Strength-Shoes Review Challenged 
FC Hatfield II

Affiliation: Fred Hatfield MS SSC (associate strength and conditioning coach), James Madison University, Harrisonburg, VA 22807.
Correspondence: (Fred Hatfield)

Summary. This article includes copies of correspondence from the manufacturer of Strength Shoes and an abstract in which positive effects of the use of Strength Shoes are reported. Until better evidence is published, I still advise caution in the use of this training aid.
In January of 1997, we published a review of the safety and performance benefits of Strength Shoes. On the basis of available research, we concluded that gains made with the Strength Shoe were no better than those made while training with a regular athletic shoe. Furthermore, we concluded that use of strength shoes imposed a substantial injury risk due to the excessive loading on the achilles tendon and gastrocnemious muscle.

Recently, the editors of Sportscience received the following letter via email from Strength Footwear, Inc:

Fred Hatfield,

This in response to your information regarding the strength shoe and its effectiveness in increasing vertical jump, speed, anaerobic power and anaerobic capacity.

If you would have done your research you would find a recent strength shoe study by Jim R. Flarity, Ph.D. Director, Human Performance Laboratory, Department of Exercise and Sport Sciences, Tulane University. This study can be reviewed at our website

This study was published in THE PHYSIOLOGIST vol.39, num.5 Oct. 1996 and a second study will be published in the AMERICAN MEDICAL ATHLETIC JOURNAL in July of 1997. It was submitted by Jim R. Flarity,Ph.D., Mackie Shilstone,M.A., M.B.A., Tim Church, M.D. and Zachary C. Fischer, B.S.

We strongly suggest that you correct and update your website because the information that you are supplying the public is incorrect. Copies of your defaming remarks that the strength shoe has not been medically documented and can cause injury have been sent to our attorney's as well as the above mentioned names in this letter. If we see no response by July 3, 1997 we will seek legal action. I can be reached at 504-468-5412 if you would like copies of both published studies.

Best Regards,
Kevin M. Bouza
Vice President
Strength Footwear, Inc.

We took this threat seriously, not because it was grounded, but because it exemplifies an unfortunate fact: Even when sport scientists, coaches, and doctors painstakingly try to present material to the public in a fair and accurate manner, they will meet ridicule from those who have different motivations. We agree that if new evidence becomes available, we should evaluate it carefully. There were two major issues here: First, does training in Strength Shoes result in greater performance enhancement than the same training regimes performed in standard athletic shoes? Second, do the loading conditions imposed by strength shoes impose a greater injury risk relative to performing training activities in standard athletic shoes? The same issues remain.

Here is a verbatim copy of the abstract that appears on the website of Strength Footwear, Inc.

The Effectiveness of The Strength Shoe in Enhancing Sport Specific Skills.

Tulane University, Department of Exercise and Sport Science, Laboratory of Human Performance, New Orleans, LA. 70118-5698.

By Jim R Flarity, Ph.D., Director, Human Performance Laboratory Tulane University, Department of Exercise and Sport Sciences

This study analyzed the effectiveness of the Strength Shoe in improving sport specific variables. Male athletes ages 18-22 yr of age (n=20) were studied to assess the effects of 9 weeks of training with the Strength Shoe. The manufactures training protocol emphasizing proper warm-up, cool-down and progression was followed. Subjects trained three days per week for approximately 45-60 minutes per session. The subjects were randomly divided into a control group (wearing the Strength Shoe). Analysis of variance (ANOVA) was used to determine significant differences between the control and treatment group (p<.05). Statistical analysis demonstrated a significant improvement the following variables; relative anaerobic power (.19 W/kg-1 v. 1.22 W/kg-1), relative anaerobic capacity (.25 W/kg-1 v. .67 W/kg-1), Vertical displacement (2.01 in v.3.58 in) and 40 yd. speed (-.06 v. 1.14). No significant difference was found in upper and lower leg size, and lower back strength. In addition, this study found no significant difference in the report of injuries between the control and treatment groups. This study, which is experimental by nature is consistent with that of previously reported subjective data. The results of this study strongly suggest that the use of the Strength Shoe is indeed and effective training tool in the development of anaerobic power.

This abstract represents the entirety of "new published evidence." The second study mentioned by Mr. Bouza was not published at the time of this update, so we cannot report data from that investigation. Published abstracts are always weighed more cautiously than the results of complete published papers. Full papers provide more information for the reader to use when interpreting the data. When the published abstract is not peer reviewed, and funded by a financially motivated third party, then that caution is magnified.

Neither the above abstract nor any other other studies have reported improvements in performance or calf girth anything like the improvements claimed by Strength Footwear, Inc. In particular, no study has ever shown that the Strength Shoe can add 5 to 9 inches on one's vertical jump. Such claims are made in testimonials sent in by customers of Strength Footwear, Inc., according to the producers.

In our first review of literature on the Strength Shoe, we reported a study by Porcari et al., 1996. This study, conducted at the University of Wisconsin - LaCrosse, reported 8 out of the 24 trainees who used the strength shoe dropped out because of injury as opposed to 2 out of 24 trainees who trained with regular shoes. In response to this report of a higher injury rate among Strength Shoe subjects, Kevin Bouza issued this statement:

I would like this letter to serve as my written response pertaining to the injured subjects of the Wisconsin - LaCrosse Study and the implication that these subjects were injured while training in the Strength Shoe.

After I received my copy of the study, I called Kevin Ward and John Porcari and asked them about the dropouts in the Strength Shoe group due to injury. John Pocari told me that the injuries were not the result of training in the Strength Shoe, but, the result of other physical activities by participants in the study. He apologized for the misrepresentation of the Strength Shoe due to these facts. I informed him that the damage has already been done due the the fact that the study has been published without necessary clarification of how these injuries occurred.

Strength Footwear Inc. has provided us with many other studies which are not peer reviewed, including graduate thesis, studies performed by coaches, and studies "soon to be published". Given all of the data presented from a variety of sources, we can only conclude that they are equivocal. This equivocality is highlighted by two studies performed by different groups at the same university (Tulane), whose findings regarding the safety and efficacy of training with Strength Shoes are in direct opposition. Based on the comments of several outside reviewers, we remain convinced that the eccentric loading conditions imposed on the achilles-soleus-gastroc complex by Strength Shoes impose a substantial risk of injury, especially if not used correctly. Of course, this can be said with any training device. Coaches must decide on what risk is acceptable when imposing any training modality on their athletes. The claims made by Strength Footwear Inc., that the Strength Shoe can add 5 to 9 inches on your vertical jump and take 0.2 seconds off your 40 yard time are not substantiated by the published data, though there is some non-peer-reviewed evidence that training in Strength Shoes can result in much smaller but significant improvements. Until further research indicates otherwise, our position remains that the risk of injury may be too great and the magnitude of training effect too small, on average, for us to recommend Strength Shoes to coaches and athletes. 
Edited by Stephen Seiler. Webmastered by Jason Nugent. Last updated 24 July 1997. · · Homepage · Copyright ©1997