March 2011

Unstable shoe designs: Functional implications

Photo courtesy of MBT

By Scott Landry, PhD

Whether it is a fad supported by strong marketing or an actual paradigm shift,1 the concept of barefoot locomotion is becoming increasingly popular, particularly in the past five years. Many shoe companies have recently introduced “barefoot” functional footwear into the marketplace, with the most common brands being Masai Barefoot Technology (MBT), Nike Free, Reebok Easytone, Sketchers Shape-ups, VivoBarefoot, and Vibram Fivefingers. The increased popularity of this functional footwear has many biomechanists, clinicians, coaches and even the general public wanting to i) better understand the long term implications of using these shoes on the body, and ii) determine whether these barefoot shoe concepts are helping to alleviate pain, prevent injuries and possibly enhance performance.

For the majority of our existence, humans have been unshod2 and it was not until the running boom in the 1970s that shoes began to incorporate cushioned soles and specific stabilizing features, thought to be important for protection of the foot and body during ground contact.3 However, injury rates do not appear to be influenced by these shoe modifications4-6 and it is now being suggested that the enhanced cushioning and stability built into the modern shoe might be overprotecting and thereby weakening important stabilizing structures, specifically the smaller intrinsic and extrinsic muscles at the foot and ankle joint complex. The current theory proposed by many is that barefoot training can strengthen and condition the smaller neglected muscles of the lower limb, thereby helping to enhance performance and prevent injuries.1,7

Figure 1: Sagittal view of unstable MBT shoe showing the anterior-posterior curvature and cushioned heel sensor.

Originating from Switzerland in 1996 and coming to North America in 2003, the unstable MBT shoe is the original “barefoot” function shoe. This shoe has a rounded sole in the anterior-posterior direction and a cushioned sensor under the heel area that creates a natural degree of instability in both the anterior-posterior and medial-lateral directions (Figure 1). The basic concept behind the unstable shoe is to transform flat and hard surfaces (e.g. pavement and concrete) into uneven surfaces (similar to grass or sand) and provide some of the benefits ributed to barefoot locomotion. This shoe features are thought to specifically activate, strengthen and condition the smaller neglected extrinsic foot muscles (e.g. tibialis posterior, flexor and extensor hallucis longus, flexor and extensor digitorum longus and peroneus brevis and longus) while standing or walking in the shoe.7 By activating these neglected muscles, posture and gait could be improved and loads or stresses on the lower limb joints may be reduced to help prevent injuries and reduce pain.

Nigg7,8used a computer simulation model of an ankle joint with strong and weak small extrinsic foot muscles to theoretically show that stronger extrinsic muscles could more effectively control ankle joint movements and potentially reduce ankle joint loading. It was proposed that joint loadings are reduced because the small extrinsic muscles have shorter moment arms and are anatomically closer to the joint’s movement axis compared to the larger muscles of the triceps surae (soleus and gastrocnemii), thereby allowing for a quicker and more effective reaction to movements and perturbations.

Of all the different types of “barefoot” functional footwear on the market, the unstable MBT shoe has been the most extensively investigated, both biomechanically and clinically, in peer reviewed medical and scientific journals.9-20 MBT has also financed a number of other studies, found on their website (http://us.mbt.com/Home/Benefits/Studies.aspx), and while many of these do not appear in peer reviewed journals, the research has been performed by well-respected biomechanists throughout the world. An ample amount of anecdotal evidence and personal testimonials also supports the effectiveness of the unstable MBT shoe for relieving joint pain and helping to reduce and/or manage lower limb or back injuries.

However, not all studies have supported the effectiveness of the unstable shoes. A recent study commissioned by the American Council on Exercise compared physiological measures (oxygen consumption, heart rate, rating of perceived exertion and caloric expenditure) and activity levels of selected larger muscles while walking on a treadmill with three unstable footwear products (MBT, Skechers Shape-Ups, and Reebok EasyTone) and a traditional athletic shoe (New Balance running shoe). No consistent or notable differences between the four types of footwear were found;21 however, it is important to highlight that this study focused on muscles proximal to the ankle joint and not on the smaller extrinsic foot muscles thought to be neglected when using a more traditional shoe. The study also did not report on joint pain relief, one of MBT’s main claims that is supported by both scientific13,14 and anecdotal evidence.

Peer reviewed studies of clinical relevance on the unstable MBT shoe have focused on obesity,15 knee osteoarthritis (OA) pain,13 low back pain,14 diabetes,22 balance in women over 50 years of age,16 and balance in children with developmental disabilities.10 Several studies have investigated specific biomechanical and neuromuscular changes introduced while walking or standing in the unstable MBT shoes9-17,19,20,22 and one study has addressed changes while running in the unstable MBT shoes.18

Pain relief

In a study involving 126 individuals with moderate knee OA, wearing an unstable MBT shoe was effective in reducing subjective pain after as few as three weeks, and this pain reduction remained after 12 weeks of MBT usage.13 A moderate knee OA control group that received a pair of high-end walking shoes (New Balance 756 WB model) also experienced a similar reduction in pain after 12 weeks of wearing the more traditional control shoe. Although this study was not designed to examine the mechanisms responsible for the reduction in pain, it was suggested that the MBT shoe may have strengthened the small muscles, thereby resulting in reduced joint loading and pain, as described in Nigg’s simulation model.7,8 The same research group from the University of Calgary also showed in golfers with low back pain that wearing unstable MBT sandals throughout the workday for a six-week period reduced pain and did not affect golf performance in comparison to a low back pain group that wore more stable footwear.14 Golfers wore the unstable footwear while hitting golf balls on the driving range but not during actual rounds of golf.

Walking biomechanics and neuromuscular function

Biomechanical and neuromuscular changes have been identified while walking with the unstable MBT shoe, particularly at the foot and ankle. Healthy adults walking in the unstable shoe have a more dorsiflexed ankle during foot contact and throughout early stance compared to walking in a more traditional stable shoe.9,11 Walking in an unstable MBT sandal also significantly increases ankle range of motion in both the frontal and sagittal planes compared to walking barefoot. 23 The clinical or therapeutic relevance of this increased range of motion, however, is still not clearly understood. Initial ground contact for the unstable MBT shoe occurs closer to the midfoot than the heel, as evident from a foot pressure distribution study that showed a reduction in mean plantar pressures for the posterior half of the foot and an increase under the forefoot and toes for a group of healthy individuals, compared to flat-soled training shoes.12

Unlike high heels that elevate and shift loads and pressures to the medial side of the forefoot, the design of the unstable MBT shoe more evenly distributes plantar pressures across the width of the entire forefoot or metatarsals, more closely resembling the pressure patterns observed during barefoot walking.12 Although further research is required, it has been suggested that the shifting of pressure distributions for the unstable MBT shoe could be beneficial to patients with diabetes at risk for rearfoot ulcerations or patients with plantar fasciitis where relieving pressure in the midfoot and longitudinal arch is required. Maetzler et al22 studied peak plantar pressures in patients with diabetes wearing the unstable MBT shoe, and while their patterns were slightly different from a healthy population,12 peak pressures were reduced near the first metatarsal head and rearfoot. It was concluded that the MBT shoes could be effective in diabetic patients at risk for foot ulcerations at these particular locations.

Figure 2: A) Close up of the 15 channel EMG circumferential linear array, positioned proximal to the ankle joint. B) EMG linear array measuring muscle activity levels of extrinsic foot muscles while standing in the unstable MBT shoe.

Although walking in an unstable MBT shoe does not significantly reduce joint loadings (net moment impulses) at the ankle,9 trends and significant changes for the larger muscles crossing the ankle joint have been identified.9,11 Nigg et al9 reported a trend of increased gastrocnemius activity and decreased tibialis anterior activity during late and early stance while walking in an unstable MBT shoe compared to a stable shoe. Similarly, Romkes et al11 demonstrated that wearing the unstable MBT shoe significantly increased gastrocnemii activity between terminal stance and midstance, decreased tibialis anterior activity during initial contact, and increased tibialis anterior activity during the entire swing phase. The lack of statistical significance in the study by Nigg et al might be attributed to the fact that walking speeds were controlled to be the same for both footwear conditions, or to the relatively small sample size of eight subjects. Romkes et al had their subjects walk at preferred speeds for each footwear condition, which resulted in a reduced cadence, stride length, step length, and walking speed for the unstable MBT shoe. Although further research is needed, a speed reduction while walking in the unstable MBT shoe could be one contributing factor as to why some individuals experience pain relief, as joint loadings have been shown to decrease with decreasing walking speeds.24, 25

The scientific peer reviewed literature suggests that walking in the unstable MBT shoe alters the biomechanical and neuromuscular function of the knee and hip to a smaller degree than the ankle joint. Nigg et al9 reported no significant biomechanical changes (joint angles and moments at the hip and knee) and only trends for neuromuscular changes when wearing the unstable shoe compared to a stable shoe. Romkes et al11 identified slightly larger knee flexion angles and increased quadriceps activity during mid-to-late stance while walking in an unstable MBT shoe and cautioned that the increased activity could be problematic for individuals with knee issues due to possible increases in joint contact forces. In another study, however, overweight male subjects walking in the unstable MBT shoe experienced a reduction in peak knee adduction moment,15 a measure that has been linked with the presence,26 severity,27 and rate of progression of medial compartment knee OA.28 It was suggested that the unstable MBT shoe could be of clinical relevance for reducing the risk of knee OA initiation in overweight individuals. However, it was also found in the same overweight subjects that the unstable MBT shoe increased vastus lateralis activity and vastus lateralis/medial gastrocnemius co-activation during the second half of stance. This finding should not be overlooked given that co-activation potentially increases joint contact loading,29 a measure that one might want to minimize over the long term in a joint affected by OA.

Standing: Neuromuscular Function

The natural degree of instability incorporated into the unstable MBT shoe is also designed to provide training benefits while standing, similar to using a wobble board.30-32 Studies have identified increased postural sway while wearing the MBT shoe, as evidenced by increased center of pressure excursions while standing on a force platform.9,19 Muscle activity levels of some larger lower limb muscles (tibialis anterior, gastrocnemius, vastus medialis, biceps femoris, and gluteus medius) have also been quantified while standing in the unstable shoe; the only muscle showing a significant increase in activity level was the tibialis anterior.9 This muscle is responsible for generating a dorsiflexion and inversion moment to cope with the increased instability of the MBT shoe.

Backing MBT’s claim of activating neglected muscles, a recently published study was able to confirm that standing in the unstable MBT shoe does activate some of the smaller extrinsic foot muscles to a greater extent than standing in a stable control shoe.19 A custom designed electromyographic (EMG) linear array, positioned just proximal to the ankle joint and wrapping around the circumference of the lower leg (Figure 2), was used to measure muscle activity levels for the smaller extrinsic foot muscles—including the flexor digitorum longus, peroneus muscles (brevis and longus) and anterior compartment muscles (tibialis anterior, extensor hallucis longus and extensor digitorum longus)—along with the larger soleus muscle.  Measurements were made with the EMG linear array before and after a six-week accommodation period of wearing the unstable shoes at the participants’ workplace. Magnetic resonance imaging (MRI) was also used to help with positioning and repositioning of the EMG linear array between the two testing sessions and to determine which muscle was being measured by each of the 15 surface electrode pairs directly below the surface of the skin (Figure 3).

Figure 3: MRI showing cross sectional view of lower leg musculature along with the position of the 15 electrodes along the circumference of the lower leg. Muscle activity levels were measured for FDL (flexor digitorum longus), SOL (soleus), PR (peroneus group) and AC (anterior compartment group).

Activity levels for all the smaller extrinsic muscles investigated were significantly greater while standing in the unstable MBT shoe compared to the stable control shoe. No differences between footwear conditions were identified, however, for the larger soleus muscle.  Activity levels for the smaller muscles remained elevated after the accommodation period despite postural sway (defined by center of pressure excursion) decreasing with continued MBT shoe usage. The enhanced activation of the neglected smaller muscles is again thought to strengthen these muscles over time and lead to reduced loading, as simulated by Nigg’s ankle model with small and large muscles.7,8

Balance

Another proposed benefit of standing and walking in MBT shoes is improved balance or postural stability, through the mechanism of challenging and training the muscles and proprioceptive system. Some form of balance improvement after wearing the unstable MBT shoe has been identified in older but healthy female subjects,16 children with neurological deficits,10 and patients with knee osteoarthritis.13 In the aforementioned group of golfers with low back pain, however, wearing the unstable footwear for six weeks did not improve balance ability.14 This may be related to the superior baseline balance abilities of the golfers (two to five  times higher than the knee OA patients, depending on the type of balance test) and the study duration (six weeks for the low back pain study vs 12 weeks for the knee OA study). Despite the positive effects of the unstable shoe on balance in these different populations, Ramstrand et al16 cautioned against prescribing unstable shoes to individuals whose balance may be compromised, as there could be an increased risk of falls associated with the shoes. In able-bodied individuals, however, the risk of falling is negligible and should not be a concern.

Conclusion and recommendations

Both anecdotal and scientific evidence supports the claim that the unstable MBT shoe is capable of alleviating pain at specific joints. From a neuromuscular and biomechanical perspective, the unstable MBT shoe appears to have one of its greatest effects at the ankle joint, with standing in the unstable shoe leading to increased muscle activity for the smaller and often neglected extrinsic foot muscles. Not all beneficial claims made by the “barefoot” functional footwear companies are necessarily backed by peer reviewed scientific journals; however, companies such as MBT have made it a priority to be supportive in having their products extensively studied by various biomechanists and clinicians throughout the world.

The barefoot movement is gaining momentum in recent years and the notion does seem reasonable from a biomechanical and injury prevention viewpoint. Long-term biomechanical follow-up studies must be carried out on the various types of “barefoot” functional footwear before a comprehensive understanding is able to guide us as to who should and should not be wearing this type of footwear. The long-term implications of wearing such footwear on the human body also requires further research, and it is imperative that people realize that these barefoot products do not replace living a healthy and active lifestyle.

Scott Landry, PhD, is an assistant professor in the School of Recreation Management and Kinesiology at Acadia University in Wolfville, Nova Scotia, and an adjunct professor in the Schools of Health and Human Performance and Biomedical Engineering at Dalhousie University in Halifax, Nova Scotia.

Disclosure: For many of the studies discussed in this review article, Masai Barefoot Technology (MBT) provided the unstable shoes and financial support. MBT did not, however, have a role in the study design or in the interpretation and presentation of the data.

References

1.             Wallden M. Shifting paradigms. J Body Mov Ther 2010;14(2):185-194.

2.             Trinkaus E. Anatomical evidence for the antiquity of human footwear use. J Archaeol Sci 2005;32(10):1515-1526.

3.             Shorten MR. The myth of running shoe cushioning. Presented at IV International Conference on the Engineering of Sport, Kyoto, Japan, September 2002.

4.             Knapik JJ, Trone DW, Swedler DI, et al. Injury reduction effectiveness of assigning running shoes based on plantar shape in Marine Corps basic training. Am J Sports Med 2010;38(9):1759-1767.

5.             Richards CE, Magin PJ, Callister R. Is your prescription of distance running shoes evidence-based? Br J Sports Med 2009;43(3):159-162.

6.             Ryan MB, Valiant GA, McDonald K, Taunton JE. The effect of three different levels of footwear stability on pain outcomes in women runners: a randomised control trial. Br J Sports Med 2010 Jun 27. [Epub ahead of print]

7.             Nigg B. Biomechanical considerations on barefoot movement and barefoot shoe concepts. Footwear Science 2009;1(2):73-79.

8.             Nigg BM. Schuh und seine biomechanische/therapeutische Wirkungsweise (the MBT shoe and its biomechanical and its therapeutical effects). Med Orthop Technik 2005;3:77-78.

9.             Nigg B, Hintzen S, Ferber R. Effect of an unstable shoe construction on lower extremity gait characteristics. Clin Biomech 2006;21(1):82-88.

10.             Ramstrand N, Andersson CB, Rusaw D. Effects of an unstable shoe construction on standing balance in children with developmental disabilities: a pilot study. Prosthet Orthot Int 2008;32(4):422-433.

11.             Romkes J, Rudmann C, Brunner R. Changes in gait and EMG when walking with the Masai Barefoot Technique. Clin Biomech 2006;21(1):75-81.

12.             Stewart L, Gibson JN, Thomson CE. In-shoe pressure distribution in “unstable” (MBT) shoes and flat-bottomed training shoes: a comparative study. Gait Posture 2007;25(4):648-651.

13.             Nigg BM, Emery C, Hiemstra LA. Unstable shoe construction and reduction of pain in osteoarthritis patients. Med Sci Sports Exerc 2006; 38:1701-1708.

14.             Nigg BM, Davis E, Lindsay D, Emery C. The effectiveness of an unstable sandal on low back pain and golf performance. Clin J Sport Med 2009;19(6):464-470.

15.             Buchecker M, Wagner H, Pfusterschmied J, et al. Lower extremity joint loading during level walking with Masai barefoot technology shoes in overweight males. Scand J Med Sci Sports 2010 Aug 30. [Epub ahead of print]

16.             Ramstrand N, Thuesen AH, Nielsen DB, Rusaw D. Effects of an unstable shoe construction on balance in women aged over 50 years. Clin Biomech 2010;25(5):455-460.

17.             Stoggl T, Haudum A, Birklbauer J, et al. Short and long term adaptation of variability during walking using unstable (Mbt) shoes. Clin Biomech 2010;25(8):816-822.

18.             Boyer KA, Andriacchi TP. Changes in running kinematics and kinetics in response to a rockered shoe intervention. Clin Biomech 2009;24(10):872-876.

19.             Landry SC, Nigg BM, Tecante KE. Standing in an unstable shoe increases postural sway and muscle activity of selected smaller extrinsic foot muscles. Gait Posture 2010;32(2):215-219.

20.             Nigg BM, Tecante KE, Federolf P, Landry SC. Gender differences in lower extremity gait biomechanics during walking using an unstable shoe. Clin Biomech 2010; 25 (10):1047-10 52.

21.             Porcari J, Greany J, Tepper S, et al. Will toning shoes really give you a better body? American Council on Exercise 2010. Available at: http://www.acefitness.org/getfit/studies/toningshoes072010.pdf Accessed Feb 23, 2011.

22.             Maetzler M, Bochdansky T, Abboud R. Pressure distribution of diabetic patients after sensory-motor training with unstable shoe construction. Clin Biomech 2008;23(5):714-715.

23.             Roberts S, Birch I, Otter S. Comparison of ankle and subtalar joint complex range of motion during barefoot walking and walking in Masai Barefoot Technology sandals. J Foot Ankle Res 2011;4:1.

24. Rutherford DJ, Hubley-Kozey C. Explaining the hip adduction moment variability during gait: Implications for hip abductor strengthening. Clin Biomech 2009;24(3):267-273.

25.             Kirtley C, Whittle MW, Jefferson RJ. Influence of walking speed on gait parameters. J Biomed Eng 1985;7(4):282-288.

26.             Baliunas AJ, Hurwitz DE, Ryals AB, et al. Increased knee joint loads during walking are present in subjects with knee osteoarthritis. Osteoarthritis Cartilage 2002;10(7):573-579.

27.             Sharma L, Hurwitz DE, Thonar EJ, et al. Knee adduction moment, serum hyaluronan level, and disease severity in medial tibiofemoral osteoarthritis. Arthritis Rheum 1998;41(7):1233-1240.

28.             Miyazaki T, Wada M, Kawahara H, et al. Dynamic load at baseline can predict radiographic disease progression in medial compartment knee osteoarthritis. Ann Rheum Dis 2002;61(7):617-622.

29. Lewek MD, Rudolph KS, Snyder-Mackler L. Control of frontal plane knee laxity during gait in patients with medial compartment knee osteoarthritis. Osteoarthritis Cartilage 2004;12 (9):745-751.

30.             Emery CA, Cassidy JD, Klassen TP, et al. Effectiveness of a home-based balance-training program in reducing sports-related injuries among healthy adolescents: a cluster randomized controlled trial. CMAJ 2005;172(6):749-754.

31.             Waddington GS, Adams RD. The effect of a 5-week wobble-board exercise intervention on ability to discriminate different degrees of ankle inversion, barefoot and wearing shoes: a study in healthy elderly. J Am Geriatr Soc 2004;52(4):573-576.

32.             Wester JU, Jespersen SM, Nielsen KD, Neumann L. Wobble board training after partial sprains of the lateral ligaments of the ankle: a prospective randomized study. J Orthop Sports Phys Ther 1996;23(5):332-336.

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2 Responses to Unstable shoe designs: Functional implications

  1. Harvey Johnson says:

    Using an aggressive rocker shoe like the MBT on a diabetic person is a recipe for disaster. Even minor neuropathic sensory loss combined with an unstable shoe makes the diabetic even more unstable. Well balanced running shoes with good total contact stable shock absorbent orthotics is far superior than any unstable shoe. Using a unstable shoe for cross training might benefit the average walker/athlete but it should not be used as the only shoe for training. As for using unstable shoes on the little old arthritic lady you end up with similar problems that I have seen with diabetic. These shoes are far to challenging for mildly disabled people.
    Harvey Johnson
    Eno River Orthotics

  2. Tom says:

    I bought mbt karib shoes a year ago and was advised not to go in sand or mud so i just wore them in very clean areas on occasions in order to “spare” them and subsequently bought another pair online from China for less than half the price, they were not original (so i was told) but they were no different when worn.
    I wore the originals for approximately 6 weeks in total and the heals just broke up and came apart and became unwearable.
    The agent told me it was fair wear and tear and guarantee had been exceeded, but were repairable for a price.
    For me MBT shoes are a definite no no.

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