I

Intensity

Intensity is often used in aerobic endurance training and reflects “how hard the tasks really is”. In this case the intensity (for example in a running task) is set to a specific amount of % of maximal heart rate and the player therefore need to run at a pace that equals that intensity (or the % of maximal heart rate). However, this is impractical in resistance training and therefore the term “load” replaces intensity here.

Isometric (muscle contraction)
Muscle length is constant while resisting an object/movement. This type of contraction is not often used in football (compared to eccentric and concentric muscle contractions).

Isotonic (muscle contraction)
Same muscle tension, while muscle shortens/lengthens. Isotonic muscle contractions are not used in football. However, it might be used in strength training to provide greater resistance at end range motion.