Active insufficiency occurs when a multi-joint muscle reaches a length (shortened) where it can no longer apply an effective force. To demonstrate active insufficiency one can fully flex (bend) the knee on one leg while simultaneously trying to bring that leg back to achieve full hip extension. Hip extension will be limited because the hamstrings are unable to shorten enough to produce a complete range of motion. Some will also notice a cramping in the hamstring muscles during this maneuver. By the same token, if you try bringing back your hip into a hyper-extended position (bringing your leg behind you), and then bending your knee, you will find that your knee flexion is limited. The hamstrings can only perform one of these functions well at one time. When both are attempted at the same time, the muscle essentially goes "slack" and is unable to contract effectively because it is already well shortened. Straightening the leg (extending the knee) should restore full range of hip extension motion and the difference will be significant. Active insufficiency reflects the inability of a multijoint muscle to apply an adequate force in all degrees of motion.
Another example is the finger flexors. You may have noticed that you cannot produce a tight flex with your wrist bent (in flexion). This is because of active insufficiency of the finger flexors. They can only make a tight flex with the wrist in a neutral position. See passive insufficiency.
The gastrocnemius of the calf plantar flexes the ankle but also crosses over the knee joint. Therefore, if the knee bent, shortening the muscle at that joint, it can no longer apply as effective a force in plantarflexion. This fact is taken advantage of during calf raises, when, if done seated with the knees bent, shifts the focus from the gastronemius, which is actively insufficient, to the soleus, making it the prime mover in seated calf raises. When the calf raises are done standing, the gastrocnemius can apply a full force.
Need more information like this? See Applied Biomechanics: Concepts and Connections
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This page created 06 Sep 2011 00:42
Last updated 18 Jul 2016 19:04