Can a machine be used to replicate the biomechanics of human stabbing performance?

A human performance trial was conducted where the stabbing actions of 40 participants were investigated. A range of factors were assessed such as the action used when the blade shape and familiarity of the knife changed. A qualitative evaluation of these factors was presented in an earlier paper to precede the analysis of quantitative kinematic data in this paper. The data from participants was then compared to that obtained from a stabbing machine. A motion analysis system was used to measure parameters such as the joint angular displacement and absolute linear velocity at impact. Motion data was collected using a 12-camera Vicon MX motion analysis system. Results indicated that the mean absolute linear velocity of overarm stabbing actions was consistently higher than the underarm actions. Forensic textile damage examinations often involve testing of edged weapons by manually performing stabbing actions. An underlying fundamental limitation of manual simulation experiments is the natural variation inherent in human performance. Sufficient control measures can be implemented to successfully use manual simulation experiments in forensic testing and therefore this is the accepted practice. However, performance can be improved using a stabbing machine. A custom-built instrumental stabbing device has been shown to operate within the same parameters of the human participants and is therefore suitable for use in casework.

An evaluation of human stabbing performance to inform the standardisation of textile damage examinations: Do simulation trials correlate to reported stabbings?

Forensic textile damage examinations are commonly requested in cases such as stabbings. These requests often involve the testing of knives or other weapons submitted to determine if they could have caused the damage to the evidential garment. Currently a forensic practitioner conducts this testing by manually performing the stabbing action. A biomechanics performance trial was conducted to evaluate how a range of human factors contribute to the creation of textile damage by stabbing actions. Surveys of sharp force fatalities and clinical penetrative injuries reported the chest and abdomen as the most frequent target location for stab wounds. The location of the cut-type damage recorded during the trial was found to correlate to the location of stab injuries incurred during actual stabbing cases. The type of weapon had an impact on the actions undertaken. Participants mostly utilised the smaller utility and hunting knives in underarm thrusting or overarm hacking actions, whereas an overarm hacking action, or combined hacking/slashing action was performed when using the machete. The familiarity of the knife, shape of the handle and perceived risk of injury determined how the handle was held. Participants frequently stabbed into the target immediately in front of their dominant hand, however care should be taken in interpreting this in a casework scenario. The machete was used with the highest mean velocity, and the utility knife the lowest.

Australian forensic textile damage examinations - Finding a way forward since PCAST.

Textile damage examinations are requested in a range of crime types such as assault, sexual assault and homicide. They typically involve the examination of clothing for damage such as cut, tear or thermal damage, often then followed by experimental scenario testing to help ascertain the cause of the damage. Understanding the underpinning science is central to the accurate interpretation of the complex mechanism of damage formation. In a stabbing incident for example, an understanding of the dynamic relationship between the knife blade, fabric and skin (or skin simulant) is critical. Recent reports, including the President's Council of Advisors on Science and Technology (PCAST) report, have scrutinised forensic feature-based comparison techniques. Whilst textile damage was not a focus area, it can be considered a feature-based evidence class, and one which is currently largely reliant upon a practitioner's opinion, experience and professional judgement. This paper will review the current state of textile damage examinations in Australia and survey research being conducted to address the issues raised in the context of the PCAST report. The central contribution of observational data to the evidence class of textile damage will also be explored, as well as some practical measures to counter the effects of cognitive bias.