The path curvature of the body centre of mass during walking as an index of balance control in patients with Multiple Sclerosis

BACKGROUND AND AIM: The path curvature of the centre of mass (CM), mechanically representative of the whole body system, may provide hints to detection of fall risk during walking. Here, an example is taken from results of an ongoing controlled study. It shows the comparison between the CM path in a healthy subject and in a fully autonomous patient with Multiple Sclerosis (MS). METHODS: A representative healthy subject (woman, 26 years, 1.55 m tall) and a MS patient (woman, 34 years, 1.65 m tall, with very mild left hemiparesis) are presented. Subjects walked on a force-sensorized treadmill (1) at 0.6 m/s. Data were averaged across 6 subsequent strides. The 3D displacements of the CM were computed via double integration of the ground reaction forces (Cavagna's Method). The path curvature of the CM during one stride was computed according to the Frenet-Serret formula (2). The instantaneous efficiency of the kinetic-potential, pendulum-like energy transfer of the CM was also computed (percent recovery, R: 100%=complete recovery, i.e. fully passive CM translation) (3). RESULTS: The left and right panels refer to the control and the MS subject, respectively. In the upper set of panels the human sketches on top of the figure help identifying the stride phases (% cycle) and give a frontal and a sagittal perspectives. The first and second rows of curves from the top give the instantaneous R and the path curvature of the CM during one stride. Each step begins with the single stance of the front leg (R=right; L=left). The horizontal bars under the curves mark the double and the single stance phases (continuous and dashed lines, respectively; grey tract=left step). The lower set of panels (closed curves) gives the planar projections of the CM path during the same stride. The space-time correspondence between the 2 sets of curves is facilitated by the shared A-D labeling of peak curvatures and the shared graphic conventions (dashed line=single stance; gray tract=left step). In both steps the curvature is peaking when R suddenly drops from 100 to 0, demonstrating that the passive pendulum-like mechanism of translation is briskly substituted by a short lasting, fully muscle-driven, propulsion. The highest peaks (A and C) are coincident with the lateral redirection during single stance. Of note, the patient's CM path is characterized by a 10-fold higher C peak (single stance, paretic-to-unaffected side redirection). This may be interpreted as a feature of "escape" limp, barely perceivable by clinical observation, when seen from the perspective of the body CM on the horizontal plane (bottom curves). CONCLUSIONS: Increased curvature peaks may reveal the attempt to shorten the stance on the affected side yet, placing at risk the lateral stability of the body. REFERENCES:[1] Tesio L, Rota V, Am J Phys Med Rehabil 2008;87:515-526 [2] Tesio L et al, J Biomech 2011;44:732-740 [3] Cavagna GA, J Appl Physiol 1975;39:174-179.