HUNTER, JOSEPH P.; MARSHALL, ROBERT N.
A drop jump involves stepping from a small height, landing, and then explosively jumping (usually, but not always) vertically. Various restrictions can be set when performing such jumps, ultimately affecting jump technique and the height obtained. An example of such a restriction is to instruct the athlete to minimize ground contact time while still attempting to jump as high as possible. To aid in achieving this, feedback of the ratio of “jump height to ground contact time” can be given after each jump (11). The jump technique used with such restrictions has been referred to as a “bounce drop jump” (i.e., a quick rebound style of jump) of which the antithesis is a “countermovement drop jump” (i.e., using a relatively larger countermovement) (2).
When restricted to using a bounce drop jump or countermovement drop jump technique, the highest jump heights are achieved with the latter (11). However, when a group of athletes perform unrestricted drop jumps and are instructed to jump for maximum height, the technique will range from that of a bounce drop jump to a countermovement drop jump (2). That is, there exists a “jump technique continuum” in which technique ranges from use of a small amplitude countermovement, production of high lower-limb stiffness during the eccentric phase of the jump, and a short ground contact time to the converse at the other end of the continuum.
The existence of drop jump technique variation and the effects of instructions have been documented (1,2,11). However, the effects of various training methods on jump technique (with regard to the jump technique continuum) remain undocumented. For example, lower-limb stiffness used during the eccentric phase of a stretch-shortening cycle jump is likely to have a large influence on both jump technique and the height jumped. In the interest of making more informed training program decisions, it would be valuable to know the changes in stiffness associated with various training programs. The suggested optimal level of stiffness for a stretch-shortening cycle task has ranged from stiff (7) to compliant (10). This issue remains unresolved, and it is possible the authors were referring to different “sources” of stiffness (see the Discussion for more on this). However, if there is an optimal stiffness to use during the eccentric phase of a jump, then it is possible, when unrestricted, subjects will adapt to using such a technique. Therefore, the purpose of this study was to assess the effects of power training and stretching on unrestricted jump performance, in particular the effects of training on the technique used with regard to the jump technique continuum.
Approach to the problem and experimental design.
This study included four randomly allocated groups: a power training group (P), which trained to increase vertical jump performance; a stretching group, which trained to increase flexibility (S); a combined power and stretching group (PS); and a control group (C). The study was approved by The University of Auckland Human Subjects Ethics Committee, and all subjects attended a test familiarization session at which written informed consent was obtained. A pre-training testing session was followed by 10 wk of training and then a post-training testing session. The tests included stretch tolerance of the hamstrings and quadriceps, and four variations of vertical jumps.
The jumps included drop jumps from 30-, 60-, and 90-cm drop heights (DJ30, DJ60, and DJ90) and also a countermovement jump (CMJ). These jumps were chosen due to the wide range of eccentric loading experienced by the performer. The performance goal for all jumps was to obtain maximum height. With the intention of allowing the subjects to optimize their own jump technique, no restrictions were placed on the time spent in contact with the ground and the magnitude of the countermovement used. All jumps were performed with hands placed on hips. In addition to jump height, the technique variables measured (with regard to the jump technique continuum) included eccentric lower-limb stiffness, magnitude of countermovement, and, in the case of drop jumps, ground contact time.
Training took place over a 10-wk period. Power training consisted of two supervised sessions per week. Stretch (flexibility) training consisted of one supervised session and three unsupervised sessions per week. Subjects who performed the stretch training kept a log of date and duration of the unsupervised sessions. Control subjects were requested not to start any specific lower-body power or stretch training for the duration of the 10 wk. Throughout the duration of the study, all subjects were requested to maintain their sporting involvement at the volume and intensity as it was for the weeks before the study. Therefore, the sole form of intervention was the addition of a training stimulus of higher intensity than that usually experienced.
All subjects were male, from a variety of sporting backgrounds (but primarily basketball and volleyball), and generally had little consistent resistance training experience. None of them had been involved in any structured plyometric program or stretching program (designed to increase flexibility) in the past year. The mean age, height, and body mass (± SD) of all the subjects were 24 ± 4 yr, 1.82 ± 0.06 m, and 82.93 ± 11.71 kg, respectively.
From an initial 60 subjects that were randomly assigned to each group (i.e., 15 per group), one subject discontinued from each of groups PS and C, and four subjects discontinued from each of groups P and S. This left a total of 50 subjects who completed the study requirements (PS = 14, P = 11, S = 11, and C = 14). Part way through the study, one subject in group PS suffered discomfort in one knee when performing drop jumps. In the interest of safety, his program was slightly modified and drop jump data were not obtained in the final testing session. Nonetheless, the subject did perform CMJ and all other test requirements.
Group P performed a power training program, group S performed a stretching (flexibility) program, and group PS performed a program combining power training and stretching. The “double volume” of training that the PS group undertook was considered irrelevant due to the fact that the two programs aimed to improve different aspects related to performance. All subjects kept a logbook of their required tasks.
The power training program involved twice a week training performing a combination of resistance training exercises and plyometrics (see Table 1). The resistance training sessions comprised of a deadlift/squat hybrid exercise and weighted countermovement jumps with dumbbells held in the hands. The plyometrics included drop jumps and countermovement jumps.
The deadlift/squat hybrid exercise was performed with a bar that allowed subjects, standing with arms hanging by their sides, to squat down and grasp two fixed parallel handles slightly wider than shoulder width. Although similar to a conventional barbell deadlift in concept, the actual movement pattern is more like that of a squat due to the unobstructed path of the bar about the knees. The bar was positioned on blocks to allow the subject to start from a knee angle of approximately 90°. The advantage of such an exercise was the limited amount of equipment required to perform the exercise in safety.
The weighted and unweighted countermovement jumps and drop jumps were all performed without the use of the arms (i.e., either hands on hips or holding dumbbells) and with the only goal being to jump for maximum height. All jump exercises were performed with maximum effort right from the beginning of the program.
The lower-body stretching program was performed 4 d per week. One of the sessions was supervised whereas the other three were unsupervised. The program included a variety of common static stretches for the hamstrings, quadriceps, hip extensors, hip adductors and abductors, and plantarflexors. Each stretch was held at the point of only mild discomfort with an emphasis on relaxing the entire body. As the discomfort subsided, the subjects were instructed to slowly and very gently take the stretch further. If required, subjects were permitted to take small breaks (a few seconds’ duration in which the stretching tension was released) during each stretch repetition. From week 4 onward, some proprioceptive neuromuscular facilitation (PNF) stretching was included only in the supervised sessions (using intermittent bouts of 10-s submaximal contractions followed by a relaxed stretch). See Table 2 for information regarding sets and repetitions of the stretching program.
At the initial testing session the control subjects completed a form to indicate their physical activities during the past 3 wk. During the 10 wk between their initial test and final test, each control subject was required to fill out a daily physical activity log. This provided a means of checking for any significant change in physical activity over the 10 wk.
Adherence to programs.
All power training sessions were supervised, and if a subject could not attend a session, a replacement appointment was made. Only one from every four stretching sessions was supervised; however, each subject kept a logbook of the unsupervised sessions. Subjects in the stretching groups were encouraged to adhere to the programmed unsupervised sessions but also to record a true reflection of the stretching that was performed. The information recorded in these logbooks and insight gained from regular discussion with each subject suggested that in any particular week at least three-quarters of the set program was completed. If a subject did not achieve these standards on a regular basis, then their involvement in the study was discontinued. There existed no notable differences in adherence to stretching programs between groups. From logbooks and contact kept with the control subjects, it was inferred that these subjects did adhere to their requirements of maintaining involvement in their regular activities.
Each subject was tested before and after the 10 wk of training. On each occasion, subjects were requested to refrain from any physically exhausting exercise 48 h before the testing session and to have done no previous exercise on the day of the test. After obtaining measures of body mass and height, there were two main sections to the testing session: a) stretch tolerance test for hamstrings and quadriceps and b) four variations of a vertical jump.
Measures of stretch tolerance of the hamstrings and quadriceps were preceded by a warm-up on a cycle ergometer (Monark 818, Varberg, Sweden) for 5 min at 120 W. One experimenter performed all of the flexibility measures on all subjects. The procedures used to measure stretch tolerance were designed specifically for this study; however, some of the methods were similar to those suggested by Hubley-Kozey (6). The subject’s body (particularly his pelvis) was held still by strapping the subject to a specially designed table. A Leighton Flexometer (Leighton Flexometer, Spokane, WA) was used to measure joint angle while moving the joint through its range of motion. All stretches were passive; that is, an external force (provided manually by the experimenter) moved the joint through its range of motion. This was performed slowly yet progressively over three periods of 30 s while gaining feedback from the subject regarding the tolerance of the stretch. The subjects were informed that they were in control of how far the stretch was performed and at the end of the last 30-s period they should aim to reach a point at which they can tolerate the stretch being held for at least 10 s yet feel that they would not be injured. For each trial, the maximum amount of flexion (hip or knee) was measured and used as the approximate starting point for the following trial. The maximum range of motion measured on the last trial was used as the measure of stretch tolerance. Great care was taken to ensure continual feedback from the subjects to safeguard against injuries.
To test the stretch tolerance of the hamstrings, the subject laid supine on a table with one strap across the anterior superior iliac spines and another across the upper thigh of one leg. A flexometer was attached to the lateral side of the “free” leg, just above the knee. The flexometer was set to zero while the knee was extended and the leg flat against the table. The subject was requested to keep his leg completely straight while the experimenter raised it in the subject’s sagittal plane. Both left and right legs were tested, and the average of the two used for analysis.
To test the stretch tolerance of the quadriceps, the subject laid prone on a table with knee joints protruding just over the end of the table. Two straps fastened the subject tightly to the table: one strap across the upper thighs immediately below the buttocks and one strap across the 5th lumbar vertebrae. A flexometer was strapped to one ankle and faced laterally. While the subject laid with his chest against the table, the leg to be tested was fully extended, completely relaxed, and the flexometer was set at zero. For the first two trials, the subject laid flat with the chest against the table as the experimenter applied a force to flex the subject’s knee. Before the third trial, the subject rose up on his elbows in a “sphinx position” with his upper arms vertical and forearms flat against the table. This was to aid holding the subject’s pelvis flat and prevent any hip flexion. This was only done on the last trial as some subjects found it too uncomfortable to remain in this position for longer periods of time.
All jumps were performed on a force plate (Bertec 6090; Bertec Corporation, Columbus, OH) used to collect vertical ground reaction force data. The force plate amplifier (Bertec AM6-3) was interfaced via an A/D MacLab System (ADI MacLab Systems, Dunedin, New Zealand) to a Macintosh computer (Power PC 8100,100AV) running Chart version 3.4/s. A two-point calibration of vertical force application was performed with known weights immediately before each test. The sampling rate was 400 Hz.
After a jump-specific warm-up, subjects performed four variations of a vertical jump (all with hands placed firmly on hips) in the following order: CMJ, DJ30, DJ60, and DJ90. Three maximal effort trials of each jump variation were attempted, preceded by one or two submaximal trials to become familiar with the “new” jump. It was considered more appropriate not to randomize the order of the jumps but rather to allow the subjects opportunity to optimize their technique by performing each jump variation in consecutive order. Approximately 1–2 min were taken between individual jumps. The only outcome goal was maximal height; that is, there were no restrictions placed on the magnitude of countermovement or ground contact time (in the case of drop jumps).
Jump heights for all jumps were calculated, and the greatest height for each of CMJ, DJ30, DJ60, and DJ90 were used. For CMJ, it was assumed the vertical velocity of the body mass center, before performing the jump, was zero. Therefore, the vertical velocity of the body mass center at take-off was calculated using the impulse-momentum relationship. Jump height (h) was then calculated using h = v2/(−2a) where v is the velocity at take-off and a is acceleration due to gravity.
Jump heights of the drop jumps (DJ30, DJ60, and DJ90) could not be determined from the vertical impulse method as the velocity of the initial landing from the drop height box was unknown. An alternative method of calculating jump height from flight time was used. Jump height (h) can be estimated from flight time using:h = (a ·t2)/2 where t is equivalent to 1/2 flight time and a is acceleration due to gravity. The assumption made with this calculation is that the height of the body mass center is identical for both the instants of take-off and landing. To help achieve this, subjects jumped with hands on hips; however, it was clear that for some subjects this method overestimated jump height. As a result, a corrected flight time method was used to calculate drop jump height.
From an assumption that the expected error (of the flight time calculation method) for the drop jumps would be similar to that of CMJ, the expected error was derived by calculating all three CMJ heights with both the flight time method and the impulse method, subtracting the former from the latter and then taking the mean of the three differences. This expected error was then subtracted from the flight time calculated heights of the best drop jumps.
Stiffness of the combined lower limbs produced during the eccentric muscle action phase (i.e., eccentric lower-limb stiffness) was calculated for the highest jumps of each of CMJ, DJ30, DJ60, and DJ90. Stiffness was calculated from the ratio of change in vertical ground reaction force to the simultaneous change in height of the subject’s body mass center. This was obtained via double integration of the vertical acceleration data measured from the force platform (3,5). In this process, knowledge of the velocity at some point in time is required. For the CMJ, this was obtained from where the subject stood still before initiating the jump (i.e., velocity equalled zero). For DJ30, DJ60, and DJ90, the velocity point used was that of the instant of jump take-off (v) calculated using:v = ( −2a·h)0.5 where a is acceleration due to gravity and h is the height jumped. Also, to calculate absolute position data, the position of center of mass at some point in time was required. For CMJ, the height of the subject’s body mass center when standing erect was defined as zero; for all drop jumps the height of the subject’s center of mass at initial contact (after dropping from the drop height) was defined as zero. Note that each individual athlete was capable of performing jumps with a wide variation in eccentric lower-limb stiffness. Therefore, it should be kept in mind that this measure of eccentric lower-limb stiffness is that which each subjects “chose” to produce for each particular jump variation so as to obtain maximum jump height.
Figure 1 shows representative graphs for vertical ground reaction force as a function of vertical position of the body mass center. Note that lowering of the center of mass has been defined as positive. Figure 1A is that of a CMJ. The start of the CMJ eccentric phase was defined as the point where acceleration reached a minimum; the end was when the subject’s body mass center reached its lowest position. Typically, the eccentric phase contained a relatively linear portion when excluding the first and last 5 cm of vertical displacement. It was considered that the slope of this “middle” portion would best describe the predominant level of eccentric lower-limb stiffness for the CMJ.
Figure 1B shows vertical ground reaction force as a function of vertical position of the body mass center for three different subjects all performing drop jumps from a 60-cm drop height. Unlike the CMJ, there was typically no distinct linear portion for the eccentric phase of the jumps. Eccentric lower-limb stiffness for the drop jumps was calculated as the change in vertical ground reaction force over the change in position of the body mass center for the complete eccentric muscle action phase during ground contact. This approach permitted the detection of differences in “average” stiffness produced as indicated by the three subjects in Figure 1 (with the slope of the straight lines representing lower-limb stiffness).
From the body mass center position data, derived via double integration of the vertical acceleration data, the vertical displacement of the body mass center during the eccentric phase of each highest jump was calculated. The starting reference point for CMJ was the position of center of mass while standing erect and for DJ was the position of center of mass at initial ground contact. For both CMJ and DJ, the end of the eccentric phase was defined as the lowest position of the body mass center.
Ground contact time was measured for the highest jumps of each of DJ30, DJ60, and DJ90. This variable was measured from the instant of landing to the instant of take-off as detected by the vertical ground reaction force data.
The statistical methods of this study were chosen so as to allow sample size to be kept at a practical level and to maintain a reasonable level of statistical power (80% chance of detecting an effect size of 0.35 in vertical jump height with P < 0.10). This involved using a limited amount of preplanned contrasts with no adjustments made for multiple tests. It was determined that 14 subjects in each of the four groups would be sufficient. Subject numbers did drop slightly below this figure for some of the groups, which would have slightly decreased the statistical power in detecting small effect sizes for contrasts between two groups. However, many of the contrasts did involve combining groups (see explanation in next few paragraphs), which ensured ample subjects for these contrasts.
The differences between pre- and post-training measures for each variable were analyzed with a one-way ANOVA and with a limited number of orthogonal preplanned contrasts (Table 3). These preplanned contrasts were designed to answer three specific questions for each of the variables. They were: a) what effect did the power training have, regardless of whether stretching was performed or not; b) what effect did the addition of the stretching program to a power training program have; and c) what effect did the stretching program have in contrast to a control group? Note that the contrast related to the first question combined both groups which performed power training (groups PS and P) and both groups which did not (groups S and C). These are later denoted in the text as CPG (combined power groups) and CNPG (combined nonpower groups), respectively.
The exception regarding use of these contrast coefficients was one other preplanned contrast between groups PS and S combined (CSG = combined stretching groups) and groups P and C combined (CNSG = combined nonstretching groups) for stretch tolerance of the hamstrings and quadriceps. This was designed to answer the question: Was the stretching program effective (regardless of whether power training was performed)?
Alpha was set at 0.10, however, the level of evidence of an effect (8,9) is indicated in the tables with either *P < 0.10, **P < 0.05, or ***P < 0.01. Any contrasts that “approached significance” are also indicated in the tables.
Analyses on changes in stretch tolerance were made between all subjects who performed stretching (CSG) and all those who did not (CNSG). For the hamstrings stretch tolerance test, there were 25 subjects in each group. Due to some subjects reaching the maximum level of knee flexion for their initial test (and thereby not included in analysis), the quadriceps stretch tolerance analysis included only 22 subjects in the CSG and 20 subjects in the CNSG. Stretch tolerance results are shown in Table 4.
Fifty subjects were included in all the CMJ test analyses, whereas only 49 were included in the drop jump analyses (due to knee discomfort experienced by one subject during drop jumps). Jump height results are shown in Table 5.
For DJ30, DJ60, and DJ90 eccentric lower-limb stiffness, two or three outliers existed (i.e., subjects who landed with markedly greater stiffness than the others). Therefore, analyses were performed with and without these outliers removed to see whether the same conclusions were revealed. Results for when the outliers were removed are shown in Table 6. When outliers were not removed (and the data were transformed to satisfy the assumptions of ANOVA), the statistical inferences were relatively similar except in the case of DJ90 where there was no evidence of a difference.
Results for vertical displacement of the body mass center are shown in Table 7. Ground contact time was determined for DJ30, DJ60, and DJ90 only and are shown in Table 8.
Often research is conducted with the goal of determining if one form of training is superior to another. However, in many cases, training effects other than performance outcome (e.g., effects on technique) remain unknown. This current study assessed the effects of power training and flexibility training on jump technique, in addition to jump performance. Insight gained regarding the effects of the training methods on technique could possibly contribute to a more informed choice of training methods by coaches and athletes. From the results of this study, it is likely that if the training goal for drop jumps is maximum jump height alone, technique will change in the direction of a lower eccentric leg stiffness, greater depth of countermovement, and possibly a longer ground contact time, whereas for a countermovement jump eccentric leg stiffness and the depth of countermovement may both increase. Depending on the desired outcome, these changes may be either detrimental, advantageous, or irrelevant to the athlete’s sport. The decision remains with the coach and athlete.
The results from this study also suggest that stretching has little effect on the jump technique variables measured but may offer some performance outcome benefits, at least in the case of CMJ. Other researchers have also reported improved stretch-shortening cycle performance with the addition of stretching to a strength program (in subjects previously unfamiliar with regular flexibility training; (10). At this point in time, these results suggest that stretching should not be ignored as a possible source of improvement in stretch-shortening cycle tasks, at least for those athletes who are inflexible. However, further research is required.
The level of eccentric lower-limb stiffness produced during the best (highest) jump for each jump variation was used for analysis. Although each subject was capable of producing a variety of stiffness levels (i.e., acute regulation of stiffness), the level produced during the highest jump was considered that which allowed optimal technique so as to maximize jump height. Subsequently, because of the direct effect of eccentric lower-limb stiffness on jump technique, it was included (with magnitude of countermovement and ground contact time) under the term “technique variables.”
Farley et al. (4) measured lower-limb stiffness produced by subjects when performing two-legged hopping in place at a preferred frequency. These authors described the behavior of the lower limbs as being “spring-like.” That is, the vertical ground reaction force was dependent on the displacement of the body mass center. In another similar study (5), they explained their force displacement graphs of two-legged hopping as being “. . .approximately linear at high levels of force and displacement.” This was not the case for the drop jumps in the current study, although CMJ were mostly linear (see Fig. 1 for an example). As a result of this, the reader must be aware, especially in the case of the drop jumps, that the measures of lower-limb stiffness are calculated simply from force and position differences over two points in time. Nonetheless, even for the drop jumps, these measures of lower-limb stiffness were considered to sufficiently describe the differences in jumping technique used by the subjects. To emphasize that stiffness was calculated over the eccentric phase of the jump the term “eccentric lower-limb stiffness” has been used in this paper.
Interestingly, an inconsistency existed in the effects of training on the level of eccentric lower-limb stiffness produced during CMJ and the drop jumps. Power training, regardless of whether stretching was performed or not, was shown to increase the level of eccentric lower-limb stiffness produced during CMJ but to decrease that of DJ30, DJ60, and DJ90. No evidence existed to suggest that stretching had any effect on the level of eccentric lower-limb stiffness used. CMJ and drop jump eccentric lower-limb stiffness were derived from the vertical force-displacement graphs with slightly different methodologies. However, it is unlikely that such slight variation in the methodologies would explain the extreme differences of the effect of the training on the level of eccentric lower-limb stiffness produced (i.e., an increase for CMJ versus decreases for the drop jumps).
Before discussing the possible reasons of the effect of power training on the level of eccentric lower-limb stiffness produced, it might be helpful to consider the changes in the other jump-technique variables. Evidence suggested that power training, regardless of whether stretching was performed or not, resulted in a greater vertical displacement of the body mass center. Furthermore, there was some evidence that ground contact time increased for the drop jumps. Just as with eccentric lower-limb stiffness, there was generally no evidence to suggest that stretching had any effect on vertical displacement of the body mass center and ground contact time.
When only considering the drop jumps, it appears that the power training enabled the subjects to decrease the level of eccentric lower-limb stiffness used and to absorb the early ground reaction forces over a greater distance and time. It is possible the subjects, at the initial testing session, performed the drop jumps with suboptimal technique. However, with habitual jumpers chosen for the study, the familiarization process used, and the best jump of each jump variation used for analysis, this inference seems unlikely. A more likely explanation would be that the training adaptations allowed the new technique to evolve. Just as in testing, during training there were no restrictions placed on the depth of countermovement used and ground contact time. Without these restrictions, the subjects were free to modify their technique so as to optimally use their current neuromuscular capabilities.
In contrast to the drop jumps, there was strong evidence to suggest that the level of eccentric lower-limb stiffness produced during CMJ increased for all those subjects who performed power training (regardless of performing stretching or not). Despite this, there was strong evidence that vertical displacement of the body mass center increased. That is, these subjects used a greater magnitude of countermovement.
The question remains, “Why did the subjects appear to benefit from the addition of a stretching program for CMJ but not for any of the drop jumps?” A possible explanation might be the changes in jump technique that were adopted over the 10 wk of training. Komi (7) has suggested that greater lower-limb stiffness during the eccentric phase of a stretch-shortening cycle task might be an advantage by allowing greater storage and release of elastic energy. Note that for all those subjects who performed power training, the level of eccentric lower-limb stiffness produced during CMJ increased. CMJ was the only jump variation in which this occurred and was the only jump variation in which group PS appeared to benefit (with a greater jump height improvement) over group P, likewise for group S over group C. It is possible that the greater eccentric lower-limb stiffness (supposedly via acute regulation of the contractile components of the musculature involved) did allow greater utilization of elastic energy, particularly in the subjects of group PS. Wilson et al. (10) have reported benefits (to a rebound bench press exercise) gained from stretching program. The authors presented evidence of a stretching program causing a decrease in series elastic component stiffness and proposed that it allowed greater storage of elastic energy, thereby resulting in greater stretch-shortening cycle performance. The subjects in group S of the current study may have benefited in a such a way. Group S did not increase their level of eccentric lower-limb stiffness produced during CMJ; rather, it remained approximately the same. Greater compliance of the series elastic component (due to increased flexibility; (10) with the same eccentric lower-limb stiffness would have still allowed more of the eccentric force to be stored in the series elastic component, supposedly resulting in benefits due to recoil of elastic energy. However, this is speculation and requires further research.
In the cases of the drop jumps, the level of eccentric lower-limb stiffness produced decreased, and ground contact time and magnitude of the countermovement used increased. Furthermore, there was no apparent advantage to performing the stretching program. This agrees with the above hypothesis in the following way. Greater storage (and then utilization) of elastic energy is only possible through increased eccentric lower-limb stiffness, despite any effects that stretching might have had. That is, all subjects in groups PS and P decreased the level of eccentric lower-limb stiffness produced (possibly to help absorb the high impact forces and prevent Golgi tendon inhibition) and in doing so decreased their opportunity to store elastic energy. If this was the case, then the stretching program would possibly not be of the same advantage as when training for CMJ (or any other stretch-shortening cycle task in which eccentric lower-limb stiffness was at least maintained over the 10 wk of training).
Another possible explanation of the decreased drop jump eccentric lower-limb stiffness (and increased countermovement and ground contact time) for CPG is that, with training, strength was developed in the deeper countermovement position, allowing for higher vertical ground reaction forces to be produced over a longer time period (i.e., greater impulse). It is possible that the benefits to be gained from such technique were greater than the benefits that would be gained from maintaining or increasing lower-limb stiffness in an attempt to utilize more elastic energy.
In attempting to explain the training-induced changes in jump technique, it must be considered that the maximization of one single factor of performance (e.g., utilization of elastic energy) is probably not optimal. It is possible that the technique used in a jump that does maximize utilization of elastic energy might result in suboptimal concentric performance by the contractile component. In a drop jump, say, maximization of elastic energy use might occur with very high levels of eccentric lower-limb stiffness; however, this means the subject would only be taking a relatively small countermovement and, therefore, have less distance and time over which to produce impulse in the concentric phase. More benefits are likely to be gained from using a technique that allows the optimal combination of elastic utilization, impulse produced by the contractile component, and other important factors.
In conclusion, it was found that the variables used to measure jump technique (with regard to the “jump technique continuum”) changed with 10 wk of power training designed to increase vertical jump height in which maximal height was the only goal. Power training was attributed with the following changes in drop jump technique: decrease in eccentric lower-limb stiffness, and increases in magnitude of countermovement and ground contact time. Stretching (flexibility training) appeared to offer no added benefits to drop jump height and had no significant effect on drop jump technique. Power training was associated with the following changes in CMJ technique: increases in eccentric lower-limb stiffness and the magnitude of countermovement. There was evidence that stretching (flexibility training) did aid in increasing CMJ height, and possible explanations were discussed. However, stretching appeared to have no significant effect on CMJ technique. Coaches and athletes should consider these technique changes when deciding on the instructions (i.e., outcome goals) associated with jump training. Finally, it is proposed that the technique changes that occurred were a result of attempting to optimize a complex combination of factors (and their changes as a result of the training programs) involved in jumping. Further research is required to investigate: a) the reasons behind such changes in technique, b) the possible benefits a stretching (flexibility) program may have to stretch-shortening cycle tasks, and c) the effects on technique of a jump training program with instructions to “minimize ground contact time while still trying to jump as high as possible.”
This research was funded by grant no. 9154 3498044 from Sports Science New Zealand.
Address for correspondence: Joseph P. Hunter, Department of Sport and Exercise Science, The University of Auckland, Private Bag 92019, Auckland, New Zealand; E-mail: email@example.com.