Response to the Letter to the Editor: Is Barefoot Running More Economical?

Response to the Letter to the Editor by Rodger Kram and Jason R. Franz

We would like to thank Dr Kram and Mr Franz for their comments regarding our study on the oxygen cost of barefoot running. Our research showed that in 10 recreational male and female runners, VO₂, heart rate (HR) and rating of perceived exertion (RPE) were all significantly lower in the barefoot compared to shod conditions. Though not significant, we also saw 5.7% higher VO₂ overground and 2.0% on the treadmill in the tested runners. This was the first study to our knowledge that has investigated barefoot running on an overground track. Previous studies have used either a treadmill or a short runway across a force plate in a laboratory setting.

The main argument in the “Letter to the Editor” pertained to the calibration of the Nike+ device (Nike, Inc., Beaverton, Oregon) before testing. This device is relatively new and has not been used extensively in scientific research. The manufacturer states that the device without calibration is “quite accurate” but the user can “improve the accuracy” by calibrating to their running style [4]. The calibration procedure consists of having subjects run a known distance (0.25 miles/402 meters) at a steady, natural pace. The software then uses a proprietary algorithm and applies that setting to future runs. A third-party review has found the system to be around 94.6% accurate [3].

Dr Kram and Mr Franz state that the Nike+ system assumes horizontal distance moved forward during contact time for a given individual “does not change much at all for a range of level running speeds” but do not reference any peer-reviewed studies to support this. There have been no studies to date that have validated different calibration procedures of this device, i.e., barefoot vs. shod calibration. Only one previous research study performed any validation of the device, Kane et al. 2010, and they did not have a barefoot running condition [2].

Dr Kram and Mr Franz also state that “5.7% is the greatest difference in VO₂ ever reported for barefoot vs. shod running”. This referenced percent difference was found by us on an overground track. Again, this was the first study to our knowledge that has investigated barefoot vs. shod running on an overground track.

Divert et al. state in their study that there is likely “a rise in elastic energy storage during barefoot running and could be an explanation of the greater efficiency found in barefoot running” [1]. Perhaps this energy storage is greater when running overground than while running on a treadmill; there is not yet sufficient evidence to make any determination. They also state in their conclusion that “although oxygen consumption was mostly influenced by the mass of the shoe, ηnet (net efficiency) was influenced exclusively by the shoe” and that “the lower (ηnet) reported when running shod could only be due to the wearing of shoes which induced mechanical alterations”.

Dr. Kram and Mr Franz mention 2 studies in which barefoot running was shown to have a shorter contact time. The first study [5] used 8 male subjects who were all very experienced with barefoot running. In fact, 3 of their subjects had run a marathon completely barefoot. The second study [1] also used all male subjects. These studies both used force data to assess contact time; the Nike+ device is an accelerometer and has not been shown yet to accurately identify contact time specifically. The contact time during barefoot running may be decreased compared to shod running; however, studies have not shown that using accelerometry to assess contact time is a valid method. Further, in our study we had both male and female subjects; most were very unfamiliar with barefoot running and this may have increased the variability of their stride lengths. If the subjects ran with the same stride length as they did while shod this would negate any contact time differences.

• References

• 1 Divert C, Mornieux G, Freychat P, Baly L, Mayer F, Belli A. Barefoot-shod running differences: shoe or mass effect. Int J Sports Med 2008; 29: 512-518
• 2 Kane NA, Simmons MC, John D, Thompson DL, Basset DR. Validity of the Nike+ Device during walking and running. Int J Sports Med 2009; 31: 101-105
• 3 Next-Generation Running Gear: Track Test Retrieved December 13, 2011, from http://www.popularmechanics.com/outdoors/sports/technology/4212167, 2009
• 4 Nike + iPod User Guide Retrieved December 13, 2011, from http://nikerunning.nike.com/nikeplus/v1/pdf/English.pdf, 2008
• 5 Squadrone R, Gallozzi C. Biomechanical and physiological comparison of barefoot and two shod conditions in experienced barefoot runners. J Sports Med Phys Fitness 2009; 49: 6-13