BioMechanics
April 1998

Materials:
Do soft soles improve running shoes?

By Steven Robbins, MD, Edward Waked, PhD, and Gad Saad, PhD

There is evidence that amplitude of vertical impact in sports involving running and jumping accounts for many injuries.^1-4 Therefore, it seems sensible for practitioners to attempt to reduce impact amplitude through footwear design. We reviewed earlier published reports dealing with the effects of shoe “cushioning” on impact, and provide an update of new developments.

Cushioning consists of highly resilient materials of relatively low hardness (soft) used as an interface between foot and support surface, mainly in shoe midsoles. Reports indicate that machine testing of footwear is invalid because it fails to account for intense behavioral responses, or alterations in impact-moderating behavior, in relation to a change in hardness of shoe soles.^5-10 Accordingly, we will consider only data obtained from impact measurement when subjects run and jump, not data obtained through machine testing.

Does “cushioning” cushion impact?

Snel et al¹⁰ examined impact when 10 male recreational runners each ran over a force platform wearing one of nine pairs of shoes randomly selected from retail outlets. Impact did not vary significantly between footwear samples that had different machine impact testing results. The authors concluded that no difference in impact could be anticipated in relation to either footwear manufacturer or price of footwear.

“When subjects were tested with soft versus hard shoes, no difference in impact force was found and curiously, the second, propulsive peak in the vertical ground reaction force was actually higher with soft shoes,”¹¹ said E.C. Frederick, then director of Nike Sport Research Laboratory, in 1986, presumably based on internal Nike investigations.

Kaelin et al⁸ investigated impact when 11 runners each ran with one of 12 pairs of custom shoes with sole thickness between 18 mm and 43 mm, and midsole hardness between Shore A 30 and Shore A 60. Again, differences in cushioning did not influence impact.

Nigg et al¹² examined impact when subjects ran wearing shoes with midsoles that spanned a wide range of hardness. This was the first report to find significant differences in relation to cushioning. Impact was significantly higher with the softest soled shoes (Shore A 20). Above Shore A 30 in hardness, cushioning did not influence impact (Figure 1).

Cushioning during impulsive landing

In summary, under laboratory conditions, impact remains constant when humans run and jump with cushion-soled shoes of Shore A 30 or harder, regardless of manufacturer, model, or shoe price. Shoes with relatively soft sole material (Shore A 30 or lower) are associated with amplified impact. An increase in cushioning causes a reduction of impact-moderating behavior. Humans are unable to estimate vertical impact with precision when wearing athletic footwear, therefore changes in impact-moderating behavior are not a direct response to it.^14-17 The notion that impact can be moderated through use of cushioning is invalid in humans, and use of the term “cushioning” when talking of vertical impact moderation during human locomotion and jumping is misleading.

Effect of deceptive advertising

In blind testing, impact is similar with all athletic footwear, regardless of manufacturer and price.¹⁰ One would therefore anticipate that injury frequency in relation to cost would be similar for all athletic footwear. However, expensive athletic shoes have been shown to account for 123% greater injury frequency than the cheapest ones (see table).¹⁸ This led us to suspect that users of more expensive shoes experience higher impact after the shoes are purchased. We tested the hypothesis that deceptive advertising creates a false sense of security with users of expensive athletic shoes, inducing attenuation of impact-moderating behavior, elevated impact, and injury.¹⁹

In the experiment, 15 young healthy male volunteers with a mean age of 31 years (SD = ±4.2) confronted four surfaces: one a bare force-moment platform, and three with this platform covered by identical shoe-sole material made to appear different and advertised divergently. Advertising messages suggested superior impact absorption and protection (deceptive message), poor impact absorption and high injury risk (warning message), and unknown impact absorption and safety (neutral message). Ground reaction forces were recorded for 10 barefoot footfalls, according to a protocol requiring stepping forward from a perch to a surface 4.5 cm below.

Results indicated that impact varied as a function of advertising message (p < 0.001). The deceptive message equaled the neutral message in eliciting higher impact than the warning message and the bare platform. Differences grew with repetitions (p < 0.001).

These data provide a plausible mechanism to explain higher injury frequency in users of expensive athletic shoes. This was the first report to suggest that deceptive advertising of protective devices may represent a public health hazard and may have to be eliminated, presumably through regulation; and there is a tendency in humans to be less cautious when using new devices of unknown benefit because of overly positive attitudes associated with new technology and novel devices.

Our study on deceptive advertising found that cheap shoes do not require warning signs because people use them with extreme caution, as evidenced by the substantial decrease in impact demonstrated in our study.

Effect of instability on vertical impact

We later discovered the mechanism accounting for this instability.²⁶ Softer and thicker sole materials caused an increased amplitude of frontal plane foot movement, which caused imprecise postural adjustments and a decline in foot-position awareness.²⁶ Poor foot-position sense has also been implicated as the cause of ankle sprains. This suggests that cushioned footwear accounts for many ankle sprains in athletes.

Whereas humans are relatively unaware of the vertical impact to which they are subjected, we found that relative stability is estimated precisely. Therefore, we suspected that the reduction in impact-moderating behavior related to soft-soled shoes might be a response to instability produced by the footwear.²⁷ Furthermore, as previously mentioned, reports have indicated that behavior that reduces impact, such as flexion of hip and knee, declines as sole material becomes softer, which explains why wearing softer and thicker soled shoes does not result in diminished impact.^12,13,28 This further supported our notion that the reduction in impact-moderating behavior associated with soft materials underfoot was an attempt to improve equilibrium in people destabilized by soft-soled footwear.²⁷ To test this, we hypothesized that this hard landing strategy is an attempt by the user to improve stability by momentarily transforming the material through compression to a less destabilizing variety.

In an experiment we examined impact and balance when subjects landed impulsively and when they stood on one leg on materials consisting of ethylene-vinyl acetate (EVA) foams of varying stiffness, identical to those used in the soles of most athletic footwear. The experiment was based on a randomized-order, crossover, controlled comparison, and blind design. Subjects were volunteers selected from the general community. They consisted of a random sample of 12 healthy male subjects of mean age 30 years, SD ±6. Additional selection criteria were absence of disabilities influencing ability to walk, run, and balance, and no history of frequent falls.

By this method, impact testing and stability measures were all done on the same test day. Ground reaction forces were measured for 10 barefoot footfalls per subject. The protocol required stepping forward from a perch to a surface 4.5 cm below. Stability testing was done with one-legged standing. Barefoot subjects placed left foot on top of right for 30 sec with their eyes open, gaze straight ahead, and arms at their sides. Subjects confronted four surface conditions presented in random order: first the bare rigid platform, then this surface covered with one of three 2.5-cm-thick materials.

Results indicated that steady-state vertical impact was a negative function of interface stiffness, with the softest interface producing the greatest vertical impact, and the stiffest interface producing the least vertical impact. Vertical impact and stability measures were also negatively related, with the strongest correlation obtained with the softest interface (r = -0.87, p < 0.001). No relation between these variables was obtained for the rigid surface.

We conclude that balance and vertical impact are closely related. This supports our view that landing hard on soft surfaces is an attempt to compensate for the instability associated with the interface. According to our findings, currently available sports shoes and mats are too soft and thick, and should be redesigned if they are to protect humans performing sports.^29,30 Even thin layers of soft, highly resilient material, such as insoles, destabilize humans, causing a decline in foot-position awareness and reduction in impact-moderating behavior. For optimal stability, shoe soles must be relatively thin and above Shore A 50 in hardness. However, subjects find shoes of this hardness to be less comfortable than those with softer soles. We are investigating whether soft-sole materials of low resiliency will allow for high comfort while retaining stability.

Conclusion

After considering footwear advertising, additional factors appear to influence impact-moderating behavior. Recent reports also indicate that humans reduce impact-moderating behavior, thereby amplifying impact, when they are convinced that they are well protected by the footwear they are wearing. Advertising that suggests good protection results in higher impact, whereas advertising that suggests injury risk attenuates impact.¹⁹ Deceptive advertising, suggesting that expensive cushioned footwear offers advanced technology that protects against impact, accounts for the 123% greater frequency of injuries with the most expensive shoes found by Marti.¹⁸

Public health could be advanced through truth in advertising of footwear products with cushioned soles. Furthermore, footwear must be required to provide good balance. Current athletic footwear undoubtedly causes falls, since footwear with thick yielding soles destabilizes humans by as much as 300% compared with hard-soled shoes.

Now that the destabilizing nature of cushioned footwear is well established, continued manufacture of these hazardous items without explicit warning labels represents risk for liability claims from users who are injured from falls and ankle sprains while wearing them. In the context of this report, footwear that provides superior balance will probably be effective at attenuating vertical impact. Clearly, highly resilient materials must be removed from shoe soles for many reasons. This move will portend better health through improved stability and fewer injuries from excessive repetitive impact in sports.

Steven Robbins, MD, and Edward Waked, PhD, are with the Centre for Studies of Aging, McGill University, Montreal, Quebec, Canada. Gad Saad, PhD, is with the marketing department, Concordia University, Montreal.

References

1. Andrish JT. Knee injuries in gymnasts. Clin Sports Med 1985;1:100-120.
2. Brunet ME, Cook SD, Brinker MR, Dickson MA. A survey of running injuries in 1505 competitive and recreational runners. J Sports Med & Phys Fit 1990;30:307-315.
3. Clement DB, Taunton JE, Smart GW, McNicol KL. A survey of overuse running injuries. Phys & Sportsmed 1981;9:47-58.
4. James SL, Bates BL, Ostering LR. Injuries to runners. Am J Sports Med 1978;6:40-50.
5. Clarke TE, Frederick EC, Cooper LB. The effects of shoe cushioning upon selected force and temporal parameters in running. Med Sci Sports Exer 1982;14:144.
6. Clarke TE, Frederick EC, Cooper LB. Effects of shoe cushioning upon ground reaction forces in running. Int J Sports Med 1983;4:247-251.
7. Clarke TE, Frederick EC, Hamill CL. The effects of shoe design parameters on rearfoot control in running. Med Sci Sports Exer 1983;5:376-381.
8. Kaelin X, Denoth J, Stacoff A, Stussi E. Cushioning during running—material tests contra subject tests. Proceedings of the Fourth Meeting of the European Society of Biomechanics 1985:651-656.
9. Nigg B. Factors influencing kinetic and kinematic variables in running, In: Nigg B, ed. Biomechanics of running shoes. Champaign, IL: Human Kinetics Publishers, 1986:139-160.
10. Snel JG, Delleman YF, Heerkens YF, van Ingen Schenau GJ. Shock-absorbing characteristics of running shoes during actual running. In: Winter DA, Norman RW, Wells RP, et al, eds. Biomechanics IX-B. Champaign IL: Human Kinetics Publishers, 1985:133-137.
11. Frederick EC. Mythology and the biomechanics of running shoe design. Proceedings of the Annual Meeting of the American Society of Biomechanics 1986:21-22.
12. Nigg BM, Yeadon MR, Herzog W. The influence of construction strategies of sprung surfaces on deformation during vertical jumps. Med Sci Sports Exer 1988;10:396-402.
13. Mcnitt-Gray JL, Yokoi T. The influence of surface characteristics on the impulse characteristics of drop landings. Proceedings of the 13th Annual Meeting of the American Society of Biomechanics 1989, Vermont:92-93.
14. Robbins SE, Gouw GJ. Athletic footwear: Unsafe due to perceptual illusions. Med Sci Sports Exer 1991;22:217-224.
15. Robbins SE, Gouw GJ, Hanna AM. Running-related injury prevention through innate impact-moderating behavior. Med Sci Sports Exer 1989;21:130-139.
16. Robbins SE, Hanna AM, Gouw GJ. Overload protection: Avoidance response to heavy plantar surface loading. Med Sci Sports Exer 1988;20:85-92.
17. Robbins SE, Hanna AM, Jones L. Sensory attenuation induced by modern footwear. J Test Eval 1988;16:412-416.
18. Marti B. Relationship between running injuries and running shoes. In: Pforringer W, Segesser B, eds. The shoe in sport. Chicago: Year Book Publishers, 1989:256-265.
19. Robbins SE, Waked EG. Hazard of deceptive advertising of athletic footwear. Br J Sports Med 1997;31:299-303.
20. Magnusson M, Enbom H, Johansson R, et al. Significance of pressor input from the human feet in anterior-posterior postural control. Acta Otolaryngol 1990;110:182-188.
21. Ring C, Nayak USL, Isaacs B. The effect of visual deprivation and proprioceptive change on postural sway in healthy adults. J Am Geriatr Soc 1989;37:745-749.
22. Teasdale N, Stelmach GE, Breunig A. Postural sway characteristics of the elderly under normal and altered visual and support surface conditions. J Geront Biol Sci 1991;46:238-244.
23. Robbins SE, Gouw GJ, McClaran J. Shoe sole thickness and hardness influence balance in older men. J Am Geriatr Soc 1992;40:1089-1094.
24. Robbins SE, Waked EG, Gouw J, McClaran J. Athletic footwear affects balance in men. Br J Sports Med 1994;28:114-122.
25. Waked EG, Robbins SE, McClaran J. The effect of footwear midsole hardness and thickness on proprioception and stability in older men. J Test Eval 1997;25:143-148.
26. Robbins SE, Waked EG, Allard P, et al. Aging in relation to optimization of footwear in older men. J Am Geriat Soc 1997;45:61-67.
27. Robbins SE, Waked EG. Balance and vertical impact in sports: Role of shoe sole materials. Arch Phys Med Rehab 1997;78:463-467.
28. McKinley P, Pedotti A. Motor strategies in landing from a jump: The role of skill in task execution. Exp Brain Res 1992;80:427-440.
29. Robbins SE, Waked EG. Foot position awareness: The effect of footwear on instability, excessive impact, and ankle spraining. Crit Rev Phys Rehabil Med 1997;9:53-74.
30. Robbins SE, Waked EG. Factors associated with ankle injuries: Preventive measures. Sports Med 1998;25:65-72.