Morphology of trunk muscles and their potential role in active living and quality of life of older adults

\\({Background}\\) The muscles of the trunk are essential for an individual's ability to perform normal functional activities such as standing or walking and are involved in the control of balance and posture. Decreased trunk proprioception, muscle imbalance and functional decline of these muscles all have the potential to alter posture and balance, leading to an increase in the risk of falls and deterioration in physical function and quality of life. The trunk muscles investigated in this study are the rectus abdominis, transversus abdominis, internal oblique, external oblique and the lumbar multifidus (MF) muscles. The external and internal abdominal oblique muscles are torque producers of the trunk, while the transversus abdominis and MF muscles are tonically active during weight-bearing activities. While peripheral skeletal muscle mass and strength deteriorate with age (sarcopenia), the effect of ageing or pathology on muscles of the trunk, and the potential effects these changes have on physical function and quality of life are not fully understood. \\({Aims}\\) This thesis aimed to: 1. Summarise the evidence in older adults for: i. changes in function, composition and morphology of the abdominal and MF muscles and the effects of these changes on physical function. ii. validity and reliability of electromyographic and imaging measurements of abdominal and multifidus muscles among adults aged 50 years and older. 2. Determine the test-retest reliability of ultrasound imaging (USI) for the assessment of abdominal and MF muscle thickness and CSA at the L2 - L5 vertebral levels in older adults. These results will determine the appropriateness of using these measures in cross-sectional and longitudinal studies (aims 3 and 4), particularly as outcomes for randomised controlled trials (RCT) 3. Determine the effect of 12 months of vitamin D supplementation, on morphology and function of the abdominal and MF muscles of adults aged 50 to 79 years with low serum 25(OH)D levels. 4. Determine the associations between abdominal and MF muscle size and function and measures of physical activity, physical function and quality of life among older adults. \\({Methods}\\) The first part of this thesis provides a comprehensive and systematic assessment of the current literature investigating abdominal and multifidus muscle in older adults (Study 1). This study helped to inform the studies in the second part of this thesis (Studies 2 ‚Äö- 4) that aimed to fill some of the gaps in the literature identified in the systematic review. Studies 2 - 4 used data from an ultrasound imaging sub-study (n=217) of the Vitamin D Effect on Osteoarthritis (VIDEO) clinical trial, conducted between June 2010 and December 2013. The participants were community-dwelling adults aged 50-79 years with ongoing symptoms of knee osteoarthritis and serum 25(OH)D levels between 12.5 and 60 nmol/L. \\({Key}\\) \\({findings}\\) The key findings and evidence gaps identified by the systematic review (Study 1) were: ‚Äö Research on abdominal and MF muscles in older adults was limited ‚ÄöThere was limited evidence that imaging modalities were reliable tools for the assessment of abdominal and MF muscles of older adults ‚Äö There was no evidence for the reliability of test-retest measures of abdominal or MF muscles using ultrasound imaging in older adults ‚Äö There was limited evidence for an age-related decrease in abdominal and MF muscle size and an increase in intramuscular fat infiltrations that had the potential to affect physical function of older adults ‚Äö There was limited, but consistent evidence of detrimental effects on abdominal and MF muscles by conditions that affect physical function such as various spinal conditions and low back pain (LBP) ‚Äö There was limited or no evidence on the effect of other conditions such as stroke or vitamin D deficiency on abdominal or MF muscles ‚Äö There were no studies in healthy, older adults investigating associations between trunk muscle measures and any aspect of physical activity or physical function nor identifying modifiable factors that could mitigate age-related changes in abdominal or MF muscles Study 2 determined the test-retest reliability for measurements of abdominal and MF muscle thickness and CSA. The estimates of reliability were as follows: ‚Äö Substantial (ICC 0.87-0.98) for all measurements of abdominal muscles, except for the right internal oblique muscle in the contracted state which was moderate (ICC 0.75) ‚Äö Substantial (ICC 0.84-0.91) for all measurements of MF muscle CSA ‚Äö Fair to moderate (ICC 0.55-0.74) for all measurements of MF muscle thickness Establishing the test-retest reliability of USI for assessing abdominal and lumbar multifidus muscles confirmed the appropriateness of using these measures in longitudinal studies and particularly as outcomes for randomised controlled trials (RCT) We used this technique to assess muscle outcomes in the VIDEO RCT. Twelve months of vitamin D supplementation in vitamin D deficient older adults with knee osteoarthritis alone was not an effective means to improve or maintain abdominal or MF muscle size of active, community dwelling adults aged 50-79 years (Study 3). Given the lack of effect of vitamin D on muscle size, we proceeded to investigate another important gap in the literature described in section 1.4 of the introduction of this thesis and also identified the systematic review, namely a paucity of studies investigating associations between trunk muscle measures and any aspect of physical function in healthy older adults. This cross-sectional analysis revealed no cross-sectional correlations between abdominal or MF muscle size or function (assessed by changes in muscle thickness on contraction) and measures of physical activity (International Physical Activity Questionnaire and pedometer), physical function ( functional deficit subscale of the Western Ontario and McMaster Universities Arthritis Index and quality of life (Assessment of Quality of Life instrument) in healthy, active, community-dwelling adults with knee osteoarthritis (Study 4). However, such effects cannot be ruled out due to the limitations in the measures used to assess muscle function, physical activity and physical function available in VIDEO. \\({Conclusion}\\) Age and various spinal conditions have detrimental effects on abdominal and MF muscle size, strength, activation and muscle quality, reflected in an increase in intramuscular fat infiltrations. However, how these detrimental factors affect physical function and healthy ageing are not clearly understood. The findings in this thesis have contributed to fill some of the identified gaps in the literature. Particularly important are the finding of the RCT on the effect of vitamin D supplementation on abdominal and MF muscles suggesting that vitamin D supplementation alone is not an effective therapy to improve or reserve the size or ability to contract of postural muscle of the trunk. A question that was not investigated in this study and remains unanswered is the effect of vitamin D supplementation on other aspects of trunk muscle function such as strength, power or physical function. Study 4 contributed to a limited body of evidence on the associations between abdominal and MF muscle size and measures of physical function. Although our negative results are consistent with findings in the systematic review published in 2017 and some of the studies in the systematic review update in this thesis, the evidence is not robust enough to rule out associations between abdominal and MF muscle size and measures of physical function in older adults. Given that the evidence base remains limited, further research is definitely required, to further address important persisting gaps in the literature. These include: ‚Äö what other factors affect trunk muscles of older adults? ‚Äö what are the long-term effects of declines in muscle size on physical activity, physical function and quality of life of older adults? ‚Äö what strategies can be implemented to mitigate the effect of age-related changes on these muscles? Finally, the literature in the systematic review update suggest that abdominal and MF muscle strength and muscle quality are important determinants of physical function in older adults. Therefore, these two areas of research would also appear to be promising for future research investigating the effect of age-related changes in abdominal and MF muscles on physical function and quality of life of older adults.