Abstract
The development and output of muscular power is very important in the training of athletes, no matter what the level of play. Mechanical power can be described as the rate of doing work, or how much work is done in a specific amount of time (16). There are many studies that have looked at power production during different activities, but many of them do not compare the activities or methods to each other for their abilities to cause such adaptations in power. The goal of this review paper was to look into the research that has been done on power development to find what training methods are the best for producing the most gains in this area of muscle physiology/performance, and how these variables affect power development. The review of the literature shows that there are many training variables to consider and they should all be taken into account when developing a program. These variables can be combined to develop a maximal amount of power in athletes of all levels. The most important thing a coach can do is to find what works best for the specific population in question and fit that “best” program into the time available. Future research should compare as many differing methods as possible and try to match the best variables that produce the most power. This is a continuing area of research that can only improve as technology and training methods become more advanced.
Introduction
The amount of force generated in the smallest amount of time possible is becoming greater and greater as training methods become more advanced. The development of muscular power is very important in the training of athletes, no matter what the level of play. Mechanical power can be described as the rate of doing work, or how much work is done in a specific amount of time (16). Power includes two components, force production and velocity of movement (12).
Crewther et al. referenced that in order to adequately train for power, movements producing high power outputs with high contractile velocities are thought to be important (6). Kawamori and Haff’s work concurred with Crewther et al., stating that because power is the product of force and velocity, both components must be addressed in a program to develop power (12). The higher an athlete’s force production or velocity of movement, the more power that athlete can produce. Using these two components, a strength coach can develop an effective program to increase power output.
The importance of program development is often overlooked as a factor of improving athletic performance. Correct programming sets the stage for an athlete’s development on all levels. It is the author’s belief that training methods involving Olympic weightlifting movements and other movements involving the stretch shortening cycle; high force, low velocity strength movements (i.e., back squats and deadlifts); hypertrophy periods; sufficient rest periods between sets and between training sessions; the use of optimal training loads and velocity of movement for the specific training outcomes desired (increasing force development or velocity of movement), and using the proper amount of repetitions per set will induce the biggest performance gains in power development/output.
Research has shown that Olympic weightlifting movements are an effective means of training when the outcome desired is increased power (9, 10, 12). There have been many studies done on heavy resistance exercises and their effect on power development that have reported positive outcomes (5, 19, 26). Squats are an effective heavy resistance exercise for increasing power (5, 26). These examples may be effective because they both require the use of type II muscle fibers, which are known to produce higher forces than type I muscle fibers (24). Resting for sufficient lengths of time is needed in order to recover adenosine triphosphate (ATP) and phosphocreatine (PCr) stores (23). If rest is too short, the energy systems will not recover and the muscle’s force production will be markedly decreased, especially if using higher loads.
The load used in each training set is one of the most important variables that can be investigated. If too much load is used, the force-velocity curve will be shifted too far on the force side. If too little load is used, the curve will shift too far towards the velocity side. There has to be an optimal load that allows for maximal power production. Also, the optimal load really depends on the exercise being performed. This optimal load training is called maximal power training (15).
Lastly, while performing an exercise, one has to perform the proper amount of repetitions in order to gain the desired benefits. If too many repetitions are performed, the wrong energy systems (oxidative over PCr) and muscle fiber types (type I over type II) will be utilized, which will not allow the proper adaptations to take place. Therefore, the best range of repetitions for increasing muscular power is between one and five repetitions (3). However, there have been studies that have used higher than normal repetition schemes for power development (6+ reps) (18).
There are some researchers who believe that plyometric programming along with heavy resistance training is enough to elicit muscular power development (10, 19, 26). Others believe that there are optimal loads to develop power that fall in the range of 30-70% of 1RM (15), while others believe it is higher (>80%), or lower (10%) (12).
The purpose of this review paper is to look into what research has been done on different variables of power development and bring these variables together. With the most important variables brought together in one review, we can look at how they affect power development. We can also see which variables are more important to the development of power and which variables are still being debated. Consequently, all of this information will show us what program variables need to be used together to develop the most power in the athletes we train.
Olympic Weightlifting
The Olympic weightlifting (referred to as weightlifting from here) movements are thought, by many coaches, to be highly transferable to various athletic skills. This is especially true of any sport requiring a vertical jump (9) because of the similar joint kinematics involved, or any sport requiring the simultaneous extension of the hip, ankle, and knee (triple extension). Some background on the weightlifting movements is warranted.
Garhammer has found weightlifting to involve very high power outputs for both men and women (9). The relative power output (watts per kg body mass) has been recorded at about 34.3 W/Kg during the entire snatch or clean pulling movements (10). That is a great deal of power, considering that the relative power outputs for the deadlift (similar to the pull in weightlifting) and bench press have only been measured at about 12 W/Kg and 4 W/Kg, respectively (10). The competition lifts (Snatch and Clean and Jerk) and their component lifts (Snatch Pull, Hang Clean, Box Jerks, etc.) are often used for power development because the required combination of strength and velocity of movement is unmatched compared to other lifting movements. However, Kawamori and Haff (12) claim that very few studies have actually measured the power outputs of differing loads during the weightlifting exercises.
The snatch starts with a wide grip on the bar in a deadlift-type stance. In one fluid motion the bar is lifted from the floor straight overhead, as the athlete pulls himself/herself under the bar into a squat position, locking out the elbows. Then, once steady, the athlete rises from the squat position and returns the bar to the floor.
The clean and jerk begins in the same position as the snatch, except the hands are moved in closer to each other. From the starting position, the bar is lifted to the clavicle, where the athlete pulls himself/herself under the bar into the squat position. The athlete then rises from the squat, takes a breath, dips the hips and knees, and then violently explodes upward and jerks the bar overhead, locking out the elbows at the top.
There are six phases of the snatch and clean: Pre-lift-off, Preliminary acceleration, Adjustment, Final acceleration, Unsupported squat under, and Supported squat under. The jerk also has six phases. They include the Start, the Dip, the Braking phase, the Thrust or Explosion, the Unsupported squat under, and the Supported squat under (7).
Tricoli et al. compared a weightlifting group and a vertical jump training group, which included different plyometric exercises, in the effectiveness of the training styles to elicit power production and found that the weightlifting group produced more performance improvements than the vertical jump group in physically active subjects (18). This is an excellent example of why it is beneficial to use the weightlifting movements, rather than just plyometric movements (vertical jump training), to elicit power output. The movements are very similar, but the weightlifting movements have the added benefit of increasing both factors, force and velocity, of the power equation.
Unless an athlete would be carrying an extra load while performing plyometrics, they are only going to be working towards enhancing their velocity of movement. More research should be done to examine the best loads to use during weightlifting, depending on the sport and time of season.
Stretch Shortening Cycle
Elastic energy use, also known as plyometrics, have been known to be a large contributor to power production (4, 15, 25). After a muscle is stretched, using the mechanical energy stored as elastic energy is very important to increased force output (4). The stretch shortening cycle (SSC) uses elastic energy, much like a rubber band, to create rapid force after being stretched. Wilson and Flanagan defined elasticity as “a measure of how readily a body will reform after it has been deformed by being stretched, compressed, or twisted” (25). So, after a muscle is lengthened (stretched) it has the ability to shorten or contract (reform) back to normal length. This creates a large, quick production of force, which we all know as power.
A successful SSC requires three critical elements, including “a well timed preactivation of the muscle(s) before the eccentric phase, a short and fast eccentric phase, and an immediate transition (short delay) between stretch (eccentric) and shortening (concentric phase)” (25). Blakey and Southard describe this SSC process in greater detail:
The first phase is called amortization and occurs as a result of yielding work forcing a rapid stretch of the lower body extensor muscles. In the second phase, muscles perform a reactive switch from yielding work to overcoming work to initiate a positive vertical velocity. The third phase is the phase of active take-off. The extensor muscles contract to perform the jump. The first phase stretches the extensor muscle groups, the second is a reactive recovery, and the third uses the benefit of a reciprocal increase of force during contraction (4).
There have been many studies on the outcomes of power development, including groups combining resistance training and plyometrics, resistance training alone, and plyometrics alone. Blakey and Southard did an 8-week study on the effects of depth drops, a plyometric exercise, combined with resistance training on lower body power development (4). They found that the combined programming of these modes (resistance training + plyometrics) improved leg strength and power. They also found that the height of the drop jumps did not alter the resultant training effects. This finding, in itself, is helpful to know because this could save athletes from taking the beating of increasing ground reaction forces (GRF) when a coach increases the height from which they have to drop.
Lyttle, Wilson, and Ostrowski performed a similar 8-week study, comparing a maximal power training group with a combined resistance and plyometric training group (15). They found that both groups were equally effective in improving a variety of performance measures such as jumping, throwing, cycling, and lifting. In contrast, Kubo et al. (13) investigated the effects of plyometric and weight training protocols on jumping performances. The study lasted 12 weeks, training four days per week. Each subject performed plyometric training on one leg and weight training on the other. They compared baseline measures to the results at the end of the study in both legs to find which training method worked best to improve performance in several jumping-related tasks. Their results show that jump performance was improved only by plyometric training, with no changes from weight training.
Plyometrics are shown to be an effective way of using the stretch-shortening cycle for enhanced power output, but more research is needed in this area to find the benefits over resistance training alone. While some studies (4) show a significant benefit to combining these two modes, others show no difference (15) or even a benefit to just performing plyometric training (13).
High Force, Low Velocity Movements for Increased Power
Because power is the product of force and velocity, it is quite obvious that strength movements of high force and low velocity would help, in some way, to increase the development power. Several studies have looked into this theory and there are contradicting results.
Wenzel and Perfetto quoted Verkoshansky, stating “it has been established that absolute muscle strength has a negative effect on movement speed and on the ability of a muscle to display explosive effort” (20). However, Young, Jenner, and Griffiths found that performing a set of squats with a 5-RM load prior to a set of loaded countermovement jumps dramatically improved power (26). This contrasting of heavy and light loaded movements is called postactivation potentiation (PAP). PAP is defined as “referring to the enhanced neuromuscular state observed immediately after a bout of heavy resistance exercise” (19).
Weber et al. performed a study similar to Young, Jenner, and Griffiths, looking into the effects of PAP on squat jump performance (19). They found that performing heavy-load back squats before a set of consecutive squat jumps may enhance acute performance in average and peak jump height, as well as peak GRF. Chatzopoulos et al. investigated the PAP effects after a heavy resistance stimulus (back squats) on running speed. They found that the heavy back squats improved 10 and 30 meter sprint performance when performed five minutes after the squat set (5).
In contrast, Wenzel and Perfetto studied the effect of a speed group (speed of lifting) versus a non-speed group (traditional repetition velocity) in developing power and found that there were no differences between the two groups on any performance measures (20). This means that the low velocity group/non-speed group did not gain any significant edge in power or strength compared to the speed group.
Kawamori and Haff claimed a study by McBride et al. found Olympic lifters who used heavy training (low velocity, high intensity) and explosive-type (high velocity, low intensity) training gained greater results in jump height and muscular power measures than power-lifters who only used heavy resistance training (12). The majority of studies have documented increased performance in power with increasing levels of maximal strength, but there is still much research needed in this area of performance.
Hypertrophy
Hypertrophy is the enlargement of the physiological cross-sectional area (PCSA) of a muscle or, simply put, the growth of muscle fibers. The ability of a muscle to produce force is directly related to its physiological cross-sectional area (1) regardless of age (2). Because power involves a strength component, strength gain from hypertrophy could lead to a gain in power production (12). However, there has been discussion on how much this relationship really does affect force production.
Akagi et al. established a new index of muscle cross-sectional area with its relationship to isometric muscle strength (1). They discovered that muscle thickness (MT) multiplied by circumference (C) reflects muscle cross-sectional area and can be used as an index for determining muscle cross-sectional area. They also found that when this index was measured at maximal voluntary contraction (MVC) it was more closely related to force production than when measured at rest. This would indicate that muscles tend to be more closely related to their force production capabilities when they are examined in their fully contracted state.
Another study by Akagi et al. (2) looked at similar cross-sectional characteristics in older individuals (age range 51-77 years) and found the same results: muscle cross-sectional is more closely related to muscle force production when examined at MVC than at rest. This is helpful to know because of age-related changes in muscle performance. We now know, at least from a PCSA point, that force production relationships do not change with age.
Type II muscle fiber proportion has been significantly correlated with training-induced hypertrophy and increases in strength. It has been suggested that type II fibers have a greater specific tension and combining that with their greater hypertrophy response would likely contribute to increases in whole-muscle specific tension (8). This means that type II fibers have a greater potential to contract harder (greater specific tension) and they are able to hypertrophy at a greater rate than type I fibers.
More research is needed in this area to find how muscle size and strength compare in different movements, within individual muscles, and within different sports. It is well-known that more muscle leads to more strength, but sometimes getting bigger is too much of an expense to an athlete because of weight classes. If researchers can find how to improve strength at a greater rate without too much hypertrophy, it will help athletes in all sports with weight categories involved.
Sufficient Rest
It is a problem in the strength and conditioning world when a coach has a group of athletes and a limited amount of time to train. The goal is to get the best results in the shortest amount of time, but to also be able to let the athletes have the proper rest periods as well. This has been a double-edged sword in the strength and conditioning world for quite some time and much research has been done to find answers to this problem.
Willardson states that since blood flow to the muscle is occluded at intensities as low as 20% of maximal strength, a rest period is essential to reestablish intramuscular blood flow and oxygen delivery that allows for the replenishment of phosphocreatine stores, restoration of intramuscular pH, removal of metabolic end products, and return of muscle membrane potential to resting levels. He also goes on to say “the selection of appropriate rest intervals becomes crucial to maintain high velocity, rate of force production, and power throughout a set” (22). Resting for sufficient lengths of time is needed in order to recover adenosine triphosphate (ATP) stores as well (24). If this rest does not take place, there won’t be enough recovery of these muscular energy components, which means that there will not be enough intramuscular stores to produce the required force/power necessary for the remaining sets of a training session.
Larson Jr. and Potteiger compared three different rest intervals between several sessions of squats (14). They mentioned that, as a general guideline, rest intervals between sets are progressively reduced as the body adapts to certain training loads, and lengthened as training loads increase. The three rest intervals they used were a fixed three minute time interval, achieving a specific postexercise recovery heart rate (post-HR) of 60% age-predicted maximum, and a fixed work : rest ratio of 1:3. During the three exercise sessions, the subjects performed four sets of squats to voluntary exhaustion with 85% of their 10-RM (10 repetition maximum). This study found no significant differences between repetitions to exhaustion, blood lactate concentrations, or RPE among the three rest conditions.
Willardson and Burkett also did a study on three rest interval components (21). They compared rest intervals of 30-seconds, one minute, or two minutes on five consecutive sets of the squat and the bench press (15RM for both). The purpose was to find which rest length had the best sustainability on the number of repetitions performed. They found that that, for each exercise, the number of repetitions performed on the first set was not sustained throughout all five sets, no matter which rest interval was used. They thought that the intensity (weight) should be lowered with each subsequent set to be able to sustain the desired amount of repetitions.
While both of these studies do well on questioning which rest intervals are best to improve performance, neither of them found any significant results. Also, they are not exactly comparable to training for power results because of the load and repetitions that were used. Nevertheless, they do give us knowledge into specifications about rest intervals for strength gains in secondary exercises such as back extensions, reverse hyperextensions, and abdominal training that use higher repetitions per set.
Studies with heavier loads and fewer repetitions are needed in order to apply results to power-type training. One such study focused on comparing squat strength gains and volume components with rest intervals of two and four minutes between sets over several mesocycles (23). In this study, 15 trained men were randomly assigned to either the two minute group or the four minute group. Both groups performed the same training program, with the only difference being the rest interval. Each week had two sessions, a heavy session (70-90% 1RM) and a light session (60% 1RM). The groups made significant strength gains, but there were no differences between groups. However, the four minute group did demonstrate significantly higher total volume for the heavy workouts. This finding is noteworthy because if higher volumes of high intensity (heavier weight) can be obtained, this will lead to continued increases in strength gains. The primary finding of this article was that large strength gains can be achieved with a minimum of two minutes rest between sets, and little additional gains are made from resting four minutes between sets. Coaches can use this information for their programs and significantly cut the amount of rest time between sets, which will give them more time to add extra exercises into the program for increased volume.
Again, significant rest must be allowed in order for the best adaptations to take place. However, coaches have limited time to make these adaptations occur with training because most athletes do not have all the time in the day to train. Thus, rest research and findings should be followed heavily when trying to train athletes toward a certain goal. Without resting enough or resting too long, the desired adaptations will not take place and training will be a waste of time.
Optimal Training Loads
It cannot be stressed enough how important using proper loads is on training results. This training variable has a direct impact on both components of power development (force and velocity of movement). Mentioned earlier was the fact that certain loads elicit the best results for power (maximal power training/loads). This is where a person produces the most power in a certain movement. Once this load is figured out, a coach can use this load as a baseline to start training.
Harris et al. declared that some researchers claim the use of 80% of 1RM is recommended to improve power characteristics, while others suggest 50-60% of 1RM and below (11). Kawamori and Haff (12) agreed with Harris et al., stating that there is inconsistency in the optimal load to produce the highest power. They claimed that some studies that used untrained subjects, single joint exercises, and upper-body exercises reported 30-45% of 1RM, while others using trained subjects, multi-joint exercises, and lower-body exercises reported 30-70% of 1RM.
It appears that trained subjects and lower-body exercises produce the greatest power outputs at higher percentages of 1RM than untrained subjects and upper-body exercises. More research is needed in this area because of the large range of percentages still being used at this current point in time.
Because of the similar kinetics and kinematics, Tricoli et al. (18) compared vertical jump training and Olympic-style weightlifting and their affect on power output. Although they did not measure percent of 1RM used, the weightlifting group improved on all of the performance tests (10- and 30-meter sprint, agility test, and clean and jerk) except one (1RM half squat). This shows that using an external load is more beneficial than just using body weight in improving power output.
There is much controversy surrounding this area of training among strength and conditioning coaches. That is why it is so important that researchers find definitive values for what training loads improve power output the best. At this point in the literature, there is a large range of percentages that is being tested on power output. Once the correct load parameters are discovered, coaches can put research into practice.
Conclusions
Athletes must possess the ability to produce large forces in short amounts of time to plateau at their highest potentials. This is why power is so important in sports. Strength and conditioning coaches need information at their disposal to invoke the best adaptations to power training in their athletes as possible.
There are many training variables to consider when developing a program and much controversy surrounding these training variables. The development of a program should question whether all aspects are being considered. Most importantly, what types of lifts are being done in the program; is there any use of the stretch-shortening cycle; are there high-force, low-velocity movements to develop maximal strength; is there a hypertrophy period; are the athletes resting the appropriate amount of time; and are the athletes using the optimal loads to develop power?
These variables can be combined to develop a maximal amount of power in athletes of all levels. As mentioned throughout this review, a program that utilizes the weightlifting movements, squats, and plyometrics, with the proper loads and rest periods, as well as some hypertrophy exercises could lead to a superior amount of power development. Thus, leading to superior athletes thanks to the proper combination of training variables in a program.
Coaches can get carried away with all of the training methods at their disposal. The most important thing is to find what will work best for the specific population in question and being able to fit that “best” program into the time available. Working smart is just as important, if not more important, than working hard. Remember, there are certain things that are hard to get wrong in training, but there are even more things that are easy to get wrong. It is a waste of everyone’s time if things are being done wrong in a program. Programs should be developed so that every aspect can be backed by peer-reviewed research.
Practical Applications
It is the author’s recommendation that all strength and conditioning coaches review the current research material on all of the aspects of training that are available to them. It is better that a coach have the research to back up what he is doing if their methods are questioned. They could be blindly developing programs, not knowing what or why they are doing certain things with their athletes. Education is always going to be a crucial part of a coaches’ repertoire.
Future research should compare as many differing variables as possible and try to match the best variables that produce the most power. This is a continuing area of research that can only improve as technology and training methods become more advanced, but it will only improve if strength and conditioning coaches allow their methods to change with time.
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