Human locomotion: right/left symmetry in 3D trajectory of the body centre of mass

SUMMARY A mathematical 3D representation of displacement of centre of mass of human body (BCOM) during locomotion on treadmill has been used to assess, in each movement direction (forward, vertical and lateral), the symmetry of BCOM trajectory between the two stride phases (steps). CONCLUSIONS Differently from what expected, healthy human locomotion is rather asymmetrical. The illustrated methodology, by analysing even subtle changes in 3D BCOM trajectory, quantitatively describes the individual gait signature and represents the basis for evaluation of gait anomaly/pathology. INTRODUCTION Current literature lacks an accepted method globally describing the displacement pattern of BCOM during locomotion. The main purpose of this study was: to apply a mathematical function representing the 3D displacement of BCOM over time domain [1], identify and clarify its symmetry, and build up a comprehensive database of equations coefficients, designed to be the reference for healthy locomotion. PATIENTS/MATERIALS AND METHODS Seventy healthy subjects (35 men and 35 women) were recruited according to 7 different age groups (from 6 to 65 years). Each subject was requested to walk and run on treadmill at different speeds (0.83-1.94 m.s-1 for walking and 1.94-3.06 m.s-1 for running) on level. Each data capture lasted 1’. A motion capture system with 8 infrared cameras recorded kinematic data of relevant joints of body at 100 Hz. From them, 3D displacement of BCOM was calculated [2] and Fourier Series (1 coefficient amplitude and 1 phase per each harmonic, per 3 spatial coordinates, per 6 harmonics) and Lissajous contours graphically defined the Digital Locomotory Signature (DLS; see figure), a global index of BCOM dynamics. Harmonic coefficients were used to calculate the Symmetry Index (SI, 0: no symmetry, 1: complete symmetry), namely representing the spatial differences, in BCOM trajectory, between two steps. Differences between age groups (over speed) and between speeds (over age) were assessed by using t-tests. RESULTS No significant differences were found in SI between males and females. In each movement direction, SI was lower in young children (6-13 yrs; 0.79±0.03, in walking; and 0.75±0.03, in running; p<0.01) and in elderly adults (56-65 yrs; 0.82±0.04, in walking; and 0.80±0.03, in running; p<0.05) than in all the other age groups (pooled 14 to 55 yrs; 0.84±0.01, in walking; and 0.82±0.01, in running). Walking forward and vertical SI, in each age group, increased in the speed range 0.83-1.67 m.s-1 (p<0.001); medial/lateral direction SI slightly decreased (p<0.05). Running forward SI increased with speed (p<0.001). DISCUSSION Although no gender differences were found, human ‘healthy’ gait is rather asymmetrical. It is interesting to note that, in each testing condition, right and left steps are mostly symmetrical in the medial/lateral direction. Also, global asymmetry is more pronounced at extreme ages: while at early stages of lifespan this result could be ascribed to the process of gait development, old age asymmetries are probably due to structural wearing of musculo-skeletal system. REFERENCES [1] Minetti A. E. (2006) IMEC, Banff. [2] Minetti A. E. et al. (1993) J. Physiol. 471: 725-735.