At any point in time, the performance of skeletal muscle is affected by its contractile history. The most obvious effect of contractile history is fatigue, which impairs performance. This review is about another effect of contractile history—postactivation potentiation (PAP). In contrast to fatigue, PAP serves to improve performance. The phenomenon of PAP and its mechanism(s) have been studied for many years, but its application to human performance has received less study. The purpose of the review is to consider the possible roles of PAP in human performance and, in particular, to discuss the hypothesis that PAP may offset fatigue in endurance exercise, increase rate of force development, and, thus, improve speed and power performance.
PAP is an increase in muscle twitch and low-frequency tetanic force after a “conditioning” contractile activity. In research on PAP, the conditioning activity (or contractile history) most commonly includes a series of evoked twitches (staircase or treppe), an evoked tetanic contraction (posttetanic potentiation (PTP)), or a sustained maximal voluntary contraction (MVC). In this review, potentiation, no matter how induced, will be referred to as PAP. An example of PAP is shown in Figure 1. The principal mechanism of PAP is considered to be phosphorylation of myosin regulatory light chains, which renders actin-myosin interaction more sensitive to Ca2+ released from the sarcoplasmic reticulum (12,13). Increased sensitivity to Ca2+ has its greatest effect at low myoplasmic levels of Ca2+, as occurs in twitch and low-frequency tetanic contractions; in contrast, increased sensitivity to Ca2+ has little or no effect at saturating Ca2+ levels, as occurs in high-frequency tetanic contractions. Thus, PAP “raises” the low- but not high-frequency portion of the force-frequency relation (1,14). In fact, the conditioning activity (e.g., 10-s MVC) can, concurrently, increase (due to PAP) and decrease (due to fatigue) low- and high-frequency force, respectively ((12);Fig. 2).
Most studies of PAP and the force-frequency relation have involved isometric contractions. It is important to recognize that the type of muscle contraction affects both the force-frequency relation and the range of frequencies over which PAP occurs. In concentric contractions, especially those at higher velocities, the force-frequency relation is shifted to the right compared with isometric contractions; that is, higher frequencies are needed to evoke a given percentage of maximum force (1). In addition, PAP extends to higher frequencies in concentric versus isometric contractions (1). Most activities involve primarily concentric (e.g., swimming, rowing, cycling) or coupled eccentric-concentric contractions (e.g., running, jumping, weightlifting); therefore, PAP may have a performance-enhancing effect beyond what would be expected based on its effect on isometric contractions. The interaction between the force-frequency relation, PAP, and contraction type is illustrated in Figure 3. Note the potentially greater role of PAP in concentric compared with isometric contractions.
A notable feature of PAP is that it is greater in fast, Type II muscle fibers because fast fibers undergo greater phosphorylation of myosin regulatory light chains in response to a conditioning activity (13). Accordingly, muscles with a higher percentage of Type II fibers (e.g., gastrocnemius vs soleus), and people with a higher percentage of Type II fibers within a muscle (e.g., vastus lateralis, (7)), exhibit greater PAP. A person’s fiber-type distribution is determined primarily by genetic factors, but may also be influenced by age and activity level.
It might be expected that PAP offers the greatest potential for performance enhancement in brief, maximal intensity activities requiring maximal strength and speed (and the product of these two, power), activities that depend on fast fibers. But in these activities, motor unit firing rates are likely to be at their highest, in the very range where PAP’s effect on force is smallest or absent. Taken in this light, greater PAP in fast fibers almost seems a “waste.” It would be better for slow fibers to have greater PAP, because they are typically involved in low-intensity activities in which motor unit firing rates are relatively low, the range in which PAP is greatest. However, it will be shown later that PAP has another effect on muscle (in addition to increasing force), an effect that is beneficial for speed and power performance, even when motor units are firing at high rates. This beneficial effect is also most pronounced in fast fibers.
Endurance performance typically consists of submaximal contractions that are repeated for prolonged periods. From the beginning of performance the contractions themselves would activate the mechanism(s) responsible for PAP (12). Because, in these submaximal contractions, the recruited motor units would be discharging at relatively low rates, the force output of the motor units should be increased by PAP (Fig. 2). If this happens, and if a constant force has to be maintained, motor unit firing rates would have to decrease to compensate for the increased force (or alternatively, some motor units could be de recruited). In fact, motor units have been observed to decrease their firing rates in sustained, constant force, submaximal contractions without altered motor unit recruitment (2). A decrease in motor unit firing rate, by reducing the number of nerve impulses and muscle action potentials per unit time, may delay impairment of “central drive” to motoneurons, neuromuscular transmission, muscle action potential propagation, and excitation-contraction coupling, all possible sites and mechanisms of fatigue. It should be noted that, as fatigue develops in sustained submaximal contractions, motor units already recruited will eventually have to increase their firing rates to compensate for fatigue, and depending on the exercise intensity and the muscle group, additional motor units will be recruited.
PAP may have a special role in compensating for the impaired excitation-contraction coupling that occurs with fatigue. Impaired excitation-contraction coupling is responsible for low-frequency fatigue (LFF), a disproportionate loss of low-frequency tetanic force (12). This is the exact opposite of PAP, which is a disproportionate increase in low-frequency tetanic force. Thus, PAP can serve to compensate for LFF. Many endurance activities (e.g., running, cycling, swimming) consist of repeated brief concentric or eccentric-concentric actions in which motor units discharge briefly at fairly high rates; however, it should be recalled from Figure 3 that in concentric (vs isometric) actions, the force-frequency relation is shifted to the right, extending the frequency range over which both LFF and PAP would be acting.
In contrast, PAP cannot compensate for so-called high-frequency fatigue, the force decline when motor units are firing at very high rates, because PAP cannot increase high-frequency force ((12);Fig. 2 and 3). Thus, in fatiguing exercise, low-frequency force can increase or be maintained (by PAP) at the same time as high-frequency force is decreasing (Fig. 4). With reference to Figure 4, a scenario in an endurance event can be imagined in which an athlete is faring well while maintaining a steady submaximal pace, because of the beneficial effect of PAP. However, if the intensity must be increased (hill climb, strategic pace increase, etc.), with the accompanying high motor unit firing rates, the situation would be as shown by the vertical dashed line in Figure 4. By changing from low to high motor unit firing rates, the beneficial effect of PAP would be lost and the full effect of fatigue would be felt.
During recovery after fatiguing endurance exercise, PAP decays within minutes whereas LFF can persist for at least several hours, if not days (12). The prolonged depression of low-frequency force may partly explain the “dead” leg complaint of endurance athletes during a period of high-volume training. From rest, when the athlete begins to do “easy” daily activities such as walking and climbing a flight of stairs, the leg muscles feel weak and are weak in response to the low motor unit firing normally employed to generate the required force. Faced with the LFF force deficit, central drive to the motoneurons must increase to recruit more motor units and to increase the firing rate of those already active, to compensate for the deficit. The athlete perceives this neural adjustment as increased effort. Perhaps surprising to the athlete, when the activity has been underway for a short time, it begins to feel easier. The explanation for this is that the activity has reactivated PAP, offsetting the effect of LFF. (This explanation assumes that motor unit firing rates are still within the range affected by PAP.) An athlete might also experience this transition from initial fatigue to “feeling better” during a training session, if LFF has persisted from the previous training session. It would be of interest to monitor both LFF and PAP in endurance athletes during a training program.
STRENGTH AND SPEED PERFORMANCE
Strength and speed performance typically require, in a brief maximal effort, that all relevant motor units be recruited and fire at maximum possible rates. It would appear, with reference to Figure 3, that PAP could offer little benefit when motor units are discharging at very high rates, because PAP cannot increase high-frequency force. Furthermore, PAP does not increase maximum unresisted shortening velocity (for review, see (4)). Therefore, with reference to the force (load)-velocity relation, the two extremes of this relation, peak isometric force and maximum shortening velocity (Vmax), cannot be altered by PAP (Fig. 5). However, PAP has an additional effect; it can increase rate of force development, even at very high stimulation frequencies where force is not increased by PAP ((1,14);Fig. 6).
The effect of PAP on rate of force development raises the prospect that PAP may, by increasing rate of force development and hence the acceleration attained with loads between zero (Vmax) and peak isometric force, increase the velocity attained with these loads. Thus, PAP would shift the load (force)-velocity relation upward and rightward (making it less concave) without changing the endpoints, as depicted in Figure 5. If this were to happen, activities like jumping, kicking, and throwing might be improved if the muscles were in a state of PAP. There are reports of improved (5,11,15) and unaffected (4,9,10) performance of this nature after a conditioning activity such as isometric MVCs or a set of repetitions with a heavy weight. The inconsistent results of the studies could be the result of variation in the performance to be improved, the conditioning activity, and the time interval between the conditioning activity and the performance. In only two studies (4,10) was the presence or absence of PAP monitored. The selection of the conditioning activity and the interval between it and performance are part of the strategy for exploiting PAP (see next section).
In experiments with the muscle of a small mammal (rat gastrocnemius), PAP was shown to increase the force and power of isovelocity concentric contractions, particularly at the higher velocities tested (up to ∼70% of Vmax). PAP did not increase isometric rate of force development or concentric force at the highest stimulation frequency used (1), indicating that, like isometric force, there is a limit (albeit higher than for isometric force) to the stimulation frequency at which PAP can have an enhancing effect on rate of force development. Whether this limit is reached with the motor unit firing rates attained in human performance is presently unknown.
STRATEGIES FOR EXPLOITING POSTACTIVATION POTENTIATION (PAP)
In exploiting PAP to enhance performance, two dilemmas must be resolved. First, a more intense and prolonged conditioning activity may activate the PAP mechanism to a greater extent, but it also produces greater fatigue (Fig. 7). The second dilemma, also illustrated in Figure 7, is that the longer the recovery period between the end of the conditioning activity and the beginning of performance, the greater the recovery from fatigue, but also the greater the decay of the PAP mechanism.
The two dilemmas can be resolved only by trial and error. In one study (4), the recovery period chosen was only 15 s, so that performance would begin when PAP (assessed by the force of twitch contraction) was still near its maximum (see Fig. 7). However, performance (velocity attained with a given load in concentric knee extension) was actually depressed because of fatigue induced by the conditioning activity (10-s isometric MVC). This was another example of low-frequency (isometric) force being increased at the same time as high-frequency (concentric) force was depressed. If a longer recovery period had been selected, say 3 min, performance may have been improved, provided that fatigue had dissipated at a faster rate than PAP had decayed (Fig. 7). There is some evidence that this is what happens when longer recovery periods are used (3,5,15).
An additional consideration is that when the performance is a series of contractions, the contractions themselves have a cumulative effect in mobilizing the PAP mechanism (4,10). This may partly explain the progressive increase in performance observed in a series of jumps (5) or dynamic knee extensions (4,10). It has also been shown that the effects of a conditioning activity and repeated performance have an additive effect on the magnitude of PAP, at least over a few repetitions of the performance (4,10). In fact, if the performance consists of enough trials, the PAP induced by the trials themselves may rival that of the conditioning activity, making the latter unnecessary (10). Again, all of this has to be sorted out by trial and error experiments.
The emphasis has been on strategies for exploiting PAP in strength/speed performance. For endurance performance, the optimal strategy may be simply to begin the event. PAP will be quickly induced by the first several contractions. It might be argued, however, that preliminary (conditioning) “warm-up” activity, apart from other possible benefits, would allow the athlete to benefit from PAP at the beginning of the event. As in strength and speed performance, a balance has to be struck between the PAP and any fatigue induced by the conditioning activity.
EFFECT OF TRAINING ON PAP
Some adaptations to strength training could possibly increase PAP induced by a conditioning activity such as a 10-s MVC. One adaptation would be increased ability to activate, during the conditioning activity, the high threshold, fast motor units, whose muscle fibers exhibit the greatest PAP. Because the magnitude of PAP is dependent on the extent of activation in the conditioning activity, greater activation (recruitment and increased stimulation rate) of these fibers would contribute to greater PAP in the whole muscle. A second adaptation is the preferential hypertrophy of fast versus slow muscle fibers commonly observed with strength training. If, after training, fast fibers comprise a larger proportion of the whole muscle, and PAP is greater in fast fibers, PAP of the muscle as a whole would be expected to increase. A third possible adaptation would be increased capacity for PAP, brought about by altered myosin light chain composition (see discussion of endurance training below). As a specific example, the regular performance of jumping drills might improve jump performance in repeated jumps as a result of increased PAP from the adaptations outlined. Of the few longitudinal training studies done to date (for review, see (6)), only one showed an increase in PAP, after 12 wk of weight training in elderly subjects (8). In the one cross-sectional study, PAP was greater in recreational weight trainers than sedentary subjects (6). When an increase in PAP has been observed, the adaptation responsible for the increase has not been identified.
In contrast to strength training, endurance training is likely neither to increase the ability to activate high threshold fast motor units in brief maximal contractions, nor to cause preferential hypertrophy of fast twitch fibers. Nevertheless, the one published study related to endurance training found PAP, induced by a 10-s isometric MVC, to be greater in endurance athletes than sedentary control subjects. The enhanced PAP was only present in the trained muscles, pointing to a training adaptation rather than a genetic trait (6). In addition to the absence of adaptations that might increase PAP as a result of strength training, the endurance athletes likely possessed a greater than average percentage of slow twitch fibers, which would promote reduced rather than amplified PAP.
What adaptations could account for the increased PAP in the endurance athletes? One adaptation would be increased fatigue resistance, which would allow PAP to prevail over fatigue immediately after the conditioning activity. In fact, in the study cited above (6), the endurance athletes suffered less fatigue during the 10-s MVC used to induce PAP. A second possible adaptation is an increase in the content of “fast” myosin light chains in Type I fibers (for review, see (6)), which might increase the capacity for myosin light chain phosphorylation, the principal mechanism of PAP. The significance of this adaptation, if it occurs, is that the capacity for PAP has been increased, whereas the other possible adaptations described in response to strength or endurance training act by increasing activation of an existing capacity for PAP by a conditioning activity (increased motor unit activation), increasing the proportion of the muscle occupied by fast fibers (which produce the greatest PAP), or by allowing the PAP mechanism to dominate over fatigue (increased fatigue resistance). Figure 8 is a schematic showing how PAP might increase after combined strength and endurance training. Throughout the test depicted in Figure 8, all adaptations previously discussed may have contributed to the increased PAP, although increased fatigue resistance would have played a relatively greater role as the test progressed. For a specific sport example, the increased PAP from strength training may enhance acceleration at the start and during sprint phases of a cross-country ski event, whereas the increased PAP from endurance training may improve efficiency by reducing the motor unit firing rates needed to maintain a competitive pace.
PAP and its mechanism(s) have been studied for many years, but their application to human performance has received less study. There is some evidence that PAP can improve high-velocity strength and power performance, but more research is needed to determine the best strategy for exploiting PAP. Theoretically, it can be imagined how PAP could enhance endurance performance, but again, more research is needed to clarify PAP’s role. The few studies to date suggest that both strength or endurance training may increase PAP, but the extent of increase and the responsible adaptations await further study. Finally, research on PAP must also consider other effects of contractile history. For example, an increase in muscle temperature would, by increasing rate of force development, act synergistically with PAP to improve power performance. On the other hand, increased temperature shortens twitch duration and therefore shifts the force-frequency relation to the right, an effect opposite to that of PAP. Thus, PAP’s possible enhancement of endurance performance would be opposed by increased muscle temperature.
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