The Role of Plyometrics in the Scope of a Periodized Training Model
Robert Pettitt Mount Union College
The role of plyometrics in the scope of a periodized training model. J. Perf. Enhan. 1999 1(1):11-20. Plyometric drills are often periodized across an athletes competitive year according to their levels of difficulty or intensity. The periodized model assists the performance enhancement professional in structuring a program that prevents injury and overtraining and enables athletes to peak in muscular performance at a more critical point in the competitive season. A reduction of the amortization phase, greater cross sectional recruitment, and a threshold elevation for the inverse stretch reflex are among the physiological adaptations that occur from plyometric training. Several criteria are used in evaluating the intensity level of a plyometric session. These criteria help in determining the appropriateness of plyometrics throughout a competitive year and should be considered in conjunction with the athletes previous training history.
Key Words: stretch-shortening cycle; training intensity; training volume
Plyometrics is a common training methodology used by competitive athletes to develop speed and power. Jumping, bounding, skipping, throwing, or basically any recoil movement which ballistically stretches muscles are characteristic of plyometric drills and are characteristic of motions found in virtually every sport. The acquisition of a more rapid and forceful contraction is the fundamental basis for engaging in plyometrics training. As with most forms of exercises there are varying degrees of difficulty or intensity. This perpetuates the need for careful programming to avoid injury and overtraining. One of the more common means employed by performance enhancement professionals to accomplish this goal is through a model called periodization.
The periodized training model breaks the competitive year (or macrocycle) of an athletes training program, into three distinct mesocycles (17). For this particular review, the off-season mesocycle is referred to as the period of time in which athletes are in no way officially engaged in their chosen sport. The pre-season mesocycle progresses from the first official practice to the first regular season contest, while the in-season mesocycle lasts from the first regular season contest to the last official contest up to and including post-season competition. Some periodized models have gone on to divide mesocycles into smaller periods of training termed microcycles (17,23) however this review will focus on distinguishing the broad use of plyometrics during off-season and in-season.
Periodization involves the systematic fluctuation of training volume, intensity, and sport specific skill. According to many periodized models, training should progressively become more intense and encompass less volume as athletes approach the end of their in-season mesocycle (17,19, 23). The focus for developing a strength base lies primarily in the off-season and pre-season and the use of exercises such as plyometrics become more prevalent as the regular season unfolds. When the end of the regular season and post-season competitions approach, elevation of training intensity should be coupled with a further decay of training volume (17,23). Individual training sessions become much more focused on the elements of the sport with greater frequency of game-like situations (i.e., scrimmages, timed sprints or relays, etc.).
Athletes devote a lot of time to the weight room during the off-season and pre-season for the purpose of developing a strength base. This has been recommended not only for the rigors of sport (4,5,17), but also for safe engagement in plyometric drills. The ballistic nature of plyometric drills can be quite taxing on the musculoskeletal system. Muscles used during plyometrics are rapidly lengthened and shortened lending to another name for plyometric drills: stretch-shortening cycle (SSC) exercises (21,22).
The physiology of a SSC can be defined as the reflexive shortening of a muscle to a rapid eccentric stretch. The governing mechanism for the SSC is the myotatic stretch reflex (21). This myotatic stretch reflex is one of the most simple and consequently most rapid of reflexes in the human body (16). The reflex can be broken down as followed: Structures called muscle spindles, which sense changes in muscle length, are activated by a rapid eccentric stretch (8). There is a brief period of time, termed the amortization phase (21), in which a nerve impulse is sent to the spinal cord from the muscle spindle and directly signals a motor nerve attached to muscle fibers being stretched. Once the motor nerve is signaled, the stretched fibers respond in a concentric manner. It has been stated the amortization phase of the myotatic stretch reflex can be reduced through plyometric training (4,20) and thus may increase the speed of SSC movements in sport.
The forceful nature of the eccentric load in a plyometric drill can also yield potential training benefits. Coined as "irradiation", Sherrington (1,18) described that a greater cross sectional recruitment of muscle fibers can occur in situations of phasic myofiber stretching. Mero and Komi (13) confirmed this phenomenon empirically evaluating sprinters towed at supra-maximal speeds. The notable payoff for invoking irradiation during training is that a strong eccentric load occurring during a brief period of time forces the body to recruit more muscle fibers. Consequently, ballistic movements will result in the muscle exerting more force and the ability to reproduce this extra force on a systematic basis may carry over to the playing field (5).
One other force limiting neurological factor that can be trained with plyometrics is the Phasic Golgi Tendon Organ or inverse stretch reflex (15). The inverse stretch reflex provides a safe limit for what we know as a muscles repetition maximum or 1RM. Under normal conditions, when 1RM is exceeded and enough tension is placed on the musculotendinous unit, the threshold for the inverse stretch reflex is surpassed and nerves controlling the muscle under tension are shut off (11,15). This provides the musculoskeletal system with an important safeguard but can limit peak force production. It has been suggested highly ballistic training, such as plyometrics, can progressively elevate the threshold for triggering the inverse stretch reflex (14).
REVIEW OF LITERATURE
Plyometrics form only part of an intricate fold of training for promoting success in competitive athletics. The literature suggests this mode of training is more effective if used in conjunction with heavy resistance exercise training (4,5,7,10). As with heavy resistance training, the intensity of plyometrics should be progressed across a competitive season with careful consideration.
Drills for plyometric training are often described in two forms: box form drills and bounding drills. The first categorization, box form drills, is a bit misleading. The word box implies this categorization is limited to exercises or drills using a box. In actuality, a drill in which an athlete will at some point negotiate over one or several obstacles of certain heights can fall into this category. Any obstacle such as a stair, a log or a hurdle will suffice. The second categorization of bounding drills is subject to wide interpretation as well. Bounding drills can range from stationary exercises such as skipping rope, in-place jumps, to a variety of line-drill techniques such as alternate bounds, carioka steps, and repeated single leg long jumps. Emphasis on one categorization of drills over another should be based upon an athletes performance deficit.
The performance enhancement professional should administer a battery of tests for speed, agility, and power prior to and occasionally throughout an athletes training macrocycle. Some of the more common tests include timed sprints, shuttle side-step tests, where the number of lines crossed are counted within a given time, as well as measures of explosive power such as the vertical jump (VJ) and the standing long jump (LJ) (6). Specialists in sprint training have also described assessments that evaluate an athletes stride length and stride rate (5). Athletes having a weakness in stride rate, a weakness in assessments examining the number of ground contacts, or a VJ weakness, are recommended to place more training emphasis on box form drills because these attributes emphasize vertical displacement. Conversely, training emphasis on bounding drills are recommended for athletes having a weakness in stride length and/or the LJ for these attributes emphasize horizontal displacement.
A number of strength and conditioning coaches have taken a stance against the use of plyometrics in training athletes due to the ballistic nature of these activities and the potential for orthopedic injuries. As with any exercise there is an assumed risk for injury. If plyometric exercise programs however are structured and progressed with care by qualified performance enhancement professionals, the risks for overtraining and acute injury can be minimized. The structure of safe weight training programs includes the development of proper lifting mechanics prior to progressing resistance. Along similar lines, athletes need to develop their ability to maintain proper control of form in response to impact forces prior to engaging in drills requiring greater intensity. There are a number of variables in plyometrics that influence the extent of the intensity in these drills. Following Newtons 2nd Law, these variables can be broken down into components of mass and acceleration.
Both box form drills and bounding drills can range in intensity based on mass and acceleration. Examples for factors influencing mass and/or acceleration are summarized in Table 1. In box form drills factors such as the height of the obstacle influences the acceleration of the load experienced given the constant of gravity (12). The use of weighted vests, which add mass, and/or the inclusion
Table 1. Sample Variables Influencing the Intensity of a Plyometric Drill
|Variable||Low intensity||Moderate intensity||High intensity|
|Flat||Inclined surfaces; lower gradient hills||Higher gradient hills|
|Boxes||Aerobic Step||Small Box||Large Box|
|Stairs||Alternate Run||Ascending Bounds||Descending Bounds; Single Leg Bounds|
|Elastic Tubes||N/A||Elastic Resisted||Elastic Towed|
|Hurdles||N/A||Low Hurdle||High Hurdle; Low Hurdle on Inclined Surfaces|
|Weight Vest||N/A||Low/Moderate Intensity
|Moderate/High Intensity Drills Vested|
|Amplitude of Movement||Short Pulsed Movements||Double Leg Stretched Positions: i.e., deep squats, swim starts.||Single Leg Stretched Positions: i.e., straddles, lunges.|
of lateral movements can also influence the intensity of box form drills. The intensity of bounding drills can be modified by considering factors such as training surface (foam mats vs. natural grass), the use of hills, the inclusion of lateral movements and the use of towing systems. Progressing to drills that use only a single limb also adds intensity to both forms of plyometrics.
In the context of programming, the volume of plyometrics should be carefully monitored. Frequency in plyometrics is often measured by accounting for the number of foot contacts in a training session. Changing the mass and/or acceleration varies the intensity of a movement. A session consisting of low intensity drills but having too many foot contacts can result in poor form and lead to an acute injury or result in cumulative breakdown. Likewise an acute injury can result from a box drill of extreme height (12) due to excessive intensity. Other factors to consider when setting up a program are the number of drills, the number of sets for each drill, and the length of rest between drills. These training variables are all given attention in the periodized model and are alluded to in the recommendations.
The Transfer of Learning Controversy
Authors (3,9) citing motor learning research have suggested complex lifts such as the clean and jerk are technically specific to the specific lifting event and are not effective in training explosiveness for sport. While this argument has merit with regard to complex lifts, extending this to non-specific motions of plyometrics seems counterproductive. As stated earlier if athletes increase resistance in weight training using improper mechanics, they run the risk of injury. Likewise, if athletes push themselves to fully engage in ballistic sport motions their training has not adequately prepared them for, there is a potential for causing harm as well. Further from a performance standpoint, Basmajian (2) described a major benefit from training ballistically is the inhibition of excess muscle fiber activity "that floods into play when one first attempts to produce the required response".
Examining the transfer of learning controversy further one can say if a sprinter wants to "shave down" their sprint times, instead of engaging in plyometrics they should simply sprint more. Unfortunately, more of anything including plyometrics is not better. Any intense athletic event such as sprinting on a consistent basis leads to overtraining. Hence the purpose of training the exclusive event solely for a brief period of time (i.e., end of the in-season mesocycle) in the periodized model (23). Plyometrics training allows for work on specific muscles used in a skill or sport (17,19). In the context of sprinting for example, an athlete can devote time to developing explosiveness of the hip flexors or plantar flexors with various plyometric drills (4,5). There is undoubtedly a fine line between the effectiveness and safety of certain plyometric drills (10). Using the periodized model armed with the knowledge of what contributes to the intensity of a plyometric drill helps bring into prospective these considerations.
Setting up a periodized program for an athlete includes the use of several exercise modes including plyometrics. Plyometrics should be progressed across a macrocycle considering the parameters of any exercise mode. These include: frequency of sessions, number of drills, number of sets per drill, rest between sets, repetitions or foot contacts of each drill, and the intensity of each drill.
In a majority of periodization models, the off-season and pre-season is a time period devoted to the development of a strength base or improving an athletes 1RM. This arguably can be the most important factor for an athletes conditioning given the strong relationship between strength and power. The improvement of 1RM corresponds with an increase in the tensile strength of muscle and aids in minimizing the potential for injury (1). The use of plyometrics during this critical period of training should not compromise the development of strength. While there are a number of neuromuscular benefits derived from engaging in plyometrics, the potential for these gains to influence power are marginal in comparison to the potential strength development has to improve power.
As covered in the review, a battery of performance tests should be conducted to gain an appraisal of strength and power (6). It is prudent that the athletes weaknesses in all areas be identified early for the purpose of designing goals for developing an adequate strength base for ballistic motions. Plyometric drills in the early off-season should be used sparingly (i.e., one session per week). Low intensity drills should be initially introduced for the purpose of letting an athlete develop control over ballistic movements. Training emphasis should be on minimizing contact time with the ground (4). In fact, Wilson et al. (22) demonstrated SSC motions with pauses significantly reduced force output. Gradually increasing the foot contacts of these drills and progressively lengthening the training session may enable the athlete to develop attributes of a rapid SSC. Keeping drills lower in intensity, the athletes endurance to ballistic motions can be improved by systematically reducing the duration of rest between drills (19).
As the in-season mesocycle approaches, medium intensity drills should be introduced into the plyometric training sessions, however emphasis in the total program is should still focus on improving strength. Incorporating motions involving greater mass and acceleration helps prepare the athlete for the rigors of their sport. The number of foot contacts for the medium intensity drills should be progressed with greater caution than the low intensity drills.
A new parameter can be utilized as the regular season unfolds. Goals for strength development begin a gradual shift to strength maintenance allowing for an increase in the total number of plyometric training sessions per week (i.e., two to three sessions per week). The volume shift of plyometrics training however remains static and gradually reduces as increases in the number of training sessions are counteracted by a reduction in the total number of foot contacts and number of drills per session (23). It is important to reiterate that volume should be monitored more carefully as training sessions per week are increased. The difference in the scope of this mesocycle relates to a shift in progressing intensity (i.e., progressing from medium to high intensity drills) (17). Training sessions devoted to strength development should at this point should be minimized to three to five sessions across a two week span.
A number of periodization models advocate an emphasis of strength training in the off-season and pre-season. Strength training sessions as the regular season unfolds are reserved for the purpose of maintaining the gains in 1RM worked for in the off-season and pre-season. Curiously, the rationale for maintaining the neuromuscular adaptations gained from plyometrics during the previous macrocycle are not followed in many periodization models (17,19,23). Athletes using plyometrics devote a lot of time in the regular season to improving attributes such as the rate of force development (17). Their training has enabled them to perform intense SSC activities with proper form and control. Given this training history, it is conceivable certain athletes can engage periodically during the off-season in exercises beyond the level of low intensity without compromising the goal of improving their 1RM. Younger athletes without this training history and musculoskeletal development however are best advised to progress through plyometric drills in the periodized manner previously described.
5. T F Training volume in the periodized model should peak towards the end of an athletes competitive season.
GLOSSARY OF TERMS
Amortization phase: Wilk et al. (21) defined the amortization phase as "the amount of time between undergoing the yielding eccentric contraction and initiation of the concentric force".
Irradiation: spread of electrical activity in the spinal cord resulting in the recruitment or activation of additional muscle fibers (1,18).
Macrocycle: largest phase of training in the periodized model. A macrocycle is typically a calendar year.
Mesocycle: an increment phase of training in the periodized model. There are typically three mesocycles within a macrocycle.
Microcycle: Subsets of training periods that vary in volume and intensity which form a mesocycle. A microcycle typically consists of a two or three week time span.
Myotatic (stretch) reflex: a simple reflex in the spinal cord forming a single synapse between a sensory and motor neuron.
Stretch shortening cycle: a muscular behavior characterized by a rapid eccentric lengthening sequentially followed by a rapid concentric shortening.
Training intensity: a term used to represent the difficulty of an exercise. Movements that are more rapid or have greater resistance are considered to have higher training intensity.
Training volume: A term used to represent the quantity of work performed. It is the product of repetitions and the number of sets.