An EMG and biomechanical investigation of co-activation of antagonistic muscles during high-speed movements of male lower limbs
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.