I was wondering what the effect of carrying a few extra pounds was on running times. I found on the web that scientists have done treadmill time trials with runners who wore known amounts of excess weight. They found that on average runners lost 23 metres distance per 1% excess weight per 12 mins running time. See runnersconnect.net/running-...

This implies (I think) that if you know how much excess weight you are carrying and your current time for 5k you should be able to calculate roughly what your 5k time might be if you lost the excess weight. Its a bit complicated but here goes...

1. Divide your current weight by your target weight to find the excess weight %

2. Divide your current 5k time in minutes by 12 to find your time factor

3. Multiply your time factor by your excess weight % and then by 23 to find the number of metres you are losing over 5k

4. Divide that number by 5000 to find the % distance you are losing

5. Multiply your current time by 1 minus the % distance you are losing to get your potential time (multiply the decimal by 60 to get accurate seconds).

Example 1

Current weight = 160lbs (11 stone 6lbs)

Target weight = 140lbs (10 stone)

Current 5k time = 36 mins

1. Excess weight % = 160 / 140 = 1.12 ie 12% overweight

2. Time factor = 36 mins / 12 = 3

3. Metres lost = 3 x 12 x 23m = 828m

4. Proportion of 5k lost = 828m / 5000m = 0.17

5. Potential 5k time at target weight = 1 - 0.17 (0.83) x 36 mins = 29.88 = 29mins 53secs

Example 2

Current weight = 1901bs (13 stone 8lbs)

Target weight = 160lbs (11 stone 6lbs)

Current 5k time = 40 mins

1. Excess weight % = 190 / 160 = 1.188 ie 19% overweight

2. Time factor = 40 mins / 12 = 3.333

3. Metres lost = 3.333 x 19 x 23 = 1456m

4. Proportion of 5k lost = 1456 / 5000m = 0.29

5. Potential 5k time at target weight = 1 - 0.29 (0.71) x 40 mins = 28.4 = 28mins 24secs

I hope that this gives real encouragement to those amongst us who are doing C25K as part of a weight loss programme. Let me know how you get on running your own figures through the calculation. Or leave your own figures for current weight, target weight & current 5k time below and I will work it out for you...

ChrisL

PS: The above calculation is simply about the effect of carrying the excess weight. It makes no allowance for any improvement due to increased fitness as you lose weight nor to any improvement in movement or running technique over the time. So you should actually run faster than it predicts if you get fitter as well as lighter (I think).