Introduction Many studies have been published concerning overuse and acute injuries of the lower extremity with respect to individual risk factors.1 Based on the studies by Isman and Inman2 it has been speculated that inclination and deviation of subtalar joint axes (STA) could be used to estimate the risk to chronic or acute injury. In this novel approach a method has been used to determine a set of parameters, describing the axes of rear foot movement with respect to the tibia. The aim of the study was to measure STA of runners with and without chronic symptoms of Achilles tendon (AT).

Methods The Zebris motion analysis system has been adapted for STA estimations. By postulating a pure rotational movement arround the STA the system could be reduced to only one ultrasound marker which was fixed rigid to the rear foot complex by a special mountig device. The ankle has been moved to maxium dorsiflexion. Then rearfoot has been moved from inversion to eversion six times. A set of six finite axes between the end positions was calculated in real time and then averaged in order to minimise errors from data capturing. A total of 614 axes were calculated from 307 high level runners.

Results and discussion Data of this study revealed an average inclination of 42° and an average deviation of 11° from 614 ankles. Significant differences have been found between deviation angles from ankles with AT problems (18° ± 23°) compared to those without AT problems (10° ± 23°). Compared to data of Isman and Inman2 (23° ± 11°) it seems that the average deviation of the STA is smaller and therefore more parallel to the longitudinal axis of the foot, which is also in line with the findings of Lewis et al3 (18° ± 10°). It has been speculated that AT problems might be related to asymmetrical loading during running and this is supported by the findings of this study. However, the range of data and SDs are actually higher than published in previous studies. Further investigations are necessary to shed light on anatomical factors and other reasons for AT problems in runners.

Conclusions This method can be used to estimate a functional joint axis which is closely related to STA orientation. This might be useful, especially to investigate large sample sizes in order to detect individual risk factors to chronic AT overuse in running.