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BackgroundThe angle-dependent torque capacity of the subtalar pronators and supinators is important to maintain dynamic ankle stabilisation. Based on the peak torques during maximum voluntary isometric pronation and supination across the subtalar range of motion, the strength curves of younger and elderly males and females were investigated.MethodsMaximum voluntary isometric subtalar pronator and supinator strength tests were administered to 30 younger and 30 elderly volunteers (each 15 male and 15 female subjects). Total active subtalar range of motion and peak pronator and supinator torques were measured in five anatomical subtalar joint angles using a custom-built apparatus with two force transducers. Furthermore, relative torques (normalised to the individual peak torque) and pronator-to-supinator strength-ratios were also calculated.ResultsPronator-to-supinator strength ratio, and peak pronator and supinator torques are affected by age and by joint angle x age interactions. All supinator strength curves show a steadily descending characteristic from the pronated to the supinated positions. The pronator strength curve had an inverted U-shaped characteristic, except for younger women of whom 47 % exert highest peak values in the end-range pronation angle. Both relative pronator and supinator strength are dependent on sex (P < 0.05). Relative pronator strength is also affected by joint angle x sex (P < 0.0001) and joint angle x sex x age (P < 0.05) interactions. Beside age effects on all range of motion parameters, pronation range of motion is influenced by a sex x age interaction (P < 0.05).ConclusionsAge- and sex-related differences in both subtalar strength profile and range of motion have to be considered when testing strength across subtalar range of motion. Younger females have higher pronator strength capacity in the most pronated joint angle, which may be due in part to their greater subtalar joint range of motion compared to the other groups. In the most supinated position both pronator and supinator strength capacity is reduced in younger females compared to younger males.Electronic supplementary materialThe online version of this article (doi:10.1186/s13047-015-0125-2) contains supplementary material, which is available to authorized users.

<b><i>Background:</i></b> Associations between lower limb muscle strength and balance performance in adults have previously been reported. However, the function of the foot muscles for postural control has not been understood, yet. <b><i>Objective:</i></b> The purpose of the present study was to investigate associations between pronator and supinator muscle strength, subtalar range of motion (ROM) and postural stability while standing under various conditions in young versus old adults. <b><i>Methods:</i></b> Using a custom-built apparatus equipped with a force transducer and an electrogoniometer, maximum voluntary isometric subtalar pronator and supinator strength as well as ROM tests were administered to 30 young (mean age: 25.1 years) and 30 old (mean age: 65.2 years) volunteers. Total active subtalar ROM, peak pronator and peak supinator torques were measured. While standing on a force plate, limits of stability (LOS) were determined during anterior–posterior (AP) and medio-lateral (ML) leaning tasks. Furthermore, sway distance and velocity during single-legged standing were measured. Correlation and regression analyses were conducted. <b><i>Results:</i></b> In both age groups, subtalar pronator muscle strength was related to AP-LOS (young: <i>r</i> = 0.36; old: <i>r</i> = 0.49). In young adults, subtalar supinator muscle strength was associated with ML-LOS (<i>r</i> = 0.41). The regression analyses revealed that summed subtalar muscle strength predicts 13 and 20% of the variance of AP-LOS in young and old adults, respectively. Summed subtalar muscle strength was found to predict 18% of the variance in ML-LOS in young but not in old adults. There were no correlations and no predictors found concerning subtalar muscle strength and postural sway during single-legged standing for both age groups. <b><i>Conclusions:</i></b> Longitudinal studies have to proof whether pronator muscle strength training might positively affect balance performance during AP leaning, specifically in old adults.

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