Age-related change in muscle mass, strength and function in healthy adult Irish women

Sarcopenia refers to the age-related decline in muscle mass. The criteria for the identification of sarcopenia lag some way behind the established criteria for the diagnosis of bone health (osteoporosis). European and American working groups have recommended diagnosis of sarcopenia to include components of muscle mass, strength and function. Diagnostic criteria which can track degradations in muscle function across the lifespan are urgently needed as population demographics demonstrate the number of older adults will triple to approximately 1.5 billion by 2050. The aim of this research was to measure age-related change in lean tissue mass (LTM), muscle function and performance of tasks related to activities of daily living (ADL) in a sub-sample of healthy older (50-70y) women. Subsequently, the aim of this research was to evaluate these measures ability to track change due to progressive resistance training (PRT) in a sub-sample of women supplemented to ensure adequate dietary protein intake. In Chapter 3, median whole body lean tissue mass (LTM) was observed to be lower in older (50-70y) women (-1.6kg, p<.001) compared to their younger counterparts (18-29y). The greatest absolute (-0.7kg, p<.001) and percentage (15.9%, p<.001) decline came from the upper leg LTM (ULTM). In order to quantify muscle quality (MQ) a measure of the force generating capacity of the upper leg was required. Muscle quality (MQ) can be expressed as strength per unit LTM. In Chapter 4, maximal voluntary isometric strength of the knee extensors (KE) and knee flexors (KF) provided a valid and reliable measure of the force generated by the upper leg LTM. A significant learning effect (5.1%, p<.001) in the measurement of KE and KF-PT, was observed between testing sessions held 7 days apart (Chapter 4). The combined measure of KE and KF-PT declined at a rate of 14.4% between the 5th and 6th decade (p<.001). The majority of this decline came from KE-PT (15.1N·m (p<.001) vs. 2.4N·m (p=.234)). Furthermore, the KE were found to provide a more reliable measure of maximal voluntary isometric leg strength. KE-PT declined at 3 times the rate of upper leg LTM (18.3% (p<.001) vs. 6.1% (p=.102) per decade). The decline in MQ was greatest when expressed as KE-PT per kg ULTM (9.3%, p=.029). KE rate of torque development (RTD) declined at almost double the rate of KE-PT (33.1% vs. 18.3% per decade, p<.001). Chapter 5 sought to investigate the age-related decline in the ability to perform tasks related to ADL and to determine associations between laboratory measures of muscle function and performance of ADL. Those in the 6th decade required an extra 53 seconds (p=.001) to complete a kilometre and completed 2 less chair rises in 30s (p=.043) than those in the 5th decade. MQ (KE-PT per kg upper leg LTM) had the strongest association with 1km and chair rise performance (r=.409 and .382 respectively, both p≤.001). Finally, Chapter 6 aimed to evaluate the response of muscle function to PRT in women supplemented to ensure adequate dietary protein intake. Upper leg LTM and KE-PT improved due to PRT in a ratio (1:3) that mirrored age-related decline between the 5th and 6th decade. A 3.6% (p=.011) increase in upper leg LTM was accompanied by a 12.7% (p=.008) increase in KE-PT. Baseline dietary protein intake did not mediate changes in upper leg LTM (p=.596) due to PRT nor did compliance to the supplement (p=.286). MQ demonstrated a 9.0% increase in those engaged in PRT. Those engaged in PRT had a 26 second (6%) improvement in 1km time (p=.001) but the number of chair rises remained unaltered (p=.367). Relative changes in KE-PT (r=-.515, p<.001) and MQ (r=-.512, p<.001) had the strongest association with relative change in 1km performance. The greater age-related and therapeutic rate of change in KE-PT suggests it may be of greater clinical significance than upper leg LTM in the identification and treatment of sarcopenia. Furthermore, only indices of muscle function which included KE-PT were associated with the performance of ADL.