The purpose was to quantify the effects of mid-flight whole-body and trunk rotation on knee mechanics in a double-leg landing. Eighteen male and 20 female participants completed a jump-landing-jump task in five conditions: no rotation, testing leg ipsilateral or contralateral (WBRC) to the whole-body rotation direction, and testing leg ipsilateral (TRI) or contralateral to the trunk rotation direction. The WBRC and TRI conditions demonstrated decreased knee flexion and increased knee abduction angles at initial contact (2.6 > Cohen’s dz > 0.3) and increased peak vertical ground reaction forces and knee adduction moments during the 100 ms after landing (1.7 > Cohen’s dz > 0.3). The TRI condition also showed the greatest knee internal rotation angles at initial contact and peak knee abduction and internal rotation angles and peak knee extension moments during the 100 ms after landing (2.0 > Cohen’s dz > 0.5). Whole-body rotation increased contralateral knee loading because of its primary role in decelerating medial-lateral velocities. Trunk rotation resulted in the greatest knee loading for the ipsilateral knee due to weight shifting and mechanical coupling between the trunk and lower extremities. These findings may help understand altered trunk motion in anterior cruciate ligament injuries.
Anterior cruciate ligament (ACL) injuries are common severe injuries in athletes (Kay et al., Citation2017). While females demonstrated increased incidence rates of ACL injuries compared to males in several sport events (Kay et al., Citation2017), males suffered the majority of ACL injuries in the general population (Gianotti, Marshall, Hume, & Bunt, Citation2009; Granan, Bahr, Steindal, Furnes, & Engebretsen, Citation2008). Following ACL injuries, individuals demonstrate abnormal neuromuscular function, elevated risk for secondary injuries and increased risk of knee osteoarthritis (Ingersoll, Grindstaff, Pietrosimone, & Hart, Citation2008; Kamath et al., Citation2014; Luc, Gribble, & Pietrosimone, Citation2014). ACL injuries frequently occur during jump-landing, cutting and pivoting tasks and are characterised by small knee flexion, increased knee abduction and increased knee internal/external rotation (Dai, Mao, Garrett, & Yu, Citation2015; Koga et al., Citation2010; Krosshaug et al., Citation2007; Olsen, Myklebust, Engebretsen, & Bahr, Citation2004). These injury characteristics are generally consistent with ACL loading mechanisms (Dai, Mao, Garrett, & Yu, Citation2014), although knee external rotation may decrease ACL loading (Utturkar et al., Citation2013). Consequently, jump-landing training has been focusing on soft landing with increased knee flexion and minimised knee abduction and rotation (Dai, Garrett et al., Citation2015; DiStefano, Padua, DiStefano, & Marshall, Citation2009; Welling, Benjaminse, Gokeler, & Otten, Citation2016).
Compared to knee kinematics and kinetics, the biomechanical association between trunk motion and ACL loading is less clear. A more upright trunk and a more posteriorly positioned centre of mass were observed when basketball players sustained ACL injuries (Sheehan, Sipprell, & Boden, Citation2012). This position was likely to load the ACL due to the increased quadriceps forces required to prevent falling backward (Sheehan et al., Citation2012). Similarly, female basketball players demonstrated a more upright trunk and tended to exhibit increased lateral trunk bending and knee abduction to the injured leg in ACL injury events (Hewett, Torg, & Boden, Citation2009). Lateral trunk bending was likely increasing internal hip adduction moments, potentially moving the knee medially, and therefore increasing external knee abduction moments (Hewett & Myer, Citation2011). Two general scenarios were identified when female netball players sustained ACL injuries (Stuelcken, Mellifont, Gorman, & Sayers, Citation2016). In the first, players experienced a mid-flight perturbation followed by an unbalanced landing. The second scenario consisted of lateral trunk bending towards the injured side with trunk rotation away from the injured leg. The trunk was also more likely to rotate away from the injured leg when male soccer players experienced ACL injuries (Walden et al., Citation2015). These studies suggest altered trunk motion may play a role in ACL injury events and support the correlation between poor trunk control and ACL injury risk (Zazulak, Hewett, Reeves, Goldberg, & Cholewicki, Citation2007). However, most previous studies have analysed trunk motion in ACL events using two-dimensional videos captured from uncalibrated cameras, which may introduce significant errors (Dai et al., Citation2015; Koga et al., Citation2010). Further validation of the relationship between trunk motion and factors associated with ACL loading is warranted.
Several studies have quantified the effect of trunk motion on landing kinematics and kinetics in a lab setting. Active trunk flexion increased peak knee and hip flexion and reduced vertical ground reaction forces (GRF) and quadriceps activation during a double-leg landing (Blackburn & Padua, Citation2008). Added trunk-load increased estimated knee anterior shear forces in participants who increased trunk extension but not in participants who increased trunk flexion in a double-leg landing (Kulas, Hortobagyi, & Devita, Citation2010). These two studies support that positioning the trunk centre of mass closer to the knee in the anterior-posterior direction is likely to decrease ACL loading (Blackburn & Padua, Citation2008; Kulas et al., Citation2010). In regard to trunk bending, Kimura et al., (Citation2012) quantified single-left-legged landing mechanics after an overhead stroke following left or right back-stepping in female right-handed badminton players. The left back-stepping, which involved greater lateral trunk bending towards the left leg, resulted in increased knee abduction angles and moments compared to the right back-stepping. In addition, peak knee abduction moments were positively correlated with lateral trunk bending and rotation towards the landing leg in a single-leg landing after catching a ball (Dempsey, Elliott, Munro, Steele, & Lloyd, Citation2012). Furthermore, a side-step cutting study also found lateral trunk bending and rotation towards the cutting leg to increase knee abduction and internal rotation moments, respectively (Dempsey et al., Citation2007). Recently, Hinshaw et al. (Citation2018) showed mid-flight lateral trunk bending resulted in re-positioning of body segment centre of mass, and subsequently increased impact forces, knee internal rotation, and abduction angles for the leg on the bending side. These four studies support the connection between lateral trunk bending and increased ACL loading (Dempsey et al., Citation2012, Citation2007; Hinshaw et al., Citation2018; Kimura et al., Citation2012). Regarding trunk rotation, increased impact GRF and force couple indexes were observed when soccer players landed with two feet after mid-flight whole-body rotation (Harry, Barker, Mercer, & Dufek, Citation2017), but this study was limited to force analyses and whole-body rotation. A previous study showed that individuals demonstrated decreased knee flexion angles and increased knee moments and knee valgus and internal rotation angles when the testing leg was placed on the lateral side of the jumping direction during an anticipated landing-lateral-jump task, suggesting the two legs may load differently during a horizontal landing task (Stephenson et al., Citation2018). However, the effects of whole-body and trunk rotation on double-leg landing kinematics and kinetics are unknown. Studying double-leg landing is particularly important for identifying factors that may cause increased loading for one leg, as bilateral asymmetries have been identified as risk factors for ACL injuries (Hewett et al., Citation2005; Paterno et al., Citation2010).
Therefore, the purpose of the current study was to quantify the effects of mid-flight whole-body and trunk rotation on knee kinematics and kinetics in a double-leg landing in five conditions: no rotation (NR), testing leg ipsilateral or contralateral to the whole-body rotation direction, and testing leg ipsilateral or contralateral to the trunk rotation direction. The whole-body rotation condition involved a forward jump with 90-degree whole-body rotation, which would place the leg contralateral to whole-body rotation to the lateral side of the landing direction. Based on the literature (Dempsey et al., Citation2012; Stephenson et al., Citation2018), it was hypothesised that the leg contralateral to whole-body rotation and the leg ipsilateral to the trunk rotation would demonstrate less knee flexion angles and greater landing forces, knee moments, and knee abduction and internal rotation angles compared to other three conditions.
Based on previous studies (Dempsey et al., Citation2012; Stephenson et al., Citation2018), a medium to large effect was expected for the comparisons between landings with or without rotation. Based on an effect size of 0.5 for a paired t-test, a sample size of 34 was needed to achieve a power of 0.8 at a type-I error level of 0.05. Eighteen males and 20 females volunteered to participate (age: 21.2 ± 2.3 years; height: 1.72 ± 0.10 m; mass: 72.0 ± 13.0 kg). Participants had experience in jump-landing sports or exercises and participated in sports or exercise at least two times for a total of 2–3 h per week at the time of testing. Individuals were excluded if they (1) had a major lower extremity injury that required surgical treatment; (2) had a lower extremity injury that kept them from participating in physical activities for more than 2 weeks in the previous 6 months; (3) possessed any other conditions that prevent participation at maximum effort; or (4) were pregnant. This study was approved by the University of Wyoming Institutional Review Board. Participants signed informed consent forms prior to data collection.
Data collection was performed in a biomechanics lab. Participants performed a 5-min jog and a standard dynamic stretching protocol (Dai et al., Citation2018). Spandex shirts and pants and standard shoes (Ghost 5, Brooks Sports Inc. Seattle, WA, USA) were provided. Retro-reflective markers were placed on the 7th cervical vertebra, superior sternum, left and right acromioclavicular joints, iliac crests, anterior superior iliac spines, posterior superior iliac spines and greater trochanters. On the testing leg (preferred leg to jump for a further distance), markers were placed on the lateral and anterior mid-thigh, medial and lateral femoral condyles, tibial tuberosity, anterior inferior shank, lateral shank, medial and lateral malleolus, calcaneus, first toe, and first and fifth metatarsal heads.
After a static trial, participants completed a jump-landing-jump task in five conditions (). For the NR condition (), the participant jumped from a 30-cm box located half the participant’s height away from the landing area. A standard men’s basketball was located on a tripod at the participant’s elbow height. The ball was placed half the participant’s arm length directly in front of the participant’s toes. For the two whole-body rotation conditions, the ball was placed at the same location as the NR condition, but the box placement was moved 90 degrees around the force platform. As such, the testing leg was either ipsilateral (WBRI, ) or contralateral (WBRC, ) to whole-body rotation direction. For the two trunk rotation conditions, the box was placed at the same location as the NR condition, but the ball placement was moved 90 degrees around the force platform. Similarly, the testing leg was either ipsilateral (TRI, ) or contralateral (TRC, ) to the trunk rotational direction.
For all five conditions, participants were instructed to jump forward from the box, reach for and hold the basketball as early in the movement as possible, land with feet on the targeted area, then perform a maximum vertical jump. When jumping off the box, participants were instructed to minimise the height they jumped to reach the targeted landing area, but the exact jump height was not standardised. For the whole-body rotation conditions, participants rotated their whole-body after they jumped from the box and landed with both feet pointing to the same direction as the NR condition. For the trunk rotation condition, participants rotated their trunk to reach the ball while they landed with both feet pointing to the same direction as the NR condition. A trial was repeated if a participant failed to land on the targeted area, paused before performing the maximal vertical jump, or if there was a significant delay between reaching for the ball and landing. Participants had a minimum of two practice trials followed by three official trials for each condition. A minimum of 30 s rest between trials was given. The order of the five conditions was randomised for each participant. Kinematic data were recorded using eight Vicon Bonita 10 cameras at a sampling frequency of 160 Hz (Vicon Motion Systems Ltd, Oxford, UK). GRF data were collected using one Bertec FP4060–10 force platform at a sampling frequency of 1600 Hz (Bertec Corporation, Columbus, OH, USA).
GRF and kinematic data were filtered via a fourth-order, zero-phase Butterworth filter with a low-pass cut-off of 100 Hz and 15 Hz, respectively (Stephenson et al., Citation2018). Joint centres and segment reference frames for the pelvis and lower extremities were defined as previously described (Dai, Heinbaugh, Ning, & Zhu, Citation2014). A trunk reference frame was also defined by the left and right acromioclavicular joints and the centre of the anterior superior iliac spines and posterior superior iliac spines. Cardan angles with an order of rotation of flexion (+)/extension (-), adduction (+)/abduction (-), and internal (+)/external (-) rotation were calculated between the thigh and shank reference frames for knee joint angles, and between the trunk and pelvis reference frames for trunk joint angles. Joint angles during the static trials were subtracted from those during the jump-landing-jump trials. Joint resultant moments were calculated using an inverse dynamics approach (Kingma, de Looze, Toussaint, Klijnsma, & Bruijnen, Citation1996). Segment mass, centre of mass and moments of inertia were based on a previous study (de Leva, Citation1996). Knee joint resultant moments were expressed in the tibia reference frames as internal moments. Joint moments were normalised by the participant’s body weight and height. GRF were normalised by the participant’s body weight.
Trunk rotation angles were assessed at initial contact and 100 ms after landing with the targeted rotational direction defined as positive. Similar to previous studies (Kristianslund & Krosshaug, Citation2013; Stephenson et al., Citation2018), both initial and peak knee flexion, abduction and internal rotation angles during the first 100 ms of landing were identified. Peak vertical GRF, knee adduction, extension and external rotation moments during the first 100 ms of landing were also extracted. Jump height and stance time were calculated to quantify jump performance (Dai et al., Citation2015). These calculations were performed using customised subroutines developed in MATLAB (MathWorks Inc. Natick, MA, USA).
Data for the three official trials were averaged for analysis. Dependent variables were compared among the five jump-landing-jump conditions using repeated measures analyses of variance (ANOVA). Significant ANOVAs were then followed by paired t-tests. A type-I error rate of 0.05 was used for the ANOVAs for statistical significance. The study-wide false discovery rate for all the paired t-tests was controlled at 0.05 (Benjamini & Hochberg, Citation1995). The effect sizes of changes between two conditions were quantified using Cohen’s dz, with Cohen’s dz < 0.5 considered ‘small’, 0.5 ≤ Cohen’s dz < 0.8 considered ‘medium’ and Cohen’s dz ≥ 0.8 considered ‘large’ (Cohen, Citation1988). Statistical tests were performed using SPSS Statistics 24 software (IBM Corporation, Armonk, NY, USA).
Significant ANOVAs were found for all variables ( and ). The largest p value for a significant paired t-test was 0.033 after the adjustment for the false discovery rate. Jump height was significantly greater for the NR condition than the WBRC and TRC conditions with small effect sizes. The TRI and TRC conditions demonstrated significantly longer stance time and greater trunk rotation at both initial contact and 100 ms after landing than the other three conditions with mostly large effect sizes.
For landing kinematics, the WBRC and TRI conditions demonstrated significantly decreased knee flexion angles at initial contact with mostly small effect sizes and increased knee abduction angles at initial contact with large effect sizes compared to the other three conditions. On the other hand, the WBRI and TRC conditions showed knee adduction angles instead of knee abduction angles at initial contact. The TRI condition also showed significantly greater knee internal rotation angles at initial contact than the other conditions with large effect sizes, while the WBRI and TRC conditions demonstrated knee external rotation angles instead of knee internal rotation angles at initial contact. The WBRC condition had significantly less peak knee flexion angles during the 100 ms after landing than the other conditions with large effect sizes, while the WBRI and TRI condition showed significantly less peak knee flexion angles than the NR and TRC conditions with medium-to-large effect sizes. Peak knee abduction and internal rotation angles during the 100 ms after landing were the greatest for the TRI condition with medium-to-large effect sizes and the least for the WBRI and TRC conditions.
For landing kinetics, the WBRC and TRI conditions demonstrated significantly greater peak vertical GRF and knee adduction moments than the other three conditions with mostly medium-to-large effect sizes. The TRI condition also showed significantly greater knee extension moments than the other conditions with mostly large effect sizes. In contrast, the TRC condition showed the least peak vertical GRF and peak knee adduction moments, while the WBRI condition demonstrated the least peak knee extension and external rotation moments.
Discussion and implications
The purpose of the current study was to quantify the effect of mid-flight whole-body and trunk rotation on double-leg landing mechanics. Whole-body and trunk rotation was imposed by tasks that simulated catch and shoot manoeuvres in basketball and netball. The increased trunk rotation at initial contact and 100 ms after landing confirmed that trunk rotation was initiated in mid-flight and persisted during early landing for the trunk rotation conditions. On the other hand, the small trunk rotation angles for the whole-body rotation conditions suggested that the body was rotated together in mid-flight.
The results generally support the hypothesis that the leg contralateral to whole-body rotation and the leg ipsilateral to the trunk rotation would demonstrate less knee flexion angles and greater landing forces, knee moments, and knee abduction and internal rotation angles compared to other three conditions. The whole-body rotation condition was characterised by decreased knee flexion angles, increased knee abduction and internal rotation angles, and increased GRF and knee moments for the contralateral leg compared to the ipsilateral leg, associated with increased ACL loading for the contralateral leg (Dai, Mao et al., Citation2014). As the participants jumped forward, rotating the body 90 degrees in mid-flight, the two legs were placed parallel to the direction of the approaching velocity, resulting in a posture similar to a lateral landing. The preference for the contralateral leg to decelerate medial-lateral velocity was consistent with a previous study, showing individuals preferred to use the contralateral leg to land and generate a horizontal velocity in lateral jumps (Stephenson et al., Citation2018). For example, when a participant lands with an approaching velocity directed towards the right, the right leg is more likely to play a dominant role in generating a deceleration towards the left. This limb preference could be caused by stronger hip abductors than hip adductors (Sugimoto, Mattacola, Mullineaux, Palmer, & Hewett, Citation2014), as the medial-lateral decelerating force would impose internal hip abduction moments for the contralateral leg and hip adduction moments for the ipsilateral leg. A previous study has shown greater GRF and force couple indexes during landings with 180-degree whole-body rotation compared to landing without rotation (Harry et al., Citation2017). However, participants only jumped vertically without a horizontal velocity, and the individual role of the two legs was unclear. The current findings suggest that whole-body rotation may not affect the two legs equally. A forward jump with 90-degree whole-body rotation would impose greater loading for the leg that was mainly used for decelerating the medial-lateral velocity.
For the trunk rotation conditions, the ipsilateral leg in the trunk rotation condition experienced the greatest knee abduction and internal rotation angles as well as peak vertical GRF and knee moments, associated with increased ACL loading from all three planes (Dai et al., Citation2014). As the participant rotated the trunk to the ipsilateral leg, a greater percentage of body weight was shifted to the testing leg resulting in increased vertical GRF and joint moments. In addition, an internal hip adduction moment was needed to maintain postural stability because more weight was placed on the lateral side of the ipsilateral hip. Consequently, an internal hip adduction moment likely moved the knee medially and increased knee abduction angles (Hewett & Myer, Citation2011). Meanwhile, as the trunk, pelvis and lower extremities act as a kinetic chain in landing, trunk rotation could have increased external rotation of the ipsilateral femur relative to the global coordinate system. An externally rotated femur relative to the global coordinate system could have the same mechanical effect as an internally rotated tibia relative to the global coordinate system, contributing to increased local knee internal rotation angles. For the contralateral leg, these mechanical effects were reversed and could have resulted in opposite changes in knee kinematics and kinetics. These findings are consistent with previous studies, showing increased trunk rotation to the supporting leg would increase external knee abduction or internal rotation moments in a single-leg landing or cutting manoeuvres (Dempsey et al., Citation2012, Citation2007). The current study differed from previous studies by using a double-leg landing and showing that trunk rotation can cause different landing patterns for the two legs.
Previous studies have generally observed small knee flexion and increased knee abduction for ACL injuries, but the presence of knee internal or external rotation is less consistent (Krosshaug et al., Citation2007; Olsen et al., Citation2004; Stuelcken et al., Citation2016). One reason could be the difficulty in determining the time of injury and magnitude of knee rotation from uncalibrated cameras (Dai et al., Citation2015; Koga et al., Citation2010). To improve the validity of video analyses, Koga et al., (Citation2010) used a model-based image-matching method to quantify knee kinematics for ACL injuries. Small knee flexion angles along with increases in knee abduction and internal rotation were found during the first 40 ms after landing. However, the knee started to rotate externally after the injury. An in vitro study has also shown that the knee changed from internal rotation to external rotation after the ACL is ruptured under compressive loads (Meyer & Haut, Citation2008). Therefore, the knee external rotation observed in some studies might be the consequences instead of the causes of ACL injuries (Meyer & Haut, Citation2008). Kim et al., (Citation2015) used bone bruise location on the femur and tibia to reconstruct knee kinematics near the time of ACL injuries. The injured knee was close to full extension with a 5-degree increase in abduction, a 15-degree increase in internal rotation, and a 2.2-cm increase in anterior tibial translation compared to a neutral position. In addition, an in vivo study has shown that knee valgus collapse, mainly characterised by increased knee external rotation, decreased ACL length (Utturkar et al., Citation2013). These findings also support knee internal rotation being more likely to contribute to ACL injuries than knee external rotation. However, two studies observed that the trunk was more likely to rotate away from the injured leg and resulted in knee external rotation when ACL injuries occur (Stuelcken et al., Citation2016; Walden et al., Citation2015). This inconsistency could result from the limitations of qualitative video analyses, as no calibration was performed for accurately quantifying joint angles. Even with the previously mentioned image-matching technique for analysing ACL injury videos, the root mean square of differences in knee internal/external rotation in side-cutting was still around 10 degrees compared to a motion capture system with skin-mounted markers (Krosshaug & Bahr, Citation2005). Meanwhile, another explanation could be that knee internal rotation may not be a necessary factor in ACL injuries if excessive loads from other mechanisms are present. In the current study, the ipsilateral knee kinematics for the trunk rotation conditions resemble the injured knee position reconstructed with a high accuracy (Kim et al., Citation2015). Trunk motion provides a proximal-distal mechanism for knee internal rotation when the foot is typically fixed on the ground and rotated outward relative to the global coordinate system in landing. Future studies are needed to quantify trunk and lower extremity rotation with a high accuracy to further elucidate the role of trunk rotation in ACL injury events.
Several strategies may be utilised to modify factors associate with ACL loading when landing after whole-body and trunk rotation. After completing a task that involved mid-flight trunk rotation, individuals are encouraged to return to a neutral trunk position before landing. Second, individuals should have adequate proprioceptive awareness and increase both knee and hip flexion in mid-flight when whole-body or trunk rotation occurs. This strategy will increase the flight time for adjusting trunk position and preparing for a soft landing. Third, avoiding landing with excessive whole-body and trunk rotation may be considered for individuals whose priority during sports participation is not performance.
There were several limitations to the current study. First, the power analysis was performed for comparisons of different jump-landing conditions in both males and females without considering potential sex effects or interactions. Our secondary analyses including sex as an independent variable showed minimal sex effects and interactions for knee kinematics and kinetics, suggesting males and females responded similarly to trunk and whole-body rotation. Second, only the dominant leg was tested, and bilateral asymmetries were not directly quantified. Dominant and non-dominant legs were assumed to demonstrate similar landing patterns among different conditions, and leg dominance may have affected some side-to-side differences. Third, considering the injury knee at the estimated time of ACL injury only had a 5-degree increase in abduction and a 15-degree increase in internal rotation (Kim et al., Citation2015), increases of 2–3 degrees in these angles with medium-to-large effect sizes were likely to be clinically significant in the current study. However, previous studies have documented significant errors of using motion capture systems with skin-mounted markers for quantifying knee joint angles in the frontal and transverse planes compared to bone-mounted markers or biplanar fluoroscopy techniques (Benoit et al., Citation2006; Miranda, Rainbow, Crisco, & Fleming, Citation2013). Future studies are needed to quantify more accurate femur and tibia motion associated with trunk and whole-body rotation during landing. Fourth, the current task was an anticipated task. An examination without a pre-planned condition could investigate the role that trunk rotation plays on landing mechanics in a more game-like setting. Fixed ball locations were used to prevent excessive whole-body and trunk rotation. Increasing rotation may induce greater changes in landing mechanics but may also raise safety concerns. In addition, the whole-body rotation condition resulted in a lateral landing, so it was not possible to separate the effect of the lateral deceleration from that of the deceleration of the whole-body rotation. Finally, the findings can only be applied to individuals without major injuries. Including individuals with a history of ACL injuries may reveal different landing patterns and help understand secondary ACL injuries.
Whole-body rotation increased contralateral knee loading because of its primary role in decelerating medial-lateral velocities. Trunk rotation resulted in the greatest loading for the ipsilateral knee due to weight shifting and mechanical coupling between the trunk, pelvis and lower extremities. The kinematics demonstrated by the ipsilateral knee with trunk rotation resemble knee positions near the time of ACL injuries. These findings may help researchers better understand trunk rotation in ACL injury events. Athletes should be aware that mid-flight whole-body and trunk rotation may result in increased loading for one leg. Specific technique and neuromuscular training may help athletes better prepare for these situations.
No potential conflict of interest was reported by the authors.
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