August 2017

Flip Flops: Biomechanical critiques resonate with clinicians and designers

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It’s no secret by now that traditional flimsy flip flops are associated with gait alterations that can contribute to more serious issues, but for many patients, flip flops are a hard habit to break. A new generation of “comfort” flip flops offers an alternative, but clinicians remain wary.

By Shalmali Pal

Anyone who spends time in airports in the summer expects to be subjected to a sea of faces. But at floor level, there’s another sight to behold—a flood of flip flops.

The “official” shoe of summer is most often the classic, minimal flat rubber sole with the Y-shaped, or thong, strap. Sometimes it’s jazzed up with sequins, done in vivid colors, or tooled from leather. But these embellishments, of course, don’t address lower extremity practitioners’ criticisms of flip flops—that they offer little to no support for the foot, and can lead to excessive stress in lower extremity tendons and muscles, which must work overtime to hold on to the shoe (see “Flip flops: Fashionable but functionally flawed,” September 2010, page 49).

Perhaps in response to all the bad buzz, a “new generation” of flip flops has emerged that seems to offer many of the same features as comfort footwear—contoured footbeds, slightly higher heels, and thicker outsoles. The rubber has been replaced by performance materials, such as EVA (ethylene vinyl acetate). The straps are sometimes adjustable. The footbed may feature antimicrobial properties (see “Materials science targets foot odors and microbes,” LER Foot Health, June 2016, page 21).

Are these updated flip flops really any better than the traditional ones in terms of foot health and biomechanics? LER checked in with lower extremity specialists to get their current thoughts on flip flops, and whether these upgraded versions are truly a better bet.

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The fuss about flip flops

There’s no shortage of data on the effects of flip flops on biomechanics and gait. An early study from 2008 tested peak plantar pressure in 10 healthy-weight (body mass index [BMI] less than 25 kg/m2) women with size 7 feet while they walked in standard flip flops, athletic shoes, and bare feet. Flip flops were associated with higher peak plantar pressures than athletic shoes but lower pressures than bare feet. The higher peak plantar pressures associated with flip flops increase the risk of pathologic abnormalities, the authors wrote.1

A 2013 study found that, compared with sneakers, flip flops were associated with a shorter stride, larger ankle angle at the beginning of double support and during swing, smaller braking impulse, and shorter stance time.2 That study also found flip flops were associated with different peak ground reaction force variables and ankle dorsiflexion moment compared with closed-toe shoes.

Other studies have also reported kinetic effects of flip flops. A 2014 study reported that, compared with barefoot walking, wearing flip flops was associated with greater maximum ankle dorsi­flexion and greater tibialis anterior activation during swing.3 And a 2010 study found thong-style flip flops were associated with lower peak vertical force at heel contact than sneakers.4

“Wearing flip flops does alter gait kinetics,” the authors of the 2010 study wrote. “The observed altered flip-flop gait may lead to compensation or unusual stresses that flip-flop wearers do not encounter while wearing a more traditional shoe such as an athletic sneaker.”4

Wendi Weimar, PhD, a coauthor on that study, explained to LER in an email that “because the flip flop and the foot are not firmly connected, the toes have to be more engaged to keep the foot and flip flop together. This increases the forces across the ankle and foot and causes compensatory joint positions during gait and standing.”

Biomechanics experts are just beginning to understand the long-term effects of these compensatory foot positions, said Weimar, who is an associate professor and the director of the Sport Biomechanics Laboratory in the School of Kinesiology at Auburn University in Alabama.

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“The longer you are in [flip flops], the more potential for low back and hip issues,” noted Howard Osterman, DPM, a partner in Foot and Ankle Specialists of the Mid-Atlantic in Washington, DC, and team podiatrist for the Washington Wizards of the National Basketball Association.

“With those dollar-bin flip flops, where the strap just comes across the front of the foot, the toes have to grip all the time to keep the shoe from falling off,” said Osterman, who serves on the executive board of the American Academy of Podiatric Sports Medicine. “That’s not great for any kind of extended walking. The increased ankle angle and shorter stride is because, without that, a person will just walk out of the shoe.”

Peter Morcom, CPed(C), president of the Pedorthic Association of Canada and manager at Kintec Footwear and Orthotics in Vancouver, pointed out that chronic problems may develop over time, or may be related to a one-time change in activity.

In his experience, using flip flops on and off on a daily basis doesn’t necessarily cause problems. But when flip-flop wear time increases drama­- tically, that can be problematic. Morcom said patients who normally have no issues with flip flops will come to him with complaints of pain and discomfort, explaining that “I went on this vacation to Disneyland or Vegas, and I wore flip flops ten hours a day.”

The injury list

Of the chronic issues related to flip flops that specialists see clinically, plantar fasciitis topped the experts’ list. This may be because the lack of structure in conventional flip flops allows for excess stress through the soft tissues of the foot, Morcom said.

Next on the list is Achilles tendinopathy. Again, this may be an overuse issue related to the repetitive stress of gripping and the shoe’s lack of structure, Morcom noted.

Then there is metatarsalgia, which can occur because of lack of shock absorption provided by the flip flops’ very thin outsole.

“If there’s no contoured arch contact between the shoe and the foot, especially in the pes cavus foot, you’ll see a lot more of those impact forces, especially when walking on pavement. The thin outsole provides very little shock absorption,” Morcom said.

Just as long-term use of flip flops can cause these lower extremity disorders, people who already have these conditions should stay away from this footwear, the experts agreed.

“Those conditions are related to the biomechanics,” said Seamus Kennedy, CPed, BEng (Mech), president of Hersco Ortho Labs in Long Island City, NY, and spokesperson for the American Academy of Orthotists and Prosthetists.

Even after treatment, incorrect footwear may increase the risk of recurrence, Kennedy said.

“You can’t say conclusively, ‘If you wear flip flops, the conditions will come back.’ But you are certainly putting yourself at a higher risk if you are wearing flat shoes with no arch support,” he said.

Kennedy also said he would vote against flip flops in patients with neuropathy, including diabetic patients, as it puts them at risk for chafing, foreign bodies getting in the shoe, or other types of
microtrauma.

Sometimes limited use of flip flops can be allowed in people with lower extremity issues, depending on the condition and its severity, Morcom said.

“What muscles are activated more when you are using a flip flop? If you have an anterior tibialis tendinopathy or tendinitis, every time you take a step in the flip flops, that muscle is firing a bit more.3 Flip flops wouldn’t make sense for those patients, at least in the acute stages,” he said.

But if the person is being treated for the condition, and is not in a substantial amount of pain, then the occasional use of flip flops may be acceptable, though Morcom cautioned a more structured shoe will give the person better lower extremity control, which in his experience is more conducive to faster and quicker healing.

Morcom said he’d put people with what he called “two extreme foot types” on the list of those who would be better off avoiding flip flops.

“A pes cavus foot lacks the ability to absorb impact. Regular pronation is how the foot absorbs impact, so when there’s a lack of it, you need to find it somewhere else, which is usually the shoe or another joint in the body,” he said. “The other extreme is the excessively pronated foot. When you put that foot into a flip flop, it will just flatten out or overextend, and you’ll run into some of those repetitive stress injuries.”

Lastly, patients at risk for trips and falls should be cautious about wearing flip flops, and they are definitely not the preferred footwear for people who require walkers or other assistive devices, Kennedy said. This includes patients with lower extremity disorders that impact ambulation and people with neurological disorders who have trouble with balance.

“They need stability and shoes that aid proprioception,” he said. “If the foot is covered and the shoe has a snug closure that is holding it to the foot, it activates sensors [in the foot] so the body knows where it is in space. If a patient requires an assistance device, such as a walking stick or a walker, that’s an indication that the balance has gotten to a point where a flip flop would no longer be appropriate.”

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A bad rap?

Despite the potential problems associated with flip flops, experts say the reality is that most people without major lower extremity disorders who wear flip flops on a regular basis will do just fine.

“It’s not that flip flops are good or bad,” Kennedy said. “A solid majority of the population doesn’t have foot issues, so they can wear whatever they want.”

Osterman agreed.

“I don’t actively discourage flip flops,” he said. “What we try to do is guide patients toward better shoe options. The people who come to see me have a reason for being there in terms of their foot health, so we want to help them find something appropriate.”

Not all research has been weighted against flip flops. A 2016 Irish study of 15 healthy adults (eight men; mean age 27 years) found no differences in temporal-spatial parameters for walking with flip flops versus barefoot, suggesting successful adaptation by the flip-flop wearers.5

However, that’s not to say the shoes didn’t render some changes, including an increase in peak knee flexion during swing and peak ankle dorsiflexion at terminal swing. The authors also noted that maximum peak separation between foot and flip flop was 8.8 cm, and occurred at preswing. Finally, minimum toe clearance of the hallux measured 4.2 cm during barefoot walking versus 1.6 cm in flip flops.

The findings suggest that, despite some spatio­temporal adaptation, the separation of the flip flop from the foot led to increased knee flexion and ankle dorsiflexion during swing, most likely to maximize adherence to the foot. Also, the flip flop’s lower minimum clearance (versus barefoot) could increase the risk of tripping over uneven ground, the authors noted.5

Flip flops 2.0

That brings the discussion to the next generation of flip flop designs. The websites of various flip flop manufacturers feature claims related to arch support, midsole shock absorption, and stress reduction on the feet, ankles, and knees. Others describe their flip flops as having ergonomic arch contouring and foam impact pillows to cushion the balls of the feet. There are flip flops that supposedly react to body temperature and conform to the foot. Finally, there are companies that will design flip flops with custom footbeds based on digital scans.

Research conducted at the University of Salford in the UK supports some of these claims.3,6 The British researchers studied an enhanced flip flop that featured a thicker, ergonomic, multidensity EVA midsole and a thicker strap positioned closer to the ankle, relative to a standard flip flop.

A 2013 study from the Salford group found that areas of high plantar pressure associated with a standard flat flip flop were lowered when an enhanced flip flop was used, particularly peak pressure under the heel and pressure-time integral at the first metatarsophalangeal joint. Contact area under the midfoot was nearly 20% greater in the enhanced flip flop than the flat version, and time spent gripping with the hallux was significantly reduced.6 A 2014 study by the same group found the enhanced flip flops were associated with a lower rate of loading at heel strike and less frontal plane motion at the ankle during stance, compared with flat flip flops.6

The footwear experts interviewed by LER agreed that, biomechanically speak­ing, “comfort” flip flops are preferable to the flat rubber flip flops that can be found piled in a bin at the local discount store.

“I don’t necessarily think the new generation of flip flops will completely mitigate the gait and biomechanical changes that these shoes present, but they are a better option than the standard, flat flip flop,” Osterman said. “It is also relative to what you are using the flip flops for. Are they appropriate as an all-day walking shoe? No. Are they all right for a trip to the beach or just a few hours of wear at a special event? Probably.”

Kennedy agreed these enhanced flip flops are better than those with flat rubber soles, thanks to the slightly elevated heels and contoured arches.

However, he said, “I wouldn’t go so far as to call these biomech­anical flip flops. Raising the heel can alleviate the potential for a number of biomechanical issues, such as plantar fasciitis and or Achilles tendinitis, [but] they are not as stable as a sports shoe or a hiking shoe.”

Weimar also concurred that comfort flip flops do offer improvements over flat ones, but noted some of the limitations of flip flops still have not been addressed.

“Specifically, the main issue of the thong flip flop has not been fixed, and I am not sure that it can be,” she wrote. “When someone chooses a flip flop, it is because they want to feel unencumbered, they want to feel ‘free.’ Unfortunately, this lack of support—foot-to-shoe contact—that most people desire is at the heart of the injuries that can result.”

Marcom elaborated on that issue: that even with an enhanced flip flop, the foot still works to hold on, which increases demand on the tibialis anterior and extensors in the foot. Mitigating the amount of work the foot has to do to stay in the shoe comes down to proper fit, and ensuring the top strap fits as securely against the foot as possible.

“When you look at the supportive flip flops, it’s a lot like buying off-shelf insoles. If you are going to invest in these shoes, it’s worth going to a store with a certified pedorthist or anyone with some biomechanical knowledge that can help fit you into the right flip flop,” he suggested.

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Flip flops for Rx?

In 2011, researchers at Rush Medical College in Chicago compared the effects of several shoe types, including flip flops, on knee loading in 31 patients (21 women; mean age 57 years; mean BMI 29.3 kg/m2) diagnosed with knee osteoarthritis (OA), all of whom could walk without assistance.7

Shoes tested were clogs, stability shoes, flat sneakers, and flip flops with a heel height of 15 mm and a weight of 91 g for all sizes. The study’s primary endpoint was peak external knee adduction moment (KAM), which the authors described as “a gait parameter that reflects the extent of medial compartment knee loading during walking.”

The authors reported that the clogs and stability shoes were associated with the highest peak KAM during walking, anywhere from 7% to 15% greater than the flat sneakers, flip flops, and barefoot walking. However, knee loading while wearing flip flops was not different than barefoot walking, they stated. Possible reasons could have been that even the minimal heel on the clogs and stability shoes contributed to the higher knee loads, the authors suggested. In addition, the soles on these shoes may have imposed a level of “stiffness” not present in the flip flops, they noted.

Finally, the authors noted the flexible, lightweight flip flops could be considered a type of barefoot-like minimalist footwear, and that barefoot walking has been associated with lower peak knee loads than conventional footwear.8

“It may be that the flexible movement of a bare foot is biomechanically advantageous,” the authors stated.

The same researchers later designed a flat, flexible mobility shoe—with more structure than a flip flop—specifically for patients with knee OA.9 But can flip flops themselves be beneficial for patients with knee OA? Experts say to proceed with caution.

The lower KAM associated with the flip flop is not surprising, Morcom noted.

“When you go into a minimalist shoe, and usually it has a lower stack height, as that drops, it makes the lever arm for knee adduction moment less. So, when that happens, it lowers that knee adduction moment and should lessen some of the issues associated with knee OA,” he explained.

Morcom emphasized, however, there’s more to knee OA than knee adduction moment. Tibial rotation, for example, can also play a role.10,11

“Although a minimalist shoe may help reduce knee adduction moment, which everyone thinks is good, it may increase internal rotation through the tibia,” he said. “Especially with diseases like OA, it is important to have a knowledgeable clinician analyze your gait to see what footwear features will benefit you the most.”

Accommodating deformities

Beyond knee OA, are there other lower extremity condition that may be a good match with flip flops? Patients with bunions often have a tough time finding shoes that don’t irritate the condition, so they may be happier in flip flops, Kennedy noted.

“But in the case of a very severe bunion, the joint will literally hang out over the [out]sole, so they need to be aware that they are putting themselves at risk for injuries, such as knocking the foot on a table leg,” he added.

Patients with severe hammertoes, or clawed toes, may be in the same position in terms of needing footwear that will not blister the tops of the toes, so the enhanced flip flops may be a viable option, Kennedy said.

Compromises and common sense

The bottom-line: Flips flops may not be the best footwear, but they may not be all that bad. Either way, they aren’t likely to lose traction as popular footwear.

“I’ve been [practicing] for twenty-six years, and have learned that people are going to do what they want to do,” Osterman said. “Of course, flip flops aren’t as good as tennis shoes or other more supportive footwear. But the reality is, people are going to wear them.”

It would be a plus if lower extremity practitioners would guide people to more supportive flip flops, experts say. He encouraged practitioners who also have retail businesses to carry a variety of supportive flip flops—different arch heights, amounts of shock absorption, and strap configurations—so patients can be properly fitted.

Despite the considerable price differential between standard flip flops and their enhanced counterparts, Morcom said, “I’d say that you’ll get more longevity out of the supportive flip flops, so it would be a better investment.”

Kennedy recommended using plain old common sense when it comes to flip flops.

“As with all things in life, listen to your body. It will tell you when you are doing something right or wrong. Flip flops may be a possible culprit for difficulties that people experience, whether it’s foot-
related or back pain,” he noted. “There are no definitive yes/no categories here. If you notice that every time you wear flip flops, your back aches, then there’s a good chance that they are not the correct footwear for you.”

Shalmali Pal is a freelance writer based in Tucson, AZ.

REFERENCES
  1. Carl TJ, Barrett SL. Computerized analysis of plantar pressure variation in flip-flops, athletic shoes, and bare feet. J Am Podiatr Med Assoc 2008 98(5):374-378.
  2. Zhang X, Paquette MR, Zhang S. A comparison of gait biomechanics of flip-flops, sandals, barefoot and shoes. J Foot Ankle Res 2013;6(1):45.
  3. Price C, Andrejevas V, Findlow AH. Does flip-flop style footwear modify ankle biomechanics and foot loading patterns? J Foot Ankle Res 2014;7(1):40.
  4. Shroyer JF, Weimar WH. Comparative analysis of human gait while wearing thong-style flip-flops versus sneakers. J Am Podiatr Med Assoc 2010;100(4):251-257.
  5. Sharpe T, Malone A, French H, et al. Effect of flip-flops on lower limb kinematics during walking: a cross-sectional study using three-dimensional gait analysis. Ir J Med Sci 2016;185(2):493-501.
  6. Price C, Graham-Smith P, Jones R. A comparison of plantar pressures in a standard flip flop and a FitFlop using bespoke pressure insoles. Footwear Sci 2013;5(2):111-119.
  7. Shakoor N, Sengupta M, Foucher KC, et al. Effects of common footwear on joint loading in osteoarthritis of the knee. Arthritis Care Res 2010;62(7):917-923.
  8. Shakoor N, Block JA. Walking barefoot decreases loading on the lower extremity joints in knee osteoarthritis. Arthritis Rheum 2006;54(9):2923-2927.
  9. Shakoor N, Lidtke RH, Wimmer MA, et al. Improvement in knee loading after use of specialized footwear for knee osteoarthritis: results of a six-month pilot investigation. Arthritis Rheum 2013;65(5):1282-1289.
  10. Duparc F, Thomine JM, Simonet J, et al. Femoral and tibial bone torsions associated with medial femoro-tibial osteoarthritis. Index of cumulative torsions. Orthop Traumatol Surg Res 2014;100(1):69-74.
  11. Vincent KR, Conrad BP, Fregly BJ, Vincent HK. The pathophysiology of osteoarthritis: a mechanical perspective on the knee joint. PM R 2012;4(5 Suppl):S3-S9.
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