As Daniel Patrick Moynihan, an American sociologist, politician, and diplomat once said: “Everyone is entitled to his own opinion, but not his own facts”. And we wholeheartedly agree. A shared set of facts is the first step to building a better world with longevity for all. In that spirit, we are creating a series that covers 101 indisputable facts about ageing, health and longevity.
We discussed in fact #43 how low muscle mass and, especially, low muscle strength, correlate with mortality. However, we know that it becomes harder to maintain muscle tissue with increasing age. Muscle mass can be accurately measured with CT or MRI scans, but these are not ideal for studying large samples of healthy people, and so most studies use dual X-ray absorptiometry (DEXA). DEXA gives an approximation of a person’s total muscle mass by measuring the fat-free mass of the appendages (appendicular lean mass (ALM)). The mass of the torso is omitted because the amount of fat there confuses the measurement. Here’s what appendicular lean mass (normalised to height) looks like during ageing:
These graphs show percentiles, meaning they show the proportion of the population that is equal to or below a given appendicular mass at a given age. For example, if you look at the line marked ‘50%’, you can see that at age 70, 50% of males have an ALM of 8 or less. For females, ALM remains relatively stable due to having less muscle mass to begin with, while in males, ALM declines starting from midlife. The graph below shows a measure that is perhaps more telling and meaningful in terms of health: average muscle strength, in this case of the dorsiflexor muscles, in males (black bars) and females (white bars).
Looking at the males in the first graph and recalling previous facts such as those on cardiorespiratory fitness and telomere length, you might notice a familiar pattern: while there is a downward trend, there is also considerable variation within age groups. The top 10% of males at age 80 still have more ALM than a considerable portion of of males at age 30, and some (though fewer) will even be stronger. So even though the age-related decline in muscle mass and strength cannot be prevented, maintaining a higher muscular fitness throughout life is still highly beneficial.
The reasons for this decline in strength and mass are various. The decline of growth-inducing hormones like insulin-like growth factor 1 (IGF-1) and testosterone lead to reduced production of important muscle proteins. The pool of muscle stem cells, with the ability to develop into new functional muscle cells, declines, as does the number of muscle fibres – the contractile units of the muscle tissue. Electrical changes may also be very important as the number and quality of connections between motor neurons and the muscle is reduced with age. This reduces the muscle’s responsiveness to signals from the brain and also reduces the delivery of signals necessary for muscle maintenance.
Reference values of body composition parameters and visceral adipose tissue (VAT) by DXA in adults aged 18–81 years—results from the LEAD cohort: https://doi.org/10.1038/s41430-020-0596-5
The influence of aging and sex on skeletal muscle mass and strength: https://oce.ovid.com/article/00075197-200111000-00007/HTML