Postactivation Potentiation: Built-in Neuromuscular Tuning

Postactivation potentiation (PAP) is a normal, regularly occurring feature of a dynamic, healthy neuromuscular system. One does not need an exercise professional, massage therapist, or any other specialist to elicit PAP any more than one needs a doctor in the room in order for blood to clot in a wound. However, with an understanding of the characteristics and purposes of PAP, professionals can use the PAP to enhance strategies for helping clients reach their goals. This is the first in a series of articles that will explore PAP and how it may be used to understand and manage aspects of exercise programming and design.

Defining Postactivation Potentiation
To clearly define and understand PAP, let’s review the basic functional unit of a muscle: the motor unit. A motor unit is made up of an alpha motor neuron (found in the spinal cord) and all of the skeletal muscle fibers to which it connects. The alpha motor neuron stimulates the muscle fibers, the muscle fibers then contract to generate force, know as tension. When an alpha motor neuron stimulates the muscle fibers that it innervates to contract, this motor unit has been recruited or activated.

Now let’s break down the words postactivation potentiation. Postactivation: “post” meaning after and “activation” meaning recruitment of a motor unit. So postactivation means after the initial recruitment of a motor unit. Potentiation is the augmented activity of something1. In this case, potentiation refers to an increase in the amount and rate of tension produced by a muscle in response to stimulation from the alpha motor neurons that innervate it. So taken all together, PAP refers to motor units that have acutely adjusted to produce even more force, more efficiently than when first called into action23. PAP is most likely not a whole muscle event. It seems to happen to individual motor units upon recruitment45. This is an important distinction between this author’s description of PAP and previously published definitions.

When, how, and why does the neuromuscular system use PAP?

First let’s take a look at three scenarios of when PAP occurs.

Scenario 1
Prior to exercise, your neuromuscular system is orchestrating motion using just enough neurological and muscular energy and tissues to perform pedestrian tasks. As you begin to exercise or increase the intensity of your activity, the neuromuscular system adapts to the increased demand in several ways. Two are relevant to our discussion. First, additional motor units are recruited to help generate more tension. Second, the added motor units potentiate. Meaning, seconds to minutes after they are recruited, PAP happens. Addition and potentiation of motor units in response to increased demand (time under tension, load, or both) continues, as needed, until all accessible motor units in the working muscles have been recruited. In this scenario, PAP does not feel like anything special; it goes unnoticed.

Scenario 2
You are holding a limb in a specific position – say hip abduction. Someone or something pushes against your leg in the direction of adduction with just enough force to overwhelm the muscle groups trying to hold that position. So, your leg and hip joint adduct. Note the amount of force and precisely where on your leg the force was applied when your leg started to move. Rest for about a minute, then reposition the hip in abduction, push against the same leg with the same amount of force and in the same place on the leg. If only6PAP occurred, your muscle groups will generate more torque and this time you will be able to maintain the position of your leg and hip joint. This can feel like a pretty dramatic experience. Remember, motor units only potentiate after they have been recruited. Therefore, in this scenario, only the motor units recruited to meet the force requirements of the external force have potentiated. There may be more motor units that have yet to be recruited and potentiated.

Scenario 3
Your goal for the day is to go for a new “personal best” for your bench press. You know that you are not ready to lift that load right away. A specific warm-up is needed. So, you prepare for this lift by doing a few sets with gradually increasing loads with a few minutes of rest in between sets. You are careful to warm-up, but not fatigue your muscles. When your “personal best” load is racked and ready, so are you! You feel strong, confident, and energized because your warm-up sets and rest allowed you to recruit as many motor units as possible and allowed them to potentiate. It’s PAP that has acutely tuned your neuromuscular system to lift more that you could when you walked in the door.

These three scenarios help us see that PAP is nothing extraordinary, although it is pretty cool and helps to maximize the efficiency of the neuromuscular system. It happens during the normal course of the neuromuscular system’s response to increased demand (time, load, or both).

But that is not the end of the PAP story. If PAP has a nemesis, it is fatigue. The next article in this series looks at the relationship between PAP and fatigue. We will take a look at why scenarios 2 and 3 may not always happen as described above and begin to explore ways that we can leverage PAP to help our clients achieve their goals.