Plyometrics are a staple of modern training programs in a variety of sports and have been proven as an effective training method to increase explosive power (9,13,20,22). Plyometrics utilize the rapid shortening of a muscle brought about by a prior stretch, known as the stretch-shortening cycle. The stretch-shortening cycle may allow greater power generation than movement without a rapid prestretch to allow greater athletic performance (24). There are a large variety of plyometric exercises, which in many cases are limited only by the imagination of the coach. Plyometrics can involve jumping activities for height, distance, or both. Perhaps the most common type of vertical plyometric exercise is the depth jump. Depth jumping is an exercise that involves dropping from boxes of varying heights and immediately performing a maximal vertical jump on landing. Studies that have used depth jumps as a primary training method have reported significant increases in the vertical jump height of participants (21,22,26,29). Many studies have investigated the kinematics and kinetics of depth jumps (2-4,17,18,30) and have shown that depth jumping will yield significantly greater power output and ground reaction forces than standard countermovement jumps (4). Significantly greater force development has been shown to be dependent on the technique used during the jump (2,4).
Along with jumping technique, other factors may be important in maximal depth jump performance. One factor frequently used to alter technique in a depth jump is the use of a goal. It has been suggested that the use of goals in plyometrics, such as an overhead target, can increase the effort level during individual training sessions (8,24). Depth jumps are often performed using a goal, such as a Vertec device as an overhead target. A hurdle is another type of performance goal used in depth jump training that has been recommended by coaches in the training of track and field athletes (14). The use of goals may also increase the vertical height of the jump and alter the jump kinematics and kinetics (8,23). Recent research has shown that the use of an overhead goal in depth jumping may create greater vertical jump heights and knee flexor moments than depth jumps with no overhead goal (8). To our knowledge, no other research has examined the use of overhead goals. There has been little research investigating the different types of jumping conditions available in depth jumping. Although hurdle depth jumps are a widely used exercise, there has been no research comparing the use of a hurdle jump to any other depth jump condition.
The purpose of the current study was to further investigate the kinetic and kinematic differences found among standard depth jumping, depth jumping with an overhead goal, and depth jumping over a hurdle. It was hypothesized that subjects will have to compensate for the hurdle clearance by a different strategy, requiring different ankle, knee, and hip joint kinematics and kinetics. It was also hypothesized that the hurdle and overhead goal conditions will create larger peak vertical ground reaction forces (VGRF) and vertical velocity of the sacral marker at toe-off (PVV) than during the control condition, which has no external goal. It was hypothesized that the hurdle group will demonstrate lower ground contact times and higher ground reaction forces than the other depth jump conditions.
Experimental Approach to the Problem
In this study a repeated-measures design was used to investigate the timing, kinematics, and kinetics of 3 depth jumping conditions in male National Collegiate Athletic Association (NCAA( Division III athletes. These depth jump conditions included depth jumping from a 45-cm box as a control condition (DJ45-C), over a hurdle (DJ45-H), and using a Vertec measuring device as an overhead target (DJ45-T). Dependent variables included ground contact time, maximal joint flexion and extension, maximal and minimal joint angular velocity, maximal vertical ground reaction force, joint moments, and maximal joint power absorption and generation. Joints analyzed in this study were ankle, knee, and hip joints in the left leg of each subject. Ground contact time (GCT) was measured in seconds (s), PVV was measured in meters per second (m/s), joint angles were measured in degrees (deg), joint angular velocities were measured in degrees per second (deg/s), VGRF was measured in Newtons (N), moments were measured in Newton-meters (Nm), and power was calculated in watts (W). Performance variables such as these have been studied by other authors to describe vertical jump performance (2,3,8,17,18,23,30). These outcome measures provide a description of distinct performance changes among the jump conditions. In pilot work of 8 participants, the reliability of several dependent measures were assessed using an intraclass correlation coefficient. These coefficients ranged from 0.75 to 0.90, indicating acceptable reliability; thus, the averages from 3 trials were assessed. With a statistical significance set at 0.05, a power of greater than 0.80 and a average correlation between repeated measurements was equal to 0.5 or more with an effect size of 0.90; a minimum of 14 participants was required.
Fourteen male (mean age, 20.4 ± 1.4 years; height, 188.6 ± 6.9 cm; weight, 80.5 ± 7.5 kg) NCAA Division III varsity athletes from the University of Wisconsin-LaCrosse athletic teams voluntarily participated in this study. All subjects were university athletes in a sport that required explosive leaping ability, including basketball, volleyball, and track and field. Each subject had at least 1 year of experience in strength training and plyometrics. The average hurdle height that subjects were able to clear from a standing countermovement jump was 112.10 ± 8.08 cm. Subjects were informed of the experimental risks and signed an informed consent document prior to the investigation. The investigation was approved by the university institutional review board for use of human subjects.
Each trial's 3-dimensional (3-D) kinematics were captured by securing 28 retro-reflective, spherical markers (diameter, 25 mm) at previously reported anatomical landmarks in a standard Helen Hayes configuration (wand markers substituted by regular markers) (15,16). The x, y, and z coordinates of the hip joint center were based on pelvic coordinates (1). To determine the knee joint center, the plane defined by the thigh marker, the hip joint center, and the midpoint between the femoral condyles were utilized to determine joint centers. To determine the ankle joint center, the plane defined by the estimated knee center, the tibial tuberosity, and the midpoint between the 2 malleoli markers were used. The 3-D motion capture system was synchronized with 2 Bertec force platforms (4060 NC, Bertec Corporation, Columbus, Ohio, U.S.A.) sampling at 1,200 Hz.
The 8 Eagle cameras (Motion Analysis Corporation, Santa Rosa, California, U.S.A.) running at 240 Hz and positioned at 45-degree intervals around the performance area were used to collect the kinematic data from the retro-reflective markers. The subsequent performance area and surrounding cameras were calibrated, which yielded mean residual errors of 1.1 to 1.53 mm over a volume of 3.5 × 1.5 × 3.5 m. The x, y, and z coordinates were identified and tracked using Eva RT (version 5.5, Motion Analysis Corporation). Motion Monitor software (Innovative Sports Training, Motion Monitor, version 7.72, Chicago, Illinois, U.S.A.) was used to create the rigid bodies of the pelvis and lower extremities based on the 3-D marker coordinate data. Marker trajectories, based on a frequency content analysis of the digitized coordinate data, were filtered at 15 Hz using a low-pass fourth-order recursive Butterworth filter that retained 95% of the original signal content. Ground reaction forces were also filtered at 15 Hz. Using the Grood and Suntay approach (10), joint angular positions were calculated from the filtered 3-D marker coordinate data with the assumption that the first rotation was flexion-extension, followed by abduction and then internal-external rotation, respectively. The standing neutral trial was used as a reference point; thus, 0 degrees at the hip, knee, and ankle corresponded to an erect, standing posture with the trunk, thigh, and lower leg in a straight line. When viewed from the sagittal plane, the foot segment was at a right angle to the leg.
Peak vertical velocity at takeoff was determined using the sacral marker. The peak velocity was determined when the force platform registered less than the 10-N threshold. In this manuscript, external joint moments were used, which are moments applied from the ground reaction force to all the structures within and crossing the joint. Combining the kinematic and force platform data with anthropometric parameters (6), the inverse dynamics approach was used to calculate the joint moments of the ankle, knee, and hip (15,16). Positive values were assigned to hip and knee extensor and ankle plantar flexor moments. Forces were recorded in Newtons and joint moments were recorded in Newton-meters, respectively. Joint powers were recorded in watts. The time series data sets were interpolated to 100 points using custom MATLAB (Version 7.0, Mathworks, Natick, Massachusetts, U.S.A.) programs during the ground contact phase (defined as the period from initial force platform contact to the participant leaving the platform) for graphical purposes only. Ground contact phase of the plyometric exercise was determined by using a 10-N threshold. Custom programs were then used to determine the discrete variables for the various kinematic and kinetic variables during the ground contact phase of the movement based on the vertical ground reaction forces.
The warm-up prior to testing included 90 seconds of cycling on a stationary ergometer (Monark 828E, Monark Exercise AB, Sweden) and 90 seconds of alternate leg step-ups onto a 45-cm bench. Following this, 2 sets of 5 reactive box jumps were performed on a 45-cm box (used during all testing conditions). The researcher then recorded the highest hurdle height subjects could clear from a standing vertical jump. For this test, subjects stood in front of a hurdle and, with no step, jumped vertically over a collapsible hurdle. The hurdle began at a comfortable height for the subject and increased in increments of 5 cm until the subject recorded 2 consecutive misses. At least 15 seconds were allowed between efforts for full recovery of jumping ability (25). This was performed to individualize the difficulty of the hurdle clearance for each subject. The height of the hurdle cleared during the standing jump was then reduced by 5 cm for the depth jump in the study. Immediately prior to data collection, subjects completed 5 drops off of the 45-cm box to practice proper landing position on the force platform and for the continuation of a specific warm-up. Subjects were then given 3 practice jumps, 1 for each type of depth jump condition.
Following the warm-up, subjects completed a series of 12 depth jumps from the 45-cm box placed 30 cm in front of the force platform in a counterbalanced, randomized order. At least 15 seconds were allotted between each jump to allow for full recovery (25). Four repetitions of 3 different types of depth jump conditions were completed. The 3 types of depth jumps included the following: DJ45-C, DJ45-H, and DJ45-T. During the control jump condition, subjects were instructed simply to land on the force platform and to jump as high as possible. During the hurdle jump condition, subjects dropped off of the box, landed on the force platform, and jumped for maximal height over the hurdle. During the target drop jump, subjects jumped as to record the highest vertical measurement possible on a Vertec overhead measuring device. The target was placed on the right side of the force platform, and subjects reached to touch the target with their right hand.
The parameters associated with each jump condition were also individualized for each participant. Subjects who demonstrated a higher vertical hurdle clearance required a greater horizontal jumping distance so as to not strike the hurdle on the way up. To accommodate this, the hurdle was aligned for each subject so that an angle of 47.5 degrees was formed from the center of the force platform to the top of the hurdle. This angle was chosen because it allowed for a smooth and efficient clearance of the hurdle obstacle. The target was set the same distance from the force platform as the hurdle for each subject. Along with the takeoff angle, a landing zone for each subject was also implemented, which was 60 cm long and set horizontally away from the target or hurdle. The center of the second landing zone was equal to the distance that the target or hurdle was set from the edge of the force platform. In all conditions, the subject had to land with both feet inside of the force platform in the depth jump and within the landing zone for the final landing. Jumps where subjects did not land within both areas were not counted and redone. After the practice jumps, all subjects were instructed not to constrain their performance and just jump as high as possible. This was done to keep the movement from becoming too complex, which could have limited the performance of the jump (12). Only the left leg data in each trial were analyzed for all participants. Using data from 8 participants, the reliability of several dependent measures across 4 performance trials was assessed using intraclass correlation coefficients. These coefficients ranged from 0.75 to 0.90, indicating acceptable reliability; thus, the average values from 3 performance trials were used in our hypothesis testing.
Data was analyzed using SPSS for Windows (version 12.0, SPSS Science, Inc, Chicago, Illinois, U.S.A.) with a repeated-measures analysis of variance (RM-ANOVA) with 3 within-subject factors (DJ45-C, DJ45-H, and DJ45-T) used. A significance level of p ≤ 0.05 was used for this study. The dependent variables of this study were the maximal and minimal values from various kinematic and kinetic parameters during the ground contact phase of the plyometric jump. Post hoc tests using the Bonferroni procedure were used to examine the effects of alternate types of depth jumping on GCT, PVV, joint kinematics, ground reaction forces, and joint kinetics.
Ground Contact Time and Vertical Takeoff Velocity
Group means and 3standard deviations for GCT and ankle, hip, and knee kinematics are found in Table 1. Ranges for GCT were 0.24 to 0.57 s for DJ45-C, 0.21 to 43 s for DJ45-H, and 0.24 to 0.55 s for DJ45-T. An RM-ANOVA determined that there was a difference in GCT among the 3 plyometric conditions (p = 0.000). Post hoc tests using the Bonferroni procedure showed GCT for DJ45-H to be significantly lower than that of DJ45-C and DJ45-T by 23%, respectively (Table 1). Significant differences were found within groups for PVV (p = 0.006). The PVV of DJ45-C was significantly lower than DJ45-H and DJ45-T by approximately 7% and 4%, respectively (Table 1).
Regarding joint kinematics, no effects among groups were found in maximal ankle plantar flexion (p = 0.76) or maximal ankle dorsiflexion (p = 0.52). Significant differences were found in maximal flexion of the knee joint (p = 0.000) among all 3 groups. Maximal knee flexion in DJ45-H was found to be significantly lower than DJ45-C by 9.6% and DJ45-T was lower by 10% (Figure 1). Differences were also seen in the maximum knee extension (p = 0.000), which was significantly lower in DJ45-H than in DJ45-C or DJ45-T. Differences in maximal hip flexion (p = 0.000) and extension (p = 0.005) were seen in the 3 jump conditions (Figure 2). Maximal hip flexion in DJ45-H was significantly lower than DJ45-C and DJ45-T by 16.3% and 18.4%, respectively. Maximal hip extension in DJ45-H was found to be significantly lower than DJ45-T.
No significant effects were found for maximal ankle angular velocity (p = 0.2) or minimal ankle angular velocity (p = 0.12). Significant differences were found in maximal (p = 0.005) and minimal (p = 0.000) knee angular velocity, respectively, in the 3 jump conditions. Maximal knee angular velocity was found to be 11.5% greater in DJ45-C than in DJ45-H. Also, DJ45-T was found to be 4.4% greater than DJ45-C and 11% greater than DJ45-H in minimal knee angular velocity. Significant differences were seen in maximal (p = 0.000) and minimal (p = 0.000) hip angular velocity, respectively, among the 3 groups. Hip angular velocity was seen to be 11% higher in DJ45-T than DJ45-H. Post hoc tests for minimal hip angular velocity showed that DJ45-H was 37.2% lower than DJ45-C and 36.6% lower than DJ45-T.
Ground Reaction Force
Group means and standard deviations for VGRF, ankle, hip, and knee kinetics are found in Table 2. VGRF was significantly different among the 3 plyometric conditions (p = 0.000). The peak VGRF of DJ45-H was significantly greater (17.2%) than DJ45-C and significantly greater (15.8%) than DJ45-T (Figure 3).
No significant effects were seen for the ankle dorsiflexor moment. Significant differences were seen among the 3 plyometric conditions in peak ankle plantar flexor moment (p = 0.000), and DJ45-H was found to be significantly greater than DJ45-C and DJ45-T by 15% and 17.2%, respectively. Significant differences were found in peak knee flexor moment (p = 0.000). Results showed that DJ45-H was significantly larger than DJ45-C and DJ45-T by 22.4% and 16%, respectively. Knee extensor moment was not significantly different among groups. No significant effects were found in hip flexor moment and hip extensor moment for all conditions.
Differences were found among the 3 conditions in maximal ankle power generation (p = 0.004) and maximal ankle power absorption (p = 0.008). In maximal ankle power generation, DJ45-H was greater than DJ45-C by 19.5% and DJ45-T by 23.5%. For maximal ankle power absorption, DJ45-H was 38.1% greater than DJ45-T (Figure 4). Significant effects were not found for hip and knee power generation and absorption.
The purpose of this study was to examine the differences among DJ45-C, DJ45-H, and DJ45-T conditions. The results of the study support the hypothesis that there were differences in the kinematics and kinetics of the 3 jumping conditions, particularly in those of DJ45-H, in which many significant differences with other depth jump conditions were observed. DJ45-H demonstrated lower GCT, higher PVV, reduced joint angles, greater VGRF, and greater ankle power than the other 2 conditions.
Ground contact time was found to be 21% lower in DJ45-H than in the other 2 conditions. This may have benefits for an athlete whose sport requires short ground contact times. Young and colleagues (31) have demonstrated that jumps requiring short ground contact intervals (0.2 s) are an entirely different motor skill than those jumps that utilize longer ground contact times (0.4 s). The possibility exists for an athlete to be proficient in 1 and not the other (11). In their work, Young and colleagues (31) found that 61% of performance in a single leg running jump for height is explained by performance in a depth jump for maximal height while keeping GCT to a minimal level. In the same study, it was found that only 38% of running single-leg jump performance was explained by performance in a depth jump for maximal height with no regard for GCT. Young and colleagues demonstrated in their 1999 study that low GCT is important (32). A 6-week training program involving depth jumping while simultaneously striving for low GCT can increase a running single-leg jump for height more so than a training program involving depth jumping for maximal height with no regard for GCT (32).
In jumping for an optimal combination of height and ground contact time, Young et al. (31,32) found 0.20 s to be the average GCT that may yield the best results for optimal single-leg jump training improvement. GCT during the DJ45-H was 0.31 s, which is 55% higher than this recommended time. Although this is a large difference, the instructions in the present study were for the participants to jump as high as possible without regard for time spent on the ground. Future studies may wish to compare the conditions in the present study while instructing minimal GCT, which may cause a different response. Average GCT in other studies where subjects were instructed to jump as high as possible without regard for GCT were 0.4 s (2,32). These results are similar to the 0.39 s GCT attained by DJ45-C and DJ45-T groups in the present study. It is important to note that not only did the athlete spend less time on the ground in the DJ45-H condition, but subjects also created greater PVV in DJ45-H than all other jump conditions. Whereas previous studies (2,30) have found that reduced GCT may come at the expense of reduced vertical velocity, DJ45-H in the present study recorded both lower GCT and greater PVV.
Peak vertical velocity of the sacrum was found to be significantly greater in both jumping conditions, which used goals vs. the control group, which had no goal. The 2 goal conditions help support the previous literature (8,24) that goals may help increase the effort level of the plyometric exercise. In this investigation, the overhead goal was different from previous work (8) in that a Vertec target was used, rather than a ball set at the participant's maximal jump height. Peak vertical velocities of the sacrum for participants in this study were much higher than those reported in previous studies, which instructed subjects to keep their hands on their hips (2,17,30). Keeping the hands on the hips likely lowered the maximal vertical velocity of center of mass in comparison to the present investigation (7).
In the present study, maximal knee and hip extension were found to be reduced in DJ45-H when compared with the other jump conditions. These values were also accompanied by a rapid drop-off in angular velocity of the knee and hip joints during the last 25% of ground contact phase. Because of the requirement for the DJ45-H group to immediately bring the knee and hip joints into a high amount of flexion after takeoff, this pre-takeoff phenomenon can be explained. Lower knee and hip flexion in DJ45-H were accompanied by lower peak knee and hip angular velocities. Similar results were reported by Bobbert et al. (2), who studied the difference between a depth jumping condition that examined jumping for maximal height with no regard for GCT (countermovement drop jump) and a depth jumping condition whose goal was the fastest possible reversal of eccentric and concentric movements (bounce drop jump group). They reported that the bounce depth jump group (0.26-s GCT) demonstrated lower peak angular velocities of the knee and hip joints when compared to the countermovement drop jump condition (0.4-s GCT). The minimal GCT condition in Bobbert's work (2) also demonstrated significantly reduced knee and hip flexion when compared to the maximal height condition. In the present study, mean knee and hip flexion were found to be up to 10.1% and 19.4% lower, respectively, in DJ45-H than in the other 2 conditions. The smaller knee flexion in DJ45-H was also accompanied by a significantly higher VGRF.
The hurdle depth jump condition produced a peak VGRF that was significantly greater than DJ45-C and DJ45-H. These results may have training implications for athletes based on their sport. Kollias and colleagues (17) determined that track and field athletes produced 11% greater peak VGRF while utilizing 26.8% less GCT than volleyball players in a drop jump from a 60-cm box. Although the volleyball players had lower GRF values, they still managed to jump 12.6% higher than the track and field group. From this information it may be assumed that track and field movements require shorter and more intense impulses, and the high force production associated with DJ45-H may be helpful for athletes in these types of sport conditions. For athletic events that require a low GCT, this may be particularly true. Track and field jumping events have a GCT during the takeoff phase between 0.11 to 0.12 seconds for the long jump and 0.17 and 0.18 seconds for the high jump (34). Bobbert et al. (2) found that in the bounce depth jump condition, the peak VGRF was 54.7% higher than in the countermovement depth jump condition. The countermovement depth jump condition demonstrated a 53.8% longer GCT than the bounce depth jump group. From the literature, and the current study, it can be assumed that lower GCT will be associated with greater VGRF so long as a maximal effort is provided. Walsh and colleagues (30) conducted research on drop jumping with increasingly lower GCT. As the GCT became lower, maximal VGRF became greater. Similar results were found in the present study in the goal conditions. In the DJ45-H condition, 58% of increased VGRF was explained by decreasing GCT (Figure 4). In the DJ45-T condition, 51% of increased VGRF was explained by decreasing GCT. During the DJ45-C condition, VGRF and GCT were not as related because only 31% of increased VGRF was explained by decreasing GCT in DJ45-C. Because of the higher relationships found with the use of the goal conditions in the present study, it is likely that higher VGRF will accompany lower GCT when a goal is present.
DJ45-H produced the greatest peak power generation and absorption in the ankle. The ankle has been shown to be the largest power absorber and generator in unilateral power production (27). Van Soest and colleagues (28) have shown that single-leg countermovement jumps generate a 47.4% higher peak power output in the ankle (1,794.7 ± 472.6 W) when compared to two-legged countermovement jumps (1,217.5 ± 323.0 W). Ankle power of the DJ45-H condition in the present study was 19.5% greater than the DJ45-C condition. Unfortunately, no comparison has been made with running single- and double-leg jumps. Although this comparison has not been made, the kinematics and kinetics of running single-leg jumps for height and running single-leg jumps for distance have been examined (27). In these 2 jumping styles, the ankle was the largest energy absorber and generator for both types of jumping, demonstrating that peak ankle power may be critical in single-leg jumps. Although ankle power differed significantly among conditions in the present study, power generated and absorbed in the hip and knee joints showed no significant differences.
Overall, DJ45-H demonstrated lower GCT, faster PVV, smaller joint angles, greater VGRF, and greater ankle power than the other 2 conditions. There are several reasons as to why the hurdle may have caused the responses that it did in the participants of this study. First, the hurdle was itself a barrier. For the athletes to clear this, the athlete's knees and hips needed to begin flexion immediately after takeoff for clearance. In this study, flexion started to occur even before the athlete left the force platform. Second, the hurdle as an external goal may be a key factor to the increased performances observed.
The difficulty of having a consistent takeoff angle for each subject was a limitation in the present study. Because the horizontal distance required to clear the hurdle caused subjects to land up to 100 cm beyond the force platform, landing distances had to be similar, but not exact, in trials for each individual subject. It could not be expected of each subject to take off and land at exactly the same points each trial, or hundreds of trials may have been required in each session of data collection. Another issue is that the DJ45-T condition took several practice attempts to land in the landing zone for some subjects. This may have indicated that some athletes had less horizontal displacement when jumping for an overhead goal, which may have affected vertical jump performance. A factor in the present study that was different than previous studies (2,3,30) but may not be a limitation because of its practical implication is the use of the arms in this study. The use of the arms has been shown to produce up to 20% of the power in a vertical jump (7). For the present study, however, the use of the arms was self selected and not measured.
The use of a hurdle in depth jumping can decrease GCT while increasing VGRF and ankle power. These results may be useful in training athletes whose sport requires lower GCT and higher VGRF and ankle power. Specifically, hurdle jumping can be useful in developing higher performance in jumping off of 1 leg. Track and field athletes may also benefit from depth jumps over hurdles because of the high rate of force development required. This may be useful for a track and field jumper where quick and explosive power from 1 leg is important. During any depth jump, the use of goals may provoke a higher level of effort than not using goals. The use of hurdles in activities such as depth jumping may prove more useful than other types of depth jumps that do not involve the tucking of the knees or clearance of an obstacle.
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Keywords:© 2011 National Strength and Conditioning Association
stretch shortening-cycle; plyometric training; depth jumps; hurdle jumps