The lower body positive pressure (LBPP) treadmill is emerging as an innovative tool for rehabilitation as it allows running in partial unweighted conditions. This technology uses small increase in air pressure to produce a lifting force via an airtight chamber applied to the runner's pelvis, leading to a substantial bodyweight (BW) reduction while the lower limbs still experience normogravity. An appropriate use of LBPP treadmill requires an improved understanding of the associated neural and mechanical adjustments of the running pattern.

Twenty healthy males performed 2 randomized runs of 9min at preferred speed on an LBPP treadmill. Each series included 3 successive running conditions of 3min at preferred speed (at 100%, 60 or 80%, and 100% BW) with progressive transitions in between. Vertical ground reaction forces (Fz) were recorded together with surface electromyographic (EMG) activity from 6 muscles of the left lower limb. The analyses were performed on the stabilized running pattern in each condition and on the progressive changes along the transitions.

Unweighting resulted in a rebounding running pattern (increased flight and unchanged contact time) and in a shift from a heel to a midfoot striking pattern. This latter modification was still observed after reloading. The transition analyses highlighted the linearity between the BW and the Fz changes, whereas the activation pattern showed adjustments which were muscle- and stride phase-dependent. Unweighting did not affect the extensor muscle preactivation, led to parallel decreases in quadriceps activation and Fz during the braking phase, and to reduced triceps surae activation during the push-off phase. Almost all changes were mirrored during reloading.

The results confirm the linearity between the BW changes and the vertical mechanical loading of the lower limb. For clinical purpose, our results highlight the need to further examine the LBPP-induced neuromechanical changes. The EMG stability during the preactivation phase suggests a preserved preparation for ground impact. Yet, eventual adaptation should be explored by protocols using repeated treadmill running sessions. The adoption of a midfoot striking pattern with a reduced quadriceps activation during the braking phase might be beneficial for specific clinical cases.