Human Force Exertion the Influence of the Direction of the Effort

Abstract

Although the concept of human force exertion appears a rather simple one, its measurement rises many problems. In a precedent work we have underlined the fact that the result of the effort exertion is widely conditioned by the experimental protocol that is used, including factors as vocal stimulation, visual control feed back, time duration or else… We retained for standard conditions of exerting a pulling effort on a fixed handle, without displacement, an arrangement that provides to the subject a real time visual display of the effort recording on a scope placed in front of him. This method gives sufficiently stable and reproductible data for a systematic study. The following step of this study is evaluating the force exerted at different locations around a seated subject. In this case, a new problem appears: what should be the direction of the pulling effort, related to a reference frame linked to the seat, which is equally a standard anatomical reference system for the subject belted on this seat. Two extreme choices are possible. The first one corresponds to classical industrial arrangements when the handle remains horizontally perpendicular to a frontal plane and constitutes an imposed condition. The other possibility is to let the subject free to choose the direction of his pulling effort. The present paper accounts for a comparison of these two conditions for five azimutal positions of the handle varied in the horizontal plane of the right acromion, at such a distance that in the final position the right upper limb is straight. The results for sixteen male subjects exhibit a significant degradation of the efficiency of pulling in the imposed procedure compared with the choosen one. This discrepancy is growing with the increase of the azimutal angle.