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APPLIED SCIENCES

Predicting the Patellar Tendon Force Generated When Landing from a Jump

JANSSEN, INA1,2; STEELE, JULIE R.1; MUNRO, BRIDGET J.1,3; BROWN, NICHOLAS A. T.2

Author Information
Medicine & Science in Sports & Exercise: May 2013 - Volume 45 - Issue 5 - p 927-934
doi: 10.1249/MSS.0b013e31827f0314
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Abstract

The quadriceps extensor mechanism, including the patellar tendon, acts as the primary anatomical structure to dissipate the kinetic energy generated when landing from a jump (8,38). In fact, each landing can expose the patellar tendon to a force as high as seven times an individual’s body weight at a rate of 55 body weights per second (10). Frequent application of this high patellar tendon force combined with the rapid rate at which this force is applied, and poor landing biomechanics, are believed to be the main causative factors of overuse knee injuries, such as patellar tendinopathy, in sports that involve repetitive jumping and landing (13,20,30).

Risk factors other than landing biomechanics are also believed to predispose an individual to develop patellar tendinopathy. For example, greater patellar tendinopathy prevalence is reported in males compared to their female counterparts across a wide variety of sports (20,39). Although the cause of the between-sex difference in prevalence remains unexplained, it is speculated that factors such as greater muscle strength (22) and lower quadriceps muscle extensibility (36) may increase tendon strain, transmit greater forces through the patellar tendon, and lead to more prevalent patellar tendinopathy in male athletes. Injury prevalence data also indicate a large discrepancy between skill levels with highly skilled athletes up to three times more likely to develop patellar tendinopathy than lesser skilled athletes (20,39). Although these and other risk factors have been associated with individuals who develop patellar tendinopathy, it remains unknown why factors such as sex, strength, muscle extensibility, or skill level predispose individuals to this injury. It is possible that these risk factors simply affect an individual’s landing biomechanics and, in turn, increase the patellar tendon force they generate when landing from a jump, whereby frequent application of this high patellar tendon force is the causative factor, although this notion has not been investigated.

As it is believed that lower limb landing biomechanics are related to the development of patellar tendinopathy, several researchers have attempted to identify landing strategies associated with the injury (2,3,10,30). For example, Richards et al. (30) characterized the landing technique of Canadian national team male volleyball players, including some players with a history of patellar tendinopathy or symptoms of the injury at the time of testing. High maximum knee flexion during landing and a rapid rate of knee extensor moment generation predicted patellar tendinopathy presence (30). In another study (3), volleyball players with patellar tendinopathy displayed less ankle plantarflexion and excursion, with greater ankle dorsiflexion velocity, when landing compared to players without patellar tendinopathy. Interestingly, although high patellar tendon force is thought to be a primary causative factor of patellar tendinopathy in sports that involve repeated jumping and landing (30,31), neither of these studies used patellar tendon force as a possible predictive factor in their models. In fact, there is a paucity of studies that have quantified the magnitude of the patellar tendon force generated during landing tasks.

One research team who has reported the patellar tendon force generated during landing tasks (10) found that athletes with a patellar tendon abnormality, a precursor to patellar tendinopathy, were exposed to similar patellar tendon forces and patellar tendon force loading rates when landing compared to control athletes with no patellar tendon abnormality. Athletes with a patellar tendon abnormality, however, demonstrated a distinctive landing technique that included greater knee flexion, greater hip flexion, and greater hip extension velocity during landing relative to the controls. Although this study provided some understanding of the landing technique displayed by male athletes with a patellar tendon abnormality, it remains unexplored whether these landing strategies affect the patellar tendon force generated during landing from a jump or whether these landing strategies are sex-specific. Identifying characteristics of the landing strategies displayed by male and female athletes and how these characteristics affect patellar tendon loading would therefore be valuable in understanding overuse knee injuries associated with repetitive landings, such as patellar tendinopathy (21).

The highest prevalence of patellar tendinopathy in sports is reported in volleyball with up to 45% of highly skilled male volleyball players, those who compete at a national level, likely to sustain this injury (12,20). As volleyball players are typically tall, anthropometric characteristics associated with a tall stature, such as lower limb and trunk dimensions, may be potential contributing factors to the high patellar tendinopathy prevalence in volleyball (20). As the trunk segment comprises approximately 50% of total body mass (35), high trunk segment mass and motion contribute to the forces generated during landing (15). Although not previously found to be a risk factor for patellar tendinopathy, recent evidence has shown that trunk motion may influence knee joint loading, and therefore patellar tendon force, when individuals perform a controlled movement such as a drop landing (4,5). Blackburn (4) and Blackburn and Padua (5) reported a reduction in vertical ground reaction forces (5) and knee extensor moments (4) when 40 participants actively flexed their trunk during landing. Tall volleyball players, particularly those with a long trunk and large trunk mass, will have a higher trunk moment of inertia and, in turn, more difficulty rotating into trunk flexion during landing compared to players with a shorter trunk length or less trunk mass. In addition, the obstruction of a net may also impede trunk motion in volleyball. However, how trunk anthropometrics and motion affect patellar tendon loading when a sport-specific volleyball movement is performed is unknown.

Although frequent and high patellar tendon force, combined with the rapid rate at which this load is applied, are believed to be primary causative factors of overuse knee injuries such as patellar tendinopathy, research investigating factors that affect the patellar tendon force generated during landing is scarce. Therefore, the purpose of this study was to identify whether factors that have previously been associated with the development of patellar tendinopathy (such as sex, skill level, quadriceps strength, and quadriceps extensibility), and selected variables characterizing landing technique (such as lower limb and trunk motion), could predict patellar tendon loading incurred by volleyball players when landing from a jump. On the basis of previous literature, it was hypothesized that patellar tendon loading would be predicted by male sex, a high skill level, high quadriceps strength, low quadriceps extensibility, and poor lower limb and trunk biomechanics.

METHODS

Participants

Fifty healthy male and female volleyball players of varying skill levels and diverse anthropometric characteristics (age 22.8 ± 4.5 yr, mass 78.7 ± 14.3 kg, height 1.83 ± 0.12 m) volunteered for this study. The cohort included 10 highly skilled male players who were members of the Australian Men’s National Volleyball Team or the Australian Institute of Sport Development Team, and 40 skilled players (20 males and 20 females) who competed in local state or reserve league volleyball competitions. This sample size was deemed sufficient to obtain adequate statistical power with an effect size of 0.4 when including 10 predictors into the regression model described below (G*Power, Germany). Participants were excluded from the study if they had an injury at the time of testing, a self-reported history of lower limb surgery, equilibrium disorders, or orthopedic or neurologic conditions that could influence their lower limb mechanics. The presence of a patellar tendon abnormality or a history of overuse knee injuries was not assessed, although all participants indicated they were injury-free at the time of testing. All testing procedures were approved by the University of Wollongong Human Research Ethics Committee (HE09/081) and the Australian Institute of Sport Ethics Committee (20100107), and before commencing data collection, written informed consent was obtained from all participants. Testing was conducted in the biomechanics laboratory at the Australian Institute of Sport.

Quadriceps Strength and Extensibility

Bilateral isokinetic eccentric quadriceps strength was assessed for each participant at 240°·s−1 using a Humac Norm Testing and Rehabilitation System dynamometer (Computer Sports Medicine, Inc., Stoughton, MA) following a standardized protocol (9). This quadriceps strength protocol was chosen to replicate the functional movement and knee angular velocity required during a block landing task, which was the experimental movement (3). To improve testing reliability, each participant performed a familiarization session 2 d before the experimental test to acquaint themselves with the dynamometer and testing procedure. Participants performed three warm-up repetitions followed by a 30-s rest period and then five maximal contractions. The highest gravity-corrected torque produced by each participant’s test limb, peak torque (N·m), was recorded, normalized to body mass, and used as input into the predictive model.

Each participant’s quadriceps muscle extensibility was assessed using the modified Thomas test (16) (Fig. 1). In brief, while lying supine on a plinth, each participant held his/her contralateral hip in maximal flexion to flatten the lumbar spine, while his/her test limb was lowered toward the floor. The angle between the long axis of the femoral and tibial segments was measured (°) and subtracted from 180°, with greater values representing greater quadriceps extensibility. All values were recorded bilaterally in triplicate to the nearest degree, with the median value for the test limb used for analysis.

FIGURE 1
FIGURE 1:
The modified Thomas test was used to assess quadriceps muscle extensibility.

Trunk Characteristics

Using a wall-mounted stadiometer (Seca, Hamburg, Germany), each participant’s trunk length was measured as the distance between the suprasternal notch and the greater trochanter while each participant was standing (7). Trunk length was measured in triplicate, with the median value used for the trunk moment of inertia calculation. Trunk mass was calculated as 43.46% and 42.57% of the participant’s body mass for male and female participants, respectively (7). Trunk moment of inertia was then calculated as the product of the participant’s trunk segment mass and the square sagittal radii of gyration (7,35). The primary investigator (I.J.) conducted all tests of quadriceps strength and extensibility and trunk anthropometry to maximize data reliability. Blinding to the participants’ group assignment was not possible because of the higher public profile of the highly skilled players.

Experimental Landing Task

Participants were led through a standardized warm-up of approximately 10 min, which consisted of jogging, lunges, squats, block jumps, and any stretches as desired. After the assessments described above and the warm-up, participants performed a series of lateral stop–jump block movements. This movement was selected as the experimental task as it is the movement most commonly performed by middle blocker players, who report a high incidence of patellar tendinopathy in volleyball (12). A volleyball net set at the regulation height of 2.43 and 2.24 m for the male and female participants, respectively, was used to simulate volleyball court conditions in the laboratory. A standard volleyball was mounted on a post and positioned with the center of the ball 0.15 m above net height and 0.15 m into the opposing court to standardize the height the participants jumped (32). To perform the lateral stop–jump, participants faced the net and, from a standing position, were required to move laterally with a one-step approach toward the ball. Once a second step was taken to bring the trail limb close to the lead limb, a two-footed jump was performed to block the suspended volleyball, followed by a final landing, which was the focus of the current investigation (Fig. 2). After adequate familiarization, the participants performed five successful lateral stop–jump block landings moving from the right and landing with each foot positioned entirely on a separate force platform. To prevent targeting, participants were not made aware of the force platform locations. Participants were provided with a 30-s rest between each trial, and each participant set his/her own starting distance away from the stationary ball, according to his/her stride length and movement technique.

FIGURE 2
FIGURE 2:
The lateral stop–jump landing task: (A) starting position, (B) one-step approach, (C) two footed take-off, (D) block the volleyball, and (E) land with each foot on the respective force platform. It was the final landing phase (E) that was investigated in the current study. The position of the two force platforms are outlined in D and E.

Kinematic data.

Each participant’s landing technique was monitored during the experimental task using retroreflective markers (14 mm in diameter), which were secured to each participant by the primary investigator in accordance with the University of Western Australia marker set (1,23). A total of 70 markers were used as a combination of single markers on bony landmarks and two- or three-marker clusters. Single markers were placed on the right and left side of the forehead, right and left side of the rear head by way of a headband, right and left acromion process, suprasternal notch, seventh cervical vertebrae, tenth thoracic vertebrae, xiphoid process, right and left anterior superior iliac spines, right and left posterior superior iliac spine, right and left head of first and fifth metatarsals, right and left calcaneus, and right and left head of third metacarpal. T-bar clusters consisting of three retroreflective markers were placed bilaterally on segments to define coordinate systems of the upper arm, forearm, thigh, and shank with a two-marker cluster on the dorsal surface of the hand that stored the joint center information during the experimental trials. To identify joint centers, single-calibration markers were also placed on the right and left medial and lateral malleoli, right and left femoral epicondyles, right and left medial and lateral epicondyles, right and left radial and ulnar styloid processes, and right and left anterior and posterior shoulders to define the ankle, knee, elbow, wrist, and shoulder joint centers. To improve the repeatability of marker placement and reduce kinematic error, the same researcher (I.J.) performed all marker placements. An anatomical static trial and functional trials were conducted to identify joint axes for the knee and hip (1). To account for individual anthropometric differences, subsequent kinematic data were normalized to a neutral position. Three-dimensional marker trajectories were collected using a Vicon motion analysis system (Oxford Metrics Ltd, Oxford, United Kingdom) with 14 cameras (MX13 and MX40) sampling at 250 Hz. A static and dynamic wand calibration procedure was conducted at the start of each testing session following standard procedures (14). Image errors for each camera were calculated to represent the accuracy of the three-dimensional reconstruction of the markers, and the system was recalibrated if any of the cameras reported a residual pixel error of greater than 0.25 pixels. Because of the high number of cameras used to monitor the markers (14 cameras), marker loss was minimal. In rare cases that data were lost, it was interpolated using Vicon Nexus software (version 1.5.1; Oxford Metrics) using the standard gap fill procedure.

Ground reaction force data.

The ground reaction forces generated by each participant during landing were sampled (1500 Hz) by two 0.60 × 0.90-m calibrated Kistler multicomponent force platforms with built-in charge amplifiers (model 9287BA; Kistler Instrumente, Winterthur, Switzerland), which were embedded adjacent to each other in the laboratory floor (Fig. 2). The ground reaction force and kinematic data were synchronized and collected using Vicon Nexus software (version 1.5.1; Oxford Metrics).

Data Analysis

Patellar tendon force.

Landing was defined from the instant the vertical ground reaction force exceeded 10 N for each force platform (initial foot–ground contact) until the time at which the peak patellar tendon force occurred for each limb independently. The kinematic and ground reaction force data were filtered using a fourth-order zero-lag Butterworth digital low pass filter (fc = 16 Hz) with the filter frequency selected after performing a residual analysis (35). Kinematic data were then combined with the ground reaction force data to calculate internal knee moments using standard inverse dynamics (35). The patellar tendon force for the lead and trail limb, normalized to body weight (BW), was then calculated as the knee joint moment divided by the patellar tendon moment arm (26), estimated by a regression equation using knee flexion angle (17). Patellar tendon force loading rate (BW per second) for the lead and trail limb, defined as the rate of loading from initial foot–ground contact until the time of the peak patellar tendon force, was also calculated.

Landing kinematics.

Although three-dimensional kinematic data were collected and used to calculate the relevant angles, only variables representing sagittal plane lower limb and trunk motion during landing were calculated for inclusion in the predictive regression equations as they were deemed more likely to affect patellar tendon force. Specifically, hip and trunk angle at the time of the peak patellar tendon force and average ankle dorsiflexion, hip flexion, and trunk flexion angular velocity from initial foot–ground contact to the time of the peak patellar tendon force were calculated and used for analyses. The angle between the long axis of the foot and the tibia represented ankle angle and the angle between the pelvis and long axis of the femur represented hip angle. Knee flexion angle, between the tibial and femoral segments, was also measured to calculate patellar tendon force (17). Modeling the trunk as two different segments, the thorax and pelvis, is closer to the anatomical architecture of the body compared to using a single segment model (29,37). For this reason, trunk motion was defined as thorax motion relative to the pelvis. Joint angles and inverse dynamic calculations were performed in Vicon Workstation software (version 4.6; Oxford Metrics) using the body segment parameters calculated according to values reported by de Leva (7).

Statistical Analysis

Consistent with previous research (24), a priori analysis revealed that the participants generated significantly greater patellar tendon force upon landing in the lead limb (5.17 ± 0.86 BW) compared to the trail limb (4.56 ± 0.85 BW; P < 0.001). Therefore, predictive models were only conducted on data derived from each participant’s lead limb. To achieve the purpose of the study, two multiple regression analyses were conducted, one for the dependent variable of peak patellar tendon force and one for the peak patellar tendon force loading rate, as both variables are thought to contribute to patellar tendinopathy (2,30). When conducting regression analyses, it is imperative to ensure low between-variable correlations to avoid violating statistical assumptions underlying the analyses (27). This, however, restricts variables that can be input into the predictive model. Acknowledging the necessary statistical restrictions, the following variables were chosen for input into the two regression analyses: sex (male or female), skill level (skilled or highly skilled), quadriceps strength, quadriceps extensibility, trunk moment of inertia, hip and trunk angle at the time of the peak patellar tendon force, and average ankle, hip, and trunk angular velocity (Table 1). As knee flexion angle and velocity were used to calculate the dependent variable of patellar tendon force, knee kinematics were unable to be included in the regression equation. Backward multiple regression analyses (criterion P-out = 0.10) were then performed to determine whether, and which factors were predictors of patellar tendon loading. Overall model and variable significance were set at α = 0.05 and performed using statistical software (IBM SPSS Statistics 20.0.0; Somers, NY). Although blinding of each participant’s group assignment was not possible, the data analysis process was blinded by using file names that deidentified the participants.

TABLE 1
TABLE 1:
Mean ± SD of the variables representing patellar tendon loading, quadriceps muscle strength and extensibility, trunk moment of inertia, and landing technique when landing from a lateral stop–jump for 50 volleyball players.

RESULTS

The multiple regression analyses revealed predictor models that were able to estimate and predict 52% (F4,49 = 14.258, P < 0.001) of the peak patellar tendon force, which was predicted by the interaction of sex, quadriceps strength, ankle dorsiflexion velocity, and trunk flexion velocity during landing (Table 2). In addition, the regression analyses predicted 70% (F4,49 = 29.329, P < 0.001) of the patellar tendon force loading rate variance and were predicted by the interaction of sex, quadriceps strength, and ankle dorsiflexion velocity during landing (Table 2). The regression coefficients were positive, indicating that male participants with greater quadriceps strength, increased ankle dorsiflexion velocity, and increased trunk flexion velocity during landing were predicted to incur higher patellar tendon force and more rapid patellar tendon force development.

TABLE 2
TABLE 2:
Multiple regression analyses of independent variable effects on peak patellar tendon force and loading rate during landing.

DISCUSSION

Although a high patellar tendon force and a high rate of patellar tendon force development when landing from a jump are considered precursors for developing patellar tendinopathy (2,30), research investigating factors that affect patellar tendon loading during landing is scarce (3,30). Results of the present study have revealed that the interaction of sex, eccentric quadriceps strength, ankle dorsiflexion velocity, and trunk flexion velocity are significant predictors of patellar tendon force and patellar tendon force loading rate when landing from a lateral stop–jump. Specifically, male volleyball players with greater quadriceps strength, increased ankle dorsiflexion velocity, and increased trunk flexion velocity during landing were predicted to generate higher patellar tendon loading, possibly predisposing these individuals to a higher risk of developing patellar tendinopathy.

The mean patellar tendon force generated by the volleyball players in the present study (5.17 BW) was similar to patellar tendon forces generated by male participants during vertical landings in previous investigations (10,11). However, the patellar tendon force loading rate observed in the current study (38 BW per second) was substantially lower than the loading rate of 55 BW per second observed for male participants during the vertical landing component of a forward stop–jump (10). These differences in loading rate are likely to be due to between-study differences in the experimental task (a forward stop–jump compared to a lateral stop–jump) or the cohort investigated.

Greater prevalence of patellar tendinopathy has been reported in male volleyball players compared to their female counterparts (20), making sex a risk factor believed to increase a participant’s susceptibility to developing the injury (33,36). However, this is the first known study to demonstrate that sex is also a significant predictor of the patellar tendon force generated when individuals land from a jump, whereby male sex was a significant factor that predicted higher patellar tendon forces and a more rapid patellar tendon force loading rate. Although it has been suggested the between-sex incidence in patellar tendinopathy may be a result of greater quadriceps strength and jumping capacity in male players exposing the patellar tendon to greater forces (22), it remains unknown why male players are more susceptible to the injury. We speculate that male and female volleyball players are likely to use different landing strategies, which contribute to a higher patellar tendon force in males and, in turn, predispose males to patellar tendinopathy, although this notion remains unexplored. For this reason, a comprehensive investigation of differences in the landing technique displayed by male compared to female volleyball players, and how these technique differences affect patellar tendon force and the patellar tendon force loading rate, is recommended.

Volleyball is an explosive sport with a substantial jump–land volume that necessitates frequent quadriceps muscle activation to eccentrically control knee flexion and decrease the body’s vertical momentum during landings (18). In this study, eccentric quadriceps strength measured at 240°·s−1 was found to be an individual predictor of peak patellar tendon loading. Although this is the first known study to identify eccentric strength as a predictor of patellar tendon force when landing from a jump, previous research has found that male volleyball athletes with patellar tendinopathy demonstrated greater eccentric force generation during ballistic jumps compared to uninjured athletes (21). Although technically a ligament, the patellar tendon is a thick fibrous tissue, extending from the patella to the tibia, that transfers the force of the quadriceps muscle to the tibia (28). It has been revealed that the force produced during eccentric quadriceps muscle contraction can be up to three times that of the force generated during a concentric contraction of the muscle, and therefore, is thought to be the primary cause of micro ruptures in the patellar tendon (34). As a result, eccentric quadriceps muscle contraction during landing can load the patellar tendon beyond its inherent strength and may cause microtearing and degeneration of tendon fibers. These results imply that players with superior eccentric strength may increase the load placed upon the patellar tendon and, in turn, increase their susceptibility to developing patellar tendinopathy. However, as it would be inappropriate to advocate for players to reduce their eccentric quadriceps strength in an attempt to decrease patellar tendon loading, it is recommended that further research investigates whether there are any technique modifications that players with greater eccentric quadriceps strength could use to reduce the patellar tendon force generated during landing.

Although previous research has suggested that decreased quadriceps muscle extensibility may increase patellar tendon strain during landing (6,36), this variable was not a significant predictor of patellar tendon loading during landing in the present study. Furthermore, skill level was not a significant predictor of patellar tendon loading, despite higher patellar tendinopathy prevalence being reported in highly skilled volleyball players (20,39). It is possible that risk factors not included in the current regression model, such as training frequency and the subsequent increase in loading (20), may be more relevant in the development of patellar tendinopathy in highly skilled players rather than the patellar tendon force per se. However, it is recommended that a comprehensive comparison of the landing technique displayed by highly skilled and skilled volleyball players when landing from a lateral stop–jump be conducted to ascertain whether there are any technique factors that contribute to high patellar tendon loading in either cohort. Further research is also encouraged to explore the associations among the frequency of patellar tendon loading, the magnitude of patellar tendon loading and the development of patellar tendinopathy, as well as whether the frequency of loading is a more controllable risk factor than the magnitude of the patellar tendon loading when developing preventative strategies for patellar tendinopathy.

Although poor landing biomechanics have previously been associated with the development of patellar tendinopathy (2,30), this is the first study to quantify the association between landing biomechanics and patellar tendon loading generated by uninjured athletes when landing from a lateral stop–jump. Ankle dorsiflexion velocity was identified as a significant predictor of patellar tendon force and patellar tendon force loading rate, whereby a faster change in ankle position from the time of initial foot–ground contact to the time of the peak patellar tendon force was predictive of higher patellar tendon loading. Although the quadriceps extensor mechanism acts as the primary anatomical structure to dissipate the kinetic energy generated during a jump (8,38), the ankle is the first joint to attenuate the impact force (38). The results of the present study are consistent with previous research that has shown that male athletes with previous patellar tendinopathy had greater dorsiflexion velocity than those without a history of patellar tendinopathy (3). Although Bisseling et al. (3) suggested that injured athletes modify their landing technique to compensate for their injuries, we speculate that some uninjured athletes might already display landing strategies that cause them to generate high patellar tendon forces, ultimately predisposing these individuals to a higher risk of developing patellar tendinopathy.

Trunk angular velocity was also identified as a significant predictor of patellar tendon loading, whereby a faster change in trunk position from the time of initial foot–ground contact to the time of the peak patellar tendon force was predictive of higher patellar tendon loading. Interestingly, trunk flexion velocity was only a predictor of the patellar tendon force loading rate when combined with sex, muscle strength, and ankle dorsiflexion velocity. Contrary to our hypothesis, trunk angle at the time of peak patellar tendon force was not a significant predictor of patellar tendon loading. Previous research has shown that trunk motion may influence knee joint loading, and therefore patellar tendon loading, when landing in a controlled movement such as the drop landing (4,5). In a sport such as volleyball, external influences can constrain how a player positions his/her body and, in turn, the joint forces he/she generates upon landing. For example, when landing from a block jump, players can be forced to keep their trunk vertical to prevent contact with the volleyball net. In the current study, participants performed a dynamic movement whereby the volleyball net is likely to have affected the amount of trunk flexion participants displayed, and this may have restricted variability in trunk flexion angle and angular velocity (Table 1). In fact, substantial trunk flexion when blocking will be detrimental to volleyball performance, as participants need to ensure they do not contact the net during competition.

Trunk moment of inertia was also not found to be a predictor of patellar tendon force at landing in the current study. Kulas et al. (19) explored the effect of increasing trunk mass by 10% of body mass on the knee joint for 21 recreationally active male and female participants who were performing a controlled drop landing. Results of the study revealed that increased body mass was accompanied by an increase in knee extensor angular impulse and knee energy absorption during landing. The authors speculated that this result was likely due to greater trunk moment of inertia requiring greater effort for the participants to flex their trunks during landing. Again, volleyball net restrictions in the current study may have limited trunk flexion and, in turn, masked potential negative effects of increased trunk moment of inertia on patellar tendon loading when landing from a block–jump.

The regression equations in the present study were able to identify significant predictors and explain more than half of the variance in patellar tendon loading. It is likely that knee kinematics would have accounted for a substantial amount of the unexplained variance as knee flexion and knee angular velocity during landing have previously been identified as patellar tendinopathy injury predictors in male volleyball players (3,30). However, measures of knee kinematics were unable to be included in the regression model as the same variables were used to calculate the predicted variable and this would have violated the statistical assumptions underlying the regression analysis. Nevertheless, the results of this study have potential to further our understanding of patellar tendon loading incurred during landing by volleyball athletes.

The limitations of the current study must be considered when interpreting the study results. Although all participants indicated they were injury-free at the time of testing, the presence of a patellar tendon abnormality or a history of overuse knee injuries was not assessed. Using an inverse dynamics analysis, assuming that each segment is rigid with mass concentrated at the center of mass, that each joint is frictionless, and that there is no cocontraction of agonist and antagonist muscles, may underestimate the patellar tendon force and contribute to errors in the net knee joint moments. Furthermore, patellar tendon force was estimated using a two-dimensional model that may oversimplify how the patellar tendon was loaded, given that the tendon is a complex three-dimensional structure. However, although three-dimensional linear regression equations exist for other lower-limb tendons (25), a three-dimensional regression equation for the patellar tendon moment arm suitable to calculate patellar tendon forces generated during landing tasks is currently not available. In addition, the regression equations used to estimate trunk mass and trunk moment of inertia were not specific for athletes, which may have increased the error in these calculations. Future work involving a comprehensive longitudinal prospective study of the patellar tendinopathy risk factors and landing technique of a group of male volleyball players is recommended to confirm the causative factors and relationship between these variables and patellar tendinopathy development.

CONCLUSIONS

Although patellar tendinopathy is the most common overuse knee injury in landing sports, our understanding of factors that affect patellar tendon loading during landing is limited. Results of the present study revealed that male volleyball players with greater quadriceps strength, increased ankle dorsiflexion velocity, and increased trunk flexion velocity during landing were predicted to generate higher patellar tendon loading, ultimately predisposing these individuals to a higher risk of developing patellar tendinopathy. Interventions designed to identify players at risk and alter their landing technique to decrease ankle dorsiflexion velocity and trunk flexion velocity may be effective in reducing patellar tendon loading and, in turn, patellar tendinopathy prevalence in this population. To tailor these prevention programs, however, a comprehensive longitudinal investigation exploring how sex and skill level influence patellar tendon loading during landing and, subsequently, affect the development of patellar tendinopathy is required.

The authors wish to acknowledge the use of the University of Western Australia BodyBuilder Model in the analysis of data.

No funding was received to support this study.

The authors have no conflicts of interest.

The results of the present study do not constitute endorsement by the American College of Sports Medicine.

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Keywords:

KNEE; TRUNK; PATELLAR TENDINOPATHY; INJURY PREVENTION; VOLLEYBALL

©2013The American College of Sports Medicine