Active insufficiency is the inability of a biarticulate muscle to contract properly because it is not stretched enough.
For example, bend you elbow and your wrist and try to make a fist. You can't. Why?
Because the flexors in your forearm are in a position in which they are shortened and thus they cannot move the joint in question (make the fist).
This of course is valid only for muscles that move more than 1 joint.
The practical effect for bodybuilding is that some exercises are not really effective when the muscle in question is not stretched sufficiently.
I'll write 3 examples:
- your hamstring is not stretched when you squat (because the bend at the hip is "counterbalanced" by the bend at the knee) thus squat is not the best exercise to hit the hamstring. Good exercises specifically for the hamstring are e.g. good morning or stiffed legs deadlift, in which the hamstring works in a stretched position.
- biceps. Biceps works on 3 joints. Consider that the short head of the biceps (the internal one) works even to flex (raise) the shoulder. Its contribution is minimal, but the point is that this short head cannot be worked effectively if you flex your elbow (curl) when your shoulder is raised.
This means that exercises like preacher curls, concentration curls, cheat curl (raising the elbow before flexing it) and even chin ups are not really effective on the short head of the biceps.
For this kind of curl much of the load is moved to the brachialis, and away from the biceps.
- triceps. The long head of the triceps (that acts on 2 joints) can be worked effectively only when it is stretched. That's why you need to do skullcrushers/extensions etc. and on the other hand, benching does not work (enough) the long head.
Feel free to comment.