PAP

Post Activation Potentiation (PAP)

This is term in exercise physiology that describes a situation when your muscles have been primed (a.k.a primed, potentiatied) more than they get tired (a.k.a fatigued).

When this happens your muscles are able to generate more force and strength.  Any baseball player on the planet would benefit from faster and stronger muscles.  Using PAP can be a trick that you use on field to increase bat speed and maybe even your throwing velocity.

Here is an article about PAP and bat speed

If you are really interested in this subject you can keep reading this literature review below that goes into great detail about PAP.  I wrote this when I first started grad school because I had the intention of researching the subject as it relates to baseball.  It is not a complete review but it should get you started.

Post Activation Potentiation Literature Review

Post activation Potentiation (PAP) is physiological phenomenon that can unlock levels of strength and power.  Muscular performance is enhanced acutely due to a preceding high-intensity action.  An individual’s ability to take advantage of PAP is determined by multiple factors ranging from age, training experience, fiber type and gender as well as training variables such as type of muscle contraction, recovery interval, intensity and specificity.  PAP has received a significant amount of attention from several researchers which has resulted in several theories attempting to discover the optimal training parameters in order to allow for optimal athletic performance.  The purpose of the present paper is to review the current literature on PAP in order to better understand the best way to apply the knowledge gained from PAP research so that each individual athlete can benefit from its performance enhancing abilities.

Mechanisms of Post Activation Potentiation

The ability of a muscle to contract is dependent on its contractile history, fatigue can affect it in a negative fashion, or muscle ability can be augmented when it is potentiated (Abbate et al., 2000)  The optimization of muscle power is the net balance between these two variables (Kilduff et al. 2011).  It is possible for these two states to co-exist (Rassier and Macintosh, 2000) however, in order to elicit some type of performance enhancement; the prevalence of the PAP mechanism must override the effects of fatigue.  Furthermore, the result and magnitude of the force output following the conditioning activity is the net balance between potentiation and fatigue (Vandenboom et al. 1995).  Potentiation can be measured by either an increase in muscle twitch force (Sale 2002) or an increase in H-reflex amplitude (Enoka et al. 1980).

Muscle Twitch

Potentiation of a muscle can be explained by the phosphorylation of myosin regulatory light chains.  Once phosphorylated, these myosin regulatory light chains cause the interaction between actin and myosin to become hypersensitive to Ca²⁺ (Sweeny et al., 1993a).  This subsequently leads to a greater rate of cross bridge attachments to an increased sensitivity of contractile proteins to ionized calcium which in turn increases twitch force and its rate of force development.  The increased sensitivity to Ca2+ is greatest when the myoplasmic levels of CA2+ are relatively low which occurs during twitch and low frequency contractions.  During high frequency tetanic contractions when CA2+ levels are already saturated, this increased sensitivity has little effect (Vandenboom et al., 1995).  Thus PAP occurs when there is an increase in muscle twitch and low frequency tetanic force following the conditioning exercise.

Hoffman (H) Reflex

A potentiated muscle has also been attributed to an increase in alpha-motorneuron excitability as reflected by the H-reflex.  H-reflex is an excitation potential generated as a segmental spinal reflex following impulses to activate the muscle contractile apparatus (Trimble and Harp, 1998).  H-reflex has been traditionally classified as a monosynaptic reflex however it has been suggested that oligosynaptic pathways, such as Golgi tendon organ, can have an effect on the amplitude of the H-reflex (Burke et al. 1984).  Research has also suggested that the levels of a pre-synaptic inhibition can alter the H-reflex amplitude (Zehr, 2002).  As in the case of the muscle twitch potentiation this conditioning activity can result in a suppressed level of potentiation.  When referring to the H-reflex, this suppressed level of potentiation is known as postactivation depression (PAD) while an augmentation of potentiation is known as reflex potentiation (RP) (Enoka et al. 1980).  Like fatigue, PAD initiates muscle relaxation and thought to reduce the number of transmitters that act at the pre-synaptic level (Hultborn et al. 1996).  A potentiated H-reflex is commonly induced by high frequency stimulation and is known as  post-tetanic potentiation (PTP).  In a study conducted by COrrie and Hardin (1964) it was reported that a frequency greater than 100Hz is needed to induce PTP (Corrie and Hardin1964).  They suggested that increased level of pre-synaptic CA2+, which in turn can cause neurotransmitter release from the presynaptic membrane is responsible for frequency induced PTP (Zucker and Regehr, 2002).  Stuides using volitional contractions as the conditioning activity are less numerous; however, a few studies have demonstrated that RP can occur after an extended recovery time of 4-11 minutes (Guillich and Schmidtbleicher, 1996).  Increases in the H-reflex amplitude due to a volitional conditioning activity are thought to rely on the basis of Henneman’s size principal.  This hypothesis stems from the fact that the recruitment of motor units by the H-relfex relies on the Henneman’s size principal (Henneman et al., 1965), which states that the recruitment of motor units will be done in a n orderly fashion from the smallest to largest.  Therefore following a conditioning activity the next motor units that would be recruited by the H-reflex would be fast motor units which would increase the rate of force development and peak force development (Guillich and Schmidtbleicher, 1996).

Fiber Type

Hamada et al. (2003) reported that subjects who had exhibited the highest amounts of PAP following a 10 sec isometric voluntary contraction had a greater percentage of type II (72+-9%) versus the (39+-7%) in their vastus lateralis of the subjects that exhibited the lowest levels of PAP (Hamada et al., 2003).  This can be explained by the fact that type II muscle fibers undergo greater phosphorylation of myosin regulatory light chains in response to a conditioning activity (Sweeney et al., 1993b).  A study by Vandervoot and McComas (1983) reported the gastrocnemius (high percentage of Type II fibers) had a greater post-tetanic twitch potentiation that the soleus which composed primarily of Type I muscle fibers.  However this study was in contrast to a study performed by Stuart et al., (1988) where no correlation was found between PAP in knee extensors and the percentage of type II fibers or in the amount of myosin phosphorylation in the vastus lateralis.

However Hamada et al. (2002) performed a study using endurance trained athletes to see if their training and fiber type had any effect on PAP.  Based on the previous studies it could be assumed that an endurance athlete, who would have less Type II muscle fibers than sedentary individuals, would exhibit less PAP.  The results of this study showed that the endurance athletes with less type II fibers had higher levels of PAP with an evoked muscle contraction 5 minutes after a 10 second maximal isometric contraction.  This would lead us to believe that training experience would have an effect on PAP.

Training Experience

Resistance training provides many benefits such as reduced fatigue (Gayda et al. 2007) and increase in the percentage of type II muscle fibers (Hakkinen and Komi, 1983).  One could assume that these attribute would positively contribute to PAP.  A study by Rixon and Bemben (2007) from the University of Oklahoma considered many different variables and their influence on PAP including training experience.  The experienced lifters had relatively higher levels of PAP regardless of gender and type of contraction (Rixon and Bemben, 2007b).  This confirmed a study by Chiu et al. (2003) which suggested that post activation potentiation could be a viable method in the acute enhancement of explosive strength performance in athletic individuals.  In their study, the athletes who were studied all played sports that were explosive in nature which would lead one to assume these subjects had a relatively higher percentage of type II fibers compared to sedentary or endurance athletes.  Furthermore, Chiu et al. suggested that the H-reflex could superimpose on the motor signal, which would increase the signal strength arriving at the muscle.  They also demonstrated that the magnitude of the H-reflex is proportional to the magnitude of the activation of the muscle.  This suggests that a greater activation in the muscles of those in the athlete group would have resulted in a greater potentiation via the H-reflex (Chiu et al. 2003).

The prevalence of fatigue could explain the result of the Hamada et al. (2000) study that reported higher levels of PAP in endurance athletes with lower percentages of type II fibers compared to sedentary who despite higher levels of type II fibers displayed lower levels of PAP.  The fact that the sedentary subjects did not train could explain that higher levels of fatigue could have masked the potentiation effect of the conditioning activity.  Smith and Fry (2001) showed that 11 recreationally trained men did not exhibit any signs of PAP 7 minutes after a 10 second MVC.

In a study regarding the H-reflex amplitude, Gullich and Schmidtbleicher (1996) had subjects who were considered trained or untrained perform 5 reps of 5 second isometric maximal voluntary contraction of the plantar flexors.  Only the trained group showed signs of a potentiated H-reflex which suggested that an adaptation, inherent to spinal reflex processing may only be found in trained subjects.

Age

Muscles differ from a structural and functional viewpoint when the body goes through puberty.  The immature muscle of a pre-pubertal boy will start to increase in the diameter fiber size (Aherne eal al., 1971) and in the number of sarcomeres coupled with the muscles becoming longer (Bowden and Goyer, 1960) as they grow to achieve mature status.  With these fundamental changes we would expect there to be some difference in PAP between immature and mature muscles.  It was demonstrated by McComas et al. (1973) that the twitch peak force of a post-pubertal boy was significantly higher than that of pre-pubertal boys (McComas et al., 1973).  Passuke et al. (2000) performed a study in which they electrically evoked the plantar flexor muscles of pre-pubertal boys (11 years old) post-pubertal boys (16 years old) and post-pubertal men (19-23 years old) to see if there was a difference in their twitch contraction properties (Paasuke et al. 2000).  Their results confirmed those of McComas (1973) given that they too demonstrated that pre-pubertal boys have lower peak twitch forces at rest when compared to those of the post-pubertal boys and men.  On the other hand, the pre-pubertal boys had higher ratios of peak twitch forces to MVC force when compared to the post-pubertal boys and men, which indicated that even though muscles change in their capacity to generate force as they go through puberty, no changes in potentiation capacity and time-course characteristics occur (Paasuke et al., 2000).

The structural and functional changes to our muscles continue to occur as we age: it is known that muscle atrophy increases as we age and that atrophy to type II fibers is higher compared to that of the type I fibers (Lexell et al., 1988).  This reduction in type II fibers was previously demonstrated to decrease PAP in a study by Hamada et al. (2003). A study performed by Baudry and Duchateau (2004) confirmed that the PAP is reduced by aging however they also demonstrated that this difference in PAP only occurs immediately after the conditioning MVC and that after one minute the differences dissipate (Baudry et al. 2005).

Gender

Men have a greater percentage of type II muscle fibers which would cause an increase in PAP; while women according to a study conducted by O’Leary and Sale (1998) suggest that a women’s tendancy for a lower twitch to tetanus ratio and their greater fatigue resistance favors a greater post-tentanic potentiation.  Thus it would appear that both sexes have attribute which could feasibly cause an increase in PAP.  O’Leary and Sale (1998) compared the post-tetanic potentiation of the dorsiflexors of both men and women; muscle contractions were evoked before and after 7 seconds of tetanic stimulation at 100Hz.  Potentiation was 42% for women and 45% for men 5 seconds post tetanus and was still present at 5 minutes post tetanus (women 24% and men 25%).

Rixon and Bemben (2007) conducted a study on PAP and how it is affected by various traits such as gender, training experience and type of contraction.  In this study, subjects were split into groups based on gender and training experience.  During the performance of the dynamic squat, the experienced women lifters outperformed the inexperienced men however the inexperienced men jump0ed higher following the conditioning activity compared to the experienced woman.  During the study men constantly outperformed the women regardless of contraction type and experience.  Comyns et al. (2006) reinforced this theory in their study which tested rest protocols.  In an attempt to find optimal rest time, they demonstrated that only male subjects exhibited PAP at any of the interval time’s however they attributed this to a lack of coordination on the part of the females, while performing the jumps (Comyns et al., 2006).

Type of Muscle Contraction

The majority of the studies about PAP have used maximal isometric contractions as the conditioning activity due to belief that this type of muscle contraction creates a positive net balance in favor of potentiation compared to fatigue.  Rixon and Bemben (2007) compared and observed the effects of maximal isometric squats had on PAP versus a maximal dynamic squat.  The isometric condition created more on a PAP effect than the dynamic squat under each condition (male and female trained & male and female untrained).  In fact, some of the female subjects showed a slight decline in jump height and peak power following the dynamic squat which indicates that even if the dynamic squat potentiated the muscles it was masked by the fatigue caused by the dynamic squat.  It was suggested that the maximal isometric squat is not as metabolically demanding as the dynamic squat which would reduce the low-frequency fatigue allowing for potentiation to take place (Caruso et al., 2003).