Lower extremity (LE) musculoskeletal injuries have a multifactorial etiology (1). More than 60% of all dancers’ injuries occur to the LE (2), similar to other athletic populations (3,4); however, 66% to 72% of the injuries in dance are overuse in nature (2,5). Consequently, only 28% to 34% of dance injuries are classified as acute (2,5), whereas acute injuries have been reported at 70% in other athletic populations (6). The common type of injuries in dance are tendinosis (32%), ligament sprain (17%), and muscle strain (15%), commonly linked with stress-related factors; also, those are the three conditions that required greatest amount of treatment (2). The injuries have short- and long-term health consequences that affect performance and quality of life with potential for extended disability (7). Intrinsic and extrinsic risk factors have been proposed (1,8). Deficits in muscle strength and/or muscle imbalance (9), decrease in neuromuscular control (9–11), and poor technique (8) are risk factors frequently associated with LE injuries. Injury prevention programs have been shown to be effective in altering modifiable risk factors and decreasing injury rates in the specific study samples (12). Still, overall LE injury rates have remained steady across multiple populations (3), suggesting that injury prevention programs need broader implementation. Identifying and understanding the modifiable musculoskeletal injury risk factors can lead to improved primary prevention programs (7,8).
Substantial research has been done to investigate modifiable risk factors during jump-landing activities (10,11,13–17). LE role during landing is to reduce and control the downward momentum acquired during the flight phase, which places a strenuous demand on the LE (18). It has been reported that a more flexed landing pattern, with higher trunk, hip, and knee flexion during the landing phase, is associated with higher shock absorption and, consequently, lower ground reaction force (11,19). Contrastingly, it has been reported that lower flexion angle during landing augments the reliance on the frontal plane motion and loads to decelerate the body center of mass (20). However, excessive frontal plane motion and loading is a common knee injury mechanism (10).
Previous research reported that dancers have a more erect trunk and LE biomechanical pattern during the landing phase compared with nondancers (ND) and other athletes (13,21). This extended pattern may hinder appropriate neuromuscular control to absorb the landing forces, resulting in higher ground reaction forces (11,14,17,19,22), potentially increasing the risk of injuries. Still, dancers compensate the extended position at initial contact (IC) with higher knee and ankle excursion (13) as a strategy to dissipate landing forces over a higher range of motion (17). Also, they may use a muscular strategy, through higher plantarflexor muscle action, to attenuate the landing forces (11,14,17,22) without a concomitant increase of peak vertical ground reaction force (PvGRF) (17). Further, dancers perform repetitive single-leg multidirectional landings with significant horizontal displacement as part of their artistic requirements. The drop-landing task has been primarily used to investigate LE biomechanics. Yet, some limitations have been suggested, as it inhibits the isolation of the landing phase from the entire jump-landing task, limiting the preparation for landing, and it has been implied as a less complex movement (23). In addition, literature reports that LE kinematic and kinetic parameters are significantly affected by the jump-landing direction which is absent in a drop-landing task (16).
During lateral jump-landing lower hip flexion, higher knee flexion, ankle dorsiflexion, and peak knee valgus angle were reported when compared with other directions (16,24). The different landing directions require distinct biomechanical strategies to dissipate the impact forces (15,25,26). Furthermore, those impact forces, during single-leg landings, are solely absorbed by one LE, with a decreased base of support and increased biomechanical requirements from the trunk and pelvis (15), with lower knee and hip range of motion (27). Thereby, the body needs to attenuate the impact distally (27), suggesting that further investigation of the distal and proximal joint control during landing is warranted. It is relevant to consider and incorporate jump-landing direction skills during intervention programs. Also, understanding the biomechanical landing patterns in professional dancers (PD) can contribute to comprehend what may facilitate shock absorption during landing. To our knowledge, there is limited research investigating the biomechanical responses during multidirectional single-leg landings between PD and ND that can contribute to the differences in acute injury rates (28). Therefore, the purpose of the current study was to investigate kinematic and kinetic differences between PD and ND, during multidirectional (lateral, diagonal, and forward) single-leg landings. It was hypothesized that 1) PD would have a more extended landing posture (lower hip and knee flexion, and higher plantarflexion), higher ankle excursion, and PvGRF during single-leg landing compared with ND; and 2) lateral jump landing would have higher knee and lower hip flexion compared with the forward and diagonal jump directions.
A single-session descriptive group comparison design was used. An a priori sample size estimation, related to landing techniques between dancers and athletes and LE biomechanics (29,30) with an effect size of 0.8, an exploratory alpha level of 0.05, revealed that a minimum of 24 subjects would be required to achieve 80% statistical power. A total of 30 participants, 15 PD (26.6 ± 7 yr, 1.69 ± 0.1 m, 57.8 ± 9.3 kg) and 15 ND (25.0 ± 5 yr, 1.69 ± 0.1 m, 66.0 ± 10.2 kg), volunteered to participate in the current study. All data were collected on the dominant side of each participant, defined as the preferred single-leg landing after performing a countermovement jump (31). To be included in this study, all participants were required to be between 18 and 40 yr old, and physically active; furthermore, PD (ballet and modern dance) had to practice a minimum of 10 h·wk−1 of dancing, whereas ND had to exercise a minimum of 3 h·wk−1 and had different sports background (basketball, running, surf, gymnasium, swimming, football, tennis, boxing). Participants were excluded if they had a recent history of LE injuries, a history of LE surgery within the past 5 yr, any recent pain to the LE that would impair the ability to jump, or any known neurological or cognitive disorder. All procedures performed in the study were in accordance with the ethical standards of the Institutional Ethical Review Committee and with the 1964 Declaration of Helsinki and its later amendments. An informed consent was obtained from all participants before the study.
Instrumentation and experimental procedures
LE biomechanical data were collected using a 10-camera high-speed three-dimensional motion capture system (Opus; Qualisys AB, Gothenburg, Sweden) sampling at 200 Hz. A Bertec force plate (FP4060-10; Bertec Corporation, Columbus, Ohio) recorded ground reaction force data, sampling at 1000 Hz.
Participants’ demographics and anthropometrics measures were obtained by the same researcher (AMA). Thirty-three retroreflective markers were placed on selected body landmarks, and a four-marker cluster was used on the thigh of the dominant side. The retroreflective markers were secured on body landmarks using double-sided tape, and the marker cluster was placed with a Velcro band around the participant’s thigh. Participants stood barefoot and wore spandex shorts. Women also wore a sports bra during testing. A 5-min self-directed warm-up period was provided. After the warm-up, a static trial was collected. The four calibration markers (medial knee and malleolus, upper posterior calcaneus, and first distal metatarsal) were removed before the recording of the jump-landing trials.
The same researcher (AMA) provided instructions to each participant on how to perform each jump. Participants stood on the nondominant leg, 70 cm away from the center of the force plate, then proceeded to randomly perform multidirectional single-leg jump landings: (A) lateral (LJ), (B) diagonal (DJ), and (C) forward (FJ) (Fig. 1). Upon landing, participants immediately transitioned into a maximal vertical jump, which was followed by a second landing also in the middle of the force plate, on the dominant leg (Fig. 2) (15,16,24,25). They were asked to perform three trials for each direction. Between each trial, 30-s rest was provided. A trial was deemed successful when participants jumped into the center of force plate, kept their hands on the hips and (i) did not lose balance, (ii) did not touch the force plate with the nondominant foot, and/or (iii) did not hop or adjust the landing foot upon contact with the force plate. Unsuccessful trials were defined as the loss of balance, hopping or stepping off the force plate, or using their arms to maintain balance. For the present study, only the first landing was analyzed.
Markers were manually identified using QTM (Qualisys Track Manager, Gothenburg, Sweden) software and then were exported to and processed in Visual 3D (C-Motion, Inc., Rockville, MD). A laboratory coordinate system was established with the positive x-axis, y-axis, and z-axis, which represented forward, left, and upward directions, respectively. From the static trial, a kinematic model was used to quantify joint motion, composed of four reconstructed segments: foot, shank, thigh of dominant extremity, and the pelvis for each participant. Knee and ankle joint centers were determined as the midpoints between the medial and lateral epicondyles and malleoli markers, respectively. The hip joint centers were estimated using a previously reported regression equation (32).
Force plate data were used to defined the IC point, wherein the vertical impact force plate surpassed a threshold of 10 N (15,26,33). The landing phase was defined as the time interval between the IC with the force plate and peak knee flexion (PKF) (13,21,33). PvGRF data were defined as the vertical ground reaction force maximum value during the landing phase. The kinematic variables of interest were sagittal and frontal planes joint angles for hip, knee, and ankle. Hip, knee, and foot joint excursion in sagittal plane was computed as the subtraction of the angle between IC and PKF during the landing phase (13). All joint angles were reported in degrees (°). The kinetic variables calculated were internal joint moments for hip and knee and vertical ground reaction force. Internal joint moments were computed using conventional inverse dynamics analysis (34) and normalized to each participant’s body mass and height (N·m·kg−1) (26), representing the internal load applied to each joint. The ground reaction force was normalized to body weight. Residual analysis on joint kinematics and kinetics was conducted to determine the optimum cutoff frequency (34). A fourth-order low-pass Butterworth filter with a 10-Hz cutoff was used on joint kinematics and kinetics. All dependent variables were measured at IC, PKF, and PvGRF. In addition, time to each peak was also computed.
All statistical analyses were performed using SPSS (IBM, Chicago, IL). Descriptive statistics and normalcy tests were conducted. A 2 (group) × 3 (jumps) repeated-measures ANOVA was conducted to determine interactions between groups and jumps, as well as main effects between the jumps (LJ, DJ, and FJ) and the groups (PD and ND) for all dependent variables. Pairwise comparisons with a Bonferroni adjustment were performed when significant differences were observed. The alpha level for statistical significance was set at P < 0.05 for all data.
Descriptive statistics (mean ± SD) of the kinematic and kinetic dependent variables, for the two groups and the three jump directions, are summarized in Table 1. An interaction effect between groups and jumps was significant for ankle, and hip angles in frontal and sagittal planes, respectively, at IC (P < 0.05). PD during the forward jump landing showed higher ankle eversion than ND during diagonal and lateral directions. Moreover, PD performing the lateral jump landing had lower hip flexion than during their forward direction, and the three landing directions of ND. Further, PD during the diagonal jump landing also presented lower hip flexion compared with the diagonal and forward directions of ND. A statistically significant main effect was found for group (P = 0.008), as well as, for jump directions (P < 0.05).
For group main effect, PD at IC had lower hip and knee flexion and higher ankle plantarflexion than ND (Fig. 3). PD, at PvGRF, also demonstrated lower hip abduction angle and higher hip abduction moment, knee abduction moment, and ankle dorsiflexion and eversion angles than ND. Higher hip and knee abduction moment, knee adduction, and ankle eversion in PD were also observed at PKF. Further, PD had significantly higher knee (PD, 41 ± 6.1; ND, 33.8 ± 8.4) and ankle (PD, 53.7 ± 3.4; ND, 38.9 ± 8.9) excursion than ND. PD took longer (0.21 ± 0.04 s) than ND (0.18 ± 0.04 s) to achieve PKF.
There was a significant main effect for jumps (Fig. 4). In particular, at IC, the LJ had higher hip abduction moment, hip abduction and ankle inversion, and lower hip flexion and ankle plantarflexion than the forward and diagonal jumps, and lower knee adduction angle than the forward jump. Further, the DJ exhibited higher hip abduction, hip abduction moment, and ankle inversion, and lower hip flexion than the FJ. It was also observed that the FJ presented higher knee adduction moment and lower knee flexion than the other two jumps, and higher knee adduction than the LJ.
At PvGRF, the FJ demonstrated higher hip flexion, hip adduction, knee adduction and ankle eversion, and lower knee abduction moment when compared with the other two jumps. The LJ was significantly higher for hip extension moment, knee flexion, and dorsiflexion than the other two jumps and had a higher hip abduction moment than the FJ.
At PKF, the LJ demonstrated lower hip flexion and knee extension moment and higher ankle dorsiflexion compared with DJ and FJ, and the DJ ankle dorsiflexion was also higher than the forward jump. The FJ exhibited higher hip adduction angle compared with the other two, and higher knee adduction angle and lower hip extension and abduction moment when compared with LJ.
Lastly, the LJ took longer to PKF than FJ (LJ, 0.21 ± 0.05 s; FJ, 0.19 ± 0.04 s), and to PvGRF than the two other jumps (LJ, 0.18 ± 0.09 s; DJ, 0.13 ± 0.06 s; FJ, 0.14 ± 0.07 s). The LJ (15.5 ± 7.7) had higher hip excursion than the FJ (12.7 ± 5.4). No other statistically significance differences were found (P > 0.05).
The purpose of our study was to investigate lower extremity biomechanics of PD and ND during multidirectional single-leg landings. We hypothesized that 1) PD would present an upright landing posture at IC, as well as higher ankle excursion and PvGRF during landing, and 2) higher knee and lower hip flexion would be exhibited in the lateral jump landing when compared with the forward or diagonal jump directions. The results of this study support our hypothesis that PD exhibited a more extended landing posture at IC, with lower knee and hip flexion, higher plantarflexion angle, and higher ankle excursion during landing. Further, each jump landing direction elicited distinct LE biomechanical responses, suggesting that sagittal and frontal planes displacements influence overall single-leg landing strategies.
LE alignment during landing
PD landed with an everted ankle position in the forward jump landing compared with the diagonal and lateral ND jump landings. This suggests that PD avoid a “sickling” pattern (35), maintaining an everted ankle position while plantarflexed. Comparatively to ND, this mechanism is most likely associated with the specific and extensive ankle training of PD to maintain the alignment of the leg, ankle, and foot. Thereby, they avoid a common mechanism of ankle sprain (ankle inversion with plantarflexion), perhaps due to an increased peroneus muscle activity. During the forward jump landing, while dancers landed in an everted ankle position, they concurrently exhibited an adducted knee angle. Accordingly, dancers’ biomechanical strategy emphasizes the LE alignment. Therefore, the typical “dynamic valgus” mechanism (36) during landing was not observed in the PD. This suggests that the relation between knee and ankle in our study may be a potential protective factor to maintain appropriate LE alignment during landing.
A primary goal during landing is to stabilize and maintain the center of mass within the base of support; the various jump directions influence the strategies adopted to control the landing (25). In our study, PD presented lower hip flexion during the lateral jump landing compared with the forward jump, as well as when compared with the three jump-landing directions of ND. The lower hip flexion posits that PD recruit the pelvic and hip stabilizers due to the increased lateral displacement. Consequently, dancers tend to prevent the “dynamic knee valgus,” associated with excessive hip adduction and internal rotation, probably due to the higher posterolateral pelvis and hip musculature demands (15). Analogous to previous literature, it appears that during single-leg landings, the impact forces were mainly attenuated distally (i.e., knee and ankle joints) due to the smaller base of support, decreasing the work demands on the proximal joints (i.e., hip joint) (27). It is likely that the pelvis and hip joint musculature of PD was primarily activated to maintain the postural control of the proximal segments, whereas the distal segments mainly attenuated the impact forces.
Dance training may influence landing pattern
At IC, PD landed in a more extended posture compared with ND. This upright position has been associated with higher PvGRF, which increases the load on noncontractile structures of proximal LE joints (27). Conversely, when using an active trunk flexion strategy during landing, decreased PvGRF, peak knee extensor moment, and quadriceps activation; higher hip and knee flexion angles; and higher hip extensor moment were reported (19,31,37). The magnitudes of the attained hip and knee flexion (51° and 82°, respectively) were suggested to be protective of joint loads (19,37). However, it is plausible that such magnitudes have a deleterious effect on performance in most athletic populations because of the excessive active trunk flexion (i.e., 96° of active trunk flexion). It is worth noting that in our study, PD exhibited a more erect position at IC compared with ND, but no difference in the magnitude of PvGRF was observed; this is most likely due to the higher distal joints excursion, which possibly contributes to offset the load on the passive structures while dissipating the external forces (13,17,18). This suggests that the knee and ankle joints can contribute nearly equally to the total shock absorption, with the hip joint contributing minimally (18). PD appropriately take advantage of their extensive training of the demi-plié movement to accomplish this landing strategy. Notably, the ankle, paramount to the aesthetics and technique in dance, plays an important role to adequately mitigate the landing forces (27). The significantly higher excursion and longer time to PKF probably increases the eccentric action of the plantarflexor muscles, which has been shown to decrease PvGRF (14,17).
PD presented higher knee adduction angle, lower hip abduction angle, and higher hip abductor moment during landing when compared with ND. Our findings are similar to the study Orishimo and colleagues (21), which reported that dancers (males and females) displayed higher knee adduction angle compared with other athletes. It is reasonable to suggest that PD had higher medial–lateral neuromuscular control of the knee joint while increasing LE musculature action rather than relying on the knee ligaments to absorb the landing forces (i.e., less ligament dominance) (36). Further, a neuromuscular link has been observed between the hip and the knee, with the hip abductors contributing to the neuromuscular control of the knee kinematics during landings (38). Although proximal muscular activity was not quantified, based on our hip abductor moment results, it is conceivable that the abductor musculature contributed to minimize the external adductor loads at the hip and to maintain the neuromuscular control of the knee (38). Overall, when considering both planes of motion (i.e., sagittal and frontal), PD most likely exhibited an improved neuromuscular control than ND during landing. The balanced neuromuscular control observed in PD is probably associated with their extensive practice and a consequence of specific requirements acquired through years, such as aesthetics, elegance, and technique (21).
Largely, PD took advantage of significant ankle and knee range of motion. These structures, particularly the foot-ankle complex, are vastly trained in dancers, such as smoothly landing and rolling from the forefoot until a heel touchdown occurs (13). It has been reported that the joint mechanics of the distal joints play an important role on muscle activation patterns to the overall shock absorption strategy (18,22,39). It is feasible that dancers’ training specificity leads to automatic preplanned neuromuscular strategies development that are protective of acute LE injuries during highly skilled activities. Previous research has suggested that initial impact forces can be mediated by automatic strategies through the LE musculature activation patterns (18,39). The integration of automatic neuromuscular responses must be acquired throughout years of training (40), most likely improving landing patterns and decreasing the deleterious effects of “dynamic knee valgus.” These factors combined indicate an efficient control of distal segments, potentially contributing to the lower rate of LE acute injuries in dancers when compared with other populations (28). A comprehensive understanding of the natural biomechanical characteristics between populations, as well as jump directions, is essential to enable the development of multifaceted training programs.
Multidirectional jump-landing patterns
Our results demonstrate that the multidirectional jumps have distinctive biomechanical profiles. These differences were more apparent between forward and lateral directions. We identified lower plantarflexion, hip flexion, and higher knee flexion, hip abduction angle during the lateral jump when compared with the forward jump. Albeit these differences, during both jumps, participants were in a knee adducted position and presented a similar range of motion at the knee and ankle joints, but a significantly lower hip excursion during the forward direction. These biomechanical patterns suggest distinct neuromuscular demands for each direction, while not necessarily placing participants at increased risk for LE injury. Previous research has primarily used a drop vertical jump to assess injury risk (10,13,17,19,20,31). However, this task, primarily vertical in nature (33), may not fully represent the demands of various jumps (e.g., vertical vs horizontal displacements) occurring in several activities (e.g., dance) and the mechanisms of dynamic postural stability (25). Recently, Taylor and colleagues (26) suggested that single-leg landings in different directions (forward and lateral) may better discriminate the biomechanical profile during landing. Injury prevention programs should consider the biomechanical patterns of each direction and adequately integrate them into training regiments.
To maintain the postural control during the lateral jump direction, due to the mediolateral displacement, higher hip and knee abductor moments were observed at PvGRF likely to provide lateral stabilization of the body and resist the external adduction loads. The multidirectional jumps are commonly performed in activities, such as dance, and sports. Often, injury prevention programs have primarily targeted a single jump direction (e.g., sagittal plane), and it is important to emphasize multidirectional tasks (e.g., multiple planes of motion) due to the interdependency between planes of motion during jump-landing activities. The lateral jump-landing direction has been perceived by the participants as more challenging (25). Anecdotally, we observed, and our participants reported, that the lateral jump-landing direction was perceived as the most difficult to perform. This suggests that the lateral jump-landing direction (e.g., frontal plane) likely constrains the movement degrees of freedom due to the fact that participants may have less perception of the landing area and increase the center of mass oscillation in the frontal plane (medial–lateral control) (16,25). Therefore, a primary application is that prevention programs should incorporate exercises to develop appropriate frontal-plane neuromuscular responses during jump-landing tasks (25).
Overall, our results suggest that dancers tend to land with a unique biomechanical strategy that may be protective of LE injuries during jump landings. The PD strategies observed in the current study, such as more erect posture and increased forefoot strategy at IC, followed by a higher range of motion at the knee and ankle joints, as well as improved alignment of the distal LE segments, should be considered for implementation in prevention programs. These landing techniques foster adequate LE neuromuscular control with more than likely limited detriment to performance. Dancers’ strategy is achieved over years of training to improve the neuromuscular system in multiple planes of motion (i.e., sagittal and frontal). Further, we also found significant differences among jump-landing directions, specifically between the forward and the lateral jump directions, which are related to the neuromuscular demands of each direction. Given the peculiar ankle pattern and that it is paramount to the aesthetics and technique in dance, future investigation should focus on the intricate foot and ankle complex to get an insight into the dynamic control mechanism for dissipation of landing forces (e.g., multisegmented foot model). Future research should also focus on quantifying muscular activation, and the relative muscular contribution and its coordination, through computational modeling of multidirectional jump landings.
The authors thank Dr. Maria António Castro, Companhia Nacional de Bailado, and Filipa Rola for their collaboration and assistance with this study. Joao R. Vaz was funded by NIH P20GM109090.
There are no conflicts of interest associated with the authors of this study. The results of the present study do not constitute endorsement by the American College of Sports Medicine and are presented without fabrication, falsification, or data manipulation.
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