Biomechanical analysis of movement strategies in human forward trunk bending. II. Experimental study

The large mass of the human upper trunk, its elevated position during erect stance, and the small area limited by the size of the feet, stress the imp ortance of equilibrium control during trunk movements. The objective of the present study was to perform a biomechanical analysis of fast forward trun k movements in order to understand the coordination between movement and po sture. The analysis is based on a comparison between experimentally observe d bending and hypothetical "optimal bending" performed on an infinitely nar row support, as presented in a companion paper. The experimental data were obtained from 16 subjects who performed fast forward bending while standing on a wide platform or on a narrow beam. The analysis is performed by decom position of the movement into three dynamically independent components, eac h representing a movement along one of the three eigenvectors of the motion equation. The eigenmovements are termed "hip", "ankle", and "knee" eigenmo vements, according to the dominant joint. The experimentally observed movem ent is characterized mainly by the hip and ankle eigenmovements, whereas th e knee eigenmovement is negligible. Similarly to the "optimal bending" the ankle eigenmovement starts earlier and lasts longer than the hip eigenmovem ent. An early forward acceleration of the center of gravity in the ankle ei genmovement is caused by anticipatory changes in the ankle joint torque. Th is clarifies the role of the early tibialis anterior burst and/or soleus in hibition usually observed in electromyographic recordings during forward be nding. The results suggest that the hip and the ankle eigenmovements can be treated as independently controlled motion units aimed at functionally dif ferent behavioral goals: the bending per se and postural adjustment. It is proposed that the central nervous system has to control these motion units sequentially in order to perform the movement and maintain equilibrium. It is also suggested that the hip and ankle eigenmovements can be regarded as a biomechanical background for the hip and ankle strategies introduced by H orak and Nashner (1986) on the basis of electromyographic recordings and ki nematic patterns in response to postural perturbations.