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How to characterise stability during gait? Application to the gait of transtibial amputees during ramp descent

Some populations suffering from locomotor deficits such as amputees can have their stability dramatically compromised during gait. However, gait stability is not uniquely defined. Some authors estimate stability deficits indirectly by analysing some variables (gait speed, single stance duration, step length) that are supposed to reflect modifications of gait required to ensure this stability (Vickers et al. 2008). Others consider that stability is reflected in movement variability and thus analysed standard deviation of gait velocity, or characterised stability through fractal dynamic stability, local dynamic stability or uncontrolled manifold analysis (Hausddorff 2007). Even if these methods provided promising results, they cannot always be applied due to the significant number of gait cycles required (Bruijn et al. 2009). Moreover, they do not always help to understand the origin of the instability. The present study is part of a project proposing to evaluate the effect of a new prosthetic ankle that proposes to improve the gait of transtibial amputees (TTAs) in specific walking conditions such as gait on stairs or on ramps. A preliminary study compared the kinematics and kinetics of TTAs’ gait in aforementioned conditions. During ramp descent, the kinematics and kinetics were not closer to the gait patterns of healthy controls but TTAs reported in a questionnaire that they felt more stable when walking with the new prosthetic ankle. The present study proposes then to investigate in more details gait stability in TTAs during ramp descent. First, it was proposed to test whether the global stability of TTAs was modified by analysing the extrapolated centre of mass (ECoM; Hof et al. 2007). This parameter is thought to be an extension for dynamic stability of the condition for standing stability. Therefore, the higher this parameter is, the more compromise the dynamic stability should be. Second, joint acceleration and the effect of the ground reaction force (GRF) on the joints were analysed in order to try to understand the origin of the feeling of stability in TTAs. 2. Methods