In an isotonic contraction, tension remains unchanged and the muscle's length changes. Lifting an object at a constant speed is an example of isotonic contractions. A near isotonic contraction is known as Auxotonic contraction.

There are two types of isotonic contractions: (1) concentric and (2) eccentric. In a concentric contraction, the muscle tension rises to meet the resistance, then remains the same as the muscle shortens. In eccentric, the muscle lengthens due to the resistance being greater than the force the muscle is producing.

Concentric

This type is typical of most exercise. The external force on the muscle is less than the force the muscle is generating - a shortening contraction. The effect is not visible during the classic biceps curl, which is in fact auxotonic because the resistance (the weight being lifted) does not remain the same through the exercise. Tension is highest at a parallel to the floor level, and eases off above and below this point. Therefore tension changes as well as muscle length.

Eccentric

There are two main features to note regarding eccentric contractions. First, the absolute tensions achieved can be 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, thereby allowing very high levels of tension to develop as can occur in isometric exercise.

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 because high levels of tension generate the signals for muscle strengthening.

Auxotonic Contraction

This is almost an isotonic contraction because, there is some fluctuation towards the end of the contraction. For example, the heart muscle ventricles contract to expel blood into the pulmonary artery and aorta. As the blood flows out, the previous built up load is now decreased and hence less force is required to expel the rest of the blood. Thus the tension is reduced.

See also

• Isometric exercise
  Isometric exercise
  Isometric exercise or isometrics are a type of strength training in which the joint angle and muscle length do not change during contraction...
  
   (contraction, no movement)
• Stretching
  Stretching
  Stretching is a form of physical exercise in which a specific skeletal muscle is deliberately elongated, often by abduction from the torso, in order to improve the muscle's felt elasticity and reaffirm comfortable muscle tone. The result is a feeling of increased muscle control, flexibility and...
  
  (passive, no contraction)