Why so slow?

They're called C-tactile afferents. They're in the skin of the back, arms, legs, not the palms — and they fire most to a slow, light stroke at roughly the warmth of skin, somewhere around three centimetres a second. Go faster and they quieten; go lighter or sharper and they don't respond at all. When researchers plotted how hard the fibres fire against how pleasant people rated the same strokes, the two curves were the same shape. The body has a channel built for exactly this kind of touch.

Unlike the nerves that tell you where a hand is and how hard it's pressing, these ones route to the insula, the part of the brain that deals in feeling rather than fact. Which is why a stroke at the right speed doesn't register as information. It registers as a feeling of being cared for.

A nervous system doesn't let long-held muscle tension dissolve just because it's been stretched or pressed hard enough. It lets go when it's convinced, through a channel it trusts, that there's nothing to fear. Slow touch has both the potential to relax muscle tension and down-regulate the whole of the nervous system.