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Footwear - Toning Shoes

The toning shoe has become one of the most talked about products in the footwear industry.  Toning shoes are sometimes called wellness shoes, fitness shoes or rocker bottom or rocker sole shoes but the toning term seems to be the most commonly used especially in the marketing of this type of footwear.  Some analysts predict that the toning category of footwear will grow into a $1 billion market in the next few years.8 Realizing that this is likely more than just a passing footwear fad, manufacturers are introducing new models to the market at a pace that is hard to keep up with.

At the high end of the market, price –wise, is MBT which effectively launched the category single handedly in the early 2000’s.  Currently, there are more than 15 manufacturers with toning models and more are being added almost weekly.  Prices vary widely from $30 to more than $250.

Advertising messages tout the benefits of the toning shoes as improving muscle strength and tone, improving balance, improving posture, burning more calories, relieving stress on joints and even eliminating back,  foot or leg pain.  While there are studies supporting some of these claims, toning shoes are not a panacea for gait problems or foot pain.  And research does not suggest that increased muscle activation with their use is adequate to replace other fitness activities.  As is typical in advertising, some manufacturers greatly overstate the benefits and do not fully disclose the risks associated with toning shoes.

Shoe Structure

The design concept of toning shoes is based on a rocker bottom outsole (see figures 1 & 2).  Rocker bottom outsoles are not a new invention.  Podiatrists and other medical professionals have been prescribing them for decades to treat gait problems, foot or ankle pain, arthritic conditions and deformities.  Typically, a podiatrist would order custom rocker bottom modifications for a patient’s shoes that would be performed by an orthotist, pedorthist or a skilled shoe repair specialist.

Toning shoes are purposely unstable.  They are designed to facilitate sagittal plane motion, or motion from heel to toe.  By rounding the outsole in this direction, the foot and lower leg are “rocked” forward as weight is transferred from heel to toe during gait.  Because of this rocking motion, the forces acting on joints are changed and muscles are activated to control some of the instability.

Instability is, of course, a concern for those who have poor balance or muscle weakness that may make them vulnerable to falls.

Figure 1: Rocker bottom shoe beveled at heel and forefoot

Figure 1: Rocker bottom shoe beveled at heel and forefoot

Figure 2: Rocker bottom shoe beveled at forefoot only
Figure 2: Rocker bottom shoe beveled at forefoot only

The geometric contour of the rocker bottom varies from manufacturer to manufacturer and of course some claim that their particular geometry is superior.  The rocker contour can be to the heel only (not pictured) the forefoot only, or to both heel and forefoot – which is called a double rocker.

The weight of toning shoes can vary significantly.  Some of the more popular models are significantly heavier than conventional shoes. This added weight can be detrimental to some users.  There is increased joint stress and muscle fatigue due to shoe weight. 

Benefits of Toning Shoes

Increased Muscle Activation
Research by Benno Nigg and collegues at the University of Calgary showed that “unstable footwear designs” result in greater muscle work load. 1 Even when standing, muscle activation is higher in rocker bottom footwear than conventional shoes – primarily because the wearer has to work harder to maintain balance.  This increased muscle activation could strengthen and condition muscles that are underutilized in some conventional footwear.  Other research has substantiated these findings.

Improved Balance
In those who wore rocker bottom shoes over period of 6 weeks, postural sway, a measure of balance during standing, decreased.1 Another study found older women after wearing unstable footwear for 8 weeks improved on three different measures of balance tests.2 The unstable footwear design challenges balance and improves proprioception.

Increased Metabolism
It has been proposed that rocker bottom shoes increase metabolic activity as a result of the increased muscle activation.  In a swiss study financed by MBT6, researchers measured oxygen consumption and heart rate in barefoot, conventional running shoes and rocker bottom shoe conditions. They concluded that MBT wearers burned slightly more calories per hour.  The greatest differences in metabolic rate occurred comparing barefoot to MBT and were more pronounced in standing vs. walking.

Altered Weight Distribution
Studies have shown that peak pressure to the plantar surface of the foot is altered with rocker bottom footwear when compared to conventional shoes. However, the pressure change effect is not always predictable. Because there is no uniform standard of rocker bottom geometry, the effect varies from brand to brand and model to model.7 Also, study subjects do not respond in a systematic way to footwear interventions. In one study, double rockers decrease peak pressure to the heel, midfoot and metatarsals but increase peak pressure to the plantar digits.5 The increased toe pressure is due to “grasping” of the digits as they attempt to stabilize the foot inside the rocking shoe.

This altered plantar surface distribution may provide pain relief to those prone to heel, midfoot or metatarsal conditions. However, one study suggested that because of individual variability, some form of plantar pressure measurement and gait training may be necessary to ensure optimal pressure changes. 7

Joint Motion Changes
Studies have shown alterations to joint motion in gait – especially in the saggittal plane.  One study showed a slightly increased flexion (less than 5 degrees) at the hip, knee and ankle during early mid-stance. The greatest changes appear to be at the ankle joint which showed increased dorsiflexion at initial contact and greater plantarflexion at toe-off.3,4 In terms of forefoot kinematics, one study showed that rocker soles offer advantages over conventional footwear by aiding in forefoot roll-off and simulating dorsiflexion of the digits at the level of the metatarsal phalangeal joints.9 This would be beneficial for those with forefoot arthritic conditions or joint fusions.

Risks and Other Considerations
The same features that can provide benefits in some users may have consequences in others.  For example, the unstable design while showing benefits in terms of balance in some users may increase the risk of falls in others.  Those with a history of falls, chronic ankle instability, vertigo or poor balance may not be candidates for toning shoes. 

Other considerations include; because of the increased ankle dorsiflexion in early stance, those with history of Achilles tendonopathy may not tolerate this type of footwear.  Also, manufacturers assume a normal limb alignment for those who may wear these shoes, therefore those with transverse plane deformities such as in-toeing or out-toeing will not have normal heel-to-toe transition in these shoes and could potentially be more prone to tripping or falling.

Mass Marketing of Toning Shoes
While MBT’s were the first to enter this market, there are many newcomers, at varied price ranges that have thrown their shoe into the ring.  Some of the newer brands have incorporated more style features in an attempt to develop shelf appeal.  And, of course, each brand emphasizes certain structural features that are unique to their product.

As mentioned above, there is significant variability in the geometry of the outsole, the weight of the shoe and in the way that individuals gait may be affected by the shoe.  For that reason, it is difficult to recommend one brand or model over another for individual needs.

Brands:

  • MBT
  • Skechers Shape Ups
  • Reebok Easy Tone*
  • Cogent
  • Ryn
  • Mephisto Sano
  • Earthshoe
  • Fitflops
  • Avia iBurn
  • Z7
  • Springboost
  • Orthaheel
  • New Balance Rock and Tone
  • Dansko
  • Chung Shi
  • Champion Pace

*The Reebok Easy Tone is not considered a true rocker sole shoe because it does not use the rocker concept but is included in this list because it is designed to be unstable to mimic some of the effects of rocker sole shoes.

Information provided by Paul Langer, DPM and Rob Conenello, DPM

Sources:

  1. 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, corrected proof online May 2010.
  2. Ramstrand, N, Thuesen, HA, et al. Effects of an unstable shoe construction in women over age 50 years, Clinical Biomechanics, Vol 25, #5, June 2010, 455-460
  3. Long, JT, Klein JP et al. Biomechanics of the double rocker sole shoe: Gait Kinematics and kinetics, Gait & Posture, Volume 40, Issue 13, Pages 2882-2890 (2007)
  4. Myers KA, Long JT, Klein JP, Wertsch JJ, Janisse D, Harris GF, Biomechanical implications of the negative heel rocker sole shoe: gait kinematics and kinetics. Gait & Posture. 2006 Nov;24(3):323-30. Epub 2005 Nov 21
  5. L. Stewart, J.N.A. Gibson, C.E. Thomson, In-shoe pressure distribution in “unstable” (MBT) shoes and flat-bottomed training shoes: A comparative study Gait & Posture, Volume 25, Issue 4, Pages 648-651 (April 2007)
  6. Gasser, BA, Stauber AM, et al. Does wearing shoes with unstable shoe construction stimulate metabolic activity in lower limbs? University of Bern Switzerland, 2008
  7. van Schie C, Ulbrecht JS, Becker MB, Cavanagh PR, Design criteria for rigid rocker shoes, Foot Ankle Int. 2000 Oct;21(10):833-44
  8. Sitek, T, Healthy Feet,  Footwear Insight, Formula 4 Media, March/April 2010
  9. Wuab, WL Rosenbaum, D,  Su, FC, The effects of rocker sole and SACH heel on kinematics in gait, Medical Engineering & Physics, Volume 26, Issue 8, Pages 639-646 (October 2004)

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