Contribution of arm swing to dynamic stability based on the nonlinear time series analysis method.

It is human nature to swing their arms at the frequency of leg motion during walking, but the contribution of arm swing to dynamic stability of human motion segments was poorly understood. Based on the nonlinear time series analysis method, the objective of this study was to investigate the effects of arm swing in three conditions (natural, active and restricted arm swing) on the dynamic stability of spine and lower extremity joints, and to further assess the contribution of arm swing to the human dynamic stability in relation with age. Gait experiments were carried out for 10 young and 8 middle-aged healthy volunteers while walking with natural, active and restricted arm swing. The maximum finite time lyapunov exponents were calculated to quantify the local dynamic stability of spine and lower extremity joints under three arm swing conditions, and the percentage change of the maximum Lyapunov exponents was compared between two groups to evaluate the effectiveness of active arm swing in relation with age. For both young and middle-aged groups, no significant difference of the maximum lyapunov exponents of all motion segments was found between walking with natural arm swing and with restricted arm swing (P>0.05). However, the maximum lyapunov exponents of all motion segments while walking with active arm swing was significantly lower than those while walking with natural arm swing and restricted arm swing, respectively (P<0.05), and the percentage decrease of the maximum lyapunov exponents for all motion segments while walking with active arm swing was significantly higher in middle-aged group than in young group (P<0.05). These results indicated that active arm swing would help to improve dynamic stability of human motion segments, especially more effective with age.