When you can deliver and use oxygen efficiently, you can do more work without undue fatigue. Muscular fitness includes strength, muscle endurance, and flexibility. Strong workers can lift and carry heavy loads with less fatigue or risk of injury. Muscular endurance enables you to continue working at otherwise fatiguing tasks. And flexibility means a better range of motion that lowers the risk of injury.
Fitness is the foundation of work capacity.
That's why resource agencies stress its importance, and why you should report to the job fit, work-hardened, and ready to take on tough field assignments. Gaining fitness is a gradual process that can take 2 to 3 months for substantial improvements. Working yourself into shape on the job is not an option. Some jobs, like firefighting, require that you pass a job-related work capacity test to ensure the capacity to perform arduous work. In field work, particular firefighting, fitness is a matter of safety both for you and your crewmembers.