Biomechanical analysis of movement strategies in human forward trunk bending. I. Modeling

Two behavioral goals are achieved simultaneously during forward trunk bendi ng in humans: the bending movement per se and equilibrium maintenance. The objective of the present study was to understand how the two goals are achi eved by using a biomechanical model of this task. Since keeping the center of pressure inside the support area is a crucial condition for equilibrium maintenance during the movement, we decided to model an extreme case, calle d "optimal bending", in which the movement is performed without any center of pressure displacement at all, as if standing on an extremely narrow supp ort. The "optimal bending" is used as a reference in the analysis of experi mental data in a companion paper. The study is based on a three-joint (ankl e, knee, and hip) model of the human body and is performed in terms of "eig enmovements", i.e., the movements along eigenvectors of the motion equation . They are termed "ankle", "hip", and "knee" eigenmovements according to th e dominant joint that provides the largest contribution to the correspondin g eigenmovement. The advantage of the eigenmovement approach is the present ation of the coupled system of dynamic equations in the form of three indep endent motion equations. Each of these equations is equivalent to the motio n equation for an inverted pendulum. Optimal bending is constructed as a su perposition of two (hip and ankle) eigenmovements. The hip eigenmovement co ntributes the most to the movement kinematics, whereas the contributions of both eigenmovements into the movement dynamics are comparable. The ankle e igenmovement moves the center of gravity forward and compensates for the ba ckward center of gravity shift that is provoked by trunk bending as a resul t of dynamic interactions between body segments. An important characteristi c of the optimal bending is the timing of the onset of each eigenmovement: the ankle eigenmovement onset precedes that of the hip eigenmovement. Witho ut an earlier onset of the ankle eigenmovement, forward bending on the extr emely narrow support results in falling backward. This modeling approach su ggests that during trunk bending, two motion units - the hip and ankle eige nmovements are responsible for the movement and for equilibrium maintenance , respectively.