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The required coefficient of friction in Parkinson's disease: people with freezing of gait.

The required coefficient of friction (RCOF) is frequently reported in the literature as an indicator of slip propensity, a consequence of the collisional aspect of legged locomotion. Little is known about the RCOF in pathological gait. Therefore, this study aimed to quantify the RCOF in subjects with Parkinson's disease (PD) and freezing of gait (FOG) during the OFF-pharma phase, and to investigate the interplay between RCOF parameters and ankle kinematic and kinetic gait variables. Fourteen subjects with PD and 14 healthy age-matched subjects were instructed to walk barefoot at self-selected speed over a force platform. The RCOF curve was obtained as the ratio between the tangential and vertical ground reaction forces. Then, the following discrete variables were identified: P1COF (the peak at the loading response phase), V1COF (the valley at midstance phase) and P2COF (the peak at push-off phase). Stepwise multiple regressions were applied to observe the influence of the gait speed and ankle kinematic and kinetic gait variables on RCOF variables. In subjects with PD and FOG the gait speed is a predictor of the RCOF in the loading response phase; plantarflexion and the plantarflexion moment are strong predictors of the RCOF in midstance; finally, push-off power is a predictor of RCOF increasing in the push-off phase. These results characterized the biomechanical strategies adopted by subjects with PD and FOG during gait in order to avoid falls.