The main aim of this study is to use electromyography to study muscle activation during natural, unrestrained movements. At total of fifty-seven male volunteers participated in the experiments. There were three main studies: vertical jumping, kicking a tethered football and isokinetic dynamometry. These experiments were designed to investigate the hamstrings activity during high-velocity knee extension movements in an attempt to relate the magnitude of hamstrings co-activation and the timing of hamstrings activity to the speed or power of the movements. It is clear that there is substantial co-activation of the three muscles in hamstrings and vastus lateralis in all three studies. Co-activation is present in almost all volunteers even in the slowest speed and lowest power movements studied. Co-activation has been reported by others during single knee extension movements on isokinetic dynamometers. This thesis reports for the first time that the extent of co-activation changes during repeated movements. Co-activation occurs during unrestrained vertical jumps across the whole range from the lowest power jumps in which the volunteer barely leaves the ground to maximum power jumps. In addition, it is commonly observed even in professional football players capable of producing very fast knee extensions during powerful kicks. The duration of co-activation of hamstrings during kicking was significantly shorter in the highly trained 15-year-old footballers that in their 11-year-old counterparts or in untrained adults. It is possible that this reflects changes in the way their kicking movements have developed with prolonged training. This is the first study of age related changes in co-activation. Interestingly, there is anecdotal evidence from the club coaches that injuries are far more frequent in the 15 year olds than in the younger teams. It may be that the increase in speed of movement achieved by reducing co-activation, places the limb at more risk.

The primary purpose of this thesis is to present a comprehensive analysis of occupant kinematics and spinal injuries, during road traffic accidents in Saudi Arabia from the points of view of statistical analysis, modeling of occupant kinematics, and biomechanics. An in-depth database containing information on 512 real world vehicle crashes was constructed. The study identifies the characteristics of the collisions and occupant spinal injuries in Saudi Arabia, and suggests measures to mitigate them. A logistic model has been presented which can be used to provide information about the crashes and spinal injuries. The model may serve as an initial prediction to establish the risk of spinal injury sustained by occupants at road crash, and a paramedic’s protocol, as part of the emergency response, could be revised according to the developed model. State of the art techniques for accident reconstruction have been demonstrated as a tool to investigate the crashes, and the probable cause of crashes, and to make recommendations to prevent crashes and/or mitigate the severity of the accidents and resulting spinal injuries. Computational simulations of crashes provide a tool for understanding the dynamics of crashes and injuries, and are being used worldwide to study dynamics of crashes and efficacy of safety devices. The work conducted here has demonstrated how crashes can be simulated to estimate the injury parameters, and the likelihood of injuries on various parts of the body. While this study presents a detailed multi-dimensional study on road traffic crashes and spinal cord injuries therein, it remains a pilot study for Saudi Arabia. It demonstrates how this type of study can have far reaching consequences and the need to collect such data and carry out this kind of a study on a regular basis at the national level.