The development of a novel method of recording centre of gravity location in bipedal stance in healthy adults

Abstract

Standing balance in humans requires that the centre of gravity (COG) remains within boundaries of the base of support (BOS). Clinically force-plate technology (for example, the NeuroCom Balance Master® 6.0, Oregon (BM)) is used increasingly to gain objective data when assessing clients’ balance. In addition to the primary outcome (sway velocity) provided by the BM in four tests of the modified Clinical Test of Sensory Interaction on Balance (mCTSIB) a representation of COG location is also provided .Currently, there is no objective method for clinicians to record COG location. The purpose of this thesis was to develop a novel method of recording COG location in bipedal stance in healthy adults. The aims were: 1, explore literature associated with COG behaviour and location in healthy adults when sensory inputs and BOS are altered; 2, develop a method for categorising COG location based on the BM results diagram and assess tester reliability for this method; 3, test the new categorical method of recording COG location for (i) between-session repeatability and, (ii) comparability between different force plate systems; 4, test whether the data obtained when applying the method of categorising COG location is sensitive to changes under the different mCTSIB conditions and age; 5, test whether this categorisation or COG location is sensitive to varying BOS test conditions of feet apart to feet together as well as single limb stance, and 6, compare the COG start location to COG mean location obtained simultaneously for the mCTSIB conditions and varying BOS conditions.A literature review incorporating the impact of ageing on sway behaviours and COG location, particularly when sensory inputs or BOS are altered, formed the basis for Study 1 which addressed aim 1. Studies 2 and 3 were a secondary analysis of data from an earlier multidisciplinary study (Longitudinal Assessment of Women (LAW), 2000) of 481 independently-mobile, communitydwelling women aged 40-80 years, assessed on the BM. A new method was developed to categorize COG location data into sectors in Study 2. This categorization method was tested for inter- and intra-rater reliability by two independent raters thus addressing aim 2. For aim 3 two pilot studies assessed the repeatability of the measure between two test sessions and the comparability of results between two force plate systems. These additional aspects of COG location have not previously been tested. In Study 3 data from the LAW study were analysed to assess whether differences in COG start location could be demonstrated under altered sensory conditions using the new method of categorization and whether these were similar to other studies reporting on the location of COG thereby addressing aim 4. Studies 4 and 5 analysed COG location in 81 healthy adults (men, N=31) aged 30-80 years collected using Kistler force-plates. A customised software programme replicated the BM set-up allowing for collection of additional information such as COG start and mean locations and additional BOS configurations. Study 5 analysed the COG start location for each mCTSIB condition and different BOS configurations (aim 5). Study 5 was designed to compare the COG start and mean locations for each test using the categorization method (aim 6). Study 2 demonstrated high inter- (κ 0.84, CI 0.82-0.86)) and intra-rater (Rater 1, κ 0.78, CI 0.74-0.79; Rater 2, κ 0.88, CI 0.86-0.90) reliability between two independent raters using the categorization method for COG start location. In Study 3 significant differences were shown for COG location in the antero-posterior plane for women when the surface was changed from a firm to a compliant surface with (p<0.001) and without (p<0.001) vision conditions. As difficulty increased from a firm surface with, then without vision, to a compliant surface with vision, a greater proportion of subjects had an anterior COG location relative to each subject’s predicted centre of balance. When both surface and vision were compromised, the proportion of subjects whose COG remained anterior decreased compared with an altered surface alone. No significant difference was detected as age increased for the firm surface, eyes open test.Study 4 (a mixed group of men and women) showed that as BOS size decreases and when vision is reduced (in feet apart and feet together tests), a greater proportion of subjects have an anterior COG location (p≤0.01). In Study 5 we found there was substantial to near perfect observed agreement between the start and mean locations (κ 0.66 – 0.87) for all tests except for the right single limb stance test (κ 0.41, CI 0.10-0.83).The findings from these studies provide support for the use of the proposed method to objectively record COG location in a range of test conditions. This removes the subjectivity from identifying COG location to assist in the interpretation of assessment findings and may then assist in the development of more targeted treatment programmes for clients with balance difficulties.