Postural Considerations in Maximum Voluntary Exertion

This paper is concerned with maximal manual exertions in the sagittal plane while standing; the whole body is therefore involved. Body weight and a choice of posture in which weight can be used to advantage constitute one factor governing exertion. Its effect is analogous to the force that a log of wood, leaning against a handle, would exert in the Dead Axis (Fig.1), perpendicular to the line between the handle and the ground support. In the ‘dead weight’ analogy, the force depends only upon the weight of the log and where its mass is centred relative to the ground support and the handle. Muscular capacity constitutes a second factor in exertion, which also depends upon posture; an analogy is the force that a Jack-in-a-Box exerts. The force is in the Live Axis (Fig. 1) which is the line between the centre of pressure at the ground and the handle. Although ded-weight and Jack-in-a-Box effects both operate in exertion, they cannot be readily separated. The log of wood retains its shape by virtue of tensions and compressions in its fibres. Man retains his posture by means of tensions in muscles and ligaments and by compressions within the skeleton. Man’s muscles are required both to retain the posture and to create the ‘Jack-in-a-Box’ forces.