Starfield:Fitness

Fitness is a novice tier physical skill that increases how much Oxygen you have, with the highest tier also decreasing how much oxygen is used by sprinting and power attacks.

Skill Increases
By overencumbering yourself, you will use up oxygen faster. Simply add too many items to your inventory, and proceed to sprint and jump until your oxygen bar is empty.

Note, it is NOT enough to simply fully empty the 02 bar (the white bar). You also have to fully fill the C02 bar (the red bar).