Contractile and elastic behaviour of human muscle-tendon complexes with inertial loading.

Voluntary movement requires that inertial and gravitational forces acting on the limbs be overcome by active muscle-tendon complexes (MTCs). A lot of information exists about the mechanical properties of tendon and the mechanical behaviour of active muscle preparations shortening against constant loads or at constant speeds. During limb movement however, muscles are not expected to shorten at a constant speed or under a constant load as contractions are performed against the series elasticity of the MTC and the inertia of the limbs. This thesis investigates the relationship between the kinetics of inertial and inertial/gravitational loads external to shortening MTCs and the mechanical behaviour of the components of these MTCs. Mathematical modelling revealed the fundamental principles governing the behaviour of MTC- inertial/gravitational load systems. MTC-load model systems behave in a manner, which is largely predictable from the load that is 'sensed' by the MTC. Other factors and their effects have also been identified. It was found that there is an upper limit on the extent to which muscle generated power can be amplified by the elastic properties of tendon. Experimental observations were made of human volunteers performing index finger abduction against purely inertial loads and standing ankle plantar-flexions against inertial/gravitational loads. A method was developed which allows the kinetic behaviour of the components of shortening MTCs to be determined from simple, non-invasive measurements of torque and angular displacement of the moving limbs around their joints. Theoretical and experimental results are compared and critically discussed