health-related fitness
The four parts of fitness that keep your body healthy: cardiorespiratory endurance, muscular strength, muscular endurance and flexibility.
cardiorespiratory endurance
How well your heart and lungs deliver oxygen to your muscles so you can keep moving for a long time.
progression
Making an activity a little harder, gradually, over time.
overload
Asking your body to do a bit more than it is used to, so it adapts and gets fitter.
specificity
You get better at the kind of fitness you train. Stretching improves flexibility; running improves endurance.
reversibility
Fitness slowly fades if you stop being active for a long time.
rest and recovery
Time for your body to repair and get stronger between hard workouts. Sleep is a big part of it.