Post Activation Potentiation (PAP)
- PART I -
Over the last several decades, resistance training has become an increasingly popular mode of exercise among many populations . Various advanced strength-training techniques and mechanisms have been employed by strength coaches, trainers, therapists, and researchers alike in order to maximize performance and function of athletes and trainees. Some of these techniques and mechanisms include reciprocal inhibition, concurrent activation potentiation, agonist-antagonist co-activation, eccentric accentuated training, concentric-only training, and post activation potentiation [2-5]. Recently much attention has been placed on the theory of Post Activation Potentiation (PAP) as a means of temporarily increasing power, torque, and force production so as to positively influence long-term training and performance [6, 7]. This is often accomplished by performing some form of intense voluntary contractions immediately prior to an explosive activity.
Although post activation potentiation has been used for several decades by strength coaches and trainers to enhance power, only recently has this phenomenon been more closely examined in controlled research settings . PAP has been described as a physiological phenomenon in which intense series of voluntary muscular contractions typically performed using heavy isotonic movements (barbell back squat) produces temporary increases in peak force and power during subsequent explosive activities . Although the exact physiological components that could contribute to this response are still debated there are several proposed mechanisms that could be attributed to this form of short-term synaptic plasticity all of which relate to increased CNS stimulation. First it would appear that intense muscular contractions produce phosphorylation of myosin light chains thus increasing the sensitivity of actin and myosin filaments to calcium . This in turn creates stronger contractions, as there is a greater response to the calcium released during the contraction process.
A second proposed mechanism that may be involved in the post activation potentiation process is based on the idea that intense muscular contractions induce greater amount of calcium released per action potential thereby increasing force and torque of subsequent contractions [11, 12]. Another theory associated with PAP is based on increased motor unit recruitment induced from heavy loads or high intensity movements. As a result of the short-term contractile history there would be an increase in the number or motor units recruited (higher threshold motor units) as well as an increase in the firing rate of those motor units .
Finally a theory predicated on proprioceptive mechanisms involving the Hoffmann Reflex (H-Reflex), suggests that prior heavy loading may increase muscle spindle activation, leading to increased discharge of type 1a sensory fibers . This would lead to increased excitability of alpha motor neurons and ultimately lead to increased innervation of extrafusal muscle fibers (increased alpha gamma-co-activation). Researchers postulate that post-activation potentiation may enhance the H-reflex, thereby increasing the firing rate and efficiency of the nerve impulse to the muscle .
General Research on PAP
Although post activation potentiation is a relatively new training technique, numerous studies have investigated its effectiveness as well as explored training variations and protocols that could maximize this form of short-term synaptic plasticity. One of the first studies to examine PAP in strength training was performed by French et al.  during which maximal voluntary contractions (MVC’s) were used to produce a potentiation effect. Results demonstrated that performing 3 repetitions of maximal isometric contractions for 3 seconds on a knee extension device induced a significant improvement in drop jump performance with an increase in jump height, maximal force, and acceleration impulse. Similarly, a study performed by Requena et al.  showed that a single 10-second MVC using a knee extension isometric produced significant improvements in vertical jump height as well as sprint time performance in professional soccer players.
Although the above examples of PAP utilized MVC isometrics, most studies have employed isotonic lower body exercises specifically the barbell back squat to induce postactivation potentiation. A study by Chatzopoulos et al.  demonstrated that heavy back squats performed as multiple sets of singles using 90% of 1RM improved sprint time in college-age athletes when performed 5 minutes prior to the sprint trial. A similar study found that performing a single set of back squats with a 3 repetition maximum (3RM) load increased vertical jump height when performed 4-8 minutes prior to the vertical jump assessment . Kilduff et al.  also observed that several sets of heavy barbell back squats (87% 1RM) produced improvements in vertical jump height and power output when performed 8 minutes prior to the jump test. Other research suggests that heavy barbell back squats not only produces a post activation potentiation response (when performed prior to explosive movements) but also offsets fatigue accumulation commonly witnessed with repetitive sprints .
Conservative PAP Methods: Less Effective
Because many studies such as those previously mentioned suggest that heavy strength training may produce a post-activation potentiation effect immediately prior to an explosive movement, multiple investigations have been attempted to reproduce similar effects via less extreme techniques such as dynamic warmups, low intensity plyometrics, and explosive strength training using lighter loads. However, most of the research demonstrates that these alternative forms of potentiation and attempts of short-term performance enhancement are either counterproductive or less effective than their heavy strength-training counterpart.
A study conducted by Lowery et al.  examined the effect of different back squat loading parameters on jump performance in fit college age males. Results indicated that moderate (70% 1RM) as well as (93% 1RM) when performed 4 minutes prior to a vertical jump test produced a significant enhancement in vertical jump performance and power. However when using the same protocol with light loading parameters (55% 1RM) there was no change in vertical jump performance.
Weber et al.  found similar results when comparing bodyweight squat jumps, a commonly performed (a movement included in many plyometric and dynamic warm-up programs) to heavy barbell back squats (85% 1RM). Results demonstrated that heavy back squats when performed 3 minutes prior to a consecutive squat jump assessment significantly increased vertical jump height and ground reaction forces. However the opposite occurred in the group performing squats jumps 3 minutes prior to assessing jump performance with vertical jump height and ground reaction forces significantly decreasing.
Other similar techniques such as bodyweight exercises, low intensity isometrics, and vibration training appear to be just as ineffective for producing short-term changes in power and force development. Research performed by Jordan et al.  examined the effects of whole body vibration training combined with bodyweight partial-squat isometrics on producing a post activation potentiation effect. This protocol failed to elicit any enhancement in measures of performance with no significant change in voluntary muscle activation or peak torque measurements. However, it should be noted that several other factors may have contributed to lack of potentiation in this investigation including the use of fatigue-inducing isometrics (60 seconds) and partial squats rather than full squats.
Conservative PAP Methods: Not Without Merit
Not all studies have concluded that light loads and explosive movements are detrimental or inferior to heavy loads for producing PAP. In fact several studies exist suggesting comparable potentiation effects. However there appear to be no current studies demonstrating light loads and explosive movement as producing superior PAP benefits to heavy resistance. At best they may be equivalent.
A study by West et al.  examined the effects of various upper body loading parameters for increasing the ballistic bench press throw. Results showed that performing heavy bench press repetitions (3 sets of 3 repetitions with 87% 1RM) produced comparable results to light-explosive bench press repetitions (3 sets of 3 repetitions with 30% 1RM). After 8 minutes of rest both conditions produced significant improvements in peak power output with the heavy loading condition producing a slightly greater improvement in performance than the light condition although this difference was not significant.
Gilbert et al.  drew similar results from their investigation concluding that power exercises (explosive movements with lighter loads) may produce similar post activation potentiation as high force movements (heavy loads). However the potentiation effect appears to dissipate more quickly in power exercises than with heavy loads. Gilbert et al. also emphasized that lighter power exercises may not induce the significant and immediate onset of fatigue experience directly after (0-3 minutes) heavy resistance protocols which may make it more suitable for certain training scenarios in which fatigue must be more closely monitored.
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