Context: There has been recent speculation that the intrinsic muscles of the foot may play a larger role in lower extremity control and injury than previously believed. Multiple studies have shown that certain intrinsic muscles increase in size and strength after transitioning to minimalist shoe running, theoretically decreasing injury risk. There are currently no studies that examine the effect that training barefoot has in other athletic populations. Objective: Our purpose was to compare the intrinsic and extrinsic foot muscle size and strength in gymnasts (who predominantly train barefoot) and cheerleaders (who predominantly train shod). Another purpose was to measure time to stability for both groups shod and unshod. Design: Observational study. Setting: Human Performance Laboratory. Participants: 16 collegiate gymnasts (height = 159.3 ± 4.9cm, weight = 56.7 ± 4.3kg) and 16 collegiate cheerleaders (height = 161.9 ± 5.4cm, weight = 58.7 ± 7.1kg) volunteered for this study. Main Outcome Measure(s): The muscle size of 6 intrinsic and extrinsic muscles of the foot were measured using ultrasound, toe flexor strength, as assessed using a custom-made dynamometer, and time to stability following a drop landing, as assessed using ground reaction force data collected with force plates. Results: There were no significant group differences in great toe flexor strength (p = 0.274), lateral toe flexor strength (p = 0.824), or any of the time to stability conditions (p = 0.086 – 0.90). Only one muscle, fibularis longus, was significantly bigger in gymnasts than cheerleaders (p = 0.017) Conclusions: Our findings suggest that the barefoot training of gymnasts may not have as large of an impact on the foot musculature and strength as running barefoot or in minimalist shoes has on these factors.

Bakgrund: De intrinsica fotmusklerna (IFM) har potential att påverka idrottslig prestation som hopp, sprint samt balans direkt genom påverkan av muskelstyrkan eller indirekt genom påverkan på det mediala-longitudinella fotvalvet. Ett växande antalet studier visar samband mellan träning av IFM och idrottslig prestation, dock råder ingen konsensus och motstridiga resultat finns. Syfte: Syftet med studien var att kritiskt granska litteratur och undersöka det aktuella vetenskapliga kunskapsläget för sambandet mellan muskelstyrka i IFM och friska individers sprint- och hopprestanda samt balans. Metod: En litteratursökning med noga valda sökord genomfördes i fyra databaser. Efter genomgång av in- och exkluderingskriterier inkluderades 10 studier vilka kvalitetsmässigt bedömdes med Joanna Briggs Institute granskningsmall för tvärsnittsstudier. En narrativ syntetiserande analys genomfördes. Resultat: Kvalitén i inkluderade studier bedömdes i genomsnitt vara medelgod. Det som främst drog ner betyget var hanteringen av confounders och användandet av ej valida mätinstrument. Generellt visade inkluderade studier på ett samband mellan tåflexor styrka/storlek och hopp- och sprintprestanda samt balans. Svårigheten att isolerat mäta IFM försvårar möjligheten att utreda IFMs betydelse för inkluderade studiers utfallsmått. Slutsats: Genomförd litteraturstudie visar möjliga samband mellan styrka/tåflexorstorlek och prestation i hopp och sprint såväl som balans. För att nå en ökad förståelse för hur IFM påverkar idrottslig prestation och balans behövs en större kunskap om hur IFM styrka direkt eller indirekt kan mätas isolerat och hur muskelstorlek kan översättas till styrka. Genomförd litteraturgranskning stödjer ett redan identifierat behov av valida mätinstrument för att nå en ökad förståelse av IFMs betydelse för idrottslig prestation och balans. / Background: The intrinsic foot muscles (IFM) have the potential to improve sprint, jump and balance performance by direct muscle power or indirectly by supporting the medial longitudinal arch. A growing number of studies show the association between exercising the IFM and athletic performance. However, there is no consensus if athletes can benefit from IFM training and inconsistent results are published. Aim: The aim of this study was to conduct a critical systematic review to evaluate the current knowledge of the association between IFM strength and performance in jump sprint and balance in healthy individual’s. Methods: Four data bases were searched for eligible studies. After screening for exclusion and inclusion criteria ten studies were included. The scientific quality was analyzed using the Joanna Briggs Institute ”Critical Appraisal tools” for Cross Sectional Studies, and a narrative synthesis was conducted. Results: The average quality score for the included studies was “moderately good”. The main reason for the low scores was insufficient attention to confounders and the use of non-valid instruments. An association was found in most of the studies between toe flexor strength/size and jump, sprint and balance performance. Difficulties in selectively measuring the IFM activity complicated the possibility to evaluate the impact of IFM on studied outcome measures. Conclusion: The presented study shows an association between toe flexor strength/size and athletic performance. However, to understand how IFM strength training affects athletic performance, both directly and indirectly, more studies focused on how to measure the IFM using valid methods for isolated IFM measures of strength and size is needed.