Which of the following describes an isotonic exercise?

An isotonic exercise refers to a type of strength training where the muscle tension remains the same throughout the range of motion while the muscle length changes. This occurs as the muscle contracts, resulting in either shortening (concentric contraction) or lengthening (eccentric contraction).

In this context, the correct answer indicates that the resistance varies with speed of movement. In isotonic exercises, while the muscle works against a constant load (like lifting a dumbbell), the ability to overcome that load can change depending on how quickly the muscle is contracting. For instance, if you lift a weight quickly, the muscles may have to exert more force initially to overcome inertia. This helps to enhance muscle strength and endurance effectively, reflecting the characteristic variability seen in isotonic movements.

The other statements describe characteristics that are not representative of isotonic exercises. For instance, if resistance is fixed and unchanging, as suggested in another choice, it might imply isometric exercises, where there is no movement, hence not fitting the definition of isotonic. Similarly, statements about muscles contracting without movement or shortening without changing length would not accurately represent the essence of isotonic exercises.