Final answer:
Heredity is the element of skill-related fitness that cannot be altered to improve an individual's fitness level, unlike practice and specificity, which can.
Step-by-step explanation:
Within the context of skill-related fitness, heredity is the influence that cannot be changed to improve fitness levels. While practice and specificity (deliberate practice aimed at particular skills) can certainly help an individual enhance their skill-related fitness, the genetic factors they inherit from their parents (heredity) set certain physiological limits that cannot be altered through training or lifestyle changes. It's important to note, however, that while heredity can set certain predispositions, one's fitness can still be greatly improved with the right training and lifestyle choices.