The influence of ageing and exercise training on the ability in control of force and postural stability in relation to hamstrings: quadriceps strength ratio

The overall aim of the research was to determine whether the hamstring:quadriceps strength ratio (HQR) played an important role in postural control and effects of different training modes on the HQR and its relationship with the ability to control force and posture in older adults. This thesis reports the outcomes of two related studies. Study One was a cross-sectional study aiming to determine the age-related changes in the knee flexor (KF) and knee extensor (KE) isometric muscle strength ratio and the relationship between the HQR and measured variables in the control of posture in young (YG, 18- 30yrs), middle-aged (MG, 40-50yrs) and older adults (OG, 60-77yrs). Study Two was a training study for older adults aiming to determine the effects of two training programs, home-based-exercise (HBE) and electromyostimulation-superimposed-over-voluntarycontractions (EV), on maximal voluntary contraction (MVC), HQR, muscle steadiness (ST) and postural control. Study One indicated that the MVC of the KF and KE decreased at different rates, 34.7% (~1.4 N.m/year) between MG and OG and 35.6% (~1.5 N.m/year) from YG to OG, respectively, and HQR was significantly higher in MG than OG (p\u3c0.05). MG showed a better control of posture than the OG in anterior-posterior and planar directions. Moderate negative correlations were found between HQR and stabilogram parameters (r=-0.29, p=0.05). The intervention study indicated that MVC of the KE increased in both groups (p\u3c0.01), and that of the KF increased in the EV group (p\u3c0.001), however, the HQR did not change in either group, after six weeks of training. The HBE was effective in improving KE isometric ST and the EV improved KF isometric ST which was a novel finding (p\u3c0.05). The stabilogram parameters changed significantly after training in HBE and EV whilst no differences were found between groups. The traininginduced changes in MVC were not correlated with either ST or postural control. It was iv found that the knee joint stability and the control of force and posture were maintained until the sixth decade of life. The EV training program was effective in improving muscle strength and control of force and posture for older adults. However, the mechanisms responsible for the increased MVC did not appear to be shared by those for the improved control of force. These findings suggest that the HQR may be a valid indicator in assessment of postural control. Further research that focuses on improvement of HQR and its relation to postural control is needed to validate this implication of HQR. No clear trend on the fluctuation of force and muscle contraction intensity relationship was observed. Therefore, to improve the ability in controlling muscle force, more attention should be given to the skill learning rather than maximum strength. No difference was found in the pre to post training changes in controlling upright position between the two types of training examined in this project. Future investigations examining the training effects on postural control should involve unstable stance positions and perturbed balance. Future research should also include the recording of EMG data for analysis of the neural mechanisms responsible for the age-related changes and training effects in control of force and posture, and increased duration of the training program