Isotonic (exercise physiology)

In an isotonic contraction, tension rises and the skeletal muscle's length changes. Lifting an object off a desk, walking, and running involve isotonic contractions.

There are two types of isotonic contractions: (1) concentric and (2) eccentric. In a concentric contraction, the muscle tension exceeds the resistance and the muscle shortens. In eccentric, the muscle lengthens due to force greater than which the muscle can produce.

Concentric
This type is typical of most exercize.

Eccentric
There are two main features to note regarding eccentric contractions. First, the absolute tensions achieved are very high relative to the muscle's maximum tetanic tension generating capacity (you can set down a much heavier object than you can lift). Second, the absolute tension is relatively independent of lengthening velocity. This suggests that skeletal muscles are very resistant to lengthening.

Muscle injury and soreness are selectively associated with eccentric contraction (Fridén et al. 1984; Evans et al. 1985; Fridén and Lieber, 1992). However, muscle strengthening may be greatest using exercises that involve eccentric contractions.