The aim of this study was to analyse the kinematics and kinetics of the lower extremities in the sagittal plane, when running under unstable surface conditions. It was hypothesized that 1) a greater effect of the unstable surface would occur in the gastrocnemius, soleus, and tibialis anterior muscles, contributing to plantar- and dorsi-flexion, compared to muscles involved in hip and knee movements, and 2) the step-to-step absolute variability would be larger in the unstable condition. Eleven male-subjects completed running trials on stable and unstable surfaces in a laboratory setup. Inverse kinematic and dynamic analyses were conducted to calculate kinematics and moments at the lower extremity joints. Additionally, muscle force and activation related variables were calculated for six lower limb muscles using musculoskeletal modelling. Furthermore, the individual SD was calculated for all the variables as a measurement of absolute step-to-step variability. The unstable surface led to a decrease in joint ROM of the knee and ankle by 8.3% and 11.4%, and a decrease of 13.3% on average in force development of the ankle plantar-flexor, which also was reflected by decreasing muscle peak forces of Soleus and Gastrocnemius of 10.3% and 10.8%. Furthermore, an increase of force of Biceps Femoris and activation of Vastus Lateralis were found during the unstable condition. The step-to-step variability increased up to 158% when changing to the unstable condition. In conclusion, the findings revealed for the first time, lower ankle muscle forces mostly reflecting biomechanical adjustments to the surface conditions as well as larger absolute variability when running on the unstable surface.
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