The impact of pregnancy on postural control variables and their predictive value for pelvic girdle pain

Commonly it is acknowledged that relaxin levels and altered postural control play an important role in the development of pregnancy-related pelvic girdle pain (PPGP). However, studies are conflicting regarding the association of relaxin levels to PPGP and there is little evidence for associations between altered postural control and PPGP. Altered postural control in late pregnancy is primarily associated with alterations to the centre of mass due to anthropometric changes, related to increased body weight and foetus development. However, an increase in relaxin levels may lead to increased joint relaxation response and potentially altered somatosensory system. As relaxin levels are highest in early pregnancy there is some evidence to show that the somatosensory system might be affected and this may disturb postural control. The primary aim of this thesis was to assess postural control during early pregnancy and explore whether these variables were associated with development of PPGP. Initially two systematic reviews were conducted to explore the associations of: (a) PPGP and relaxin levels and (b) PPGP and biomechanical factors. Findings from these systematic reviews demonstrate no association between relaxin levels and PPGP; and a positive association between biomechanical factors (e.g. altered postural control and pelvic mobility) and PPGP. There were two specific aims for this thesis: (1) compare muscle onset, centre of pressure (COP) displacement and velocity as well as pelvic movement pattern of pregnant women in early stages of pregnancy with non-pregnant women and (2) assess whether muscle onset, COP displacement and velocity and pelvic movement patterns represent risk factors for developing PPGP. To investigate these aims first two further laboratory-based cross-sectional studies were conducted to assess the validity and reliability of determining initiation of heel lift during the active straight leg raise through force and centre of pressure (COP) data; and the reliability of detecting postural control parameters (muscles onsets, initiation of heel lift and weight shift) using a combination of automated and visual inspection methods. Results confirmed that the use of the medio-lateral COP displacement waveform is a valid and reliable method for identifying the initiation of heel lift. The between and within-tester reliability for identifying postural control parameters was found to be excellent. To address aim (1) of this thesis, one laboratory-based study was conducted to assess postural control differences between pregnant women in early stages of pregnancy (10th to 15th weeks) and healthy non-pregnant controls. To address aim (2) a prospective cohort study was then conducted using postural control as a risk factor for determining PPGP during pregnancy. The results from the case-control cross-sectional study showed that pregnancy affects muscle onsets of the Multifidus muscles (both sides) and the right Biceps Femoris muscle during the single leg lift task with eyes closed. In addition, increased COP displacements were observed in the pregnant group regardless of the condition (e.g. eyes open and closed). Increased pelvic mobility in the pregnancy group was associated with higher frequencies of reciprocal pelvic opening patterns in this group. After a stepwise approach four variables (right and left Biceps Femoris muscle onsets, BMI and Parity) were included in the final analysis of the prospective cohort study. Results showed that the odds of presenting PPGP during pregnancy increased between 200% to 300% when there was a 50ms delay of right and left Biceps Femoris muscle onset respectively. These findings have implications for programmes designed to prevent or manage PPGP