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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Keywords:©2002 The American College of Sports Medicine
postactivation potentiation; skeletal muscle; contractile properties; muscle endurance; muscle strength; training