Strength Shoes: Pain, No Gain?

Affiliation: Fred Hatfield MS SSC (associate strength and conditioning coach), James Madison University, Harrisonburg, VA 22807.
Acknowledgments: Dave Gerrard (reviewer); Will Hopkins (statistics, editing); Gord Sleivert (reviewer).
Correspondence: drsquat=AT=drsquat.com (Fred Hatfield)

Summary. A recent study found similar small gains (~1%) in sprint and jump performance when previously untrained young men trained for 10 weeks in regular shoes or Strength Shoes. Injury rate was much higher with Strength Shoes, probably a result of the inappropriate training program prescribed by the manufacturer. An earlier study of more experienced athletes showed trends towards better performance and less injury with normal shoes. I therefore cannot recommend Strength Shoes.
Reviewers' comments.

The Strength Shoe is a sneaker with an extra platform (see picture). The idea behind the design, according to the manufacturer, is to "cause the calf muscles to support 100% of the body weight. This overload works the calf muscles in addition to the Achilles tendon." The manufacturer also makes the unsupported and untrue claim that "the Strength Shoe has been medically documented to be the most effective way to increase vertical jump and speed".

A study of the effects of training with the shoe has been reported recently (Porcari et al., 1996). I summarize that study here, and deal briefly with an earlier study.

In the recent study, 72 college-age men not previously involved in athletics or fitness training were randomly assigned to three groups of 24: a control group who did no training, a control group who trained in regular shoes, and a group who trained in Strength Shoes. The training groups trained three times a week for 10 weeks. They followed identical programs prescribed by the manufacturer. All subjects were pre- and post-tested for 40-yard sprint, vertical jump, standing broad jump, and right and left calf girth.

There were two dropouts in the no-training group, eight in the regular-shoe group, and 10 in the Strength-Shoe group. Seven of the Strength-Shoe dropouts and one of the regular-shoe dropouts were due to injury that could be attributed to the training.

The changes in performance and calf girth are summarized in the table. Both training groups showed a tendency to improve their sprinting and jumping by 0.5-1.6%. The Strength-Shoe group tended to do better than the regular-shoe group in the sprint and vertical jump, while the trend was reversed for the broad jump. There was also a tendency for the Strength-Shoe group to develop bigger calf muscles. None of the differences between any of the three groups was statistically significant, so we can't say whether any of the observed differences are likely to be true for athletes in general.

In their promotional literature, Strength Footwear Inc. claim that up to 0.2 seconds can be taken off the 40-yard time (about 4%), nine inches can be added to the vertical jump (about 40%), and calf girth can be increased by two inches (about 15%). These claims were clearly not supported in this study. In fact, any slight gain that might be possible with Strength Shoes would appear to be more than offset by the higher risk of injury.

One of the problems with this study is that the subjects were not trained athletes. It's possible that athletes with well-conditioned muscles and joints would not get injured with Strength Shoes.

Another problem is that the training program might not have been optimal.. It's possible that some preparative weight training should have been included in the program. Another possibility is that the intensity was too high for untrained individuals (ballistic shock is not well tolerated by untrained muscles or connective tissue), and that the volume was too low. A better balance of intensity and volume might have increased the gains in both training groups and increased the gains in the Strength-Shoe group relative to the regular-shoe group.

A final problem is that the study did not have sufficient power to detect the small gains that are important to athletes. The differences between the groups in some of the tests was around 1%--often a winning margin--yet these were not statistically significant. (In fairness to the authors, there were more subjects in their study than is usual in studies of performance enhancement.)

There is only one other peer-reviewed report of the effects of training in Strength Shoes. Cook et al. (1993) randomized 12 intercollegiate track and field participants to a Strength-Shoe group and a normal-shoe group. After 8 weeks of a training program supplied by the manufacturer, the normal-shoe group showed a tendency to improve more than the Strength-Shoe group on all performance measures: 40-yard dash (8.3% vs 6.9%), vertical jump (9.2% vs 3.3%), strength (torque) at slow speed (16% vs 10%), and strength at fast speed (13% vs -5%). Only calf circumference tended to get bigger in the Strength-Shoe group (2.3% vs 0.2% in the normal group). Two of the six athletes in the Strength-Shoe group complained of anterior tibial pain, and one of them had to drop out of the study because of the pain. None of the normal-shoe group complained of pain or dropped out.

You should also check out an amusing, no-nonsense report of a less formal experiment with these shoes by seventh graders. It's part of CBC Television's award-winning Street Cents site. Their conclusion: borrow a book about training if you want the most cost-effective way to enhance your performance.

I have had considerable experience with Strength Shoes, and with training athletes using them. My impression up until now has been that these shoes might enhance performance of experienced athletes if they are used with the right kind of training program. But the research to date indicates that the right kind of training program might produce even better results with normal shoes. I cannot recommend Strength Shoes.

Hatfield, F.C. (1997). Strength Shoes: pain, no gain? http:// www.sportsci.org/traintech/strengthshoes/fch.htm Sportscience Training & Technology

Cook, S. D., Schultz, G., Omey, M. L., Wolfe, M. W., and Brunet, M. F. (1993). Development of lower leg strength and flexibility with the strength shoe. American Journal of Sports Medicine, 21: 445-448.

Porcari, J. P., Pethan, S. M., Ward, K., Fater, D., and Terry, L. (1996). Effects of training in strength shoes on 40-yard dash time, jumping ability, and calf girth. Journal of Strength and Conditioning Research, 10, 120-123.

I believe the injuries in the Strength-Shoe group could be attributed to the shoes primarily because:

• Being forced to exercise on the toes (in continual plantar flexion) would increase the eccentric muscle activity in the posterior compartment, which could lead to stress in the calf-muscle and achilles tendon.
• This position could also translate strain to the shin area and contribute to medial tibial stress syndrome ("shin splints").
• Exercising in this position (on toes) would decrease proprioceptive stimulus (because the whole foot would not be in contact with the ground) and this could lead to ankle injuries. I believe it would be an unstable position.

...The nature of [the] plyometric and sprint training certainly would expose untrained subjects to a reasonably high [risk of] injury. I would have liked to see more discussion of the paper using the trained athletes... It is probably the more important of the two studies.

In addition, I disagree with the conclusion that the intensity was too high and volume too low. I believe that both volume and intensity were too high and the rest intervals were too short. The short rest intervals probably meant that subjects were performing the exercises in a fatigued state and would be more susceptible to injury (as well as less quality in their training).

In the beginning program, subjects were completing about 750 yards of sprint and jump work. Most plyometric experts recommend twice a week training, not three times a week