Flip-flops might seem to fall under the umbrella of minimalist footwear. But research suggests that walking in flip-flops is far from normal, and that only more structured models can approximate the elusive barefoot-like gait.
By Justin F. Shroyer, PhD, CSCS, NSCA-CPT, and Wendi H. Weimar PhD
There is much anecdotal evidence that flip-flops are not conducive to the health of individuals’ lower legs and feet; however, when individuals are asked why they wear flip-flops, “comfort” is the usual response. Even though there is a negative stigma attributed to flip-flops by medical professionals with regard to foot and lower leg health, the masses still wear the light airy footwear. On any given sunny summer day, one can observe the popularity of the flip-flop footwear for all ages.
But if one asks foot experts, practitioners would conclude that flip-flops are not always the best choice of footwear. One podiatrist at New York’s Hospital for Special Surgery, Rock Positano, DPM, has stated, “Flip-flops have singlehandedly caused more problems with people’s feet in the last couple years than probably any other type of shoe.”1
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Why are flip-flops so bad? Podiatrist Greg Cohen, DPM, from Long Island College Hospital in Brooklyn, NY, postulates, “Flip-flops don’t really hold on the foot like most shoes do, so we use the tendons and muscles to hold them on.”2 Interestingly, Cohen seems to be suggesting that more muscle involvement may be a bad thing, yet one company (FitFlop Ltd) has sold thousands of flip-flops claiming that their model tones and firms the hamstrings and buttocks by increasing the demand placed on the muscles of the foot, leg and thigh.
Though flip-flops are often worn for comfort, the excessive wearing of flip-flops has been linked to discomfort. In fact, according to the American College of Foot and Ankle Surgeons (ACFAS) in 2006, an increase in usage of flip-flop sandals by teens and young adults has led to an increase in heel pain.3 ACFAS spokesperson Marybeth Crane, DPM, has reported that practitioners are seeing more heel pain more than ever in patients 15 to 25 years old, noting that heel pain is a marker of plantar fasciitis which accounts for 15% of all adult foot complaints.3 Furthermore, the ACFAS recommends that patients with heel pain should avoid flat shoes with paper-thin soles and should also avoid walking barefoot since wearing flat shoes (including flip-flops) and walking barefoot provides little cushioning as well as little to no arch support.3 This lack of arch support and cushioning of the heel while wearing flip-flops seems to exacerbate any abnormalities in the biomechanics of foot motion, and may perpetuate heel pain and inflammation.
While the causal relationship between flip-flops and lower leg discomfort seems to be accepted clinically, questions remain as to the mechanism by which this effect occurs. Is there empirical scientific evidence to support the anecdotal claims by the scientific and medical communities that flip-flops are harmful?
The campus connection
Casual observation of people wearing flip-flops on college campuses was the catalyst for the flip-flop research being done at Auburn University.4-7 Anecdotally, researchers at Auburn University observed that individuals wear flip-flops beyond the structural limit of the flip-flop (i.e,. the foot bed is worn out), that flip-flops are designed and sold with a one size fits all mentality, and that individuals have a different gait while wearing flip-flops when compared to other types of footwear.
Prior to the initial flip-flop study at Auburn University,6 only two other studies had specifically investigated the thong flip-flop.8, 9 Carl and Barrett used an in-shoe pressure mapping system to assess the plantar pressures of 10 women while walking in flip-flop, athletic shoe and barefoot conditions. The results showed an increase in peak plantar pressures in the flip-flop condition when compared to the athletic sneaker condition. The barefoot condition resulted in the highest plantar pressures when compared to both of the other footwear conditions. Even though the flip-flops did provide some cushioning benefit compared to bare feet, the results indicated that flip-flops are not optimal footwear choice for dissipating ground reaction forces.8 This study could have been greatly enhanced if the researchers had evaluated different types of flip-flops and had included kinematic measures as well.
The other study did not study flip-flops directly; however, it was a case study, which found that limiting the use of flip-flops as a casual footwear choice aided in the rehabilitation of a runner with iliotibial band friction syndrome. Based on the study, wearing flip-flops was linked to excessive dorsiflexion of the great toe during the stance phase of gait, which the authors hypothesized had led to dynamic rearfoot inversion or overpronation during midstance or toe-off while running; wearing flip-flops less frequently was recommended by Certified Athletic Trainers to reduce the IT band friction syndrome.9 This suggests that flip-flops can contribute to pain in the lower extremity and when pain is present, flip-flops are counterproductive to alleviating this pain.
The initial flip-flop study at Auburn University investigated the kinematic and kinetic differences of individuals wearing flip-flops versus sneakers. The results indicated that flip-flops produced a decreased peak vertical force at heel contact in comparison to sneakers. The study also showed that female subjects had a more vertical attack angle in flip-flops when compared to athletic sneakers, while men demonstrated no significant differences between footwear. The reason for these differences between sexes is unknown and needs future investigation; however, it is clear that wearing flip-flops does alter gait kinetics. The authors indicated that 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; however, these stresses were not quantified.4
One of the more interesting preliminary findings from the research at Auburn University on gait kinematics and kinetics in individuals wearing flip-flops versus walking barefoot5 involves the activity of the tibialis anterior (TA) muscle in relation to the ankle angle during the swing phase of gait on the non-support limb. Specifically, as the non-support leg swings through, the tibialis anterior (TA) demonstrated an increase in muscle activity, yet less dorsiflexion (DF) was noted compared to barefoot walking. This finding was counterintuitive, as the TA is a primary dorsiflexor, and more activity should have been realized with an increase in dorsiflexion.
Figure 1 shows a drawing that represents peak dorsiflexion ankle angles and the average activity of the TA during the swing phase of gait across four footwear conditions. The four conditions included barefoot (BF), flip-flop 1 (FF1), flip-flop 2 (FF2) and flip-flop 3 (FF3). With regard to peak dorsiflexion, BF and FF3 produced similar results, as did FF1 and FF2, however when grouped together, BF/FF3 demonstrated increased DF in comparison to FF1/FF2. Yet, all three flip-flop conditions reported increased TA activity in comparison to the BF condition. The importance of this finding rests in the different footbed characteristics of the flip-flops utilized in this study. Specifically, FF3 had an increased heel cup and a defined toe ridge, allowing it to produce results more aligned with the BF condition. The fact that more structured flip-flops resulted in a more “barefoot like” gait seems counterintuitive, since one would expect a minimalist flip-flop to yield results more akin to barefoot walking. Anecdotally, the authors hypothesize that the structure of FF3 allowed the foot to work less to keep the flip-flop on the foot and resulted in a more natural feel; because the toes were not working as hard to keep the flip-flop on, the resulting gait was more like that of the barefoot condition.
Getting a grip
Although the activity of the flexor hallucis longus (FHL), flexor digitorum longus (FDL), and the tibialis posterior (TP) muscles was not measured, the counterintuitive finding of increased dorsiflexor muscular activity and less observed dorsiflexion angle leads the author to conclude that the increased activity of the TA in the presence of less dorsiflexion could be the result of the flip-flop wearer’s attempt to “grip” the flip-flop using the plantar surface of the foot. This mechanism of keeping the flip-flop on the foot during the swing phase would engage the toe flexor muscles (FHL, FDL and potentially the TP). Because the FHL, FDL, and TP also cross the ankle joint, these muscles would cause an implied plantar flexion moment, subsequently decreasing dorsiflexion and increasing activity of the TA. Future research is planned to attempt to isolate the activity of the FHL and FDL to see if they are more active while wearing flip-flops compared to barefoot and other types of footwear, and to confirm whether these muscles that flex the toes are causing a plantar flexion moment about the ankle.5
In addition to decreased dorsiflexion during the swing phase of gait, the activity of the toe flexors to “grip” the flip-flop may also affect the windlass mechanism of the foot. The windlass mechanism functions in the foot as the first metatarsophalangeal (MTP) joint extends. As the 1st MTP joint extends, the plantar fascia tightens. This tightening of the plantar fascia has two functions; (1) the medial arch of the foot is packed tighter and is increased in height, allowing for improved loading capacity and shock absorption, and (2) the foot becomes more rigid and has improved function as a propulsive lever (Figures 2 and 3).10 Consequently, if during the swing phase the toes are trying to “grip” the flip-flop, the 1st MTP joint is not able to extend and the windlass mechanism of the foot is compromised.
Stance phase and stride length
While the study by Shroyer and Weimar5 investigated primarily the activity and ankle angle during the swing phase, the same toe flexor activity may be occurring during single limb support of the stance phase on the foot that is in contact with the ground. If this is true, the toe flexors would be activated to once again “grip” the flip-flop to minimize movement of the foot in relationship to the footwear. In doing so, toe extension would be limited. This may be similar to the mechanism seen in
functional hallux limitus (FHL)11 however, in the case of flip-flops it would be the active contribution of the toe flexors that is causing the FHL rather than a structural abnormality. The active FHL would also contribute to the decreased extension of the 1st MTP joint and subsequently would also affect the windlass mechanism.10 As a result, the medial foot arch would be shorter and lower and the foot would not be an efficient rigid lever for push off. This decreased functionality could explain the decrease stride lengths associated with wearing flip-flops.4,6
Although decreased dorsiflexion and stride length were observed in subjects while wearing flip-flops compared to barefoot, the authors cannot conclude if the decreased stride length was a result of the decreased dorsiflexion or if the decreased dorsiflexion was a result of the decreased stride length. However, there was no statistical difference in dorsiflexion angles between barefoot and one of the flip-flop conditions, while at the same time there was a shorter stride length in the flip-flop condition. This suggests that the dorsiflexion angle observed did not have an effect on the stride lengths.Although foot strike patterns were not quantified, future research should investigate if the change in ankle angles observed in the flip-flop conditions resulted in a difference in foot strike patterns.
Although there is currently a movement in favor of minimalist footwear in the running world, the authors feel that minimalist flip-flops are not beneficial as currently designed. Based on observations and results of previous studies, the flip-flop with structure effectively functions more like a bare foot than the minimalist flip-flops. One hypothesis is that in the minimalist flip-flops (at least the ones the authors studied) individuals have to use the plantar surface of the foot to grip the flip-flop. It is this “gripping” that causes gait to deviate further from barefoot measures. Also, gripping the flip-flop is not a natural part of walking. In the running world, the minimalist footwear or barefoot running do not introduce any “unnatural” mechanisms but put the foot in a position to function as designed. Flip-flops, on the flip-side, do not.
In conclusion, the influence of footwear research on gait measures can be seen in the ever changing design of shoes. There has been a plethora of research on running shoes and the effects of medial arch supports and orthotics to minimize foot motion and decrease overuse injuries in runners.12-17 As previously mentioned, few studies have investigated the effects of thong style flip-flops on gait dynamics, one of which was the initial study done by the authors at Auburn University in which several gait kinematic and kinetic measures differed between two types of footwear illustrating that walking in flip-flops alters one’s gait when compared to sneakers.4, 6 Prior to that study, only pressure mapping had been done to compare flip-flops to barefoot and sneakers.8 Given the limited research regarding the effect of thong flip-flops on gait dynamics, the purpose of flip-flop research currently being done at Auburn University in collaboration with the University of Louisiana at Lafayette is to further address the effects of flip-flops on the windlass mechanism, in addition to the effects of different styles of thong flip-flops on gait kinematics. Instead of relying on anecdotal evidence, our goal is to establish a line of scientific empirical evidence that practitioners can utilize in helping patients and clients make healthy footwear choices.
Justin F. Shroyer, PhD, CSCS, NSCA-CPT, is an assistant professor in the department of kinesiology at the University of Louisiana at Lafayette in Lafayette, LA. Wendi H. Weimar, PhD, is an associate professor in the department of kinesiology at Auburn University in Auburn, AL.
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