Qi An1, Yuki Ishikawa1, Shinya Aoi2, Tetsuro Funato3, Hiroyuki Oka1Hiroshi Yamakawa1, Atsushi Yamashita1, Hajime Asama1

1: The University of Tokyo, 2: Kyoto University, 3: The University of Electro Communications

Muscle Synergy Analysis of Human Standing-up Motion in Different Chair Heights, Motion Speeds and Strategies    

In order to understand human motor function, it is necessary to clarify the mechanism of humans to achieve adaptive movements toward different environments. The objective of this study is to employ the idea of muscle synergy and to elucidate important muscle coordination structure among various conditions. In this study, a muscle synergy consists of muscle spatiotemporal patterns. Spatial pattern defines relative excitation level of individual muscles and temporal pattern defines time varying weight coefficient of each spatial pattern. Both analytic and constructive approaches have been used in order to identify muscle synergies.

In analytic approach variant and invariant muscle synergy structure is studied among different movements. In this approach, an experiment was conducted to measure body trajectories, reaction force and muscle activation of a lower limb. Ten healthy human subjects participated our study and they performed the motion from three different chair heights and in three different motion speeds. Muscle synergies are calculated from measured muscle activation using non-negative matrix factorization, and it is investigated how temporal pattern is corresponded to body movement and change of reaction force.

On the other hand, constructive approach focuses on the essential condition of muscle synergies to achieve the motion successfully rather than extracting shared modules from variant movements. In particular, neuro-musculoskeletal model has been developed and forward dynamic simulation is employed to calculate body movements from muscle synergies. Since this simulation methodology can generate not only succeeded standing-up but also failure movement such as falling down, it is possible to investigate the condition which distinguishes success and failure of movements. This study particularly examines how different start time of the muscle synergy affects the standing-up motion, and generated motion is evaluated based on the center of mass (CoM) trajectories.

As a result of analytic approach, three muscle synergies were obtained and it was found that spatial patterns of them were very similar each other regardless of different environments. It was shown that the first synergy activates tibialis anterior to move their CoM forward at the hip rise. The second synergy mainly activates vastus lateralis, biceps femoris, and gluteus maximus to extend their knee and hips to move CoM upward. The third synergy activates soleus and gastrocnemius to decelerate horizontal CoM movement. On the contrary, temporal patterns of muscle synergies were properly adjusted to the environmental change. Results showed that lowering chair heights requires larger amplitude of temporal patterns and the duration of synergies became longer when their motion speed was slower. Constructive approach investigated start time of the second synergy to show that it could achieve the success movement rather than falling forward or backward. Further analysis showed that three different motion strategies (momentum transfer, stabilization, and hybrid) could be generated among success motion only by controlling start time of the synergy. These results implied that humans could employ the same spatial patterns of muscle synergies, but they changed parameters of temporal patterns, such as amplitude, duration, and start time to generate adaptive movements.