Authors: N Gueguen T Coyle C Craig R Bootsma L Mouchnino
Publish Date: 2004/02/14
Volume: 156, Issue: 4, Pages: 471-477
Abstract
During lateral leg raising a synergistic inclination of the supporting leg and trunk in the opposite direction to the leg movement is performed in order to preserve equilibrium As first hypothesized by Pagano and Turvey J Exp Psychol Hum Percept Perform 1995 211070–1087 the perception of limb orientation could be based on the orientation of the limb’s inertia tensor The purpose of this study was thus to explore whether the final upper body orientation trunk inclination relative to vertical depends on changes in the trunk inertia tensor We imposed a loading condition with total mass of 4 kg added to the subject’s trunk in either a symmetrical or asymmetrical configuration This changed the orientation of the trunk inertia tensor while keeping the total trunk mass constant In order to separate any effects of the inertia tensor from the effects of gravitational torque the experiment was carried out in normo and microgravity The results indicated that in normogravity the same final upper body orientation was maintained irrespective of the loading condition In microgravity regardless of loading conditions the same but different from the normogravity orientation of the upper body was achieved through different joint organizations two joints the hip and ankle joints of the supporting leg in the asymmetrical loading condition and one hip in the symmetrical loading condition In order to determine whether the different orientations of the inertia tensor were perceived during the movement the interjoint coordination was quantified by performing a principal components analysis PCA on the supporting and moving hips and on the supporting ankle joints It was expected that different loading conditions would modify the principal component of the PCA In normogravity asymmetrical loading decreased the coupling between joints while in microgravity a strong coupling was preserved whatever the loading condition It was concluded that the trunk inertia tensor did not play a role during the lateral leg raising task because in spite of the absence of gravitational torque the final upper body orientation and the interjoint coupling were not influencedFor each subject in each loading condition the inertia tensor of the entire upper body was determined relative to the trunk’s geometrical axes ie the tensor is initially not diagonalized about the hip joint of the supporting leg The x axis corresponded to the anterior/posterior direction the y axis to the vertical and z axis to the lateral direction The moments of inertia of the upper body segments head upper torso mid torso lower torso upper arm forearm and hand the last two being held horizontal behind the mid torso were calculated from the subject’s total mass and height using the mass and moment of inertia tables established by Zatsiorsky and Seluyanov 1983 The parallel axis theorem was then applied to account for the axis of rotation located at the hip using displacements derived from the subject’s shoulder width and hip width measurements Similarly the products of inertia were calculated but as the centers of mass of the segments were considered to be aligned on the yz plane ie x=0 the products of inertia Ixy and Ixz involving the displacement in x were reduced to zero It was also assumed that the masses were added in such a way so that the position of the center of mass of the upper arm was coincident with the center of mass of the additional charge This calculation was performed by a Matlab routine and the tensor was subsequently diagonalized to determine the eigenvalues using the Matlab eig function and the orientation of e3 was determined for each subject and for each loading condition
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