Unstable shoes: functional concepts and scientific evidence

The purpose of this study was to discuss (a) the conceptual idea behind unstable footwear and (b) the validity and scientific support of some selected claims made with respect to unstable shoes. The concept is that unstable shoes are built to provide a training device that uses instability as a strategy to train and strengthen muscles in the human locomotor system. Specific claims are: (1) evidence shows that unstable shoes currently on the market produce a substantial and significant increase in instability. The effects are most evident during standing but are also apparent in walking. (2) Unstable shoes increase the activity in certain muscles in about 80% of the population. The affected muscles change between different subjects. The highest relative increases were found in the small muscles crossing the ankle joint complex. (3) ‘Muscle toning’ is not defined and experimental data associating ‘muscle toning’ with unstable shoes are not available. (4) There is evidence that unstable shoes improve the static balance performance of users whose balance skills are low. (5) There is indirect evidence that unstable shoes reduce forces in the joints of the lower extremities. (6) There is evidence that unstable shoes can reduce the level of perceived pain. This has been confirmed in subjects suffering from pain in the knee joint and for subjects with low back pain. Based on these results, it seems that unstable shoes are associated with several possible benefits. However, the effects are not consistent between different subjects. In our experience, positive effects can be shown for about 80% of the test subjects.