All efforts to maintain muscle, bone density, tendon health come down to a key measure : the amount of force you can apply.

What we think of as weakness, fragility or frailty is the inability to produce enough force when you need it. The force could be proactively or reactively exerted. But the capacity to produce it exists in our muscles. And the capacity is one that is lost without practice. And it can be honed with frequent use.

Takeaway

For any exercise to produce a useful adaptation, the effort will require you to produce some force. A muscle trying to produce force against resistance to get work done is the minimum requirement for rehab, gaining strength, packing on muscle or just giving the muscle a reason to exist.

You can stretch, you can wave your limbs around, you can hang off towels, but muscles live to produce force. And to live long and prosper, you need to keep making the muscles produce force.