Some interesting notes on Active Insufficiency from Bill DeSimone.

Muscle insufficiency is the weakness that comes from positioning, i.e. either over-shortening or over-stretching a muscle, as opposed to the weakness that comes from fatigue, atrophy, or overload. You can simply, and only, correct the insufficiency by repositioning the joints involved, so that the muscle is positioned near its “favorable length”.
As one textbook (Brunnstrom’s, 5th edition, pp 140-141) explained, “The body is designed so that such weak positions are avoided in normal activities requiring great force…Favorable length-tension relationships are maintained by movement combinations whereby the muscle is elongated by motion at one joint while producing motion or force at another joint.”
Period. The end. Or it would be, if Exercise Experts past and present hadn’t muddied things up.
The muscle magazines from my formative years (1970s) wrote about the Peak Contraction Principle, attaching some magic benefit to the sensation that you felt near the end of certain exercise (e.g., concentration curls, leg extensions). A great metaphor, and far more motivating than what the sensation really was: positioning the joint so that the load was heaviest where the muscle was weakest.
Later, I read in the Nautilus material, and especially in the articles of Mike Mentzer, about the importance of “the fully contracted position” as well as using “a full range of motion”. In that unfortunate video recorded at the end of his life, Mentzer continued to repeat the “fully contracted” line, while the subject’s muscles visibly relax in the fully contracted position of the leg press and incline press, making its importance a lot less convincing.
Today, “active insufficiency” is used by trainers and bloggers when they want to sound smarter than you. “That machine/exercise puts you in active insufficiency”, as if that somehow damns it beyond salvation, when, actually, at best, it’s just descriptive. It may put you at risk for a cramp, or it may put you at risk for an injury, but that has more to do with the effect of the insufficiency, not the insufficiency itself. An overhead press puts your deltoids into active insufficiency, and while overhead presses may present some joint complications, muscle cramps and strains aren’t among them. Seated heel raises put the gastrocnemius in active insufficiency for much of the rep; no harm comes from that. Put the hamstrings into an insufficiency, however, and you put your lower back at risk.
Or the other way it has been used is to suggest that with your exercise selection, you can put parts of a muscle (group) in insufficiency and selectively load the non-insufficient heads of muscles. While I’m not convinced that this approach is definitive, there is more substance to it than to the previous approaches.
But rather than try to reconcile all the different EE interpretations of insufficiency, or to analyze every muscle and joint position of every exercise to see how insufficiencies might be in play, it may be more useful to start with the basic muscle science, look at a few examples, and then come up with some general guidelines for how insufficiencies may affect your workout.
The take away points:
• You can’t correct insufficiencies with partial reps, or with specially designed machines or exercises, because the condition comes from a fundamental aspect of muscle physiology.
• Loading the too-shortened muscle position may or may not cause a cramp in the muscles, but that’s just about the only negative aspect to loading this position.
• Conversely, there’s not necessarily magic to loading this position; loading the joint angle of peak muscle torque is far more important.
• Loading the over-stretched position is more problematic, not only for the muscle but also for the effect on the joints.
• My personal conclusion is that the possible negative effects of exercise selection on the joints is far more critical than the potential positive effects on the muscle.