Barefoot-like designs challenge footwear conventions

The idea of going barefoot means different things to different people. To some, the absence of footwear means one less barrier between man and nature, feeling the warmth of sand between one's toes or the coolness of dewy grass tickling one's soles. To others, it means one less layer of protection from the heat of a sun-scorched sidewalk, the bits of broken glass lurking in the sand or the animal droppings that help keep the grass so green. Some think of going barefoot as losing not just protection, but also the stability and cushioning that footwear provide. And some think of it as an opportunity to use muscles in the foot that would otherwise lie fallow.

As one might expect, there aren't a lot of footwear manufacturers in the last category. But two shoe designs, the Nike Free and the Masai Barefoot Technology shoe from Swiss Masai, are now being marketed to individuals who want the benefits of going barefoot-both the feeling of freedom and the biomechanical challenge-along with some degree of protection. Research confirms that both shoes alter gait patterns and increase muscle activity. It remains to be seen, however, whether those changes result in injury prevention or improved performance. Practitioners and research also remain unclear as to just which athletes or patients might benefit from the use of the footwear, and for which individuals the unorthodox designs might pose a risk.

Barefoot background

Barefoot running, made famous by such athletes as trackster Zola Budd and marathoners Abebe Bikila and Charlie "Doc" Robbins, not only has something of a cult following among hard-core runners but also has long been used as a training tool by coaches. And barefoot walking isn't just for the beach any more, judging from the number of Web sites extolling the delights of shedding shoes for everything from driving to hiking.

Given this level of popularity, surprisingly few recent studies have specifically examined the biomechanics of barefoot walking or running-with much of what is known in this area dating back more than a decade. A February 1985 study published in Medicine and Science in Sports & Exercise found that barefoot running was more economical than shod running in terms of aerobic cost, and was associated with less angular displacement of the knee. In the April 1987 issue of MSSE, Canadian researchers documented that barefoot running was characterized by deflection of the medial longitudinal arch not seen in shod runners. An April 1991 study by Swiss researchers, published in the same journal, reported that running shoes decreased torsion angles and increased pronation angles compared to running barefoot, but those results were disproven in a November 2000 study that used more accurate marker placements. The follow-up study, published in the Journal of Biomechanics, found no significant differences in calcaneal or tibial movement between barefoot and shod running.

Following the lead of researchers from the 1980s, more recent studies have also offered evidence for active adaptations on the part of barefoot runners. Belgian researchers reported in the March 2000 issue of the Journal of Biomechanics that barefoot running was associated with flatter foot placement and lower peak heel pressures than shod running, and suggested that the latter change was the result of the former. In the September 2005 issue of the International Journal of Sports Medicine, French researchers found greater preactivation of the triceps surae muscles and lower impact peaks associated with barefoot running. And in the August 2003 issue of the Journal of Biomechanics, investigators from the University of Calgary reported that the timing and intensity of the tibialis anterior muscle firing was altered with the use of running shoes.

Recent studies have also focused on the effect of barefoot running on shock absorption further up the kinetic chain. In a study published in the August 2001 issue of IJSM, Austrian researchers found that running shoes significantly decreased shock transmission and improved muscle response at the spine while jogging compared to a barefoot condition. But a July 1995 German study of a single subject with instrumented hip implants, published in the Journal of Biomechanics, revealed that loads at the hip were lowest while the subject was barefoot and that softness of footwear materials did not translate into load reductions. And a September study from Rush Medical College in Chicago found that peak joint loads at the hip and knee were significantly lower in 75 patients with knee osteoarthritis while walking barefoot than while walking in shoes; those findings were published in Arthritis & Rheumatism.

"The concept is that shoes take over certain functions, so some of the muscles don't have to do the work that they would normally do," said Benno M. Nigg, PhD, director of the Human Performance Laboratory at the University of Calgary. "They can provide intrinsic stability, and because small muscles usually have a lever arm that is smaller with respect to the joint, loading is reduced when these muscles are active. This should have an effect on performance."

Free thinkers

The idea to make a Nike shoe for people who prefer not to wear running shoes came, in a roundabout way, from Vin Lananna, then-head coach of the Stanford University track and field team. Since the university had a partnership with Nike, company designers and researchers were on hand during workouts to observe how their shoes were being used-and noticed that, at times, Lananna had his athletes run barefoot, claiming that it improved performance. Likely taking into account that Lananna wasn't the only coach enamored of barefoot training, Nike gave researchers the go-ahead to see if they could develop a footwear product that would serve the same purpose.

On a sunny day in June 2001, the researchers used high-speed video cameras and pressure sensors to analyze 10 male and 10 female competitive runners on a natural grass field, running barefoot at a pace equivalent to a seven minute, 30 second mile. They found that, relative to running shod on a hard surface, barefoot running on grass resulted in differences in timing of foot contact, pressure progression pattern, distribution of contact area, magnitudes of peak pressures, foot angle relative to the surface just prior to contact, metatarsophalangeal joint range of motion, and range of plantar flexion right before toe-off.

"The foot was in control," said Jeff Pisciotta, senior researcher and biomechanist at Nike's Sports Research Laboratory in Beaverton, OR. "I had thought variables at the knee would have changed due to lack of cushioning, such as more flexion for shock absorption, but all the changes were at the foot and ankle. That made us start thinking that when you put a shoe on, it starts to take over some of the control."

The Nike Free 5.0 running shoe, introduced in 2004, was designed to provide about half (i.e., five out of 10) the stability of a conventional running shoe-stopping short of allowing full range of plantar flexor or MPJ motion, for example. It features a very thin upper material, no heel counter, and an outsole segmented by deep grooves to allow for flexion. A cross-training version of the 5.0 is now available, along with a 7.0 version for heavier athletes who need more stability; a 3.0 version is in the pipeline.

Two clinical studies of the prototype Nike Free were conducted by researchers at the University of Cologne and presented in the summer of 2005. In one study, presented at the International Society of Biomechanics' Symposium on Footwear Biomechanics just prior to the July 2005 meeting of the ISB in Cleveland, OH, 25 athletes used the Nike Free shoes for warm-up only (then switched to conventional training shoes for the majority of a workout) three or four times per week over a five-month training period. A control group of 25 athletes used only conventional training shoes for the same period. At the end of the intervention, the researchers found that MPJ flexor strength, plantar flexor strength, dorsiflexor strength, and inversion moment had significantly increased from baseline in the experimental group but not in the control group. In addition, the cross-sectional area of the flexor hallucis longus and flexor digitorum longus significantly increased in the experimental group.

In the second study, presented at the 2005 ISB meeting, an experimental group of 50 subjects wore the Nike Free shoes while performing a set of specific exercises (20 to 30 minutes) four times per week for six months; a control group of 50 subjects did the same exercises in their own footwear. The results indicated a significant (20%) increase from baseline for toe flexor strength in the experimental group, but no change in the control group. The path of motion of the MPJ (in degrees of dorsiflexion) decreased by 7% in those who wore the Nike Free but did not change significantly in the controls. And a significant 5% increase in flexor hallucis longus muscle volume was seen in 25 randomly selected subjects from the experimental group, with increases of 4% and 5% in the abductor hallucis and tibialis posterior approaching significance. Neither group demonstrated a significant change in MPJ range of motion.

Neither the German investigators nor Nike's research team have looked specifically at whether those documented changes translate into improved performance. But research from the University of British Columbia presented in June at the annual meeting of the American College of Sports Medicine suggested the shoe could be useful for rehabilitation following an ankle sprain (see "Nike's barefoot-like Free shoe could play role in rehabilitation," August, page 59).

"The hope is that a stronger, more flexible foot in a traditional running product will perform better and have less risk of injury," Pisciotta said. "We have not really tried to follow that up, but we do get a lot of positive feedback."

Unstable mates

Like the Nike Free, the Masai Barefoot Technology concept challenges the small muscles of the foot to work harder by providing less stabilization. In this case, the minimalist design of the Nike Free is replaced by a thick rocker bottom sole with a negative heel, providing the same type of instability while standing as a wobble board. In fact, standing in the MBT shoe is likely more beneficial than walking or running in it, Nigg said.

"When you walk or run in the MBT shoe, you have slightly less muscle activity. It's like riding a bicycle; the faster you ride, the more stable you are," Nigg said. "Since you stand a lot during the day, that's a training effect that can be quite substantial."

An eight-subject study published by Nigg and colleagues in the January 2006 issue of Clinical Biomechanics confirmed that center of pressure excursion was greater in the MBT shoe than in a control shoe and that EMG activity increased in all tested muscles, particularly the tibialis anterior. A prospective study (submitted for publication) confirmed that balance time more than doubled after six weeks of wearing the shoes.

Looking at effects of the MBT shoe during gait, Swiss researchers found that subjects took smaller steps than in traditional shoes and demonstrated EMG increases in the tibialis anterior during swing phase, the gastrocnemius muscles from terminal swing to midstance, the vastus medialis and lateralis muscles from midstance to toe-off, and the rectus femoris muscle in midstance. The study, published in the September issue of Clinical Biomechanics, also found increased ankle dorsiflexion at initial contact, a continuous plantar flexion moment throughout stance phase, and increased knee flexion.

And, consistent with the Chicago study that found benefits of barefoot walking for patients with knee OA, Nigg and colleagues reported in the October issue of MSSE that knee OA patients randomized to wear MBT shoes had greater pain reductions between three and six weeks than did those who wore control shoes; both groups experienced significant pain reductions over the 12-week study period.

Like going barefoot, shoes that mimic the barefoot experience are not for everybody. For one thing, they're not cheap: the Nike Free runs about $100, the MBT shoe more than $200. Both Pisciotta and Nigg noted that similar benefits could be achieved less expensively by simply running barefoot on sand or standing on a wobble board. And conditions such as ankle instability, metatarsalgia, or diabetic neuropathy are likely contraindications to both shoes.

"I would be concerned about the use of the MBT shoe in patients who had limited triceps surae length or who had equinus foot," said Michael T. Gross, PT, PhD, professor of physical therapy at the University of North Carolina in Chapel Hill. "This would especially be a concern in an older individual who then might be predisposed to falling backwards with the negative heel configuration during the early part of stance phase of walking gait. If a tight triceps surae were a contributor to a patient's plantar fasciitis, that might be another reason to stay away from this shoe."

Irene Davis, PT, PhD, professor of physical therapy and director of the Running Injury Clinic at the University of Delaware, said she would generally not recommend the Nike Free shoe for patients with plantar fasciitis, high arches, or severely pronated feet. However, she said, one of her patients who had plantar fasciitis and flat feet went ahead and bought a pair for himself, and to her surprise the plantar fasciitis symptoms abated and the patient was able to run short distances in the shoes.

"This is how we often learn things, when patients don't listen to us," said Davis, who added that reading up on barefoot running studies has motivated her to incorporate more barefoot walking in her own life. "I think perhaps the widespread plantar fasciitis in this country is partly due to the fact that we really don't allow the muscles in our feet to do what they are designed to do."

Jordana Bieze Foster is a freelance writer based in Massachusetts and former editor of BioMechanics.