Easy O2 depletion for fitness skill

If you're looking to increase your fitness skill quickly you can over encumber yourself in your ship (your oxygen will deplete faster over encumbered) and run into a wall over your bed, every time you rest it will reset your O2. If you press the sprint button and hold forward after starting your rest you will wake up with your oxygen still depleted and you can sprint until your CO2 fills and it will count towards the O2 depletion needed for the fitness skill.

Edit: It's not hitting the sprint after pressing rest, but resting while still sprinting right before your CO2 fills. (It counts as full O2 depletion a little before your CO2 is fully filled)