Here is a finding that surprises most people, including plenty of regular gym-goers. When researchers tracked nearly 1,900 older adults (aged 70-79) over three years, those who held on to their muscle mass, and even those who gained some, still lost strength. Leg strength declined about three times faster than muscle mass.

That gap matters because it tells you something the mirror does not. Legs can look much the same while the force they produce quietly drains away. It also explains why so much advice about aging misses the mark. The question is not whether you are losing anything, because you are. The question is which losses you can do something about.

Some things are genuinely non-negotiable. Your maximum heart rate falls steadily with age, and a large analysis of more than 18,000 people found this happens at essentially the same rate whether you are an athlete or entirely sedentary. Training has little effect.

The same goes for much of the plumbing. With age, the large arteries thicken and stiffen as elastic fibres give way to collagen and calcium, which is why systolic blood pressure (the top number in a blood pressure reading) tends to creep up.

The heart’s early filling rate – how quickly it draws in blood in the first moments after each beat – drops by 30 to 50% between the third and ninth decades, though the upper chambers compensate. This means it is not the heart’s pumping volume that limits older people during exercise. It is the falling peak heart rate and the reduced ability of the working muscles in the arms and legs to pull oxygen out of the blood delivered to them.

Peak aerobic capacity, the amount of oxygen your body can use when working hard, declines alongside all this. The Baltimore Longitudinal Study of Aging found the decline is not steady: roughly 3 to 6% per decade in your 20s and 30s, accelerating past 20% per decade after 70. Sobering detail: that acceleration happened at much the same rate in active and inactive people alike.

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Senior Lecturer in Clinical Exercise Physiology, University of East London; Senior Lecturer, Strength and Conditioning, University of East London. | This article is republished from The Conversation under a Creative Commons license.