Update:
Passive insufficiency has value as loaded stretch exercises lead to a unique form of growth via high passive tension.
Muscles have an ideal length according to the length-tension relationship. A good exercise challenges you closely to this length, which occurs in the middle of the movement. Any exercise that forces the muscles to deviate too far from this length is a poor exercise.
Many exercises that isolate muscles or put you into odd positions require you to work at far weaker lengths. These lengths are especially weak and even harmful when you enter active or passive insufficiency.
Muscles that attach to multiple joints change length depending on the positions of all the associated joints. When possible, the body attempts to shorten at one joint and lengthen at the other or vice versa. This keeps them working close to medium length.
When the muscle attempts to either lengthen or shorten over all the joints, you enter into passive and active insufficiencies.
The body normally seeks to avoid these positions, though we sometimes force it to do otherwise.
Passive Insufficiency
Passive insufficiency occurs when the muscle fails to extend far enough. This happens since a full stretch at multiple joints cannot occur. This position is the longest possible length of a muscle.
The muscle can generate hardly any active tension, the tension created when your muscle contracts, while remaining in this position since the contraction sites place too far apart. It does generate great passive tension though, which results from the stretching of the tissue.
Anytime you feel a deep stretch in the muscles, such when reaching to touch your toes, you experience passive insufficiency. Improving flexibility through stretching often places muscles in passive insufficiency, though with more control than you would have under a heavy load.
Active Insufficiency
Active insufficiency occurs when the muscle fails to shorten enough. You may feel this as a cramping sensation because the muscle bunches up like a ball of yarn. This is the shortest possible length of a muscle.
The excessive overlap of the potential contraction sites will result in little active or passive tension.
Flex your wrist completely. Your palm should approach the soft belly of your forearm. Attempt to tighten your fist. You will notice that it feels impossible it to close your fist fully or with much force compared to closing your hand in a slightly extended position, as happens when normally grasping an object. The multi-joint long finger flexors enter active insufficiency when the wrist flexes.
Consequences of Active and Passive Insufficiency
- Maximal shortening of a muscle does not lead to a high force production.
Many experts refer to this as the position of full contraction, but it actually is the position of full shortening for an exercise. For example, this occurs at the top of a curl when the biceps begin to smash against your forearm, especially if you lift your elbows. This shortens the biceps over both the elbow and the shoulder blade.
Instead of acting as the best position, and trainees will often purposely squeeze for up to a few seconds here, it actually ranks the weakest position that creates the least tension. Tension is the main stimulus for more size and strength.
A poor exercise has the weight feel heavier in this position. Many machines modify the mechanics to make this position harder, making the movement worse.
- Avoid machines.
Some machines can place you into these insufficiencies.
A leg extension machine without an inclined seat that drops your hips will allow active insufficiency, since the hip flexes too much while the knees extend. A muscle of the quadriceps, known as the rectus femoris, enters active insufficiency in this position. It shortens at both the hip and the knee.
This can occur with the leg curl as well, as too much hip extension with knee flexion will harm the hamstrings.
The seated calf or heel raise places the gastrocnemius into active insufficiency, since the knee flexes too much while the ankle performs plantarflexion. Instead of isolating the soleus, it just disables and potentially injures the gastrocnemius.
- Avoid isolation.
Isolation will harm you as well. The cable triceps extension can allow active insufficiency if your elbows to drift too far back. Wrist curls place the fingers into active insufficiency. These exercises have many problems, but insufficiencies show how unnatural they are.
- Avoid unnatural exercises.
Some exercises that may seem decent, such as pull-overs (an analogue for the roll-out), allow passive and active insufficiencies throughout the range of motion since they rely on artificial movement patterns.
This occurs with the hip thrust, which causes active insufficiency of the hamstrings and possibly the calves too.
- Avoid extreme ranges of motion.
Going too deep in the squat can overstretch the hamstrings and force the lower back to round.
- Choose better exercises.
You should mostly avoid entering these positions if you restrict yourself to exercises that load your body through gravity.
Elbow flexion combined with extension of the shoulder maintains the best length-tension for the biceps. Shoulder flexion combined with elbow extension maintains the best length-tension for the long head of the triceps. These partnerships occur when we pull or push, such as on the bench press or the row.
A squat maintains the best length-tension for the quadriceps, hamstrings, and calves.
Active and Passive Insufficiency in Lifting
By restricting yourself to free weight, compound exercises that rely on natural ways of moving, you will prevent active and passive insufficiency.