The improvement of an athlete’s vertical jumping ability is a vital contribution to overall sports performance. Basketball players execute a vertical jump (VJ) on an approach to the basket to execute a layup or dunk, whereas a volleyball player may use a rapid countermovement jump (CMJ) to achieve a block or spike a ball. Swimmers perform an extension of the hip, knees, and ankles (triple extension), as executed in a VJ, when they propel their body out of the blocks from a static start, and a rugby player needs to jump to catch the ball on a pass. These VJ performances occur in multiple situations from a static start, countermovement action, approach, and in response to landing. The methods by which power is assessed may influence the data collected, and thus, consideration should be made on the mode (12). The VJ from static start is referred to as squat jump (SJ), and the lowering of the body with a change in direction upward is called a CMJ. An approach jump (AJ) is where a CMJ is preceded by one step to multiple steps, and a VJ in response to a landing is a depth jump (DJ). The specifics of each of these types of jumps will be discussed later in the article, but the first objective is to provide background information on the muscular actions that occur to create a VJ.
All VJs use a concentric action of the leg muscles to create the displacement of their body. This concentric muscular action needs to be forceful and rapid enough to achieve maximal height. Initial fast and forceful concentric muscle actions need to be at maximum muscular effort as in a ballistic squat to achieve the greatest possible height (29). The greatest amount of force production will be determined by muscle fiber type (type II versus type I), neurological control and activation (rate coding, recruitment, and inter- and intramuscular coordination), Ca+2 phosphorylation on myosin regulatory light chains, muscle cross-sectional area, muscle length, joint angle, contraction velocity, joint angular velocity, strength-to-mass ratio, and fiber arrangement (1,28,36). All of these factors will contribute to force production capabilities within a VJ. However, only some of them can be manipulated by training methods. One example is the increase in neural responses to resistance training, as discussed by Gabriel et al. (16).
Increasing the VJ height can also be achieved by the addition of a countermovement (CM) action before the concentric action (6). This CM action is part of a greater muscular action referred to as the stretch-shortening cycle (SSC), which can be further defined in 2 types of SSC actions. The SSC can be defined as a long SSC, which has a CM action that is greater than 250 milliseconds along with a large angular displacement of the involved skeletal joints, whereas the short SSC has a CM action less than 250 milliseconds and small angular skeletal joint displacements (31). The SSC as described by Komi (20) contains the preactivation, eccentric, and concentric phases. Additionally, the period of time between the eccentric and concentric phases are referred to as amortization (27). The amortization phase is the critical differentiation between a long SSC and a short SSC with the physiological adaptations dependent on which one is emphasized in training. The SSC in general has the storage of elastic energy during the eccentric phase, which will be released as kinetic energy during the concentric phase. However, there are 2 important distinctions with the amount of time during the eccentric phase increasing an athlete’s VJ height. The greater time to develop force during the eccentric phase may be the largest contributor to higher VJ height in a CMJ (6). A short eccentric phase typically results in a shorter amortization phase and lesser VJ height; thus from a performance perspective, there needs to be an analysis of sport movements that require specific strength and conditioning exercises. The differences between a long SSC and a short SSC are important when evaluating and implementing the appropriate exercises to determine an athlete’s jumping performance. The SSC has a number of variables that contribute to VJ performance. These SSC variables are the storage and release of elastic energy, the amount of time to develop force (i.e., long SSC), rate of force development (RFD) in the concentric phase, and the possible contribution of a stretch reflex (6).
A stretch reflex involves the stimulation of a muscle spindle by sensing an increase in muscle fiber tension, which sends an action potential through the type Ia afferents (27). This action potential enters the spinal cord, continuing through an interneuron into the efferent alpha motor neuron, which signals the muscle to contract. For the stretch reflex to increase its contribution to jumping performance, the intensity of the change of direction from the eccentric to concentric phase needs to be greater than what is imposed on the muscles during a CMJ. An example would be multiple hurdle jumps following a DJ where the heights of all obstacles are just below the athlete’s CMJ height. The distance of the drop from the box and hurdles can invoke the stretch reflex when the athlete would be attempting to rapidly navigate the obstacles. As athletes progress from pubescence to higher competition levels, there will be a necessity to incorporate exercises that will address the jumping attributes for their respective sports.
Squat jumps are defined as a VJ executed from a static start position at a predetermined depth, with the athlete’s arms in shoulder extension (see Figure 1A) or with the hands on the pelvic spine (see Figure 1B) followed by rapid triple extension in a vertical direction. Utilization of the SJ is to improve an athlete’s RFD, and thus, the descriptors of the SJ will rely on the athlete starting from a static point. For example, an athlete starting in 3-point or 4-point sprint start position could gain the ability to develop force as rapid as possible. Wilson et al. (41) demonstrated that male athletes who performed an SJ starting from 110° and 150° knee flexion had the greatest RFD maximum (RFDmax) in comparison with isometric at the same knee angles and CMJs. One point that should be noted is that there was a large difference in the coefficient in variation (CV%) (27.8–60.3%) for RFDmax from these tests, with the lowest CV% observed in the SJ (27.8%). The large CV% raises caution in using RFDmax because McLellan et al. (24) observed large CV% in their assessment of the RFD values. However, it should be noted that McLellan et al. (24) observed what they referred to as peak RFD, which may vary in the RFDmax, as measured by Wilson et al. (41). Additionally, the execution procedures of the 2 studies may contribute to variations in RFD. Furthermore, there is literature that suggests that elastic tissue properties only contribute to approximately 35% of the total energy for both CMJ and SJ (4). If the elastic properties are equal for the 2 jumping formats, then the difference in performance may be in the amount time to develop force, which is greater in CMJ execution (6). The specific contributions of elastic energy and kinetic energy may vary, much like muscle activation of different jumping styles varies between men and women (14).
The improvement in initial force development using an SJ may be further enhanced by adding external load and equipment. Loaded SJs may prove beneficial if loads between 10 and 40% of concentric 1-repetition maximum (1RM) back squats are used (33). Along with the added load, the placement of the load by using a barbell, hex barbell, weight vest, or a weight belt may result in different VJ kinetics and kinematics (35). Strength coaches should consider that as the load gets closer to 60% 1RM, the velocity will diminish and affect whether the goal is to maintain high velocity or rapid displacement of a maximal load. Additionally, incorporating a power rack or a jerk table can provide a static starting point that may be close to an athlete’s starting position (see Figure 2). For example, setting power rack safety catch pins for a shot putter at a half-squat to quarter-squat position may replicate similar motions between the delivery and release of the shot. Setting a bar at a predetermined height with power rack safety catches or jerk blocks is a method to allow an athlete to perform the SJ in the absence of any CM action. A critical factor for exercise application for improving the SJ is the utilization of a static start position followed by the execution of a maximal concentric muscular action that uses triple extension. Any inclusion of a CM action changes the adaptations and classification of the SJ to a CMJ.
The CM jump is a movement where by definition it has “… a movement in a direction opposite the goal direction” (6) with an amortization phase that consists of the change of direction from the downward movement to the ascending phase. An athlete begins standing in an upright position, which is followed by a descending action of the body via ankle dorsiflexion (eccentric action of posterior calf muscle), knee flexion (eccentric action of quadriceps muscles), and hip flexion (eccentric action of gluteal and hamstring muscle groups). Depending on the objective of the athlete, the amount of time between the eccentric and concentric actions will vary, but the final phase of the CMJ is rapid muscle contraction of the leg muscle that creates vertical propulsion. The CMJ is a common and specific movement in multiple sports, such as a rebound in basketball or blocking a shot at the top of the goal in soccer.
There are a number of theories to the mechanisms of why a CMJ can produce a much higher vertical displacement than a SJ. These theories range from an increased intermuscular coordination to storage and reutilization of elastic energy to an increase in RFD (6). First of all, possible mechanisms associated with the CMJ movement, which may contribute to force production, are muscle fiber connective tissues. These connective tissues surround individual myofibrils, fascicles (bundles of myofibrils), and the whole muscle, which may allow for elastic energy to be stored and released throughout the muscle. However, the amount of contribution from elastic energy in increasing VJ with the CMJ may be minimal (6). Additionally, a greater ratio of muscle fiber types IIa and IIx (referred to as “fast twitch”), in comparison to type I fibers (referred to as “slow twitch”), may contribute to CMJ performance. Type II fibers demonstrate higher force production and high contractile speed as a result of higher ATPase concentration than their type I counterparts (22).
Bobbert et al. (6) examined these different theories and mechanisms for the increase in height in the CMJ over an SJ and surmised that the time to develop force was the most determinate factor. The CMJ has a rapid eccentric motion, which prepares the active skeletal muscles for maximal force development before the actual ascending phase. When the leg musculature begins their concentric actions, which moves the body upward in the CMJ, some force production had already began as an effect of stored potential energy, although the greatest contribution will be the increased time to develop force (6). The CMJ’s force production follows the principal of Newton’s third law: for every action, there is an equal and opposite reaction, which is based on the force imposed into the ground resulting in more force directed upward, resulting in a greater vertical displacement.
Furthermore, the role of RFD on VJ performance was examined by McLellan et al. (24), which used a CMJ with an arm swing. The results of the study demonstrated that almost half of the VJ displacement was contributed by peak RFD with the second most being peak force, which accounted for approximately 25% of VJ displacement.
APPLICATION OF THE CMJ
Ideally, an athlete would desire to be able to execute the entire CMJ in a minimal amount of time, with a maximum amount of force developed. The CMJ action in a sport is usually done as a reactive movement, but there are times when maximal height is desired, when a greater amount of time to develop force is allotted. Practitioners need to apply exercises that maximize the development of RFD and peak power in the active muscles during a CMJ, such as box squats (34). Furthermore, athletes also need to enhance the recruitment of the larger, high force-producing muscle fibers (types IIa and IIx). Hennemen et al. (19) explained that the larger motor units, those that contain larger number of type II muscle fibers, will be recruited last in a maximal muscular action. Based on the work by Hennemen et al. (19), to recruit these specific muscle fibers (type II), exercises should be incorporated that result in higher neural recruitment stimulated by a high threshold (i.e., external load >85% 1RM), which result in greater force production to displace the body in a shorter period.
Practitioners need to choose exercises that promote high force production in an upward movement using the same muscles and joints that are used in a CMJ, such as the Olympic-style lifts and their variations (snatches, cleans, and pulls). Although there may be minimal relationship between absolute squat 1RM and CMJ, there is a suggestion that relative 1RM squat performance may have more of a role in predicting CMJ height (29). Practitioners also need to have athletes perform exercises with a resistance that will recruit the desired muscle fibers (types IIa and IIx) required by using heavy loaded exercises, such as squats. The simultaneous utilization of high force production and increased RFD can be achieved through the implementation of loaded “explosive” movements, such as clean and snatch variations, low and high pulls, power snatches, power cleans, along with loaded SJs (8,13). The desired effect of maximum force production and maximum power production can be achieved using the same types of exercises, as peak power and peak force have been shown to be strongly related (18). These exercises will promote the triple extension (extension of the ankle, knee, and hip) that is used in the CMJ, and increase the amount of work that is accomplished at these joints (6), resulting in the increase of force production and ultimately an increase in the vertical displacement and velocity of the jump.
ONE-STEP APPROACH JUMPS
The 1-step AJ is an exercise where an athlete takes one stepping action forward into a CMJ. This additional stepping action can be observed in volleyball players during the execution of a spike, where an approach toward the net (1 step to 4 steps) is performed before vertical jumping. The utilization of a 1-step AJ may elicit greater vertical displacement because of a greater storage of elastic energy in comparison to a traditional CMJ. Although used in numerous sport actions, the literature is limited in the exact mechanisms of improving VJ performance through the possibility of a preactivation in the leg musculature that may increase muscular-tendon stiffness.
Increases of muscular-tendon stiffness have been observed in gastrocnemius muscle-tendon complex following 8 weeks of varying jumping exercises (15). However, the greater height with the 1-step AJ is created because of possibly greater time to develop force, as proposed by Bobbert et al. (6). Additionally, Turner and Jefferys (37) provided a review of possible mechanisms of the SSC that may enhance performance. The overlying question may not be “What is the cause of improved VJ height?” but “What are the possible causes of improved VJ height?” However, the specific mechanisms that impact an AJ are purely speculation because research has been primarily focused on the mechanisms of the SJ, CMJ, and DJ. At the present time, strength and conditioning coaches should continue to implement the 1-step AJ because the development of the specific motor patterns associated with these types of exercises may benefit athletes who use these actions in their respective sports.
Incorporation of any 1-step AJ should only occur after the athlete has demonstrated the ability to perform an SJ or CMJ in the absence of mechanical breakdown (e.g., valgus). The SJ and CMJ will be an important progression to the 1-step AJ because the additional motion may cause a disruption to the movement the athlete has previously learned. However, the time to maximize the benefits from an additional step could be reduced with an already established jump training program that emphasizes landings, body positioning, and mechanics. Furthermore, an athlete should not be limited to only forward stepping because most sports are not set and predetermined thus needing backward, diagonal, and lateral AJ. Figure 3A–C is a sequence depiction of an athlete performing a forward 1-step AJ, whereas Figure 4 is a depiction of the start stepping action of a lateral 1-step AJ. One point that should be clear is that this movement is not the same as a standing or running long jump, which has a horizontal and vertical displacement component. Additionally, the variations of the SJ and CMJ (e.g., weight vest) may also be applied to the 1-step AJ, but the point of diminishing return may be different in comparison to the SJ and CMJ. Furthermore, progression of the 1-step AJ may be to the inclusion of adding a drop from a predetermined height in the DJ exercise.
Depth jumps are a type of plyometric exercise that specifically uses potential energy and the force of gravity to store energy in the muscles and tendons. The DJ is performed by having the athlete step off an elevated platform, landing, then reversing the eccentric action into a concentric vertical upward action (Figure 5A–E) of at least ≥20 cm to harness potential energy (40). However, this height is a variable factor that can be manipulated to change the “intensity” of the plyometric exercise (27). Once the athlete contacts the ground after the step off, it is paramount to immediately rebound; therefore, redirecting the stored potential energy in the form of kinetic energy vertically while the amortization phase (ground contact time) must be minimal (i.e., <250 milliseconds). This whole process creates a kinetic energy system that uses the myotatic stretch reflex to generate very large amounts of muscular power, which is believed to be the link between speed and strength (10,42).
During the DJ, the SSC is performed via the drop from elevation (gravitational potential energy), ground contact (eccentric loading), and subsequent explosive jumping movement (concentric action) (21). This process involves the series elastic component, contractile component, and parallel elastic component of the musculotendinous unit as well as the stretch reflex of the neuromuscular system to generate massive force in a minimal period. During the DJ, minimizing the amortization phase time (ground contact time) is critical to harness the most stored energy and therefore generate the largest SSC force production (8,31,32).
Depth jump training is a common training modality for improving lower extremity power and speed. The DJ has been shown to improve power as measured by the VJ (11,17,25,32,38). As discussed, the DJ uses the elastic properties of the muscles as well as the stretch reflex potentiation of the muscles and nervous system to induce an enhanced training effect. Bosco and Komi (6) found that increases in VJ ability after the DJ can be attributed to the utilization of elastic energy and SSC components. Although this research concluded that the SSC and elastic energy utilization were responsible for the improved VJ, other research demonstrated that elastic energy utilization is not increased from DJ training. Gehri et al. (17) showed that although DJ training did not improve the amount of elastic energy used, it did significantly increase VJ performance. Therefore, although the elastic energy produced during a DJ may not increase from DJ training, the efficiency of the SSC may be enhanced.
The utilization of the elastic properties and nervous system potentiation used during the DJ may also produce an increased contractile protein training effect that can result in increased VJ performance. Therefore, in conjunction with the increased concentric contractile performance, DJ training decreases the ground contact time via decreased amortization phase time (9,17,21,32). In conclusion, DJ training may enhance the musculotendinous unit as well as the neuromuscular system to increase the power production during the SSC.
APPLICATION OF THE DEPTH JUMP
Gehri et al. (17) concluded that activities that involve SSC, such as the DJ, may improve jump performance when compared with CMJs specifically because of the neuromuscular specificity. Depth jump training may be modified to allow for even more sport specificity. The DJs are modified by changing the body posture during the initial phase to specifically isolate the musculature surrounding the desired joint. Depth jump specificity is important for various power athletes that necessitate differing ranges of motion of power production. For example, volleyball players may require maximum power production from a different range of motion than an Olympic lifter. The DJ may be modified to be hip dominant, knee dominant, or ankle dominant depending on the requirements of the specific sport. Andrew et al. (3) observed that modified DJs offer a greater degree of specificity for power training in athletes. This research shows the importance of training specificity even when related to jump training. To best determine the necessary DJ modification, a sport analysis is required.
Increasing the intensity of DJ training is accomplished via 2 primary variables. The first variable is drop height, which can be raised, thus altering the distance the athlete’s center of mass falls. The second variable is the athlete’s body mass, which may be changed by adding external weight (e.g., weight vest) during the performance of the jumping drill. These 2 variables can lead to increased potential energy; however, it has been shown that no extra benefits were provided by acute changes (e.g., wearing a weight vest) to a person’s body mass (23,39). The final means of modifying DJ intensity is by doing unilateral or single leg variations. This modification may not be the most efficient means of performing DJs because this is a similar method to increasing body mass, which takes the body mass and distributes it over 1 limb versus 2 limbs. Thus, the single limb must redirect all the energy harnessed during the DJ. By isolating the body weight, which is accelerated by gravity from a variable height, on a single limb, the amortization phase will be slowed, when compared with 2 limbs with the same body weight. Table 1 is a guideline for DJ progression, which requires further investigation into the validity of the specifics. Research has shown that slowing the amortization phase is not very efficient (23). The more efficient means of increasing the intensity and plyometric effect of acute training is by increasing the speed of the movement by allowing for longer acceleration by gravity (increased height of center of gravity). The increased starting height may help to decrease amortization phase time. The decrease in the amortization phase time results in an enhanced SSC and therefore increased plyometric effect. However, according to research by Walsh et al. (39), one of the most important jump parameters is ground contact time versus starting drop height. Their research concluded that minimizing ground contact time resulted in greater effects on important jump parameters (mechanical power output, work performed at the center of mass, ankle, and knee joints, and maximum vertical force values and takeoff velocity) as compared with drop jump starting height (39). Therefore, the most efficient manner of increasing DJ intensity is by increasing the drop height as well as minimizing ground contact time.
It should be noted that the intent of this article was not to promote or support any single method of training in relation to another but rather which jumping method may be “best” for obtaining specific neuromuscular adaptation. The overall combination of these methods in most cases will improve the performance of an athlete’s VJ. The completion of a plyometric training program could also demonstrate a decrease in ground contact time, which is an advantageous attribute in some sports (26). Furthermore, athletes who engage in both strength training and VJ exercises have the increased chance of improving their VJ performance to a greater degree than those who only strength train or jump train independently (2,13,20,30). Performing jumps with assisted or resisted body weight only are the methods that may be applied to a strength and conditioning plan to increase an athlete’s VJ (5). Along with the improvement in VJ performance, there is added benefit of improved sprinting time, which also is influenced by training volume, intensity, individual characteristics, and frequency (30). The exact strength and conditioning plan that is constructed needs careful consideration of physiological adaptations, phase of training, athlete status (e.g., Division I athlete), and assessment of results before the incorporation of exercises for improving VJ performance. The current article differentiated 4 VJ exercises and progressions that may be implemented into a well-rounded strength and conditioning plan with specific performance adaptations.
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