system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: why do humans become so "stiff" as they get older?
gpt
One of the main factors causing stiffness is the increase in fibrous tissue that restricts movement of muscles and joints.

Whenever we exercise vigorously at any age we produce microscopic tears in muscle and the connective tissues such as tendons, ligaments and fascia that keep our bodies in one piece. The tearing of tissue triggers inflammation. This is the process by which the body repairs itself. Part of the repair process involves laying down a scaffolding of collagen fibres, usually in a random disorganized manner rather than well aligned with the usual direction of pull on that part of the body. The matted fibres are what is called scar tissue. If we subject the newly deposited fibres to gentle movement, the misdirected fibres get either removed or re-aligned and we are left with correctly oriented fibres that allow our joints and muscle to function smoothly. However if we move too vigorously or erratically, we simply tear more fibres and re-commence the cycle of inflammation and repair.

Although the damage starts with vigorous movement, being sedentary does not help because collagen fibres also get laid down between adjacent tissues that are not moving, binding them together, so we would become completely moribund. If we then move forcefully the fibres will tear and we commence a cycle of inflammation and fibre deposition.

With age, there is an almost inevitable build-up of excessive collagen fibres and other debris left behind after multiple cycles of the repair process, and our muscle and joints become stiff.

The way to minimise this is to keep active in a manner that minimises tearing of tissues and promotes optimal repair of those tissues that do get torn. The strategies for achieving this are:

1) Build up exercise gradually ensuring that tissues are strong enough to resist tearing. Strength building exercises (either body weight or in the gym) are a useful adjunct to aerobic exercise such as running

2) Warm up at the beginning of exercise so that any poorly aligned fibres are gently re-aligned rather than abruptly torn apart creating a new cycle of inflammation.

3) Avoid erratic movements, especially movements that require strength near the limit or your current strength;

4) Massage, foam rolling or gentle stretching to remove knotted fibres can be helpful.