The season 25 pro, 23, performed with celebrity partner Jordan Fisher on Monday’s semifinals show, wearing a tan knee brace. Their first dance, an Argentine Tango, was to “Brother” by NEEDTOBREATHE, inspired by Fisher’s role as an older sibling to his brother Cory and sister Trinity.
Fellow pro Sharna Burgess, who was eliminated alongside her partner Derek Fisher earlier this season, was on standby in case Arnold was unable to dance.
“I was practicing the pro number and I literally just stepped up onto a stair and it felt like my whole leg gave out and stopped working and I thought it was just a one-time thing and then it happened a couple more times, just doing simple things,” Arnold said in the video from their practice, that was exclusively shared with PEOPLE.
In the footage, Arnold teared up during physical therapy treatment. “It’s frustrating because it’s the semi-finals and I want to dance it with Jordan, and I just hope this doesn’t stop me from doing that. That scares me,” she said.
After earning a 29/30 for their performance, Fisher revealed he suffered a minor injury during rehearsals on Monday. “I have a scratched cornea a nail in the eye,” he told co-host Erin Andrews.
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Arnold is the only pro left in the competition who has never won the Mirrorball Trophy.
“Lindsay injured her knee during camera blocking rehearsal Sunday morning while working on a number with the troupe,” a DWTS source previously told PEOPLE on Sunday.
Dancing with the Stars airs Mondays (8 p.m. ET) on ABC.