Monday, July 25, 2011

Run Faster: Shoe Weight and Performance

Most everyone wears running shoes for good reason, without them the discomfort would be too great to run for any duration at speed over frequently encountered terrain.  Shoes protect our feet from sharp objects and extreme temperatures.  This protection comes at a cost.  Shoes are a mass on our feet that take energy to bring up and forward each stride.  To maximize performance, shoe weight needs to be minimized without prematurely fatiguing the feet.

Shoe variety is a good way to combat foot related fatigue and challenge different muscles in each run.  This is part of a good mitigation strategy against overuse injuries.  Road running and trail running require different footwear and different distances dictate different footwear as well.  It is good to have a mix of different shoes suited for long distance, tempo, intervals and races.  I will wear lighter shoes for a 5k that I cannot wear for half marathon due to blistering and discomfort at longer distances.    

Every serious runner should have several different shoes for different distances, terrain, and races.  Lightweight 5k flats are a great addition to high mileage trainers, have the largest performance payoff, can be used for intervals, some tempo work, and of course 5k races.

The real question is what do we stand to gain in minutes and seconds by way of shoe selection?  What potential performance gains exist by building foot strength to use lighter shoes for longer distances?  These questions are easy to answer by way of mathematical analysis utilizing simple physics and are just as easily verifiable in application.  I will spare these details for a follow up post for those interested.  

 I have a variety of shoes at home including pairs of Nike Voomero and Free 3.0 V2 that are used in this example.  The minimalist Free is a much quicker shoe than the Voomero, provided my foot strength is up for the distance.  How much quicker?  For a 5k event the lighter Free gives me close to a 17 seconds edge and a finish time of 18:23.  For a 10K this is a 27 second advantage leading to a finish time of 38:30.  For a half marathon the advantage is a little over 1 minute for a finish time of 1:28:12.  The biggest advantage is for the 5k with a 1.5% improvement.  The 10k and half marathon have diminishing returns of 1.2% and 1.1%.

Shoe weight is just one of many factors that impact running efficiency.  The 6.7oz Free requires 2.1% to 2.4% of total effort to accelerate the shoe up and forward during a run.  For the 10.4oz Voomero this is 2.9% to 3.8%.  With respect to the just the shoe, cushioning of the sole will also reduce efficiency.  This will be a subsequent post.  Keep an eye out for a post about the math behind this analysis and additional Run Faster posts covering all aspects of efficiency and bio-mechanics.  I am happy to customize these numbers for others, let me know.

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