While a single-joint muscle can produce motion only at the joint it crosses, a multijoint muscle can produce motion at more than one joint. This is of advantage in many human movements. However, multijoint muscles can become inefficient when trying to exert full active or passive motion at two or more joints at the same time. The former results in what is called active insufficiency and the latter in passive insufficiency.
Active Insufficiency Of Muscle
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.
The gastrocnemius of the calf plantar flexes the ankle but also crosses over the knee joint. Therefore, if the knee is bent, or flexed, 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 gastrocnemius, 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
Passive Insufficiency refers to the inability of a multi-joint muscle to lengthen to a degree that allows full range of motion of all the joints it crosses simultaneously. An example of passive insufficiency would be short hamstrings, which results in the inability of the muscle to normally elongate to the degree that proper combined hip flexion and knee extension can occur, which is required for all sports.
You can demonstrate this passive insufficiency of the hamstring by lying on your back and using your arms to bring one leg toward your shoulder while keeping the knee fully extended (straight). The hamstring will experience a noticeable stretch and at some point you will no longer be able to move the leg toward the shoulder. The hamstring muscles are not cabable of lengthening to a degree that allows full hip flexion and full knee extension simultaneously. However, if you now flex (bend) the knee, you will be able to bring the leg closer to the shoulder.