(CNN) — In the middle of an intense workout, Kristina Lazzaro stepped back and heard a sound like a cork coming out of a bottle of champagne.

The 35-year-old Buffalo, New York, resident was doing a partner workout at her local gym, pushing herself to finish strong on a shuttle run in which runners sprint from one line to another and back.

Lazzaro exploded off the first line. Arms pumping, she dashed 10 yards across the gym to the other line. As she put her foot down to pivot, she heard a popping sound and crumpled to the floor — unable to move her foot at all.

“I ended up on the ground, and I didn’t know how I got there,” she said. “There was no warning, no symptoms, and nothing that suggested something like this could happen.”

Lazzaro later learned she had ruptured her Achilles tendo

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