Successfully complete the indicated task to unlock the corresponding challenge:

Jackknife challenge: Get a "Four Star" rank in the 10 Second Workout and 20 Second Workout.
Parallel Stretch challenge: Get a "Four Star" rank in the 30 Second Workout and 60 Second Workout.
Pressup challenge: Get a "Four Star" rank in the 6 Reps and 10 Reps.