• Vasilios Baltzopoulos
Keywords: modelling, musculoskeletal, joint moment, simulation, isokinetic dynamometry


Musculoskeletal modelling is widely used in sports biomechanics for the estimation of joint and muscle loading in inverse dynamics applications or the simulation and optimisation of human performance in forward dynamics simulations. The relative motion of the segments is normally modelled using three different approaches: a) as a simple pin joint allowing only rotation around a fixed axis, b) a parametric description of relative motion describing the linear displacement of one segment relative to another as a function of the rotation angle and c) as the motion of a full biomechanical model of the joint that includes mechanical models of muscles, tendons, ligaments and other restraining structures and is based on the response of the model to the applied internal and external forces. In the first two approaches that are the most common, the relative movement of the segments due to the contraction of muscles and the resulting internal forces is ignored and this can have significant implications for the output of the model, especially in more complex models of the musculoskeletal system. In forward dynamics applications with the above models, joint rotation is generated using either torque generators or Hill-type muscle models. Torque generators are functions of torque based on the joint angular position and velocity. These functions are typically calculated by measuring the joint moment at different joint positions and angular velocities using isokinetic dynamometry. In general, it is assumed that the moment measured using dynamometry is equivalent to the actual joint moment. However, it has been documented that this is not the case due to a) gravitational forces, b) inertial forces (e.g. Herzog, 1988) and c) misalignment of the joint and dynamometer axes of rotation resulting from the non rigidity of the dynamometer arm-lower leg system (Herzog, 1988; Kaufman et al., 1995; Arampatzis et al., 2004). Implementations of appropriate methods for the correction of the gravitational and inertial forces have been reported. The movement of the segment relative to the dynamometer is the main factor for the differences between measured and actual joint moments. Hence, the main purpose of this study was to use X-ray image analysis to examine the effects of the non-rigidity of the dynamometer chair, arm and lower leg system on the knee joint kimematics and the resulting joint forces calculations using inverse dynamics and the measurement of active knee extension moment-angular position relationship that is the basis for toque generator functions in forward dynamics applications.