Distance running with an upper-body amputation is a war against instability. Muscles can strain so much to compensate for the missing limb that they contort your spine, misalign your shoulders, and worsen your imbalance. In 2017, Ashley Jones—then 15 years old—faced a punishing return to high school sports following the amputation of her right arm. On the soccer field, she had collisions so violent that the pain would bring her to the brink of blackout. Later, during cross-country runs, the jostling of the amputation site intensified her phantom pain—which she’d already rated 7 out of 10 on normal days—to as...