Muscle Potentiation

Post-Activation Potentiation or Post-Activation Performance Enhancement for Acute Increases in Muscular Power… You Be the Judge

Muscular power is a key indicator of human performance and both athletes as well as beginners alike abide by periodized strength training programs to enhance power. Strength and conditioning programs to increase muscular power are designed to enhance the speed, or velocity, by which force is applied. These strength training workouts are programmed to enhance muscular strength and power chronically over time, as opposed to acutely, by promoting muscular physiological adaptations. Though, it has been proposed that acute enhancements in muscular power may be evoked via post-activation potentiation (PAP) [1]. However, there is more to this story. 

The concept of increasing muscular power and human performance via PAP has been around since the early 1900’s. Scientists first discovered the post-activation phenomenon by producing low [2] and high [3,4] frequency stimulations (using muscle stimulation) and examining muscle twitch responses. Thus, there was no outside influence of neural drive. These were referred to as staircase and post-tetanic potentiation. Staircase potentiation consisted of several low-frequency electrical stimulations that gradually increased in amplitude whereas post-tetanic potentiation utilized a brief high-frequency train of electrical stimulation. These brief bouts of electrically signalled muscle contractions are termed the “conditioning contraction” as the twitch response (potentiation) is measured after (hence, post activation) the conditioning contraction. It has been proposed that the use of conditioning contraction to increase the speed of a muscular contraction may positively impact athletic performance [5,6]. Though, it was not until the 1970’s when PAP was fully introduced to the human performance community. True PAP differs from staircase and post-tetanic potentiation in that it is defined as the enhanced contractile response (muscle twitch) evoked by voluntary muscle activation, as opposed to involuntary activation [7]. It has been speculated that the first application of this concept to human performance did not originate until the early 1980’s and may not have made its way to strength coaches until a meeting in 1986 at the Moscow Institute of Sport. Though, famous strength and conditioning specialist, Charles Poliquin, recalls the first time hearing about this phenomenon at the National Strength and Conditioning Association Conference in 1991.

Enhancing power is the single most important variable to enhancing all aspects of human performance. Therefore, it is important to not only understand the chronic mechanisms by which speed and force of contraction can be increased but also the acute mechanisms. This may be through PAP. However, as more scientific research studies are performed, and evidence is displayed, the physiological underpinnings of PAP may not be the same as the physiological mechanisms that lead to enhanced sport performance, defined at post-activation performance enhancement (PAPE) [8]. This warrants further investigation into the physiological mechanisms of acute power development to optimize human performance.

Defining Muscular Power

Muscular power is the ability of the muscle to contract forcefully and quickly [9]. Thus, the ability to produce a lot of force in a short period of time would display enhanced power output. The below equation to calculate power displays this relationship:

Power = Force x Velocity

It is well understood that greater neural drive and greater muscle cross-sectional area (CSA) equates to stronger muscle. This targets the force production aspect of the power equation. Therefore, emphasis must be placed on muscle strength and hypertrophy training. However, to increase the velocity variable in the equation, exercise training must also consist of high velocity contractions. 

Strength and/or hypertrophy training programs are designed to enhance neuromuscular facilitation and increase the size of the muscle’s CSA. This type of training typically consists of moderate to high intensity ranging from 65-95% of someone’s 1 repetition-maximum (1RM). 

Power training consists of high velocity contractions designed to increase the rate of force development (ie. faster contraction rate). The key is to move the body (via plyometrics) or weight (like a power clean) as fast as possible. Through periodized exercise prescription, strength coaches elicit positive physiological neuromuscular adaptations. 

Skeletal muscle adaptations that make the body bigger and stronger are a result of specific neural and muscular adaptations. The reason bigger muscle is considered stronger muscle is because of the cellular adaptations that happen within each individual muscle fiber. A well-designed strength and conditioning program will result in the gradual accumulation of muscle proteins through the upregulation of hypertrophic signalling pathways, release of inflammatory agents, and activation of satellite cells [10]. With an increase in skeletal muscle proteins, specifically contractile proteins, the muscle cell gets bigger and produces greater contractile force. 

In addition to the muscular adaptations, neural adaptations also take place contributing to increased force and velocity of contraction. The neural adaptations are as follows: increased motor unit (MU) activation, MU firing rate, MU synchronicity, and increasing agonist activation while decreasing antagonist activation [11,12,13,14]. A motor unit (MU) is a single alpha-motor neuron and all of the muscle fibers that it innervates and it is the alpha-motor neuron, located in the spinal cord, that sends an electrical signal to the muscle. By stimulating more muscle fibers together at a faster rate, both force and velocity of the muscular contraction may increase causing an increase in power output. 

A Fun Story and Distinctions Between PAP and PAPE

It may be possible that this is a fictitious story, but rumor has it that Canadian Sprinter Ben Johnson warmed-up with sets of maximal squats minutes prior to his 1988 Olympic 100m sprint world record of 9.79 seconds. This would be taking the concept of PAP and applying it to a competition setting where an athlete performs a conditioning voluntary contraction to increase the firing rate of his muscles. Even though his medal and world record were later revoked due to a positive test result for performance enhancing drugs, one cannot help but wonder if heavy squatting potentiated his muscles, giving him world record speed.

Researchers Güllich and Schmidtbleicher have reported that a bobsledding team did something similar to elicit PAP and it resulted in a world championship in 1995 [15]. These same two researchers have shown jump height to increase after doing multiple sets of 5 second maximal isometric contractions which leads to believing these stories a little more [15]. Though it seems as though PAP may promote enhanced muscular power and performance, the physiological mechanisms behind PAP and the performance enhancement demonstrated from muscular contractions (PAPE) are quite different.

It was not until recently that PAP and PAPE took on different definitions. Post-Activation Potentiation (PAP) is defined as an increase in the amplitude of a muscle twitch that continues a few minutes after a conditioning voluntary muscular contraction [19]. On the contrary, post-activation performance enhancement (PAPE) is an enhancement in post-contraction performance (ie. running speed, jump height, etc.) [8]. Though it may be speculated that PAP results in performance enhancement, the physiological mechanisms of PAP and PAPE are different. 

Physiology of Post-Activation Potentiation

The proposed mechanisms of PAP are governed by two theories [16]. Theory number one suggests that the conditioning contraction results in the phosphorylation of myosin and renders actin-myosin more sensitive to calcium [17]. Calcium is a key component of muscular contraction, without it, no muscle contraction would occur. Essentially, the conditioning contraction results in an increased crossbridge cycling rate, so actin and myosin are connecting and “sliding” faster resulting in more force being applied at higher velocities (ie. increased power). 

The second theory behind PAP is via the H-Reflex (named after Paul Hoffman who first described it). Strength training, which requires high-intensity voluntary muscular contractions, prior to competition increases synaptic excitation within the spinal cord [18]. Consequently, this enhances the postsynaptic potential of the motor neuron which increases the efficiency and rate of nerve impulses resulting in an increase of force generating capacity of the muscles. Thus, the motor neuron is more excitable and ready to fire! It is understandable as to how these two theories may translate into enhanced human performance and power production.

However, the half-life of PAP is approximately 28 seconds and no longer than 3-minutes [19]. This is contradictory to PAPE where performance enhancement seen after conditioning muscular contractions takes approximately 6-10 minutes and has demonstrated to still be evident beyond 15-minutes [19]. Meaning, those that have been using the physiology behind the PAP theories as a means of performance enhancement may be inaccurate in their reasoning, which makes it important to distinguish the physiological differences between PAP and PAPE. 

Physiology of Post-Activation Performance Enhancement

The key difference between PAP and PAPE is the significant difference in the time course of force enhancement which suggests different physiological mechanisms. For PAP, there is an early effect (within seconds) observed but a delayed effect (after minutes) for PAPE. Therefore, it is important to understand the physiological mechanisms underlying PAPE.

Evidence for PAPE suggests similar physiological benefits as a warm-up prior to an activity [20]. The voluntary conditioning contractions prior to a sport or activity increases the heat produced by the muscle and also increases muscle blood flow as well as muscle intracellular water content [19]. Increasing muscle temperature anywhere from 0.3-0.9 degrees celsius increases cross-bridge cycling rate (ie. faster muscle contraction) [19,21] and has demonstrated to increase muscle power anywhere between 1-10% [22,23]. Muscular contractions increase blood flow to the muscle and a subsequent shift of fluid into the intracellular space causes an increased sensitivity to calcium [24]. As previously mentioned, calcium is an essential component of muscular contraction as it binds to the protein, troponin, which ultimately allows for actin and myosin crossbridge formation. The increased sensitivity to calcium allows for greater force production and faster shortening velocity [25,26]. PAPE elicits an increase in the rate of muscular force development, meaning force produced at high speed contractions suggesting a physiological distinction between PAP and PAPE. 

How PowerDot Induces PAP and PAPE

To acutely enhance performance, it is recommended to use PowerDot in a warm-up. PowerDot has harnessed and optimized the power of NMES into a wireless bluetooth device. PowerDot is FDA approved and requires minimal equipment outside of a phone or tablet. With PowerDot’s scientifically designed potentiation setting, there is no more guesswork when it comes to enhancing acute power production. 

A proper warm-up utilizing the potentiation setting on the PowerDot app may enhance the numerous physiological benefits associated with both PAP and PAPE. The muscle stimulation may induce an increase in the muscle twitch response, as demonstrated with staircase and post-tetanic potentiation. Though, incorporating the NMES technology into a warm-up may also further increase muscle temperature as well as increase blood flow to the working muscles [27]. The physiological benefits of a warm-up include [28]: 

• Improvements in physical performance due to increases in blood flow and muscle and core temperatures
• Faster muscular contraction and relaxation
• Greater economy of movement because of lowered viscous resistance within active muscles
• Facilitated oxygen delivery and use by muscle, because hemoglobin (protein that carries oxygen) releases oxygen more readily at higher temperatures
• Facilitated nerve transmission and muscle metabolism

Using NMES as the conditioning contraction, as opposed to maximal voluntary contractions, may result in a greater potentiating effect increasing muscular torque and power [29]. This elicits faster muscle cross-bridge cycling and/or greater synaptic excitation. 

PowerDot electrical muscle stimulation provides a means of bypassing the central neural drive to elicit a muscular contraction. A specific concern with PAP is muscular fatigue [30]. By utilizing NMES with Powerdot, users eliminate the central fatigue worry associated with voluntary contractions. Therefore, PowerDot provides the most efficient and effective means of acutely enhancing muscular power using both physiological concepts from PAP and PAPE. 

If looking to gain a competitive edge or just improve overall human body performance, the PowerDot Duo is a vital piece of technology. It is simple to use, lightweight, and can make the greatest physiological impact. PowerDot provides the means for everyone to acutely enhance human performance. 

References

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