Avani Hocker Singh sat in the grass holding her knee and grimacing in pain.
Moments earlier, she’d slipped trying to jump over a hurdle — a task she’d done thousands of times before — and landed on a slab of concrete, twisting her knee in the process.
“I was just sitting there with my knee tucked, and something felt wrong,” she said. “My coaches and my teammates were just staring at me like, ‘Are you going to get up or what?’…