Peter L. Davidson

Estimating subject-specific body segment parameters using a 3-dimensional modeller program

The estimation of body segment properties is important in the biomechanical analysis of movement. Current subject-specific estimation methods however can be expensive and time-consuming, while other methods do not adequately take into account individual or group variability. We describe a simple procedure for estimating subject-specific geometric properties, independent of joint centres. The method requires only a small number of anthropometric measurements and digital images of the segment or subject, a 3-dimensional modeller program and simple mathematical calculations to estimate segment volumes and centroids. Assuming that the segment is of uniform density, its mass and moment of inertia can also be derived. Future work should include generating segment density profiles for particular populations, to increase the accuracy of the method, and comparing the accuracy of the results obtained with those produced by other techniques.

Lower limb injuries in New Zealand Defence Force personnel: descriptive epidemiology

Objective: To describe the epidemiology of lower limb injuries in the New Zealand Defence Force (NZDF).

The influence of a back support harness on spinal forces during sheep shearing

Previous research has classified the occupation of sheep shearing as heavy work where shearers flex their spine and hips for long periods of time, handle awkward loads and expend high amounts of energy. The aim of this research was to investigate the magnitude of spinal forces produced during the shearing phase of the work and to determine whether the use of a commercially available back support harness would reduce these forces. Following discussion on task complexity and risk of back injury with senior shearing instructors, three component tasks of the shearing phase were identified as posing high risk of injury and were prioritized for primary analysis. Although the dragging out of a sheep in preparation for shearing and an unexpected loss of animal control were also identified as being of high risk, technological and instrumentation difficulties precluded their analysis. Twelve experienced shearers were videotaped while shearing with and without the use of a back harness. Surface mounted retro-reflective markers placed on the trunk defined three linked segments: Pelvis, Lumbar and Head, Arms, and Upper Trunk (HAUT). A 3D, link segment, top down, inverse dynamics approach was used to describe the motion and to estimate forces involved during the identified tasks of shearing. The spinal force/time profiles of this sample of shearers demonstrated large compressive and shear forces for all three tasks that are close to the NIOSH and University of Waterloo action limits for compressive and shear forces respectively (McGill 1997, Yingling and McGill 1999, Marras 2000). The use of the back support harness reduced these forces by substantial and statistically significant amounts. This effect was consistent across all three tasks. The results of this study demonstrate the production of high levels of compressive and shear forces within the spine of shearers during the three shearing tasks studied and that the use of a back support harness can substantially reduce these forces. Therefore the use of a back harness may reduce the cumulative load on the spine during shearing thereby moderating damage to the articular structures. However it is not known whether the harness would protect the spine from a sudden or unexpected force.

Physical Components of Soft-Tissue Ballistic Wounding and Their Involvement in the Generation of Blood Backspatter*

Abstract:  Gunshot backspatter comprises biological material expelled backward through bullet entry holes. Crime scene investigators analyze backspatter patterns to infer wounding circumstances. An understanding of the mechanism of backspatter generation, and the relationship between spatter patterns and bullet and tissue characteristics, would enhance the predictive value of such analysis. We examined soft‐tissue ballistic wounding responses to determine the underlying components and how these might be relevant to the generation of backspatter. We identified five mechanistic components to ballistic wounding (elastic, viscous, crushing, cutting, and thermal), each related to mechanical disciplines (respectively, solid mechanics, fluid mechanics, fracture mechanics, rheology, and thermodynamics). We identified potential roles for these five components in backspatter formation and provide a scenario whereby a sequence of events incorporating these components could lead to backspatter generation and expulsion. This research provides a framework for the mathematical representation, and subsequent computational predictive modeling, of backspatter generation and pattern formation.

Stochastic-rheological simulation of free-fall arm impact in children: application to playground injuries

The aim of this study was to develop and pilot a stochastic-rheological biomechanical model to investigate the mechanics of impact fractures in the upper limbs of children who fall in everyday situations, such as when playing on playground equipment. The rheological aspect of the model characterises musculo-skeletal tissues in terms of inertial, elastic and viscous parameters. The stochastic aspect of the model allows natural variation of childrens musculo-skeletal mechanical properties to be accounted for in the analysis. The relationship of risk factors, such as fall height, impact surface, child mass and bone density, to the probability of sustaining an injury in playground equipment falls were examined and found to closely match findings in epidemiological, clinical and biomechanical literature. These results suggest that the stochastic-rheological model is a useful tool for the evaluation of arm fracture risk in children. Once fully developed, information from this model will provide the basis for recommendations for modifications to playground equipment and surface standards.

Examination of Interventions to Prevent Common Lower-Limb Injuries in the New Zealand Defense Force

The biomechanical mechanisms of lower-limb injuries in the New Zealand Defense Force were identified from the circumstances of the injuries, and injury prevention strategies that addressed these mechanisms examined for their applicability to a military setting. Many of the injuries were the result of rolling or twisting movements and ankle instability was a common causal factor. Ankle bracing and stability training were identified as the strategies that address this factor and are most likely to be effective in preventing the injuries. A successful intervention strategy must also take into account the particular requirements of the user group. Concerns with ankle bracing included ongoing costs, individual fitting requirements, and the inability to remain effective under extremes of physical activity and external conditions. Stability training was considered more appropriate than ankle bracing for the defense force. Stability training is low cost and has the ability to address the biomechanical mechanisms of several lower-limb injuries. However, it requires trialing in a military setting to assess the logistics of implementation and whether the reported sport-specific programs should be adjusted for the varied physical activities undertaken by military forces.

A laser-aligned method for anthropometry of hands

We have developed a laser-aligned measuring device to facilitate the measurement of linear hand dimensions. Our objective has been to make a simple, clinically applicable device for anthropometry of the hands in patients with arthritis in order to gauge the progression of hand deformities. The system provides an alternative to the use of rules, tapes, photogrammetry and least-squares devices which have previously been applied to hand measurements. The system delivers results which are not significantly different from those measured with rules and callipers and has a similar level of accuracy. The use of laser pointers for alignment facilitates measurements which necessitate picking the highest point of the hand. The device is portable and simple to use in the clinic. We have used it to measure hand dimensions in normal subjects and patients with arthritis. However, the system could equally well be used to facilitate measurement of other objects with an irregular surface profile.

Energy analysis of wrist impact and surface rebound

The mechanical interactions during impact of a falling human body onto a non-rigid surface are complex. Mechanical properties of both the impacting body and the impacted surface contribute to risk of injury. Increased understanding of these properties should provide insight into the process and how to reduce injury risk. We assessed whether modelling energy flows in the body during impact can provide useful information. As input, we used data from gymnastic tumbling mats and from children performing an exercise involving freefall onto an outstretched arm. Even basic energy transfer principles provided information not discerned by the mechanical approach traditionally used. The model identified differences between surfaces in how energy flowed through an arm and the strains imposed on the wrist during impact and rebound. Therefore, it shows promise for identifying potentially injurious human–surface interactions. Analysis of other human impact situations, and the relationship between the energy flow and injury risk, is planned.