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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 (provisional): 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.

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Alex’s Notes: As a shout-out to all my long-lived Super Humans out there, let’s review why Carl used to (and still should, in my opinion) say that muscle is metabolic currency. Impaired mobility is more often than not a precursor for functional decline as we age. This leads to frailty and disability, which in turn leads to greater healthcare costs and burdens to caregivers. In other words, not cool.

Thankfully, recent studies have supported the intuitive notion that lifelong physical activity and exercise can preserve muscle structure and function in older men (average age 70 years). In fact, skeletal muscle structure, function, bioenergetic characteristics, and phenotype of these “senior sportsmen” are actually comparable to that of active men four decades younger. But what happens when you are sedentary? Sarcopenia is a good starting place, which basically means the age-related loss of muscle. Estimates of muscle loss per year vary from 0.4% to 2.6%, while strength loss is estimated at 1-3% per year, and muscle power loss even greater still.

In the Health, Ageing, and Body Composition (Health ABC) study, the age-related decline in muscle strength was reported to be two- to five fold greater than the loss of muscle mass in older adults aged 70–79 years over a 3-year follow-up. Moreover, 25% of males and 31% of females actually gained lean mass, and yet this failed to prevent increased weakness and loss of strength. The point is that there is a disconnect between changes in muscle mass and muscle strength, which suggests that there are other factors related to muscle quality that contribute to age-related declines of muscle function and mobility.

For instance, some observations of muscle fibers indicate that there is a shift in fiber composition with age. Specifically, there is a progressive loss of fast-twitch fibers which are responsible for rapid and high force production. Of course, not all studies agree. Regardless, the loss of either type of muscle fiber with age would likely be attributable to reduced activity levels that lead to disuse and denervation. Use it or lose it, right?

Muscle architecture (the arrangement of fibers within the muscle) may also change with age, as the gastrocnemius fascicle length and pennation angle were reportedly smaller in 70-81 year olds when compared to 27-42 year olds. Fortunately, studies have also shown that 4-5 weeks of resistance training is able to improve muscle architecture in young adults. Since both young and old adults respond in a similar manner to resistance training, these results may be generalizable to the older population.

I suppose all this is moot if you can’t contract the muscle. Motor units (innervated bundles of skeletal muscle fibers) undergo constant remodeling, denervation, and re-innervation throughout life. These suckers are reduced in men over 65 years of age and further still in men over 80 years compared to 25 year olds. However, this reduction of motor units was only related to strength loss in the 80 year olds. Nonetheless, neuromuscular improvements commonly precede increases in muscle mass and greatly influence force development and muscle power. Perhaps their loss also precedes muscle mass loss. Through a reduction in power output (especially in the elderly), the muscle no longer must work as hard and begins to atrophy.

So what does everything up to now mean? That resistance training is crucial for healthy ageing. But for everyone who avoids cardio, listen up. Aerobic capacity is a strong predictor of mobility in older adults, and cross-sectional evidence from healthy men and women aged 18-90 years indicates that mitochondrial function decreases with age. Aerobic capacity is not simply cardiovascular adaptations, but also muscular adaptations that allow for greater oxygen use. As such, it is no surprise that persons who are aerobically active throughout life have greater muscle strength than their sedentary peers. Does this mean you should forgo resistance training? Hell no, but you should incorporate some of both into your routines.

And that is basically the bottom line. Muscle is metabolic currency, and anyone who gives a shit about living a quality life into old age will train hard, eat hard, and rest harder still. You may grow old, but age is just a number. It is up to each of us to preserve our youth.

 
 

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