The evaluation of joint power is an important part of training and rehabilitation in sports. Maximal vertical jump height assessment is one of the most commonly used protocols to evaluate overall joint power in the lower limbs, as it is inexpensive, easily done, and easily interpreted (12). However, the question whether maximal vertical jump height represents maximal individual joint power remains unanswered.
From the power delivered by the individual joints in the lower limb, peak hip joint power during the jump correlates best with jump performance (2). It has also been shown that a greater performance in the vertical jump is achieved by a greater effort in the hip extensor muscles (9). Also, loss of overall strength (e.g., due to aging) results in decreased jump performance primarily due to less flexion-extension in the hip joint (8,10). This may have provided the general consensus among practitioners that vertical jump performance mainly represents hip joint power.
The importance of knee joint power has also been supported. Simulation studies regarding the effect of muscle strengthening on jump performance have indicated that jump performance benefits more from strengthening knee joint musculature than strengthening hip joint musculature (5,13). Also, Tomioka et al. (16) found a good relationship between jump performance and isokinetic knee extension strength. However, studies have indicated that knee joint power during a maximal vertical jump is limited because of coordination requirements in the movement. It has been suggested that due to the forward trunk inclination in the countermovement phase of the jump, knee extension is delayed to avoid forward rotation of the body and subsequent loss of balance throughout the remainder of the push-off (6,14). In other words, if forward trunk inclination is decreased, an increase in knee joint power output is expected. This statement seems commonly accepted but the extent to which knee joint power is limited was never quantified.
The way in which performance of the vertical jump is a representation of individual joint power has important implications for trainers, rehabilitators, and researchers. The goal of the present study was to gain more insight into the role of individual joint power in jumping performance, in particular the role of forward trunk inclination on individual joint power. This was done through comparison of individual joint power output in vertical jumps with and without allowing forward trunk inclination.
Experimental Approach to the Problem
An experimental protocol was used to compare joint power in normal maximal countermovement jumps and in maximal countermovement jumps in which forward trunk inclination was restricted throughout the jump.
Twenty athletic male adults (mean ± SD: age, 19.9 ± 3.9 years; height, 180.0 ± 6.5 cm; mass, 75.4 ± 13.3 kg) participated in the study. All the participants were competitively active in sports, which ranged from playing field games to gymnastics, were not involved in specific plyometric training regimens and thus represent a broad range of subelite athletes. All were fit and injury free, and each gave written informed consent as required by the University Ethics Committee. Participants were given the opportunity to warm up with light exercise and stretching, and practiced a couple of normal jumps and jumps with restricted forward inclination of the trunk prior to the tests.
First, each subject performed 3 repetitions of a normal maximal countermovement vertical jump from stance, with arms held akimbo with the thumbs in a belt around the waist to prevent use of the arms (Figure 1, top). In order to investigate the influence of restricted forward inclination of the trunk, participants were then required to perform 3 repetitions of a maximal jump while keeping the head-arms-trunk segment as upright as possible throughout the entire jump (Figure 1, bottom). The participants felt comfortable in executing the upright jumps after only 1 or 2 practice jumps prior to the tests, regardless of the fact that this type of jump only occurs in certain ballet jumps, which none of the subjects had ever practiced (15). Data were averaged over the 3 trials for each condition.
For computation of kinematic variables, the location of reflective markers on the body were recorded with a 6-camera optoelectronic motion capture system (Proreflex; Qualisys, Savedalen, Sweden). Reflective markers were placed over the 2nd metatarsal-phalangeal joint, lateral maleolus, lateral collateral ligament, trochanter major, lateral epicondylus of the humerus, processus styloideus of the ulna, acromion process, C7, and the vertex of the head using a marker placed on top of a cap worn on the head. From the 3-dimensional position of these markers, a 2-dimensional (sagittal plane) 4-segment (feet, shanks, thighs, head-arms-trunk) model was defined in which segment orientations and joint angles were computed from segment end points. Segmental data as proposed by Dempster (7) for adult males were used to calculate segment and whole-body center of mass locations. Forward inclination of the head-arms-trunk segment was defined as the angle made by the trunk to the vertical, being 0° during upright standing and 90° while fully inclined forward.
Ground reaction forces were recorded with a force platform mounted in the floor (Kistler, Winterthur, Switzerland). Jump height was calculated according to Vanrenterghem et al. (17), comprising the increase of vertical position of the whole-body center of mass from that during stance. Combining ground reaction force and kinematic data in a linked segment model, instantaneous net joint torques and powers about hip, knee, and ankle joints (presented for 2 legs combined) were obtained. Net joint torques having a hip extending, knee extending, and ankle plantar flexing influence were defined as positive.
Data were collected for a period of 6 seconds, which allowed approximately 2 seconds of quiet standing before the jump commenced. The kinematic data were collected at 240 Hz and were smoothed using a Butterworth 4th order zero-lag filter with padded end points (18) and a cutoff frequency of 7 Hz based on a residual analysis and qualitative evaluation of the data. The force data were collected at 960 Hz. All data were electronically synchronized in time, and force data were later decreased to 240 Hz by averaging over 4 adjacent points.
To evaluate the test-retest reliability, average measure intraclass correlation coefficients (average ICCs) between 3 trials were calculated for jump height and maximal trunk inclination. After prior inspection of the time profiles of joint powers (curves of 2 conditions showed a similar evolution), peak joint power was chosen to best represent jump-related muscle performance. Pearson correlation coefficients were used for establishing correlations between jump performance and individual peak joint powers, after having screened the data for outliers. Paired-sample Student t-tests were used for establishing differences between normal and upright jump variables. All analyses were executed using SPSS 12.0 (SPSS Inc., Chicago, IL). The significance level was set at p ≤ 0.05. Bonferroni correction of α levels to take into account the effect of multiple comparisons may have been partially appropriate, but as this method is also known to overcorrect, it was decided not to use that method. Post hoc analysis did reveal that even if Bonferroni correction had been applied this had no effect on our conclusions.
Jump height was 44.4 ± 4.9 cm in the normal jumps and 39.8 ± 3.9 cm in the upright jumps. This is a decrease of 10% (p < 0.001). Participants were not able to fully eliminate forward trunk inclination in the upright jumps, but forward trunk inclination was decreased by 48.4% (p < 0.001), that is, from 44.2 ± 10.4° in normal jumps to 21.4 ± 8.3° in upright jumping. Average ICCs for jump height and maximal forward trunk inclination in normal and upright jumping ranged between 0.73 and 0.97, indicating good test-retest reliability. Total movement time (start of downward movement until take-off) was similar in both jumps (1.01 ± 0.17 and 1.03 ± 0.23 seconds for normal and upright jumps, respectively) and the transition from downward to upward movement occurred after approximately 74% of the movement time in both jumps.
Jump performance of the normal jump was correlated to that of the upright jump (Pearson correlation = 0.739, p < 0.001). Table 1 shows correlations between jump performance and individual peak joint power. For both the normal and upright jump, only maximal hip joint power was correlated significantly to performance.
The decreased maximal forward inclination of the trunk resulted in a 27% reduction of maximal hip joint flexion (p < 0.001), an 8% increase in maximal knee joint flexion (p < 0.05), and no change in ankle flexion (p = 0.67) (Figure 2). Joint torque and joint power profiles are shown in Figures 3 and 4. Hip joint torques were decreased from approximately 40% of movement time onward, having a 37% decrease in maximal hip joint torque (p < 0.001) and 37% reduction in maximal hip joint power during the propulsive phase (p < 0.001). The knee joint torque, on the contrary, was increased from approximately 30% of movement time onward, with 19% increase in maximal torque (p < 0.001) and 13% increase in maximal joint power during the propulsive phase (p < 0.001). These results indicate that decreased forward inclination allows for increased knee joint power. The ankle joint torque was increased between approximately 40% and 70% of movement time, but decreased after 70% of movement time. The maximal joint torque during the latter phase was 8% lower in the upright jump (p < 0.005), but maximal joint power was not significantly different (p = 0.129).
Participants systematically inclined the trunk segment when asked to perform a normal maximal standing vertical jump. Instructing subjects to jump with minimized forward inclination of the trunk led to a decreased jump height. For both jump conditions, only hip joint power was significantly correlated with jump performance. However, hip joint power was decreased in the upright jump, and this allowed for increased knee joint power, whereas ankle joint power remained the same.
When evaluating jump performance in the present study, our findings were comparable with findings from previous investigations. According to Luhtanen and Komi (11), rotation of the trunk segment accounted for approximately 10% of jump performance. Ravn et al. (15) reported a 7.6% decrease in performance when comparing a ballet-specific jump with minimized forward trunk inclination to a normal vertical jump (with arm swing). These results are similar to the 10% difference between normal and upright jumping in the present study. Also, joint kinematics and kinetics for the normal condition in terms of amplitude resembled data presented in previous studies (1,3,4).
A reduction of forward trunk inclination by half resulted in a 13% increase of maximal knee joint power. Already after 20% of movement time, there was an increased knee joint flexion and increased knee joint torque. This is a manifestation of increased eccentric loading, which then resulted in increased knee joint power throughout the push-off phase. These findings confirm the hypothesis that knee joint power is limited due to forward trunk inclination in a normal maximal vertical jump (6,14). However, maximal knee joint power still not correlated with jump performance in the upright jump, whereas this may have been expected. This suggests that there is still a limitation of knee extension in order to maintain balance throughout the push-off. As the maximal ankle joint power was not correlated with jump performance either, one may believe that the ankle joint has a similar role. However, ankle joint power is similar in normal and upright jumps. In previous studies, ankle joint power was found to be constant in submaximal vertical jumping, and it was suggested that the musculature around the ankle is used optimally for any jump height (9). This is likely to be the case here as well and has an important practical implication for practitioners in that the ankle joint torques and power are maximal in most jumping activities. It is important to keep this in mind, particularly for rehabilitation of ankle injuries.
Another practical implication for practitioners is that jumping while the trunk is held upright induces up to 19% higher torques in the knee joint. This can be the case in a volleyball block where the position close to the net may not allow extensive forward trunk inclination, in squat performances during weightlifting where prevention of back injuries requires keeping the trunk upright, or in ballet-type jumps. The high knee joint torques induce a high stress on the structures crossing the knee joint, and therefore care should be taken when setting the volume (number of repetitions, intensity) of jump training regimens.
Finally, our findings confirm that strength training requires a balanced training of hip, knee, and ankle musculature. If isolated knee musculature strength is trained in the belief that knee joint power is most important, then the use of this additional knee joint power in vertical jumping will remain limited due to the balance requirements. As was suggested in simulation studies, such muscle strength training requires reoptimization of jump coordination (5,13). Also, practitioners may promote isolated hip musculature strength training in the belief that hip joint power is the best predictor of jump performance. However, our results found a high correlation between maximal hip joint power and jump performance, even when hip joint flexion-extension was decreased by 27%. Therefore, the benefit to jump performance from isolated hip musculature strength training is expected to be less than in a training regimen with balanced hip, knee, and ankle musculature strength training.
In summary, the findings of this study help clarify the effect of trunk inclination on individual joint power. In normal jumps, maximal knee joint power is decreased by 13% due to forward trunk inclination. When trunk inclination is restricted, earlier and higher knee joint torque development is possible, resulting into higher maximal knee joint power. These findings have important practical implications for training and rehabilitation.
The vertical jump is a common training tool to improve and test power output of leg extensor muscles. In the present study, it was shown that keeping the trunk upright during jumping results in decreased power output in the hip joint but increased power output in the knee joint. From a training perspective, this means that when the focus is on training the knee musculature, it may be worthwhile to incorporate jump exercises, keeping the trunk upright to induce greater stress on the knee joint musculature and subsequent higher training effects. From a power assessment perspective, this means that knee joint power is not accurately tested with jump tests involving normal jumping. Jumping while keeping the trunk upright is not an alternative here, as it is still unlikely under that condition, that maximal knee joint power is achieved. Rather than using jump tests, it may be more appropriate to use joint-specific tests when evaluating specific joint power output in the context of joint-specific rehabilitation or training (e.g., the use of a dynamometer).
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