The transition of the synergy pattern in human locomotion learning was identified by kinematically quantifying the degree of synergy between arm and leg in walking backward on a treadmill with eyes closed as an example of inexperienced motion.
Eight adult men having landmarks on the body walked backward with eyes closed on a treadmill at a belt speed of' 50 m/min for five minutes, four times. Body landmark coordinates during motion were measured continuously for five minutes at 60 Hz. Joint angles for the shoulder, hip, knee and ankle calculated based on the measured 2-dimensional coordinates were used to examine the transition characteristics of the synergy between joints. Mutual correlation coefficient (MCC) between joint angle time-series data sets was used to quantify the degree of synergy between joint motions. Level of locomotion learning was evaluated by correlation dimension (CD; Grassberger & Procaccia ). The CD is a landmark to estimate the degree of freedom of a dynamical system creating the attractor. The method of Takens (1981) was used to reconstruct the attractor trajectory based on leg joint angle time-series data.
The CD in joint motion attractors significantly decreased in the shoulder and ankle joint as the trial progressed (p<0.05). Thus, the degrees of freedom for these joint motions became small as learning progressed. The CDs in joint motions for the hip and knee did not change while learning progressed. MCC for the joint motion between shoulder and hip (shoulder-hip) significantly increased as the trial progressed (p<0.05). This may indicate that shoulder flexion is synchronized with hip extension during locomotion learning. No significant change with learning level was found in MCCs for shoulder-knee and shoulder-ankle.
Within the limitations of this study, the synergy of motion between shoulder and hip may exist in the locomotion used in our study. The synergy appears to progress through a decrease in motion complexity of the shoulder with learning.