A biomechanical analysis of human strength
1. Human strength, regarded as the maximum force that subjects can exert under given test conditions has been frequently observed and reported. The influences of biomechanical factors on the results of tests of strength are both complex and obscure. Hitherto, these influences have received very little attention and they are therefore the subject of this thesis. Particular emphasis has been given to the influence of posture because of its theoretical and practical importance. 2. Three contrasting tests of strength were experimentally investigated. (i) Strength of plantarflexion was investigated as a function of posture of the ankle and knee. These observations were collated with quantitative cadaveric studies of the relevant muscles and joints. (ii) Strengths of pronation and supination were measured as a function of the posture of the forearm. The ability to transmit supinator torques using ranges of uniform cylindrical handles and the handles of commercially available screwdrivers was investigated. (iii) Strength of pulling in the sagittal plane in a variety of two handed tests was measured in 165 subjects. A multivariate statistical analysis established the extent to which performance was determined by body weight and stature. Results confirmed the predictions of a theoretical analysis of the task by means of free-body diagrams. They showed that, in any given tests, the proportion of the variance in strength which could be attributed to weight and stature was closely related to the posture of the subject. A further experiment investigated the strength of extension of the trunk as a function of posture. 3. The interaction of the physiological properties of the musculature; the transmission of stresses through the interfaces between the body and its mechanical environment; and the weight and leverage of the body is discussed.