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    Strength and power are terms that are used to describe important abilities that contribute to maximal human efforts in sports and physical activities (Baechle & Earle, 2000). Strength, as described by Baechle and Earle is the maximal force that a muscle group can generate at a specified velocity. Power can be defined as the time rate of doing work, where work is the end product of the force exerted on an object and the distance the object moves in the direction to which the force was exerted (Baechle & Earle, 2000). In sports where jumping is involved, athletes must use their strength as forcefully and quickly as possible to achieve optimal success. An increase in power is related to an improvement in the speed-strength relationship (Adams, O’Shea, O’Shea, & Climstein, 1992).

    The vertical jump is commonly considered the true test of human power. This is true because the forces of gravity affect every athlete equally. The vertical jump measures the contractile and elastic properties of the muscle, or explosive power. Vertical jumping constitutes a vital component of explosive and power performance in a variety of athletic activities (Fatouros et al, 2000). Smilios (1998) stated that the vertical jump was considered an effective field evaluation of muscular power because the height of the jump correlated significantly with the maximal power units related to body mass.

    Plyometric training is a dynamic method of improving power output and explosiveness. The ability to rapidly lengthen one's muscle causes a concentric force to be developed. The faster the muscle lengthens the more forceful movement it produces and therefore overcomes the inertia of the body. The utilization of stored elastic energy helps to explain why plyometrics can result in an increase in vertical jump ability (Smilios, 1998).

    According to Ashley and Weiss (1994) “Several variables have been proposed as contributing to vertical jump performance including, strength and power exerted by muscles at the hip, knee, and ankle. Thus leg strength may be a useful predictor of vertical jump ability” (p. 5). To provide a greater understanding of the vertical jump and how it is related to performance, this review of literature has been divided into the following sections: Power Development, Lower Body

    Characteristics, Core Strength, Measurement Section and summary.

    Power Development
    Power equals the product of force and distance divided by time (Radcliffe, 1994). Few would disagree that to apply more force would take greater strength and that to reduce the time factor would take accelerated movements (Radcliffe, 1994). Fatouros et al. (2000) compared three different training protocols and measured the effects of leg strength and vertical jump. The three protocols were weight training, plyometric plus weight training, and a control group. Weight training protocols were squats, leg press, curls, and standing calf raises. Olympic style lifts were added the last four weeks of the 12- week program. The first two weeks the loads were kept at 70% of the one repetition maximum. The preceding 10 weeks intensity maintained between 80 and 95% of their one repetition maximum. They trained three days per week. Day one was high intensity, day two was moderate intensity, and day three was low intensity. The plyometric protocol was 80-foot contacts for the first two weeks and in the preceding 10 weeks 220-foot contacts on day one, 150-170 on day two, and 120 on day three.

    Fatouros et al. (2000) found that the combination group performed the best in vertical jump height; jumping mechanical power, and flight time and the weight- training group performed the best in leg strength. Fatouros et al. (2000) explained that plyometrics should be a training modality because of the ability to generate high power output. Muscles store energy in the negative (eccentric) phase then release that energy in the positive (concentric) phase. Since the muscle fibers store elastic energy, the end product results in muscles trained at higher tempos to elicit a more powerful response. A combination of weight training and plyometric exercises may promote a more powerful training stimulus when testing the vertical jump than performing them independently (Fatouros et al. 2000).

    Smilios (1998) examined individuals divided into a high strength group and a low strength group. Fatigue was induced by repeatedly pressing loads of various percentages (50%, 70%, & 90%) of their one repetition maximum until exhaustion. Vertical jump was tested and their performance was not proportional to the initial levels of leg strength when under fatiguing conditions. The performance of the subjects decreased independently from the initial level of leg strength, although the high strength group presented higher numbers.

    To support the last study performed by Smilios, researchers Blakey and Southard (1987) demonstrated that neither leg strength nor drop of height variables altered the resultant training effects in subjects who were classified in two conditions according to leg strength and body weight ratio. After an 8-week training program that combined weight training and depth jumps the researchers found significant gains in both strength and power for both groups regardless of initial leg strength. The information obtained in these studies was clear that initial leg strength had little to do with improvements in strength and power (Blakey and Southard, 1987). Subjects were able to see gains by implementing depth jumps and weight training into their workouts.

    Thomas (1988) found subjects who performed plyometrics jumped higher, accelerated faster, did more work in less time, and were more efficient than subjects performing non- plyometric exercises. Depth jumping was defined by Thomas (1988) as the action of dropping from an elevated surface and immediately upon landing, performing a maximal jump. The depth jump is executed by having the subjects let themselves fall from a box and as soon as they land, react with a powerful movement to perform a maximal vertical jump. The dropping height varies based on the capacity of the subjects (Bosco, 1999). The starting position is standing on the box with legs straight and hands on the hips. The drop is executed by simply taking a step forward and falling. When the subject comes in contact with the ground the subject should try to bend the knees as least as possible and then explode back up to maximal jump height.

    The neuromuscular system plays an important role in depth jumping and other plyometrics. These conditions strongly stress the stretch shortening cycle, which is the most common muscle action in sport (Bosco, 1999).

    Gehri, Ricard, Kleiner, and Kirkendall (1998) hypothesized that performing depth jumps would be superior to the other training methods. For the study, three groups were formed. One group performed depth jumps and the other two groups performed the squat jump and the countermovement jump. When landing during the countermovement jumps and the depth jumps, the participants were told to have their hands on their hips, and knees flexed between 30 and 60 degrees then rebound for maximum vertical jump height. The squat group was instructed to not perform any downward movement prior to the vertical jump test and to keep their knees at 60 degrees. After a 12-week training program, the researchers showed a significant increase in the vertical jump for all groups involved. As expected, the depth jump group performed optimally by increasing their mean vertical jump by 113%. Gehri et al. (1998) attributed the increase of vertical jump to a positive energy production. Because of the improved contractile component rather than the elastic component, both countermovement jump and depth jump had significant increases in positive energy production after training (Gehri et al. 1998).

    Gehri et al. (1998) argued that the depth jumps more closely replicated sport specific jumping and may have more application to sports that involve jumping as opposed to the squat jump or simple countermovement jumps. Strength and conditioning professionals should combine the depth jump with other sport specific jumps as part of the athletes overall training program design to improve jumping ability (Gehri et al. (1996).

    Holcomb, Lander, Rutland, and Wilson (1996) contradicted the research on depth jumps by evaluating a modified plyometric program that was designed to improve the plyometric depth jump training that was currently recommended at the time for increasing power and vertical jump performance. The subjects were 51 college-age men who were randomly assigned into 5 groups: control, countermovement jump training, progressive weight training, conventional plyometric depth jump training, and modified plyometric depth jump training.

    Holcomb et al. (1996) countermovement jump training consisted of a standing vertical jump where the subject was to flex the hip, knee, and ankle prior to jumping. The progressive weight training group was made up of standing plantar flexion, knee extension, knee flexion, and the leg press. The modified plyometric training consisted of three depth jumps. The first was the ankle depth jump which required the subject to remain as erect as possible when landing except for a slight knee bend thus absorbing most of the landing force at the ankle. The second was the knee depth jump that required the subject to land with the knees flexed to beyond 90 degrees while keeping the trunk erect. The third was the hip depth jump that required the subject to flex the trunk during the fall so the trunk is flexed at 45 degrees upon landing. The subject continued to flex the trunk until it was parallel to the ground and with the knees slightly bent.

    Holcomb et al. (1996) found: two potential problems that may explain the lack of overwhelming evidence in support of plyometric training: (a) the hip contribution is very low with the typical depth jump, thus such training would neglect to train the muscles to extend the hip; and (b) the exercise volume used the depth jump training is almost always less than that of the compared methods (p. 89).

     

    Holcomb et al. (1996) attempted to correct the identified problems with plyometric training and test the effectiveness of the modified plyometric depth jump-training program. The second problem was corrected by making the volume of all training sessions as similar as possible (Holcomb et al. 1996). Holcomb et al. (1996) showed that none of the training groups were significantly better than the other based on power and vertical jump. The researchers failed to show that the modified program differed significantly from other training methods based on pre- to post test scores in power and vertical jump.

    Clutch, Wilton, McGown, and Bryce (1983) also found that depth jump training was not effective as compared to other training methods. The researchers conducted a two-fold experiment. In experiment one the groups involved had three different jumping programs: (1) maximum vertical jumps; (2) 0.3 meter depth jumps; and (3) 0.75 and 1.10 meter depth jumps. In the second experiment the groups involved were divided into two groups: (1) weight training and 0.75 and 1.10 meter depth jump training, and (2) weight training only. There were two questions that the researchers were trying to answer. The researchers attempted to determine if certain depth jump routines with weight training were better and if depth jump had an effect on athletes already engaged in training. (Clutch et al. 1983).

    Clutch et al. (1983) showed that depth jump training was no more effective than a program with regular maximal jumps. Although the group that combined weight training and depth jumps was effective in increasing vertical jump, the group was no better than the countermovement jump and weight training.  Clutch et al. (1983) concluded that a program of depth jumps adds nothing to a program that already included jumping exercises.

    Adams et al. (1992) examined the effectiveness of three training programs that would attempt to maximize vertical jump performance by increasing hip and thigh power production. The study was a six-week, twice-per-week microcyle. From a physiological and psychological standpoint this was the optimal length because the central nervous system could be stressed without excessive fatigue because of the high intensity power training. Fourty-eight male subjects participated in the study and they had little or no exposure to specific power training or plyometrics. Hip and thigh power production were tested before and after training with the vertical jump test. The subjects were also tested in the 1 repetition maximum in the parallel squat before training to help establish a training percentage to work from. They were then randomly divided into 4 groups; squat, plyometric, plyometric-squat, and control. The squat groups program consisted of a linear periodization model which increased intensity and decreased in volume as time passed. The plyometric group performed depth jumps, double leg hops and split squats in the same manner as the squat group.

    Adams et al. (1992) showed that the squat-plyometric group performed significantly better than either the squat or the plyometric groups alone in increasing hip and thigh power production in the vertical jump. Hip and thigh power production as measured by the vertical jump can be best improved with a combination of weight training and plyometrics. Implementing a squat program alone can increases vertical jumping ability because according to Adams et al. (1992) the dynamic nature of the squat is highly conducive to enhancing neuromuscular efficiency (facilitating the stretch reflex). Thus, a great transfer of power to other similar biomechanical movements that require a powerful movement of the hips and thighs, such as the vertical jump. Implementing a plyometric program by itself could also effect hip and thigh power production specific to vertical jumping. The results from the enhanced motor unit recruitment and the storage of elastic energy with in the muscles increase power production and explosive capabilities. This transfers over when performing the vertical jump, thus improving the jumping height attained (Adams et al. 1992).

    Finally, the studies performed by Adams et al.(1992), Fatouros et al.(2000), Gehri et al.(1998), Blakey and Southard (1987) and Smilios (1998) all support the concept that plyometrics, specifically depth jumps and leg strength all play a role in power production and force output when tested by the vertical jump. Holcomb et al. (1996) and Clutch et al. (1983) are two examples that contradict the previous researchers findings. Therefore, this topic of research still needs further investigation.

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