By Chris Klingenberg
Insoles with sensory-enhancing technology, previously demonstrated to help address balance and proprioceptive deficits, can also improve agility in athletes, according to research from Harvard University in Cambridge, MA.
Subthreshold stochastic resonance (SR) sensory-enhancing insoles have been shown to improve balance and proprioception in the elderly and in patients with diabetic neuropathy or stroke (see “Using subsensory noise to improve balance, gait,” May 2016, page 37). But balance and proprioception are also important for athletic performance, including agility, or changes of direction.
To test the effects of SR on agility in athletes, the Harvard investigators assessed 20 recreational athletes (eight women) as they performed a hexagonal agility drill commonly used by coaches and in sports medicine research. The athletes wore prototype SR insoles for all agility trials, but the stimulation was randomly turned on or off. Participants wore their own footwear, with the SR insoles in place of the existing insoles.
For the agility task, the researchers used tape to mark a hexagon (60 cm per side) on the floor, as well as seven smaller rectangular targets—one 18 cm outside each side of the hexagon, and one in the center. Each participant started in the center of the hexagon, then jumped to the first outside target, back to the center, then to the second outside target, and so on until a full circuit had been completed three times. Participants were instructed to jump with the feet together and land on the targets with the forefeet, and to perform the task as quickly and accurately as possible.
The athletes completed the agility task more quickly with the SR turned on than with it off, decreasing their times by .12 s on average; 14 of the 20 participants were faster with the SR on than off. This improvement was not associated with any decrease in accuracy; in fact, 11 athletes had less accumulated error (the sum of the distance by which each target was missed) with the SR on than when it was turned off. The findings were published in the May 2016 issue of the Journal of Biomechanics.
“We observed no correlation between the performance in either completion time or accumulated error during the SR-off condition and the change with the SR on,” said Danny Miranda, PhD, staff research scientist at the university’s Wyss Institute and first author of the study. “This means that the outcomes during the SR-off condition did not influence the changes observed when SR was on. This suggests that SR stimulation has the potential to improve performance outcomes independent of their baseline values.”
The .12 s improvement on the agility task in the study is consistent with an improvement of 10% to 20% in percentile ranking for NCAA Division I athletes, according to normative values published in 2006. A difference of .13 seconds on a different agility drill used at the 2015 National Football League Combine separated the top-ranked player from the eighth-ranked player. However, additional work is needed to determine if the benefits of SR stimulation in highly competitive athletes—who may have higher levels of both proprioception and agility to begin with—are similar to those experienced by the recreational athletes in the study, Miranda said.
He also noted that, along with the potential benefits, SR insoles also have some limitations.
“In regard to the pros and cons, I think the pros are pretty straightforward [sensory enhancement, balance improvement, proprioceptive improvement, and form factor to standard insoles],” Miranda said. “The cons would be charging, battery life, and vibration.”
Robert Conenello, DPM, a past president of the American Academy of Podiatric Sports Medicine and a clinical advisor for Special Olympics International, is bullish on the future of SR insole technology.
“I do believe that there is something to be said for those insoles,” Conenello said. “It is a hot topic that I don’t think we have a grasp on. I think the pros outweigh the cons. I think that the future is bright and you are going to see athletes using them more and more. People of all ages should be able to benefit from SR insoles.”
Miranda DL, Hsu W-H, Gravelle DC, et al. Sensory enhancing insoles improve athletic performance during a hexagonal agility task. J Biomech 2016;49(7): 1058-1063.
Hoffman J. Norms for fitness, performance, and health. 1st ed. Champaign, IL: Human Kinetics; 2006.