Effects of Body Weight Unloading and Treadmill Belt Surface on Tibial Acceleration-Based Loading Indicators During Running on a Microgravity Treadmill

Background/Objectives: The aim of this study was to compare tibial acceleration and acceleration-based indicators associated with tibial loading during running under full body weight (BW) conditions and with body weight support (80%, 60%, and 40% BW) on a microgravity treadmill, and to examine the effect of treadmill surface by comparison with a conventional treadmill. Methods: Twenty-six healthy, physically active adults (age 18–40 years) completed running trials at a constant speed of 8 km/h. Tibial acceleration was recorded using inertial measurement units. Results: Cadence decreased significantly with increasing levels of body weight support, while no differences were observed between a conventional treadmill running and microgravity treadmill running at 100% BW. Axial acceleration parameters demonstrated a consistent reduction in acceleration magnitude associated with progressive unloading. The magnitude of axial acceleration (MAA), as well as peak positive (PPAA) and negative (PNAA) axial accelerations, decreased significantly, with the largest reductions observed at 60% and 40% BW. Temporal parameters showed a different pattern: time to peak positive axial acceleration increased with unloading, indicating delayed peak loading. The duration of positive axial acceleration exhibited a non-linear response, increasing at moderate unloading levels (80% and 60% BW) and decreasing at 40% BW. In contrast, the duration of negative axial acceleration did not change significantly, although overall contact time increased with unloading. Conclusions: The findings of this study indicate that progressive body weight unloading during running on a microgravity treadmill leads to a significant reduction in impact-related tibial acceleration indicators associated with mechanical loading exposure, particularly in parameters describing the amplitude and dynamics of axial accelerations. Running at 100% BW on both treadmill systems demonstrated overall biomechanical similarity; however, several side-specific differences in tibial acceleration parameters suggest that treadmill surface and system characteristics may exert a subtle influence on local acceleration measurements. Moderate unloading (~80% BW) may provide an optimal balance between load reduction and preservation of natural running mechanics.