two unrelated variables, so I tested the correlation level of my weight and my race times, and found that my weight explains about 1/3 of the variance in my race times. That’s not a lot, but it does indicate that there is some impact, and as noted above, the estimated linear relationship indicates that I run faster when heavier.

What does this mean? Basically, nothing. First, it’s just a case study of one runner. Second, the weight range is actually relatively small – if I raced at X+100, I’m pretty sure that I’d be quite a bit slower. And finally, my evaluation didn’t account for any other factors, such as improved training practices.

But here’s a graph that is meaningful: the 5k equivalent time of my races over time.

And it shows that I’m generally getting faster. Because I’ve been training smart and doing a better job of matching my nutrition needs, in terms of quality and timing, to my training schedule as I’ve become more focused on running faster. My weight has been all over the place, but still, I run faster.

Which is the real take-away here. You might race faster heavier. You might race faster lighter. But you will almost definitely race faster if you train hard, train effectively, and maintain habits that support your training, such as quality nutrition, good sleep habits and the like.


  1. Charts and graphs! <3

    I totally agree with your main point. If you are at a healthy weight and train, fuel, recover, sleep, etc appropriately you will be faster. End of story.

    1. Yep. Unfortunately, it totally kills my "sub 4 hour marathon via sitting on my butt just eating celery" plan. I guess I will run. Weird.

  2. Love this post and the stats and the numbers and all. I agree with the main premise that as long as you are at a healthy body weight, your training will have more impact on your race times than a little drop in weight. However, for people who have a ton of weight to lose, I'm sure dropping pounds would lead to significant improvement in times.

  3. I was hoping the conclusion would be drink and eat whatever you want so that you gain weight and run faster. Are you sure you don't want to make broad, generalized statements based on a case study of one?

  4. This is awesome. So many runners (especially women) think that skinny=fast. It's not about what your body weighs, but it being strong enough to keep up with the demands you put on it. Love that you debunked this myth. :)

  5. I wish I had a record of my weight now so I could compare! But, I wear the same pants as I did in high school, but my times have dropped a bit, so I'm guessing it's probably due to training. Glad to know all the super skinny runners around me don't have an unfair advantage!

  6. From my own experience, my running speed has really only fluctuated with how hard I'm training and how much I'm focusing on staying healthy through proper nutrition. A lighter weight is often a product of good training and nutrition, but it doesn't necessarily mean that it causes you to become faster. I totally agree with your rebuttle to the Runner's World article!

  7. Your last paragraph says it all! I'm prolly a tad heavier right now vs this time last season but WAY faster! My secret formula? Running more. Sssshhh, don't tell my competition.

    P.S. I'm jealous of your lats ;)