Every single push.

Four thousand pushes a day for a kid in a school building.

And the shoulder is not a weight-bearing joint; it is a shallow ball in a shallow socket held together with soft tissue, and a manual chair user is walking on their shoulders for sixty years.

“Rotator cuff disease in long-term manual users runs at somewhere between a third and two thirds depending on which study you read,” she said.

“And the single biggest modifiable factor is push mechanics.