A.C. Nicol

Elbow and wrist joint contact forces during occupational pick and place activities

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 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 bodys 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-ulnar 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.

Biomechanics of knife stab attacks

Equipment, materials and methods for the measurement of the biomechanical parameters governing knife stab attacks have been developed and data have been presented that are relevant to the improvement of standards for the testing of stab-resistant materials. A six-camera Vicon motion analysis system was used to measure velocity, and derive energy and momentum during the approach phase of the attack and a specially developed force-measuring knife was used to measure three-dimensional forces and torque during the impact phase. The body segments associated with the knife were modelled as a series of rigid segments: trunk, upper arm, forearm and hand. The velocities of these segments, together with knowledge of the mass distribution from biomechanical tables, allowed the calculation of the individual segment energy and momentum values. The instrumented 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. 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. These styles were chosen based on reported incidents, providing more realistic data than had previously existed. The 95th percentile values for axial force and energy were 1885 N and 69 J, respectively. The ability of current test methods to reproduce the mechanical parameters measured in human stab attacks has been assessed. It was found that current test methods could reproduce the range of energy and force values measured in the human stab attacks, although the simulation was not accurate in some respects. Non-axial force and torque values were also found to be significant in the human tests, but these are not reproduced in the standard mechanical tests.

Method of determination of three dimensional index finger moment arms and tendon lines of action using high resolution MRI scans

High-resolution MRI scans, in conjunction with CAD software, were used to determine the three-dimensional moment arms and force vector direction cosines for 11 structures passing the interphalangeal and metacarpophalangeal joints of the index finger. The results are presented for five different angles of joint flexion for a single subject. The moment arm data obtained differ from previous studies, where results have been derived from tendon excursion techniques or geometrical models. These dissimilarities have been accounted for by the differences in experimental techniques.

Late complications after open carpal tunnel decompression

66 patients (89 hands) were assessed at least 10 months after open carpal tunnel decompression. Tender scars were found in 19% of the hands and 4% were affected by pillar pain. Grip strength was reduced in more than half of the operated hands. Hypo- and anaesthesia in the scar area were affecting 7% but were not considered disabling. In 18% of the cases there was incomplete relief of primary symptoms.

A novel force transducer for the measurement of grip force

A new strain gauge transducer has been developed to measure functional grip forces. The gripping area is a cylinder of diameter 30 mm and length 150 mm and simulates the handle of a number of devices, allowing a range of activities to be studied. The device measures radial forces divided into six components and forces of up to 250 N per segment can be measured with an accuracy of +/- 1%. The device therefore gives information about the magnitude and distribution of force around the cylinder during gripping, and has been shown to be a valuable research tool in a study of four different types of grip, providing valuable input data for biomechanical models.

The biomechanical functional demand placed on knee and hip muscles of older adults during stair ascent and descent

Age-related decline in physical capabilities may lead to older adults experiencing difficulty in performing everyday activities due to high demands placed on the muscles of their lower extremity. This study aimed to determine the biomechanical functional demand in terms of joint moments and maximal muscle capabilities at the knee and hip joints while older adults performed stair ascent (SA) and stair descent (SD). Eighty-four healthy older adults aged 60-88 years were tested. A torque dynamometer attached to a purpose-built plinth was utilized to measure muscle moments at the knee and hip joints. Participants also underwent full body 3-D biomechanical assessment of stair ascent and descent using an 8-camera VICON system (120Hz) with 3 Kistler force plates. Stair negotiation required knee extensor moments in excess of the maximum isometric muscle strength available (SA 103%, SD 120%). For the hip, the levels of demand were high, but were slightly lower than those of the knee joint. Stair negotiation placed a high level of demand on the knee extensors with demand in SA reaching maximal isometric capacity and demand in SD exceeding maximal isometric capacity. The levels of demand leave little reserve capacity for the older adult to draw on in unexpected situations or circumstances.

Interphalangeal joint and tendon forces: normal model and biomechanical consequences of surgical reconstruction

Soft tissue reconstructive surgery for rheumatoid-related proximal interphalangeal joint deformities frequently fails to produce the long-term predicted results. Detailed information on the biomechanics of this joint, under both normal and pathological conditions, is required to assess the efficacy of such surgical intervention. A biomechanical model of the proximal interphalangeal joint has been developed to investigate tendon and joint loading during real life three-dimensional activities. Based on a rigid body mechanics approach, the model uses high resolution MRI scans to obtain anatomical tendon and bone geometries in conjunction with three-dimensional kinematic and loading data. The model incorporates an optimisation routine which minimises overall maximum tendon stress in the eight individual elements considered. Radial and ulnar joint force components are included at the proximal interphalangeal joint level. Two simulated pathological versions of the mathematical model are developed to accommodate the altered anatomic relationships after tendon reconstructive surgery. Joint forces of up to 450N and common usage of the extensor mechanism during normal pinching and grasping activities are predicted. The ulnar lateral bands of the extensor tendon are generally loaded to a greater extent than the radial bands. Extensor tendon and joint forces in the simulated pathological models are significantly higher than those in the normal model. Combined with the poor tendon quality of rheumatoid arthritis patients generally, these amplified internal forces may lead to further joint deformation.

Flexible goniometer computer system for the assessment of hip function

A new computer-based system has been developed to assess function in a clinical environment. Flexible electrogoniometers were used to record the motion of both hips and both knees in total hip replacement patients. Foot switches and instrumented walking aids were also incorporated, together with a fixed length timing facility for the derivation of walking velocity, stride length and cadence. The data can be presented on the computer screen immediately following the walking test in the form of angle/time or angle/angle graphs. This facility enables clinicians to study asymmetry and other abnormalities during the clinic session. A summary version of the data can thereafter be generated and included in the patients notes. A total of 65 patients have been assessed prior to hip replacement and at 2, 6 and 12 months post-operatively. Following surgery, the patients exhibited significant improvments in range of motion, gait velocity and cadence but at 12 months post-operatively their functional performance was restricted compared to data collected for the normal subjects.

A three-dimensional finite element model of maximal grip loading in the human wrist

Abstract The aim of this work was to create an anatomically accurate three-dimensional finite element model of the wrist, applying subject-specific loading and quantifying the internal load transfer through the joint during maximal grip. For three subjects, representing the anatomical variation at the wrist, loading on each digit was measured during a maximal grip strength test with simultaneous motion capture. The internal metacarpophalangeal joint load was calculated using a biomechanical model. High-resolution magnetic resonance scans were acquired to quantify bone geometry. Finite element analysis was performed, with ligaments and tendons added, to calculate the internal load distribution. It was found that for the maximal grip the thumb carried the highest load, an average of 72.2 ± 20.1 N in the neutral position. Results from the finite element model suggested that the highest regions of stress were located at the radial aspect of the carpus. Most of the load was transmitted through the radius, 87.5 per cent, as opposed to 12.5 per cent through the ulna with the wrist in a neutral position. A fully three-dimensional finite element analysis of the wrist using subject-specific anatomy and loading conditions was performed. The study emphasizes the importance of modelling a large ensemble of subjects in order to capture the spectrum of the load transfer through the wrist due to anatomical variation.

A force transducer to measure individual finger loads during activities of daily living.

A new six-degree-of-freedom force transducer has been manufactured, with the sensitivity to measure forces in the range +/-100 N and moments of up to +/-5 Nm. The transducer incorporates two mechanical components: shear forces and bending moments are measured via a strain-gauged tubular section whilst axial forces are transmitted to a cantilevered load cell. Both components are instrumented with 350 ohms strain gauge full bridge circuits and are temperature compensated. After calibration, measurement errors are less than +/-0.3 N for direct forces and +/-0.03 Nm for applied moments. In order to measure sub-maximal finger loads during activities of daily living, the transducer has been incorporated into several housings representing objects in domestic use: a jar, a tap, a key in a lock and a jug kettle.