Biomechanics of the upper limb : applications of motion analysis and force measurement techniques
Two studies involving different applications of motion analysis and force measurement techniques are presented. The first study provides data on typical loads on the upper limb, and the muscle and joint forces which oppose them. The second presents parameters defining the biomechanics of knife stab attacks in order to specify standards for the testing of stab resistant body armour. A three dimensional, mathematical model of the elbow and wrist joints, including 15 muscle units, 3 ligaments and 4 joint forces, has been developed. A new strain gauge transducer has been developed to measure functional grip forces. The device measures radial forces divided into six components and forces of up to 250N per segment can be measured with an accuracy of «1%. Ten normal volunteers from within the Bioengineering Unit were asked to complete four tasks representing occupational activities, during which time their grip force, was monitored. Together with kinematic information from the six-camera Vicon data, the moment effect of these loads at the joints was calculated. These external moments are assumed to be balanced by the internal moments, generated by the muscles, passive soft tissue and bone contact. The effectiveness of the body's internal structures in generating joint moments was assessed by studying the geometry of a simplified model of the structures, where information about the lines of action and moment arms of muscles, tendons and ligaments is contained. The assumption of equilibrium between these external and internal joint moments allows formulation of a set of equations from which muscle and joint forces can be calculated. A two stage, linear optimisation routine minimising the overall muscle stress and the sum of the joint forces has been used to overcome the force sharing problem. Humero-uInar forces of up to 1600N, humero-radial forces of up to 800N and wrist joint forces of up to 2800N were found for moderate level activity. The model was validated by comparison with other studies. A wide range of parameters defining the biomechanics of knife stab attacks has been measured in order to specify standards for the testing of stab resistant body armour. Stab styles based on reported incidents provided more realistic data than had previously existed. A six camera Vicon motion analysis system and specially developed force measuring knife were used to measure the parameters. Twenty volunteers were asked to stab a target with near maximal effort. Three styles of stab were used: a short thrust forward, a horizontal style sweep around the body and an overhand stab. The body holding the knife u-as modelled as a series of rigid segments: trunk, upper arm, forearm and hand, and knife. The calculation of the velocities of theses egments, and knowledge of the mass distribution from biomechanical tables, allowed the calculation of the segment energy and momentum values. The knife measured four components of load: axial force (along the length of the blade), cutting force (parallel to the breadth of the blade), lateral force (across the blade) and torque (twisting action) using foil strain gauges. The 95th Percentile values for axial force and energy were 1885N and 69J respectively.