Push Press
When you have a lower body injury, sometimes you have to get creative to get your heart rate up to get intense cardio in.
Quick, explosive push presses (with lighter weight but heavy enough that sets of 25-30 reps each are challenging) are a great way to increase your heart rate during a circuit or HIIT. Incorporate these with other things you can do like intervals of ab work, push-ups, elliptical, spin bike–depends on your injury.
I use 20lbs on each side of the bar & aim for 25 reps in one minute.
In my HIIT workouts, I usually end up doing 4 sets among the other intervals.