Abdominal and lumbar muscle size, symmetry and control in an elderly population

The trunk muscles have a vital role in human bipedal gait and all functional activities in the erect posture. They provide the link between upper and lower limbs and contribute to the maintenance of spinal and pelvic stability. However, little is known about the size and activity of trunk muscles in the older adult. The overall aim of this thesis is to explore factors associated with muscle size and activation of rectus abdominis (RA), external oblique (EO), internal oblique (IO), transversus abdominis (TrA) and lumbar multifidus (MF) from L2 to L5 vertebral level, in a cross-sectional community-based sample of independently mobile older adults. The specific aims addressed in each chapter are to: 1. Determine the reliability (consistency and repeatability) of measuring ultrasound images of abdominal and lumbar MF muscle thickness, cross-sectional area and activation in our population of community-dwelling adults. 2. To identify the normative values for our data, and to assess associations of age, sex, height and weight, on the outcome measures of muscle size and activation. 3. To examine the associations of a history of low back pain (current, past and never) with the size and activation of trunk muscles in the older adult. 4. To determine the associations between muscle size and sensorimotor gait parameters and falls risk. The four studies in this thesis used cross-sectional data from participants of an ultrasound sub-study of a larger longitudinal study ‚Äö- The Tasmanian Study of Cognition and Gait (TASCOG). Participants (n=241) were aged between 60 and 86 years, and all were community-based and independently mobile. Ultrasound imaging of the abdominal and lumbar MF muscles was performed by experienced staff using previously published protocols. Demographics, relevant medical history, low back pain history and a prospective falls diary were collected by self-reported standardised questionnaires. Gait parameters were collected using the computerised GAITRite¬¨vÜ portable walkway. A range of analytical methods were used as required, including intraclass correlations, linear and multiple regressions, log binomial transformations and partial rank correlations for non-parametric measures. The first study established that ultrasound imaging can reliably measure trunk muscle size and activation in our population of older adults. Intra- and inter-rater reliability for measurements of abdominal (ICC ‚Äöv¢‚Ä¢0.97) and MF muscle thickness (ICC ‚Äöv¢‚Ä¢0.86), and MF CSA (ICC ‚Äöv¢‚Ä¢0.92) were high. These reliability indices are similar to those for measurements in younger populations and indicated that US can be reliably used to study trunk muscles in this population. The second study described the normative data for our population of older adults. Our results that age is negatively correlated with muscle size, and that men generally have larger muscles than women concur with previous studies. However, an interesting outcome was that women were able to generate a greater change in TrA thickness with contraction than men, (58% change in women versus 41% in men, p<0.01) whereas, men were able to contract the MF muscles significantly more than women. Weight was positively associated with RA, EO and TrA in women and with MF thickness in men. Finally, women had a different relative order of muscle size to older men and younger adults. Thirty percent of women had IO as the largest abdominal (IO>RA>EO>TrA) with the previously reported pattern of RA>IO>EO>TrA pattern seen in 22% of women and 31% of men. These important sex-related differences in muscle size and activation, and in the relative order of muscle size could influence future interpretation of US imaging in older people, and the potential age-related decline in superficial trunk muscles suggests that maintaining these muscles could help maintain independence and quality of life. The third study examined the effect of a history (current, past and never) of low back pain (LBP) on muscle size, symmetry and activation. The MF muscles did not show any asymmetry or decrease in size associated with low back pain status, which is contrary to findings in younger adults. The EO muscle was the only muscle to have a significant between-group difference in thickness across LBP groups, with greater muscle thickness observed in those with current LBP compared with those who have never had LBP current LBP (0.54cm vs 0.42cm respectively, p<0.001). However, participants with current LBP were less able to activate the EO muscle compared with those without a history of LBP (7.5% vs 17.4%; p<0.01). These results potentially support the need for further investigation into targeted exercise and motor retraining programmes in the older adult. Having determined the effects of age and low back pain on trunk muscle size and activation, study four investigated the effects of muscle size on gait parameters and falls risk. Larger RA, IO and L3 MF muscles were significantly correlated with faster gait speed (p<0.05) and longer step length (p<0.01), but not correlated with any other gait parameters. EO, TrA and L4 and L5 MF were not correlated with any of the gait parameters. There were no significant correlations between the ability to contract the abdominal muscles and any gait parameters. However, the ability to contract L2, L4 and L5 MF muscles was correlated with faster speed (p<0.05) and longer step length (p<0.05). The total number of falls over 12 months was not significantly correlated with muscle size. These results suggest that the larger muscles (RA and IO) may have more influence on gait parameters than the deeper intrinsic muscles, and that muscle size may have an indirect influence on the number of falls through their effect on the gait parameters of speed and step length. In conclusion, this study ascertained that older adults have unique differences in trunk muscle size and function when compared to younger adults. There were notable sex and age-related differences in trunk muscle size and activation, and LBP did not have the same impact on MF size or asymmetry as reported in younger adult literature. These morphological changes associated with the older adult suggest that further research is needed to determine the effectiveness and appropriateness of current motor control programmes and clinical treatment aims for the older adult. Being a cross-sectional study, no inferences can be made, but it does suggest that since older adults have distinct changes in muscle size and activation ability, and muscle responses to LBP, normative data and treatment suggestions for younger adults cannot be extrapolated to the older adult. This suggests that further longitudinal research is needed to investigate the effects of ageing on trunk muscle size and associations with function, gait parameters and falls risk.