Introduction: Pes planovalgus or flatfoot-associated complaints are frequent symptoms, which are thought to be caused by the foot deformity itself. Concurrently, the multifactorial weakness of the M. tibialis posterior and its tendon (trauma, systemic disease, chronic tendon degeneration by overuse) can lead to a flattening of the medial longitudinal arch of the foot. Affected patients suffer from functional impairment and pain. Less severe cases are eligible for non-surgical treatment. Foot orthoses are considered to be the first line approach. Furthermore strengthening of the arch and ankle stabilizing muscles are thought to contribute to an active compensation of the deformity. There is only limited evidence concerning the numerous therapy approaches since high quality studies are missing. One excellent report (Kulig et al. 2009) shows clear benefits by the use of foot orthoses and eccentric strengthening exercises. Beside the fact that evidence-based guidelines for therapy have yet to be developed, no data is available showing functional benefits that accompany the therapy process. This would give further insight into mechanisms behind non-surgical management strategies and how patients benefit functionally from therapy. Purpose: The purpose of this randomized longitudinal intervention study is the evaluation of the therapeutic benefit of three different non-surgical treatment regimens (foot orthoses only FOO, foot orthoses and eccentric exercise FOE, foot orthoses sham treatment FOS) in patients with Pes planovalgus and accompanying complaints. Furthermore the analysis of possible functional changes in gait mechanics (kinematic and kinetic view) and neuromuscular control (electromyographic analysis) will contribute to a superior understanding of functional changes that accompany non-operative management. The purpose of the study is to optimize non-surgical management in patients suffering from Pes planovalgus associated pain leading to an efficient use of health care system's financial resources. Methods: 60 patients with Pes planovalgus associated complaints (clinical diagnosis with plain weight bearing radiographs), M. tibialis posterior dysfunction) are included in the study. Functional impairment is evaluated pre and post intervention by the Foot-Function-Index (FFI, German version). Anthropometric data recording is followed by preparation of subject's foot anatomical landmarks with retroreflective markers and superficially detectable muscles of the ankle joint complex are prepared with surface electromyography (SEMG) electrodes. The 3D kinematic data allows inter alia the calculation of segmental angels of the lower extremity and measurement of navicular drop. The neuromuscular activity is analysed in the time (on-off pattern) and amplitude domain (gait cycle specific phases). Procedure: Potential participants are recruited via the Outpatient Clinic of the Department of Orthopaedic Surgery of the Inselspital, University Hospital, Bern. After initial screening, subjects are randomized to one of three intervention groups (foot orthoses only FOO, foot orthoses and eccentric exercise FOE, foot orthoses sham treatment FOS). FOO-subjects wear custom-made foot orthoses only. FOE-subjects wear individually accustomed foot orthoses and they will perform a combined monitored and home training program to progressively strengthen the M. tibialis posterior and accompanying ankle stabilizing muscles with eccentric exercises. FOS-subjects wear custom-made sham foot orthoses without the functional elements of the treatment orthoses (longitudinal arch support, ankle stabilizer, bowl-shaped heel for rearfoot stability). Subjects are measured pre and post intervention (12 weeks). Measurements include the primary outcome measure Foot-Function-Index (FFI, German version: total score) followed by basic anthropometric measures. Subject preparation allows then the measurement of 10 trials on a walkway and on stairs with embedded force plates in barefoot condition. An average step cycle out of 10 trials is calculated and biomechanical outcome measures are extracted. A re-test allows the calculation of intervention effects by one-factor ANOVA (group: treatment FOO vs. FOE vs. FOS) for repeated measures.