Pap

Expert Discussion Strength Training
1

Post Activation Potentiation

Introduction

When a muscle is (near) maximally contracted a phenomena occurs where for a window of time afterwards rate of force development at intermediate speeds is increased. This has been described as post activation potentiation (PAP). PAP is significant because it may be exploited to improve training stimulus and/or competition performance. PAP is induced in a muscle group either by maximal voluntary contraction or performance of an appropriate limit strength exercise. (I define a limit strength exercise here as one that permits the expression of maximal force). Exploiting PAP in strength training has been described as ‘Complex’ training. This involves a limit strength exercise followed, after an appropriate recovery, by an explosive exercise with similar kinematics, e.g. squats followed by counter movement jumps.

Protocols for Inducing PAP

Protocols used to successfully induce PAP (as measured by a significant increase in jumping performance) include 5x1r at 90%1RM (Repetition Maximum) (15); 3x3r at 3RM (3); & 1x5r at 5RM (13) In these three studies the conditioning exercise was a half squat performed on a Smith Machine (permits movement in the vertical plane only). A positive effect was also induced from three ten second maximal voluntary contractions of the leg extensor muscles (5).

The research articles that have investigated PAP have given little or no information regarding the type of warm up performed prior to testing. This is however extremely significant since if an inefficient warm up protocol were performed, the magnitude of the PAP effect cannot be accurately assessed, i.e. the conditioning exercise may simply improve the quality of the warm up.

An important variable is the actual conditioning activity selected. The obvious difference in the current research is between dynamic exercises, e.g. the squat, and maximal voluntary contractions. An exercise like the squat, activates several muscle groups including the ankle, knee and hip extensors. Due to the cumulative PAP effect across several muscle groups one might therefore expect the squat to have a greater positive effect on vertical jump performance than an isolated maximum voluntary contraction of one extensor muscle group.

There are two conflicting elements associated with the selected methodology; the positive effect of the PAP and the negative effect of fatigue. Increasing total time under tension and decreasing recovery may increase PAP, but would also increase fatigue (10). Further investigation is required to determine the optimum protocol to maximise PAP and minimise fatigue.

Volume is typically measured as the product of the number of repetitions and the number of sets. This is inaccurate however because time under tension for one repetition varies between exercises and, due to limb length variations, between subjects. Additionally, due to muscle length tension relations and lever arms, a maximum lift on a given exercise may achieve maximum force through a small range and therefore only for a fraction of a second. Maximal voluntary contractions offer an advantage in this respect because time under maximal tension can be accurately measured.

It has been suggested that a muscle must contract at greater than 85% of its maximum force in order to induce PAP (5). Lifts below the threshold should therefore be classed as warm ups. Some protocols used to investigate PAP however, may not exceed this threshold. Additionally, the relationship between intensity and the number of repetitions possible is specific to the individual at a given moment in time (16). Some athlete’s may have a 5RM equal to 80% of maximum whilst for others it may be 90%. Athletes with higher percentages of fast twitch fibres can typically perform fewer repetitions at a given sub maximal intensity (17).

Research suggests the effect of fatigue is more acute in magnitude than PAP but shorter in duration (15). Hakkinnen (6) showed that a three minute recovery was insufficient to restore maximum power output following maximal squat lifts. Other research showed that PAP was still present 18 minutes after the conditioning activity (15). Further research is required to determine the time course effect of PAP.

Muscular fatigue causes a decrease in both maximum force and rate of force development (6). One recognised cause of fatigue is an accumulation of the metabolic waste products, hydrogen and phosphate ions in the sarcoplasmic reticulum. These ions inhibit the function of the contractile proteins (12). Fatigue from high intensity, low volume exercise (e.g. weightlifting) however results in little production of these metabolites (6). Fatigue from high intensity exercise is correlated with the reduction in intracellular calcium ion concentration (due to reduced uptake) and ATPase activity (12). Alternatively, the decline in force capabilities may result from central nervous system fatigue or more specifically a decrease in rate coding (i.e. the number of nerve impulses per second) (6, 9).

Many researchers have recognised the significant correlation between the athlete’s prior strength level and the magnitude of the PAP response (5). In some cases athletes with poor strength actually showed a performance decrement in response to the PAP protocol (3). One neural adaptation to strength training is increased recruitment of fast twitch muscle fibres. As will be discussed later, the PAP response appears to increases in magnitude in correlation with fast twitch fibre recruitment. Weaker athletes would therefore benefit less from PAP because they are unable to activate fast twitch motor units (5). Additionally, the magnitude of PAP may be related to the muscle group being investigated. Muscles with a high percentage of fast twitch fibres such as the gastrocnemius are likely to show a more significant increase in rate of force development.

Typically, motor units are recruited in order of size hence the largest and fastest units are recruited last (9). Recent research, suggests appropriate training permits preferential recruitment of high threshold fast twitch motor units earlier in the recruitment order, i.e. against the size principle (8). Whilst the current research suggests that the higher the level of athlete the greater the effect of PAP, it is possible that elite power athletes are already able to recruit fast twitch fibres at high velocities and would not therefore derive a benefit from PAP. Further research is required to investigate the affects across a spectrum of standards.

Possible Mechanisms for PAP

PAP does not increase maximum shortening velocity or maximum force but shifts the Force: Velocity relationship upwards, i.e. greater force at any intermediate movement velocity. This occurs due to increased recruitment of fast twitch fibres. Fast twitch fibres have high threshold neurones, axons with high conduction velocities and fibres with large cross sectional areas. Due to increased activity of the enzyme ATPase (which breaks down ATP to yield energy), fast twitch shortening velocities are up to four times faster than slow twitch fibres (17). Increased recruitment of fast twitch fibres therefore significantly increases both force and rate of force development (8).

There are two mechanisms that have been suggested to explain the increase in fast twitch fibre recruitment, the H-Reflex and phosphorylation of myosin regulatory light chains. The former is a reflex in the nervous system that increases central drive in response to near maximal stimulation (3, 15). The increase in rate coding increases recruitment of higher threshold motor units. The magnitude of the H reflex is proportional to the degree of muscle activation (10, 11). This is one possible explanation why stronger athletes respond more positively to PAP.

Since the H-reflex occurs centrally in the nervous system it is possible that it may increase rate of force development in muscles that are not specific to the conditioning exercise performed, i.e. a heavy squat may potentiate performance in a bench throw. An investigation into such an effect may help determine the relative effect of the H-reflex versus the other mechanisms.

Phosphorlation of myosin regulatory light chains causes an increase in the sensitivity of troponin to calcium ions released from the lateral sac of the sarcoplasmic reticulum (10). When troponin binds with calcium ions it changes the position of the attached tropomyosin and allows strong cross bridge binding between actin and myosin, and hence initiates muscle contraction. This increased sensitivity reduces the neural stimulus required to activate fast twitch muscle fibres (10).

Another possible explanation for PAP is an increase in ‘intra-muscular’ coordination, i.e. increased correlation of discharge from relevant motor units within a given muscle group (8). Performing a maximal contraction may increase synchronisation by ensuring all motor units are in a similar refractory state. Research suggests that increased synchronisation particularly during initial motor unit activation could increase RFD (9).

Practical Application

PAP improves performance in speed strength exercises by increasing fast twitch fibre recruitment. Providing the increased stimulus does not lead to overreaching, its use in training may lead to a greater rate of improvement in strength and power. The protocol used is however very important. If for example, the limit strength is performed in high volumes then the fatigue induced will outweigh any possible benefit from PAP. Research by Verkhoshansky (14), albeit with novice track athletes, showed that simply switching the exercise order so that limit strength exercises were performed first actually had a negative effect on development during a twelve-week program.

Medvedyev (18) suggests the optimum exercise order in resistance training is speed strength; limit strength and finally strength endurance. It may be however that an optimum strategy is to follow this recommendation but to additionally perform very low volumes of limit strength exercises during the warm up. That way PAP is achieved without inducing detrimental fatigue and sufficient volume can be maintained in the limit exercise to induce/maintain structural and neural adaptations. Of course this approach would depend on how long a significant PAP effect is maintained.

There are a number of areas that require further research. These include investigation of highly specific populations, e.g. elite weightlifters; investigation into the minimum volume and intensity required to induce PAP; and PAP’s time course effects. Improved understanding of these variables would also be important in optimally utilising PAP in the warm up for competition in power sports, e.g. field athletics.

References

  1. Bigland-Ritchie, B., and J.J. Woods. 1984. Changes in muscle contractile properties and neural control during human muscular fatigue. Muscle and Nerve. 7: 691-699

  2. Brown, G.L. and .S. Von Euler. 1938. The after effects of a tetanus on mammalian muscle. Journal of Physiology. 93: 39-60

  3. Duthie, G.M., W.B. Young and D.A. Aitken. 2002. The acute effects of heavy loads on squat jump performance. Journal of Strength and Conditioning. 16(4): 530-538

  4. Ebben, W.P., R.L. Jenson and D.O. Blackyard. 2000. Electromyographic and kinetic analysis of complex training variables. Journal of Strength and Conditioning. 14(4): 451-456.

  5. Gullich, A. and D. Scmidtbleicher. 1996. MVC induced short term potentiation of explosive force. In New Studies in Athletics. 11(4): 67-81

  6. Hakkinen, K. 1993. Neuromuscular fatigue and recovery in male and female athletes during heavy resistance exercise. International Journal of Sports Medicine. 14(2): 53-59

  7. Harris, G.R., M.H. Stone, H.S. O’Bryant, C.M. Prolx and R.L. Johnson. 2000. Short term effects of high power, high force or combined weight training methods. Journal of Strength and Conditioning. 14(1): 14-20

  8. Moritani, T. 1993. Neuromuscular adaptations during the acquisition of muscle strength, power and motor tasks. Journal of Biomechanics. 26910: 95-107.

  9. Sale, D.G. 1988. Neural adaptations to resistance training. Journal of Medicine and Science in Sports and Exercise, 20: 135-145

  10. Sale, D.G. 2002. Postactivation potentiation: Role in human performance. Exercise and Sport Science Reviews. 30(3): 138-143.

  11. Trimble, M.H., S.S. Harp. 1998. Post exercise potentiation of the H reflex in humans. Journal of Medicine and Science in Sport and Exercise. 30(6): 933-941

  12. Williams, J.H., C.W. Ward, E.E. Spangenburg and R.M. Nelson. 1998. Functional aspects of skeletal muscle contractile apparatus and sarcoplasmic reticulum after fatigue. Journal of Applied Physiology. 85(2): 619-626

  13. Young, B.A., A.Jenner, and K. Giffiths. 1998. Acute enhancement of power performance from heavy load squats. Journal of Strength and Conditioning. 12(2): 82-84

  14. Verkhoshansky, Y., V. Tatyan. 1973. Speed-strength preparation of future champions. Soviet Sports Review. 18: 166-170

  15. Chiu, L.Z.F., A.C. Fry, L.W. Weiss, B.K. Schilling, L.E. Brown and S.L> Smith. 2003. Postactivation potentiation response in athletic and recreationally trained individuals. Journal of Strength and Conditioning. 17(4): 671-677

  16. Poliquin, C. 2001. Modern trends in strength training Volume 1, K. Goss (Ed.)

  17. Zatsiorsky, V.M. 1995. In Science and practice of strength training, R. Waashbourn (Ed.). Human Kinetics, pp. 44-50.

  18. Medvedyev, A. 1989. The methods of weightlifters’ speed strength training. In A system of multi year training in weightlifting, translated by A. Charniga Livonia: Sportivny press, pp. 80-102

2

Wow, so PAP was the mechanism Ben Johnson was exploiting when he squatted 600lbs minutes before the final in Seoul. Hehehe.

Seriously though, a very interesting article. Is PAP the same phenomena that is sometimes referred to as post-tetanic neuromuscular facilitation? Your article deals mostly with the effect of limit strength exercise performance on speed/power, but what about the effect of strength on strength? Some strength coaches prescribe wave rep schemes (e.g. 1,6,1,6,1,6) with the goal of exploiting PAP created in the 1-rep sets to improve performance in the 6-rep sets. Any evidence that this actually works?