Just read the study below, and realize that your assumptions until now have been based on rumors and speculation rather than actual science.
Foot orthoses use does not affect muscle strength
Craig Payne & Gerard Zammit
Department of Podiatry, School of Human Biosciences, La Trobe University, Melbourne, Australia
Foot orthoses are widely used in clinical practice to treat pathologies associated with what is assumed to be excessive pronation of the rearfoot. However it is not uncommon to come across unsupported statements in the lay press or from alternative health practitioners that foot orthotics or foot supports should not be used as they will, for example “act as a crutch” or “weaken the muscles that support the arch”. As there is no data on the effects of foot orthotics use on muscle strength, the aim of this project was to determine toe plantarflexion strength in subjects prior to the use of foot orthoses and again after 4 weeks of use.
Subjects were recruited from a teaching clinic who were issued with foot orthoses for symptoms that were assumed to be due to excessive pronation of the foot. The force required to plantarflex the toes of the left foot was measured using a specially designed platform. Subjects stood on a raised platform which was at a hinged part at the level of the lesser metatarsophalangeal joints, but not at the first metatarsophalangeal joint. The hinged part was held level by a force gauge. Subjects were instructed to maximally plantarflex the toes against the platform, while the examiner held their hand over the metatarsal heads to prevent them being elevated. The force gauge measured the plantarflexor force in Newton’s. The subjects were asked to lean against the wall behind them, so as not to lean forward or bend the knee when plantarflexing the lesser toes. Previous work had shown this testing method to be reliable. This test was considered to measure the strength of the intrinsic muscles and the flexor digitorum longus muscles. The test was conducted 5 times, with the mean used for the analysis. The trial was repeated at the 4 week review of the foot orthoses. The Wilcoxon signed ranks text was used to analyse the difference.
Sixteen subjects were recruited (mean age: 44.5 (±16.3); 5 male; 11 female). The mean follow up was 4.5 (±0.8) weeks. The mean plantarflexion strength of the lesser toes at baseline was 37.3 (±14.9)N and at follow-up it was 40.2 (±16.3)N (p=0.23)
This study has shown that there was no decrease in muscle strength after the use of foot orthoses for 4 weeks, giving no reason to be concerned with unsupported statements that foot orthoses can weaken muscles.
The results need to be interpreted in the context of the relatively short follow-up of 4.5 weeks and it may have been too soon to detect any weakness, however, the trend was for an increase in strength (but not statistically significant), perhaps suggesting that a longer follow up may not have shown any weakness developing.