INTRODUCTION
The ability to express high-power outputs is an important factor that can differentiate between athletes at various levels. In sports like American football, the ability to express high levels of maximum strength, power, and speed appears to explain many of the differences between levels of play, playing abilities, and position (11,45,9011,45,9011,45,90). For example, Barker et al. (11) noted that starters were stronger, more powerful, and faster than nonstarters. Similarly, Fry and Kraemer (45) reported that Division I football players were on average stronger, more powerful, and faster than their Division II and Division III counterparts. When examining high school football players, Dupler et al. (33) demonstrated that speed, power, and agility are higher in varsity high school football players when compared with junior varsity and freshman players. Similarly, as collegiate football players progress from their freshman to senior year of play, strength levels significantly increase, as does power output (69). Taken collectively, it is clear that regardless of the level of play, the ability to express strength, power, speed, and agility are central characteristics when training football players.
When considering strength, power, speed, and agility, it is important to understand that these specific characteristics are all intimately related. For example, strength should be considered the foundation for power output, speed, and the ability to change direction (Figure 1) (20,30,6620,30,6620,30,66). In support of this contention, Bret et al. (18) report that 100 m running performance is significantly related to concentric half-squat (r = 0.74, p < 0.01) and countermovement jump displacement (r = 0.57, p < 0.05), which can be considered as indicators of strength and power characteristics. Improvements in strength appear to play a role in increasing the ability to accelerate (31) in the early part of a sprint. Because improvements in strength and power-generating capacity can result in an increased ability to accelerate, it is likely that these adaptations can lead to an increased ability to change direction (127) as this ability is highly reliant on the ability to reaccelerate and apply forces rapidly. Based on this line of reasoning, there seems to be an interrelationship between muscular strength, sprint performance, and change of direction speed (66). Support for this contention can be found in the work of Hori et al. (66) where hang power clean performance was reported to be significantly correlated with jumping (r = 0.41, p < 0.05), 20-m sprint time (r = −0.058, p < 0.01), and change of direction speed (r = −0.41, p < 0.05). Additionally, the 1RM front squat demonstrates significant correlations with 20-m sprint time (r = −0.60, p < 0.01) and change of direction speed (r = −0.51, p < 0.01). Taken collectively, it is clear that the ability to express high-power outputs is dependent on the athletes overall level of strength and improvements in power output, speed, and change of direction speed are related to improvements in maximum muscular strength.
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Figure 1: Interrelationship between strength, power, speed, and change of direction speed.
The type of program used appears to strongly influence the ability to develop the performance foundation necessary for success in football. Hoffman et al. (63,6463,64) suggest that simply performing heavy-loaded resistance exercises, consistently, like those typically used in the sport of powerlifting does not optimize football performance. Instead, it is suggested that the inclusion of weightlifting exercises that target explosive strength, such as the power clean and power snatch, are essential components of the training plan and result in greater overall improvements in leg strength, explosiveness, and running speed. Additional support for these findings can be found in the work of Tricoli et al. (117) who report that combinations of heavy resistance training and jump training produce less than optimal results compared with weightlifting exercises combined with jump training when examining strength and jumping performance. As a whole, these data seem to suggest that training with a combination of activities that target the development of both maximal strength and explosive strength, such as weightlifting, is necessary for the maximization of power-generating capacity and the optimization of football playing ability (55).
Therefore, the primary purposes of this brief review are to (a) define power, (b) discuss the physiological factors that contribute to power-generating capacity, (c) examine the role strength plays in power generation, (d) discuss periodization models that can facilitate power development, and (e) present specialized methods that can be used to maximize power development.
DEFINING POWER OUTPUT
Mechanical power output can be defined as the rate of performing work or the product of force and velocity (73,11073,110). Typically, power can be calculated with one of the following equations (73):
When examining the relationship between force and velocity, it is clear that the ability to develop high levels of force is important when attempting to maximize power output. Although they are related, it is likely that there are limits in one's ability to increase unloaded movement velocity and that there is a greater potential for increasing muscular strength. Because power-generating capacity is based on the ability to express both force and velocity, both of these factors need to be considered in any training program that attempts to develop muscular power (55).
When examining muscular power, it is important that one considers the highest instantaneous power found during a range of motion, under a given set of conditions, as representing the individual's peak power. Peak power-generating capacity is typically related to success in explosive activities such as jumping, sprinting, and weightlifting movements (19,45,49,83,11219,45,49,83,11219,45,49,83,11219,45,49,83,11219,45,49,83,112). Because these activities are also related to football playing ability (11,4511,45), it is clear that training interventions designed to enhance success in football must target the development of peak power output.
In sports like football, the ability to repetitively express peak power outputs also appears to be of particular importance (89). This ability should be considered as a function of the athlete's high-intensity exercise endurance. It is important to realize that in sports like football, the ability to express high-power outputs should not be compromised by efforts to improve endurance, particularly low-intensity exercise endurance activities such as aerobic exercise or excessive conditioning. A far better approach is to engage in training activities that first elevate strength and then maximize power outputs, while engaging in conditioning activities that help to enhance the ability to express these higher power outputs in a repetitive fashion (61).
PHYSIOLOGICAL BASIS FOR THE EXPRESSION OF HIGH-POWER OUTPUTS
When examining the literature, there are a wide variety of possible neuromuscular factors that can contribute to the ability to produce high-power outputs (57,73,87,9957,73,87,9957,73,87,9957,73,87,99).
Neural Factors That Affect Power Output
The ability to recruit, rate code, and synchronize motor units plays a distinct role in the ability to generate high-power outputs (57,73,87,9957,73,87,9957,73,87,9957,73,87,99). To express higher power outputs, the ability to recruit the high threshold motor units associated with type II muscle fibers must occur. Typically, these motor units are maximally activated when the athlete performs either high-force or high-power output training activities (57). Some untrained or weaker athletes may not be able to effectively recruit these motor units (57,9957,99). The application of appropriate resistance training activities can facilitate this ability and thus result in an improved power-generating capacity (73).
The recruitment of motor units usually follows the size principle, with smaller motor units being recruited first, whereas larger motor units are recruited later; it is possible that specific training interventions can alter this basic recruitment pattern (57,9957,99). For example, the use of ballistic or explosive exercises, like those seen in weightlifting, has the potential to recruit the higher threshold motor units (57,7357,73) by improving the ability to recruit these motor units sooner or more efficiently (73). A second neural factor, which contributes to the ability to express a higher power output, is the ability to rate code or change the motor unit firing rate. One of the major benefits of explosive training using exercises such as those noted in weightlifting (57) is an ability to increase the rate coding of motor units that can lead to an increased ability to express high rates of force development, which can contribute to higher power production (73). Specifically, the greater the motor unit firing rate, the greater the force output up to a certain point (99). Once the motor unit's maximal force-generating capacity is reached, increased rate coding then continues to contribute to an increase in the rate of force development (1,991,99). The ability to increase the rate of force development is considered to be important when working with high-power movements, such as jumping and change of direction activities, because of the limited time for force application during these types of movements (78,9978,99). It is clear that the ability to increase the rate of force development is an important training adaptation, which can contribute to the ability to generate high-power outputs. In fact, the high rates of force development are often related to power outputs during jumping activities (53,5653,56) and weightlifting performance (53). One potential explanation for these relationships is that training with explosive exercise can increase the ability to synchronize motor unit firing (73), which can increase the rate of force development (104,105,119104,105,119104,105,119) and ultimately contribute to a greater power output, particularly in ballistic movements.
Muscular Factors That Affect Power Output
The muscle's cross-sectional area and fiber type appear to contribute to an athlete's ability to express higher power outputs. It is well documented in the scientific literature that an increase in muscle cross-sectional area can contribute to increased strength gains (40,86,93,12840,86,93,12840,86,93,12840,86,93,128). Conceptually, Minetti (86) suggest that increases in cross-sectional area lead to changes in force production capabilities, which then serve as the foundation for the expression of high-power outputs. Additionally, muscular architecture changes that underlie the hypertrophic changes to the muscle can play a role in the relationship between cross-sectional area and power generation (96). For example, when sarcomeres are added in series, a resultant increase in the velocity of shortening during contraction can occur, whereas the addition of sarcomeres in parallel results in a significant increase in force-generating capacity (35). The type of contraction used during resistance training can modulate how the sarcomeres are added and their contribution to the muscle cross-sectional area. Specifically, activities that use a large amount of stretch to the muscle, like plyometrics and weightlifting exercises that significantly engage the stretch-shortening cycle (SSC), have the potential to significantly increase the number of sarcomeres added in series, whereas combinations of eccentric and concentric muscle actions result in a significantly larger increase in the number of sarcomeres in parallel (96). Collectively, these data suggest that there is an orderly process by which resistance training can be used to alter the skeletal muscle size and architecture to optimize the muscle machinery necessary for producing peak powers.
A second factor that contributes to a muscle's ability to generate power is its fiber type (Figure 2) (29,3929,39). Type II muscle fibers express a higher velocity of shortening, force output, and power output when compared with type I fibers (39,6039,60). Additionally, the muscle's ability to generate peak power outputs is significantly correlated with the individual's percentage of type II fibers and particularly with total type II cross-sectional area (29). Indeed, it is quite advantageous to have a high type II:I fiber type ratio. One possible explanation for the peak power output of the type II fibers exceeding the type I fibers is related to the overall higher cross-bridge cycling rate possessed by these fibers (16,1716,17). Additionally, the higher cross-bridge cycling rate attained by the type II fibers has been suggested to be a major contributing factor in the ability to express maximal rates of force development (41). Based on these relationships, it is clear that the muscle fiber type composition is tightly related to the rate of force development (16,1716,17), the individual's maximal force-generating capacity (3), and the overall power-generating capacity.
Figure 2: Force-velocity, force-power, and velocity-power relationship.
From a training perspective, the muscle fiber type exhibits an ability to be shifted in response to training (116,124116,124), which can exert a significant impact on the overall percentage of type II fibers, as well as the concentration and size of individual isoforms of the type II fibers (type IIa and IIx). Ultimately, these shifts in fiber type can exert an influence on the ability to express maximal force, high rates of force development, and express high-power outputs. Therefore, it is important that specific training activities are logically and systematically implemented to maximize muscular adaptations that can facilitate power generation. If, for example, aerobic training (or very high volumes of any type of training) is used as a conditioning activity for football players, there will be a fiber type shift toward the type I (115), which will result in a concomitant reduction in peak power-generating capacity (36), the rate of force development (4,584,58), and maximal strength. Based on the work of Minetti (86) and Zamparo et al. (128), it seems logical to target muscle hypertrophy, then muscular strength, and then power output optimization. As such, the muscle will increase its cross-sectional area, then its maximal strength, and then its power-generating capacity.
STRENGTH IS THE BASIS FOR POWER DEVELOPMENT
When considering the ability to develop power, it is very clear that high levels of muscular strength are a key factor dictating how powerful an individual can become (108). When examining cross-sectional studies, it is evident that stronger athletes possess a superior ability to express high-power outputs compared with weaker athletes (10,26,78,83,102,103,11810,26,78,83,102,103,11810,26,78,83,102,103,11810,26,78,83,102,103,11810,26,78,83,102,103,11810,26,78,83,102,103,11810,26,78,83,102,103,118). In fact, Zamparo et al. (128) suggest that maximizing muscular strength is a crucial element when attempting to optimize power generation adaptations. Furthermore, Minetti (86) suggests that the development of maximal power-generating capacity is a 3-step process, which starts with attempts to increase the muscle's cross-sectional area and then transitions to a focus on the maximization of muscular strength before targeting power development. Recent work by Cormie et al. (27,2827,28) supports these contentions by offering evidence of the importance of developing high levels of strength before targeting specific power developing activities. Specifically, Cormie et al. (27) demonstrate that in relatively weak individuals (1RM back squat ≤ 1.5 × body mass), the performance of activities that specifically target the development of maximal strength is more effective than training which specifically emphasizes power generation when attempting to maximize the expression of high-power outputs. Additionally, Cormie et al. (28) noted that the effect of training and the magnitude of performance gain in response to 10 weeks of ballistic power training were higher in stronger individuals (back squat = 1.97 ± 0.08 1 repetition maximum [1RM] kg·body mass−1) when compared with weaker athletes (back squat = 1.32 ± 0.14 1RM kg·body mass−1) who undertook identical training interventions. Ultimately, it is highly probable that targeting maximal strength development is of primary concern with young or less developed football players (middle school, high school, and college). Some evidence indicates that to optimize power training effects, a minimum back squat (thighs below parallel) of 2 × body mass may be necessary before engaging in specific resistance training exercises that target power development (28,55,97,10228,55,97,10228,55,97,10228,55,97,102).
Although one fitness component can be emphasized at times, when training athletes, including football players, it is very unlikely that only one training factor, strength or power, is contained in the training plan. It appears that combinations of strength and power training that are performed at various loads have the potential to maximize the physiological and performance adaptations induced by the training plan (26,114,11726,114,11726,114,117). Examination of the combination of strength-power training indicates that including both strength and power training activities in the overall training plan will result in superior gains in power output and potentially result in a greater transfer of training effect. Tricoli et al. (117) clearly demonstrate that using weightlifting exercises, such as the power clean, clean and jerk, and high pulls, combined with jump training results in superior training-induced adaptive responses when compared with heavy resistance training and jump training when used by football players. Additionally, using various training loads that include both light and heavy loadings appears to further optimize power generation and movement velocities (114). Even with light loads, such as those typically used in warm-up sets, a conscious effort to move quickly or be explosive is imperative (55). Conceptually, using warm-up sets with various loads before undertaking target sets can result in training, which works various parts of the force-velocity, force-power, and velocity-power curves.
Another consideration when examining the football player's strength levels may be related to the effect of the competitive season on the athlete's strength and power-generating capacity (5,655,65). One particular concern is that if inadequate time targeting the maintenance of maximal strength is spent during the competitive season, a concomitant reduction in power output and speed can occur (5). Hoffman et al. (65) suggest that combinations of weightlifting, heavy strength training, and power training have the ability to maintain or slow the potential reductions in strength and power across a football season. In a recent article by McMaster et al. (85), it is noted that strength levels can be maintained for at most 3 weeks with no resistance training, but as time progresses past this point, there is an increased decay rate for strength. Similarly, Cormie et al. (28) demonstrated incremental decreases in muscular strength after 5 weeks of only ballistic strength training. To counteract this decline in strength, a minimum of 1–2 heavy-loaded strength training (75–85% 1RM) sessions per training week in season is recommended (85). When looking at the in-season training programs for experienced football players, it may be necessary to periodically increase training volume and/or introduce higher training loads to maintain strength and power capacities (5,1105,110). Support for this contention can be found in the work of McMaster et al. (85) who report that greater training dosages may be required periodically to maintain strength and power, whereas the base maintenance program could be centered on 3–5 sets of 4–6 repetitions performed in the intensity range of 75–85% of 1RM.
Based on the current scientific body of knowledge, it is apparent that increasing muscular strength is essential for the maximization of power development. Additionally, it appears that combinations of weightlifting-based training exercises performed in conjunction with plyometric or jump training optimize both strength and power development capacities. Finally, although these targets are typical in the off-season, it is increasingly important to incorporate training activities in-season that are designed to maintain maximal strength. Ideally, the use of periodized modeling, which systematically targets strength development and combinations of strength and power training, appear to be essential components of the off-season and in-season training practices of football players.
PERIODIZATION MODELS FOR POWER DEVELOPMENT
When examining the periodization literature, it is clearly evident that periodization is defined as a logical systematic integration and sequencing of specific training factors into mutually dependent periods of time, which are designed to result in an optimization of very specific physiological and performance outcomes at predetermined time points (14,15,52,54,67,68,89,12014,15,52,54,67,68,89,12014,15,52,54,67,68,89,12014,15,52,54,67,68,89,12014,15,52,54,67,68,89,12014,15,52,54,67,68,89,12014,15,52,54,67,68,89,12014,15,52,54,67,68,89,120). To accomplish the primary goal of elevating performance, it is essential that the periodized plan contains appropriate variation, which serves to manage fatigue while enhancing adaptive responses. Typically, variation, particularly in the resistance training literature, is considered in a very narrow scope centering on only variations in loading schemes and essentially examining programming rather than periodization (42–44,75–7742–44,75–7742–44,75–7742–44,75–7742–44,75–7742–44,75–77). Variation must be considered in a greater scope to also include the training foci, exercise selection, and the density of training (79,106,110,13079,106,110,13079,106,110,13079,106,110,130). Excessive or unplanned variation must be avoided as the application of variation in this fashion will limit the training plan's overall effectiveness (110). Ultimately, the training plan needs to vary training in a sequential and integrative fashion to modulate the training response to maximize performance.
To maximize power output, it is essential that a sequential and integrated periodized approach is used when working with football players. Based on the work of Minetti (86) and Zamparo et al. (128), it is very clear that the annual plan should have periods, which first target the development of cross-sectional area after which the training focus should transition toward strength development, followed by focusing on power development. Harris et al. (61) used this basic approach with division IAA football players to maximize training adaptations. After a 4-week strength-endurance phase, 3 different training interventions were undertaken 4 days per week across a 9-week training period. One group targeted maximal strength development, 1 targeted power development, and the third group was a combination or sequential training group, which performed 5 weeks of training targeting maximal strength development, whereas the last 4 weeks focused on high-power output training. When examining markers of performance typically associated with football playing ability, the group that sequenced high-force and high-power training resulted in superior performance gains. For example, significantly greater improvements in maximal strength, as indicated by the back squat (11.6%⇑) and 1/4 back squat (37.7%⇑), were determined for the sequenced group. Additionally, when looking at sprinting speed, only the group that sequenced training resulted in increases in sprinting ability as indicated by decreases in time across 10 yd (2.3%⇓) and 30 m (1.4%⇓). As a whole, the work of Harris et al. (61) supports the contentions of Minetti (86) and Zamparo et al. (128) in which power is optimally developed by a sequential training approach. Collectively, these data support the contention that the maximization of performance is achieved with sequential periodization models.
Although a complete discussion of periodization models is out of the scope of this article, it should be noted that the sequential models proposed by Stone et al. (107), Issurin (67,6867,68), Bompa and Haff (13), Haff and Haff (54), and Verkoshansky (121,122121,122) appear to offer methods that are well suited for the development of the football player. Ultimately, based on these works, the annual training plan should be broken into periods that target muscular hypertrophy, maximal strength, and power development. Central to the effectiveness of this training structure is the ability to integrate complimentary training factors and sequence the development of physiological and performance adaptations.
SPECIALIZED METHODS FOR DEVELOPING POWER
Regardless of the exact periodization model that is used, specific training interventions can be used to optimize power output. These methods include a vast array of activities including (a) weightlifting exercises, (b) plyometric exercises, (c) explosive exercise training, and (d) strength-power potentiating complexes (SPPC).
Weightlifting Exercises
As noted previously, Hoffman et al. (64) suggest that weightlifting movements and their derivatives produce greater performance gains when used with football players. The fact that weightlifting movements translate into greater strength/power and speed performance is not unexpected. McBride et al. (83) and Stone et al. (108) compared the strength and power-generating characteristics of weightlifters with a variety of other strength-power athletes (e.g., powerlifters, wrestlers, sprinters, etc.) and noted that the weightlifters displayed significantly higher 1RMs in the back squat and higher power outputs during jumping activities. When examined in conjunction with jump and resistance training, which targeted maximum strength development, Tricoli et al. (117) demonstrated that the inclusion of weightlifting movements resulted in greater improvements in markers of speed, agility, and power development in performance tests, which are typically used when evaluating football players (static jump, countermovement jump, 10 and 30 m sprint, and agility testing).
Collectively, these results may be expected because the exercises used by weightlifters tend to target strength-power development. For example, classic work by Garhammer (46,4946,49) suggests that during the pulling motion of the snatch and clean, power outputs on the magnitude of 80 W/kg can be achieved during the second pull while lifting considerable loads. When examining the drive portion of the jerk, power outputs on the magnitude of 56 W/kg can be generated depending on the load being used (46,4846,48). In fact, the power outputs achieved during these lifts are significantly higher than those observed in the deadlift, bench press, or squat and are comparable with those achieved during jumping motions (Table 1).
Table 1: Relative power outputs for various exercises
One important caveat that must be considered when incorporating weightlifting exercises is that time must be spent optimizing the football player's technical proficiency with each of the weightlifting movements. Specific focus must be directed at the double knee bend to maximize the athlete's ability to engage the SSC and as such maximize the transferability of the exercise to the field of play (37,7137,71). The double knee bend as a technique requires the rebending of the knee as the barbell passes across the knee joint, thus applying an eccentric stretch, which stores elastic energy in the series elastic components of the muscle before a rapid explosive extension of both the knees and hips. This technique is essential to the transferability of weightlifting movements to football playing ability. As such, lifts such as the pull, power clean, power snatch, clean and snatch performed from the floor or knee high blocks allow for the engagement of the double knee bend. Typically, football players often only perform the power clean from the hanging position at the midthigh or power position. Although this exercise is excellent, it may not be optimal for all periods of training because it does not use the double knee bend or maximally activate the SSC. Because of this, depending on the load, it is not uncommon to see significantly lower power outputs when comparing lifts from the floor to those performed in the hanging position (∼56% of maximal power) or from thigh high blocks (∼60% of maximal power) (Table 1). Therefore, strength coaches should consider that performing pulls, power cleans, power snatches, snatches, or cleans from the floor to be a critical tool in the development of power-generating capacity, which can be used throughout the training year.
When looking at the development of power and jumping performance, an often-overlooked exercise is the jerk. During the drive phase of the jerking movement, the power output achieved is comparable with that seen during the second pull position, which occurs after the double knee bend in the snatch and clean pull (46). Recently, work by Cleather et al. (24) suggests that jerking movements may have an important role in improving vertical jump performance. Specifically, athletes select either a hip or knee dominant jumping strategy based on their relative strengths and weaknesses. With hip dominant jumpers, it may be warranted to increase knee moment capacities with the use of exercises that are knee dominant, whereas in knee dominant jumpers, this performance characteristic may need to be reinforced. One exercise that appears to be effective at creating knee moments is the jerk. Similarly, the push jerk motion seems to also create a favorable knee joint loading pattern, which may be of use in developing vertical power-generating capacity. Although these data are relatively new and training studies using these methods are limited, the biomechanical similarity between the drive phase of the jerk and push jerk suggests that the jerking movements are an important training tool for the development of power.
Overall, it is very clear that weightlifting movements are superior in laying down the foundation for the expression of high-power outputs, improving vertical jumping ability, increasing running speed, and maximizing change of direction speed. Because all of these factors are related to football playing ability, it is very evident that weightlifting movements should represent a large portion of the training exercises used by football players.
Plyometric Exercises
Plyometric or SSC exercises are a very popular training tool for the development of explosive strength and power in a wide variety of athletes. Conceptually, these exercises capitalize on the ability of the SSC to augment force production and enhance performance (23). As a whole, SSC can be classified as either slow (ground contact >0.25 seconds) or fast (<0.25 seconds) (101). Regardless of the SSC classification, the application of the prestretch enhances force production and performance during the concentric muscle action when compared with situations where no stretch is applied (12,9512,95). The application of the prestretches' ability to magnify force production is largely related to (a) the time allotted for the development of force, (b) the ability to store and use elastic energy, (c) the potentiation of contractile machinery, (d) the interaction between the series elastic components and the contractile machinery, and (e) the engagement of reflex actions (23,8123,81). Although a complete discussion of these mechanisms is outside the scope of this article, it is important to understand that the contribution of these factors to the enhancement of muscle performance is different when comparing the slow and fast SSC.
The most common method to quantify the activation of the slow SSC is to calculate the prestretch augmentation or eccentric utilization ratio during jumping movements (81,8481,84). This is accomplished by performing a countermovement vertical jump (CMJ) and a static vertical jump (SJ) test and using one of the following equations to estimate the utilization of the SSC:
When examining fast SSC, an assessment of reactive strength is often used in which the drop jump height, flight time, and the ground contact time are determined and used with one of the following formulas:
As a whole, the type of plyometric exercise used in training contributes to the ability of these exercises to enhance both slow and fast SSC muscle actions (23).
Traditionally, plyometric exercises involve using body weight jumping activities and/or throwing medicine balls in an attempt to engage the SSC (81) and bridge the gap between strength and speed (23). Plyometric exercises accomplish this in a triphasic pattern in which an eccentric muscle action, an amortization/coupling phase, and a concentric muscle action are sequenced (23).
During the eccentric phase, which is sometimes referred to as the loading phase (23), a stretch is applied to the agonist muscle. This stretch serves to engage the major mechanisms associated with the SSC, including (a) muscle potentiation, where stretching an active muscle or returning the muscle nearer to its optimum length results in an increased proportion of actin-myosin cross-bridge attachments and a decrease in cross-bridge detachment rate and results in a higher force from which to start the concentric action (94,9594,95), (b) an increased activation of the muscle spindle (23) where excitatory feedback is applied to the agonist resulting in the engagement of the myotatic or stretch reflex (23), and (c) the storage of elastic energy in the series elastic components of the muscle (23,8123,81). The ability to engage these mechanisms ultimately prepares the muscle for the subsequent concentric muscle action that occurs after the athlete transitions from eccentric to concentric phases.
Once the muscle has been loaded, the transition phase, which is commonly termed the amortization/coupling phase, is initiated (23). The coupling phase is a period of time in which there is a quasi-isometric muscle action that serves to interlink or couple the eccentric and concentric muscle actions contained in the plyometric activity. If the athlete does not possess adequate strength levels, the time frame for this coupling phase can be exaggerated and results in most of the benefits of the SSC being lost (23,12523,125). Similarly, the benefits of the SSC can also be obviated when the athlete is under high levels of fatigue that can contribute to the extension of the coupling phase (38), thus dissipating the benefits of the SSC. Conceptually, the level of muscular strength may be related to the ability to break during the eccentric/loading phase, where significant deceleration forces are experienced, and then change direction during the coupling phase as the athlete moves into a concentric muscle action in a continuous fashion.
After completing the coupling phase, the athlete rapidly performs a concentric muscle action, which is often considered the propulsion phase (23). In this phase, the benefits initiated by the loading phase are used to increase the overall efficiency of force application and results in higher levels of power production. As noted previously, if the coupling phase is too long in duration, the benefits of the loading phase will be lost and the concentric power output will not be maximized. As a whole, the scientific literature clearly supports the use of plyometric training methods in an attempt to improve power output (80). It appears the effectiveness of plyometric training is dependent on the type of exercises used, its integration into the training plan, and the overall strength levels of the athletes using these exercises (81,98,11781,98,11781,98,117).
Numerous studies have clearly indicated that plyometric training, especially when coupled with weightlifting exercises, can result in improvements in neuromuscular control and movement efficiency during high impact activities such as cutting and landing and have the potential to reduce the risk for lower extremity injuries in team sports (81,9881,98). Markovic (80) suggest that plyometric training results in a 4.7% (95% confidence interval [CI], 1.8–7.6) increase in static jump height, a 8.7% (95% CI, 7.0–10.4) increase in countermovement jump height, and a 4.7% (95% CI, 0.8–8.6) increase in reactive jump performance (depth jumping). As a whole, power output can be increased by 2.4–31.3% with a properly designed plyometric program. When the plyometric program is integrated with a periodized resistance training program, the power development benefits can be significantly magnified (81,98,11781,98,11781,98,117).
Based on the literature, it is apparent that plyometric training can be an important part of a football player's preparation. Specifically, plyometric exercises create a linking between strength and speed of movement. There are numerous plyometric exercises that can be used in training, which provide a wide range of training stimuli (see Radcliffe and Farentinos (91) for examples). When looking at using these types of exercises, it is important to remember that strength levels serve as the foundation for the optimization of their use. Considering the models presented by Minetti (86) and Zamparo et al. (128), these types of exercises probably are best initiated late in the phase of training where strength begins to be emphasized and then are more important in the strength and power phase of the periodized training plan. The importance of strength for plyometric activities is also echoed in the common recommendation that football players be able to squat a minimum of 1.5–2.0 × body mass or perform a squat 5 times in 5 seconds with a 60% body weight load before being able to perform plyometrics (23,12323,123). When performing upper body plyometrics, it is recommended that the football player should be able to bench their body weight or perform 5 hand clap push-ups (123). Based on these guidelines and the underlying mechanisms associated with effective employment of plyometric exercises, it is clear that the football player should spend significant time striving to develop maximal strength levels to optimize the foundation from which power and speed are developed.
EXPLOSIVE EXERCISE TRAINING
Explosive exercises such as the weighted jump squats and bench press throws can be beneficial when attempting to elevate a football player's power-generating capacity. Overall, it is well established that stronger individuals have higher power-generating capacities and that increases in strength parallel increases in power-generating capacity (9). However, it appears that weaker individuals can increase power output without focusing on specific power-based training such as weighted jump squats (28). When looking at jump squat training, Cormie et al. (28) clearly demonstrate that with weaker athletes (<1.5 × body mass in the back squat 1RM) focusing on strength development is equally effective at improving jumping performance when compared with specialized power development training. With stronger individuals (≥1.7 × body mass back squat 1RM), it was noted that specialized power-based training (weighted jump squats) resulted in a more pronounced improvement in power-generating abilities when compared with weaker individuals.
Typically, it has been reported that power output is highest in the jump squat at a 0 kg load (25), which would essentially make the jump squat an unloaded plyometric activity (see discussion on plyometrics). Although methodological differences may play a role, several researchers have clearly demonstrated that with stronger athletes, such as advanced football players, having a back squat 1RM ≥ 1.7 × body mass, that power output is optimized at higher loads (5,8,32,83,1085,8,32,83,1085,8,32,83,1085,8,32,83,1085,8,32,83,108). For example, Baker et al. (8) report that with elite rugby players (back squat 1RM ≥ 1.8 × body mass), power output is maximized at loads between 47 and 63% of the 1RM back squat. Similarly, Stone et al. (108) reported that power output was optimized at 30–40% of 1RM in athletes whose 1RM back squat is ≥ 2.00 × body mass.
When examining the bench press throw, it is clear that strength levels also play a role in the determination of the optimal load (111). Thomas et al. (111) suggest that the optimal load corresponds to around 30% of the 1RM bench press in individuals who could bench 1.1 × body mass. When examining stronger athletes (bench press 1RM = 1.4 × body mass), the percentage of 1RM that optimal load occurs at increases to somewhere between 46 and 62% (7). In fact, well-trained or stronger athletes exhibit significantly higher power outputs than their weaker counter parts (6).
Conceptually, it is very likely that increasing muscular strength results in a rightward shift of the strength-power relationship because the stronger athlete can express higher velocities of movement with higher forces. Taken collectively, the literature clearly indicates that weaker individuals need to develop an appropriate strength base, and as they get stronger, specialized power training can be used with loading paradigms that consider the athletes overall strength. If integrated into a periodized training plan, it is likely that including these specific power-based exercises with an appropriate load can maintain power output in team sport athletes during a season (88).
STRENGTH-POWER POTENTIATING COMPLEXES
SPPC or a “complex pair,” as they are sometimes called, is a technique used by advanced athletes to maximize power outputs (62). The most basic form of an SPPC involves the performance of a high-force (51) or high-power (92,10392,103) conditioning activity before the performance of a high-power or high-velocity movement such as a plyometric exercise or an explosive lift (Figure 3, Table 2). For example, a heavy-loaded back squat can be used to potentiate a vertical jump performance (22,8222,82) or a power clean can be used to potentiate a sprint performance (103). Additionally, the combination of ascending back squats where the load is progressively raised to a high level has been shown to increase performance during horizontal plyometrics (97), vertical jumping (50,10250,102), and sprinting performance (82,12682,126).
Figure 3: Example strength potentiating complexes.
Table 2: Example strength-power potentiating complexes
In a classic study, Radcliffe and Radcliffe (92) demonstrated that incorporation of the power snatch in an SPPC resulted in significantly greater horizontal jumping performance than using back squat exercises. Similarly, Seitz et al. (103) recently reported that SPPCs that contain power cleans as the conditioning activity result in significantly greater sprint performance gains. Therefore, depending on the training phase, SPPCs can be created with either heavy load or high-power activities, serving as the conditioning activity for high-speed or high-power performances.
Overall, the volume and intensity of the conditioning will dictate the duration necessary before the performance of the subsequent high-power or speed performance. In most instances, fatigue is dominant in the first 3 minutes after a conditioning activity, but by 5 minutes, performance benefits can be noted (21). If the conditioning activity creates larger amounts of fatigue, the time frame before the high-power or speed performance can be extended to 8–15 minutes (22,7422,74). It is important to note that stronger athletes will be able to dissipate the accumulated fatigue at a much faster rate than their weaker counterparts (70,10270,102). For example, Seitz et al. (102) report that stronger individuals (back squat 1RM ≥ 2.0 × body mass) express a postactivation potentiation response as early a 3-minutes postconditioning activity, whereas weaker individuals (back squat 1RM < 2.0 × body mass) do not display significant postactivation potentiation responses until 6-minutes postconditioning activity. Similarly, Jo et al. (70) report that stronger athletes (back squat 1RM ≥ 1.5 × body mass) displayed significant postactivation potentiation responses at 5-minutes post conditioning activity. Careful inspection of the data presented by Jo et al. (70) reveals that the stronger athletes squatted between 1.5 and 1.9 × body mass, which is similar to the weaker group in the study by Seitz et al. (102). Therefore, athletes who squat between 1.5 and 1.75 × body mass may express potentiation responses in a time frame around 5–6 minutes, whereas stronger athletes ≥ 2.0 × body mass may demonstrate potentiation responses between 3 and 4 minutes of postconditioning activity.
From a mechanistic point of view, both nervous and muscle system responses to the potentiation-inducing activity explain the effectiveness of an SPPC. The postactivation phenomenon (PAP) may occur because of an increase in the phosphorylation of the myosin light chains, which makes actin and myosin more sensitive to calcium allowing for a more rapid rate of cross-bridge cycling (62). This response appears to be more pronounced in the type II fibers (2,592,59), thus suggesting that individuals, such as football players (129), who generally possess a higher percentage of type II fibers may be able to display a greater degree of PAP. Therefore, football players may receive a greater benefit from the use of SPPC because of their potential for expressing greater PAP.
An additional factor that can predicate the effectiveness of the SPPC and the PAP is the nervous system response to the PAP-inducing activity. The neural effects are primary contributors to the force production characteristics during high-velocity ballistic movements (51,10951,109). Neural factors that can impact the effectiveness of the SPPC include increased motor unit synchronization, desensitization of alpha motor-neuron input, and a decreased reciprocal inhibition of antagonists (22,5122,51).
Although the exact mechanism that predicates the effectiveness of an SPPC has not been completely defined, it is clear that 3 factors can impact the effectiveness of an SPPC. First, it appears that the athlete's strength levels play a primary role in determining the degree of potentiation that can be stimulated (22,34,82,9722,34,82,9722,34,82,9722,34,82,97) and the time course needed between the conditioning activity and the power activity (70,10270,102). Based on the available literature, it appears that football players who can back squat ≥ 2.0 × body mass will express the greatest potentiation effects, when using the back squat as the potentiation-inducing activity (97). Second, the athlete's level of fatigue appears to modulate performance effects of the SPPC (100), where lower levels of fatigue facilitate the PAP response. Interestingly, athletes who are stronger (back squat ≥ 2.0 × body mass) are able to express the PAP response much quicker than weaker athletes (70,10270,102). This response may be related to the ability of stronger athletes to develop fatigue resistance or a quicker rate of recovery to heavier training loads (22,7022,70), thus allowing postactivation potentiation responses to be displayed sooner after a conditioning activity. Finally, the athlete's overall training status appears to significantly impact the individual's ability to express a PAP response (22), with more trained individuals being able to express greater PAP responses. Based on these data, it is clear that more trained football players who possess the requisite strength levels can maximize the benefits of the use of SPPC.
There are many methods for which to use SPPCs. For example, one could perform ascending back squats, rest for 5–8 minutes, and then perform a 40-m sprint (82), repeated horizontal plyometric jumps (97), or vertical plyometric exercise such as a box jump or vertical jump. These various protocols would be beneficial to the athlete's development and based on the fact that fatigue would mitigate the ability to express the PAP effect should be considered in the context of the phase of training being undertaken. Most likely, the SPPC can begin to be used in the phase of the periodized plan that is designed to maximize power development and be used as a tool to translate strength gains into the expression of higher power outputs. Regardless of the structure of the SPPC, it is always critical to remember that this training intervention is best suited for football players who are highly trained and can back squat ≥ 2.0 × body mass when use squatting exercises as the potentiation-inducing activity.
CONCLUSIONS
The development of high levels of muscular strength is a critical goal when preparing the football player for competition. Based on the current body of literature, it is clear that a sequential approach to the training process where the focus of training leads from the development of muscle cross-sectional area to strength development and then the translation of those strength characteristics is necessary to optimize power production. Conceptually, the various phases of the training process allow for the physiological adaptations to occur that underlie the expression of power. During the various phases of training, several specialized methods of developing power can be targeted. For example, weightlifting exercises can be used in the strength-endurance, basic-strength, and strength-power phases. While it is probably best to only perform plyometric activities during the basic strength and strength power phases, the use of SPPC should be limited to the strength power phase of training. Ultimately, the literature is very clear that the optimization of power-generating capacity is established based on high levels of muscular strength. As such, the football player's development should consistently strive to increase overall muscular strength levels, while attempting to translate that strength to the expression of power and speed.
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