Impact differences in ground reaction force and center of mass between the first and second landing phases of a drop vertical jump and their implications for injury risk assessment
Introduction
Anterior cruciate ligament (ACL) injuries are catastrophic knee injuries that debilitate athletic careers, involve costly rehabilitation and lead to early onset arthritis (Lohmander et al., 2007, Lohmander et al., 2004). Investigation has identified soccer and basketball as the most frequent sources of ACL injury in adolescent athletes (Kelm et al., 2004). Female athletes incur ACL injuries at 4–6 times the rate of their male counterparts (Hewett et al., 2005, Hewett et al., 1999) such that one in every 60–80 female soccer or basketball players sustain an ACL injury (NFHS, 2002). Up to 70% of these injuries occur in non-contact situations and are associated with high-loading athletic tasks (Boden et al., 2000). Within female high school basketball, 60% of ACL ruptures are attributed to jumping or landing (Piasecki et al., 2003). Specifically, the jumping and landing task related to rebounding a ball is most frequently cited as the mechanism of ACL rupture by female basketball players (Powell and Barber-Foss, 2000).
The drop vertical jump (DVJ) task has been utilized to obtain measures related to ACL injury risk factors, including vertical ground reaction force (vGRF) (Hewett et al., 2005). Adolescent participants who drop from a box height of 30 cm generate peak vGRFs in excess of four times bodyweights (McNair and Prapavessis, 1999). VGRFs contribute to knee instability and are a primary loading mechanism of the knee joint and ACL (Hewett et al., 1996, Hewett et al., 2005, Yu and Garrett, 2007). Biomechanical factors such as increased drop height (Ford et al., 2011), decreased quadriceps to hamstrings activation ratio (Peng et al., 2011, Yeadon et al., 2010), poor neuromuscular control (Hewett et al., 2005), maturity (Lazaridis et al., 2010, Quatman et al., 2006), and increased joint stiffness (DeVita and Skelly, 1992, Myers et al., 2011) produce larger vGRFs and likely increased injury risk during landing. Greater vGRF upon landing likely enhances the probability of ACL injury as, prior to injury, participants who sustain ruptures exhibit 20% larger peak vGRFs during landing than participants who remain healthy (Hewett et al., 2005). The DVJ allows investigators to examine variation within these and other biomechanical risk factors such as joint kinetics and kinematics (Hewett et al., 2005) in order to prospectively screen athletes for potential ACL injury.
Despite the plethora of studies focused on vGRFs during the initial drop landing in a DVJ, little work has investigated the biomechanical behaviors of the second landing that follows a maximal vertical jump. The first landing of the DVJ is controlled, as athletes are provided instructions on how to initiate the drop, make contact, and position their feet. Conversely, explicit directions for the second landing are not documented in the literature. Studies have demonstrated that task instruction can immediately reduce peak vGRFs during landing (McNair et al., 2000, Prapavessis and McNair, 1999, Prapavessis et al., 2003). As vGRFs propagate through the closed kinetic chain and impart torsion moments across knee (Boden et al., 2000), increased vGRFs instigate larger moments that can create joint instability and place athletes, especially those with poor neuromuscular control, at risk of sustaining ACL injuries (Hewett et al., 2005). Therefore, relative to the first landing, the lack of instruction for the second landing in a DVJ may negatively impact neuromuscular controls and alter landing biomechanics related to increased injury risk. Coupled with a task change from drop jump to drop land, which is known to alter joint kinetics and muscle activation (Ambegaonkar et al., 2011), these factors warrant an investigation of how the second DVJ landing varies from the first.
The purpose of the current study was to determine how vGRFs and center of mass (CoM) kinematics from the second landing of a DVJ compare to those of the first landing. Our initial hypothesis was that participants would demonstrate altered vGRF and CoM behaviors between the first and second landings of a DVJ.
Section snippets
Methods
Participants included in the current study were from a cohort in a prospective, longitudinal study. A population of 239 middle school (n=162) and high school (n=77) female basketball players (mass=55.4±13.2 kg, height=1.60±0.09 m, age=13.6±1.6 years) were tested immediately preceding their upcoming season. Basketball players were appropriate participants as they generate greater peak vGRF during jump landing than soccer players (Ford et al., 2011). Participants were not divided based on
Results
Data from both the first and second landing phases are summarized in Table 1. When separated by first and second landing, participants demonstrated similar cumulative peak vGRF (p=0.45), which were all above 1000 N per leg. The absolute difference in peak vGRF between the right and left legs within the first landing was 247 N and increased to 299 N within the second landing (p<0.01; Fig. 3). When separated by leg, neither the right leg nor the left leg individually demonstrated between landing
Discussion
The purpose of the current study was to determine how vGRFs and CoM from the second landing of a DVJ compare to those of the first landing. It was found that though vGRFs are equivalent between the first and second landing, differences in CoM and asymmetry indicate that each landing enacts unique biomechanical and neuromuscular mechanisms.
Interestingly, peak vGRFs were comparable between landings, which lead us to accept the null hypothesis that vGRF would not change from the first to second
Conclusions
In adolescent female athletes, the first and second landing from a DVJ exhibit separate neuromuscular biomechanical pathways. Maximum vGRFs during impact and maximum CoM height in flight prior to landing were equivalent between landings and therefore indicated no increase in perturbation between landings. Nevertheless, side-to-side vGRF asymmetries and minimum CoM height were greater in the second landing, which indicated that athletes alter their mechanics, force distribution, and joint
Conflict of interest statement
There are no conflicts of interest.
Acknowledgments
This work was supported by NIH grants R01-AR049735, R01-AR055563, R01-AR056259 and R03–057551. The authors thank the entire Sports Medicine Biodynamics Center at Cincinnati Children's hospital for their support. The authors acknowledge Boone County, Kentucky, School District for participation in this study.
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