It is not just muscle mass: a review of muscle quality, composition and metabolism during ageing as determinants of muscle function and mobility in later life

Abstract

Worldwide estimates predict 2 billion people will be aged over 65 years by 2050. A major current challenge is maintaining mobility and quality of life into old age. Impaired mobility is often a precursor of functional decline, disability and loss of independence. Sarcopenia which represents the age-related decline in muscle mass is a well-established factor associated with mobility limitations in older adults. However, there is now evidence that not only changes in muscle mass but other factors underpinning muscle quality including composition, metabolism, aerobic capacity, insulin resistance, fat infiltration, fibrosis and neural activation may also play a role in the decline in muscle function and impaired mobility associated with ageing. Importantly, changes in muscle quality may precede loss of muscle mass and therefore provide new opportunities for the assessment of muscle quality particularly in middle-aged adults who could benefit from interventions to improve muscle function. This review will discuss the accumulating evidence that in addition to muscle mass, factors underpinning muscle quality influence muscle function and mobility with age. Further development of tools to assess muscle quality in community settings is needed. Preventative diet, exercise or treatment interventions particularly in middle-aged adults at the low end of the spectrum of muscle function may help preserve mobility in later years and improve healthspan.

Development of a bedside viable ultrasound protocol to quantify appendicular lean tissue mass

“…However, these cut‐points require validation to determine the associations with functional capacity, metabolic health, and meaningful clinical outcomes. Alongside this approach, increasing literature is demonstrating that muscle quality (fat and fibrotic infiltration, muscle architecture, and neuromuscular adaptations), more so than muscle quantity, may better identify individuals with poor muscle tissue health 41, 42. Ultrasound has the capacity to investigate muscle quality through the assessment of echogenicity and muscle architecture and may be an important aspect to investigate alongside muscle mass in the future 21.…”

Skeletal muscle mass and quality: evolution of modern measurement concepts in the context of sarcopenia

“…Vascular and neurological integrity are also essential components that ultimately contribute to a skeletal muscle's metabolic and mechanical functionality (21) . The 'quality' of a skeletal muscle is a loosely defined concept that broadly includes aspects of anatomic structure, chemical composition and metabolic and mechanical performance (13) . Our review encompasses aspects of measuring skeletal muscle mass, composition and function in relation to the evaluation of individuals and groups for the presence of sarcopenia and related conditions.…”

“…These effects are readily 31 P MRS provides information on high-energy intermediates and related metabolic processes. 13 C MRS gives information on SM triacylglycerol, glycogen and insulin resistance (84,105) …”

“…By the fourth decade populations (1,9) and individuals (10) begin to show a gradual loss in skeletal muscle mass and the rate of atrophy appears to accelerate beyond the seventh decade (1,9,11) . These later years are associated with sarcopenia and related functional loss of skeletal muscle, often referred to as dynapenia (12) , that together reflect skeletal muscle's loss in mass, altered composition (13,14) and lowered functionality as defined by strength and *Corresponding author: S. B. Heymsfield, email Steven.Heymsfield@pbrc.edu Abbreviations: ALST, appendicular lean soft tissue; BIA, bioimpedance analysis; CT, computed tomography; DCR, deuterated creatine; DXA, dualenergy X-ray absorptiometry; HU, Hounsfield units; IMAT, intermuscular adipose tissue; SEE, standard error of the estimate.…”

Polymorphisms in PTK2 are associated with skeletal muscle specific force: an independent replication study

“…maintained (McGregor et al 2014). In young, untrained adults muscle specific force varies inter-individually by 14-16% (Stebbings et al 2014;Erskine et al 2009), which might suggest some individuals are predisposed to possessing greater muscle specific force and may, therefore, be less susceptible to experiencing functional declines associated with ageing.…”