A rocker sole shoe or rocker bottom shoe is a shoe which has a thicker-than-normal sole with rounded heel. Such shoes ensure the wearer does not have flat footing along the proximal-distal axis of the foot. The shoes are generically known by a variety of names including round bottom shoes,[1] round/ed sole shoes,[2] and toning shoes,[3] but also by various brand names.[4] Tyrell & Carter identified at least six standard variations of the rocker sole shoe and named them: toe-only rocker, rocker bar, mild rocker, heel-to-toe rocker, negative heel rocker and double rocker.[1]
Rocker soles may replace regular soles on any style of footwear. Some rocker bottom shoes are purpose built to reduce the function or replace the lost function of a joint. For example, a person with a hallux rigidus (stiff big toe) may use a rocker bottom shoe to replace the flexion lost at the metatarsal joint. Rocker bottom shoes are also used to compensate for the lost range of motion, however caused, at the STJ (ankle joint). In such cases, the wearer maintains solid and stable footing while standing, but the rock of the heel assists with the propulsive phase of gait, making walking more natural and less painful to the affected joints. Beneficiaries of this type of sole modification include people suffering from arthritis or any other disorder or injury causing pain and/or loss of motion in foot joints.
The construction of most varieties of rocker sole shoes mean that the wearer's body weight is shifted behind the ankle and the wearer is required to do more work than would be required in flat-soled shoe to find their centre of gravity and remain balanced.[5]
In the 00s, a heel-to-toe rocker sole shoe for the sports footwear market was popularised by brands such as MBT, Shape Ups and EasyTone.[5]
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Rocker sole shoes have been referenced in publications as early as 1990.[6]
Branded generic rocker sole shoes were popularised for the mass market in the late 1990s and early 00s by the Swiss Masai company as Masai Barefoot Technology or MBT.[7][8] According to Swiss Masai, the market concept originated with engineer and former athlete Karl Müller who intended they would "simulate the challenge of walking barefoot on soft earth".[7][2] Various other sports footwear companies followed suit with their own branded versions of the heel-to-toe rocker targeted at the exercise equipment market.[2] A news report estimated that 200,000 pairs of modern heel-to-toe rocker sole shoes were sold in the US in 2005.[7]
Proponents of modern heel-to-toe rocker sole shoes claim that because the foot of the wearer is slightly destabilised, certain lesser-worked muscle groups in the leg, such as the core and gluteus, are challenged more than they are normally. As such they are purported to derive health benefits for the wearer, such as improved posture and tighter muscles.[5][2]
While showing some support for the use of rocker sole shoes, orthopaedic surgeon Thomas Lee suggested that the claims made about the benefits of rocker sole shoes are exaggerated. Lee also stated he disapproved of people with knee, hip or back problems wearing rocker sole shoes.[5]
While acknowledging rocker sole shoes would work muscles differently, podiatrist Jennifer Ryder said that the benefits of rocker sole shoes would not be permanent, because the muscles would adapt quickly to the change introduced. Ryder also cautioned people with heel or Achilles tendon problems against wearing the shoes.[2]
Tyrell & Carter state the prescription of rocker sole shoes in a podiatry context should be done with caution, due to the unique foot pathology of each individual.[9]
Other commentators associated with the sports shoe industry have suggested the therapeutic claims regarding rocker sole shoes are baseless and that benefits associated with wearing them are due to exercise itself, not the shoes.[5]
A 2009 study raised concern that use of rocker sole shoes may increase the risk of falls.[10]
A 2010 University of Wisconsin study commissioned by the American Council on Exercise compared exercise wearing rocker bottom shoes and regular running shoes. The study found there was no fitness benefit to wearing rocker sole shoes.[3] The study was cited in a 2011 class-action lawsuits alleging false advertising by New Balance, Reebok, and other manufacturers.[11]