Sheer simplicity. There are quite a few websites that use this formula to present online race time predictors. The final exponential element is the key. I've seen some references to this as basically a percentage of slow down (i.e. 1.06 = 6% slow down). However, the value is acutally derived from the slope of a trend line drawn though plots of velocity at different distances.
One unique and interesting aspect of Riegel's formula is it's adaptability. One can plot world record times, projected times from any of the other popular prediction tables, or even plot a trend of their own personal times. These custom trend plots result in a "best match" exponent that very closely models the input data. Anyone interested in seeing a few trend charts first hand, and/or running a trend on your own times can download the RiegelPredictor.xls Excel 2002 workbook available in the Downloads section.
I recently encorporated Pete's formula into RunPro, breathing new life and credibility into a 20 year old program. The flexibllity of the formula allowed me to closely approximate 4 different coaches' training guidelines. RunPro also allows users to set a custom value for Riegel's exponent (referred to by that program as Rx). So if you run a trend on your times using the RiegelPredictor.xls file I mention above, you can set the value you obtain in RunPro, giving you your very own personalized racing and training calculator. (Actually, the RiegelPredictor.xls workbook has a built in racing/training calculator worksheet as well).
Riegel's formula does best with distances between 5k and marathon. It deviates from other models at shorter distances. I've added a fudge factor in RunPro to compensate for this, so if you happen to compare RunPro to other predictors based on the Riegel formula, the numbers won't match up for distances below 5k.
The race time predictions are estimates of what a runner might achieve, if they train appropriately for the distance. It does not mean, for example, that if you train for a 5km and achieve a good time, then you will automatically achieve the corresponding time at the marathon distance.
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