Jump-Landing Kinematics: Establishing Normative Ranges for Male and Female Athletes

Abstract

Context

Lower extremity (LE) joint kinematics during landing tasks are important predictors of injury risk and performance outcomes in athletes.

Objective

To establish sex-related differences and normative ranges for LE kinematics during the jump-landing task in a large cohort of healthy military service academy cadets.

Design

Cross-sectional study.

Setting

US Air Force, Military, and Naval academies.

Patients or Other Participants

A total of 5308 cadets comprising 2062 female (38.8%; age = 18.57 ± 0.79 years, height = 165.61 ± 6.5 cm, mass = 62.9 ± 7.95 kg) and 3246 male cadets (61.2%; age = 18.87 ± 0.97 years, height = 178.13 ± 7.13 cm, mass = 77.59 ± 12.4 kg) participated.

Main Outcome Measure(s)

Sex-related differences in LE kinematics during the jump-landing task were analyzed using an independent-samples t test. Mean differences (MDs) and Cohen d effect sizes were reported for interpretability. Normative ranges for hip- and knee-joint angles were established separately for male and female cadets at initial contact (IC) and 50% of the stance phase.

Results

Moderate effect sizes were observed for knee–external-rotation angles, for which female cadets had greater motion at IC and at 50% of stance (MDs = −3.91° and −5.04°, respectively; P < .001; d range, 0.49–0.53) than male cadets. Small effect sizes (d range, 0.20–0.39) were observed in several measures. At IC, female cadets demonstrated less knee flexion (MD = −1.8°; P < .001) and greater knee valgus (MD = −2.2°, P < .001). At 50% of stance, female cadets exhibited less knee and hip flexion (MDs = −4.1° and −4.6°, P < .001) and greater knee valgus and hip adduction (MDs = −3.2° and 1.8°, P < .001). Effect sizes for all other joint angles were trivial.

Conclusions

We established normative ranges for LE kinematics during the jump-landing task in a large cohort of healthy military service academy cadets entering their first year. Sex-related differences in LE kinematics were observed, highlighting the importance of considering sex as a factor in the evaluation of LE movement quality and management of injury risk.

Key Points

• Female cadets exhibited stiffer sagittal-plane and greater frontal-plane (knee-valgus and hip-adduction) movement strategies at initial contact and 50% of stance of the jump-landing task.

• Normative jump-landing kinematics may help guide rehabilitation and training strategies by providing a reference standard.

Lower extremity (LE) joint kinematics during landing tasks play a crucial and growing role in the prediction of injuries,^1–3 determination for return to sport among athletes,^4–10 and assessment of performance.¹¹ The bilateral jump-landing task (also known as the drop vertical jump), in particular, has emerged as a popular method for injury screening in athletes and in the military.^1,12–16 In addition, the jump-landing task is frequently included in return-to-sport or -duty testing batteries for common LE injuries.^5,8 However, although jump-landing tasks are important and popular, a critical research gap exists in establishing normative ranges for LE kinematics during these tasks, especially within athlete and military populations. The identification of normative ranges can provide valuable insights for implementing targeted and individualized injury-prevention programs, designing rehabilitation protocols, and optimizing performance.

In general, the availability of normative data for LE kinematics during functional movements remains limited, and these types of data are even more elusive in military personnel. In their 2014 systematic review, Fox et al aggregated lower limb kinematics in females during common athletic tasks, including the jump-landing task.¹⁷ However, the aggregated sample sizes used for determining normative ranges in their review were limited to between 40 and 199 individuals, potentially restricting the generalizability and comprehensiveness of “normal” human movement patterns. Previous researchers have shown a link between aberrant LE kinematics during landing tasks and increased injury risk, particularly in the knee and hip joints.^18–21 Furthermore, sex is widely recognized to play a pivotal role in influencing injury risk, with biomechanical differences between males and females frequently regarded to moderate this relationship.^20,22 The understanding of sex-related disparities in LE kinematics during functional movements is essential, as males and females often have distinct movement patterns and differing susceptibilities to LE injuries.²² This distinction was well exemplified in the systematic review by Cronström et al, who observed greater knee valgus in women than men at initial contact (IC), peak motion, and total excursion across a wide variety of functional movements, including the jump-landing task.²³ In addition, females have exhibited smaller sagittal-plane joint angles at the hip and knee compared with males in drop-vertical jump tasks in several studies.^24,25

The primary purpose of our study was to establish sex-related normative ranges for LE kinematics during the jump-landing task in a large cohort of healthy military service academy cadets on entry into their respective academies. The secondary purpose was to investigate the mean differences (MDs) in kinematics between female and male individuals. Consistent with previous researchers, we hypothesized that female individuals would exhibit smaller sagittal-plane and larger frontal- and transverse-plane joint angles at the hip and knee than their male counterparts.^23–25

METHODS

Participants

The study cohort comprised 5908 cadets (2279 female and 3629 male cadets) from 3 US service academies (US Air Force Academy, US Military Academy, and US Naval Academy). To be included in the study, cadets needed to meet the following criteria: (1) be in their freshman year at the time of enrollment in the study and (2) have no injuries that limited participation in the jump-landing task or LE strength tests. The institutional review board (IRB) of each service academy granted approval before the commencement of the parent study.^16,26 The parent study enrolled cadets between July 1, 2005, and July 1, 2009.^16,26 During their first summer of enrollment at their respective service academies, cadets underwent a baseline biomechanical assessment. The rationale for the timing of data collection was to establish baseline movement-pattern data before the start of the cadets’ physical training program at the service academies. This secondary analysis was approved by the IRB of the University of North Carolina at Chapel Hill (IRB 23-0313) and adhered to the Strengthening the Reporting of Observational Studies in Epidemiology guidelines, ensuring comprehensive reporting and transparency in the presentation of results, methodology, and data analysis (Figure 1). All participants provided written informed consent.²⁷

Figure 1.

Strengthening the Reporting of Observational Studies in Epidemiology flow diagram for enrollment.²⁷

Instrumentation

Joint angles were evaluated using the Flock of Birds (Ascension Technologies, Inc) electromagnetic motion-analysis system via The MotionMonitor software (Innovative Sports Training, Inc). The Flock of Birds system was used to assess the position and orientation of 3 electromagnetic tracking sensors, placed on the sacrum, femur, and tibia of participants. The system uses a standard-range transmitter, comprising 3 orthogonal coils, that emits a magnetic field within the designated space. These tracking sensors capture alterations in the electromagnetic flux within the transmitted field, enabling precise tracking of participants’ movements. Previous researchers have demonstrated the accuracy^28,29 and reliability^28,29 of this system, and it has been used in several biomechanics studies.^1,13,16,30 The sampling rate for kinematic data was set at 144 Hz. Ground reaction forces during the jump-landing task were collected using an in-ground, nonconductive force plate (model 4060-NC; Bertec Corp) at a sampling rate of 1440 Hz.

Procedures

Before data collection, participants completed a baseline questionnaire that collected age, sex, and LE injury history.

For the jump-landing task, participants jumped forward from a 30-cm-high box onto a force platform positioned at a point 50% of their height away from the box (Figure 2).²⁶ They performed a double-legged landing, with the dominant foot on the force plate and the nondominant foot on the floor. After landing on the force platform, participants performed a vertical jump for maximal height. Before the task, participants received instructions and practiced to familiarize themselves with the task requirements. A successful jump was characterized by landing with the entire foot of the dominant LE on the force plate, landing with the entire foot of the nondominant LE off the force plate, and completing the task smoothly. Lower extremity dominance was determined through self-report, based on which limb would be used for kicking a ball for maximum distance.

Figure 2.

Diagram of jump-landing task with example knee-flexion waveform.²⁶ Stance phase was defined by the time between initial contact (0%) and takeoff (100%). Discrete data were extracted from 0% and 50% of stance phase for each joint motion. A, Initial jump from a 30-cm-tall box to an in-ground force plate placed at a distance one-half the participant’s height. B, Landing phase from initial contact (0%) to 50% of stance. C, Propulsion and rebound jump phase from 50% of stance to takeoff (100%) and ending with a maximum vertical jump. Illustration created with BioRender.com.

When participants were prepared, electromagnetic tracking sensors were attached to the dominant LE. The sensors were placed on specific locations of the participants’ skin, including the superior sacrum, the lateral aspect of the distal one-third of the thigh (over the iliotibial band), and the anteromedial aspect of the proximal one-third of the tibia. In addition, 6 bony landmarks, including the medial and lateral epicondyles of the femur, medial and lateral malleoli of the ankle, and left and right anterior-superior iliac spines of the pelvis, were digitized using a stylus-mounted receiver. The positions of the medial and lateral malleoli and femoral epicondyles were used to determine the ankle- and knee-joint centers, respectively. The left and right anterior-superior iliac spines were digitized to determine the hip-joint center of rotation using the method of Bell et al.³¹ Each participant performed 3 successful trials of the jump-landing task.

The kinematic data were filtered using a fourth-order low-pass Butterworth filter with a cutoff frequency of 14.5 Hz. A global reference system was established, with the x-axis representing the anterior direction, the y-axis representing the left side of the participant, and the z-axis representing the superior direction. Lower extremity joint rotations were calculated using the Euler rotation method, specifically in the order of Y, X, Z. The y-axis corresponded to the flexion-extension axis, the x-axis corresponded to the abduction-adduction axis, and the z-axis corresponded to the internal-external–rotation axis. Hip-joint motion was defined as the movement of the femur relative to the pelvis, and knee-joint motion was defined as the movement of the tibia relative to the femur. The stance phase of the jump-landing task was defined as the time between IC and takeoff for the rebound jump. Initial contact was identified as the point when the vertical ground reaction force exceeded 10 N on landing on the force plate from the 30-cm-high platform. After IC, takeoff was characterized by a vertical ground reaction force of <10 N. Ensemble-averaged waveforms (0%–100%) were constructed for the kinematic data using custom scripts in MATLAB software (The MathWorks, Inc). Averaged 3-dimensional (3D) knee- and hip-joint angles were extracted at the time points of IC (0% of stance) and 50% of stance phase, with the latter point approximating the end of vertical descent (Figure 2).

Statistical Analysis

The data were prepared, processed, and analyzed using the scikit-learn and Statsmodels frameworks within Python (version 3.10.12; Python Software Foundation). Missingness accounted for <3% of the dataset; given the large sample size, proceeding with available-case analysis was determined to be reasonable.³² Kinematic variables of interest included sagittal (flexion/extension), frontal (abduction/adduction), and transverse (internal/external rotation) planes of motion for the knee and hip at IC and the instant of maximum knee flexion during the stance phase. Negative values for frontal-plane kinematics were interpreted as knee valgus and hip adduction, with zero representing the individual’s neutral position of the joint. Negative values for transverse-plane kinematics were interpreted as knee and hip external rotation, with zero representing the individual’s neutral position. Parametric statistical tests were determined to be appropriate based on confirmed graphical normality coupled with the sample size across all variables.

To assess sex-related differences in LE hip and knee motion, we conducted an independent-samples t test. The MDs and Cohen d effect sizes were calculated and reported for interpretability. The Cohen d effect size statistic provides a standardized measure of the magnitude of the difference between 2 group means, accounting for the variability within each group.³³ A commonly accepted guideline for interpreting Cohen d effect sizes follows: trivial (d < 0.2), small (d = 0.2), moderate (d = 0.5), and large (d = 0.8).³³ These thresholds aid in understanding the practical significance and effect of the observed differences between groups.^33,34 The analysis focused on joint angles at 2 key time points: IC and 50% of stance phase. Normative ranges for joint angles were established separately for males and females at the 10th, 25th, 50th, 75th, and 90th percentiles. The α level for all analyses was set at .01.

RESULTS

Of the 5908 cadets who initially enrolled in this investigation, 206 participants (3.5%) had previous anterior cruciate ligament (ACL) or other ligamentous injury resulting in surgery. These cadets were removed from this secondary analysis to reduce the risk of introducing confounding and bias, as cadets with a previous history of LE surgery may exhibit altered movement patterns.^1,9 In addition, 394 participants (6.7%) were missing demographic or jump-landing biomechanics data and were also removed from final analysis (Figure 1). The final cohort included 5308 cadets (3246 male and 2062 female cadets). Descriptive statistics of the final cohort were calculated to summarize the sample characteristics (Table 1). Jump-landing kinematics are presented in Figure 3 as ensemble-averaged waveforms (0%–100%) for the included sample.

Table 1.

Participant Characteristics

Characteristic	Male Cadets		Female Cadets
	Mean ± SD	95% CI	Mean ± SD	95% CI
Age, y	18.87 ± 0.97	18.84, 18.9	18.57 ± 0.79	18.54, 18.61
Height, cm	77.59 ± 12.4	77.17, 78.0	62.9 ± 7.95	62.56, 63.24
Mass, kg	178.13 ± 7.13	177.89, 178.37	165.61 ± 6.5	165.33, 165.88
Body mass index, kg/m2	24.39 ± 3.13	24.29, 24.5	22.91 ± 2.37	22.81, 23.01

Figure 3.

Ensemble averaged waveforms of all included cadets. Stance phase was defined by the time between initial contact (0%) and takeoff (100%). Discrete data were extracted from 0% and 50% of stance phase for each joint motion. Blue represents the male cadets, and magenta represents the female cadets. A, Knee flexion (+). B, Knee valgus (−)/varus (+). C, Knee external (−)/internal (+) rotation. D, Hip flexion (−)/extension (+). E, Hip adduction (−)/abduction (+). F, Hip external (−)/internal (+) rotation.

Normative reference joint angles are presented as raincloud plots for the 25th, 50th, and 75th percentiles for IC and at 50% of stance in Figures 4 and 5, and specific values are provided in Tables 2 and 3. Sex-related disparities in joint angles were evident during the jump-landing task at both IC and 50% of stance. The medians and interquartile ranges for sagittal-plane kinematics at both time points were higher in male cadets, and, conversely, frontal- and transverse-plane kinematics were higher in female cadets (Tables 2 and 3).

Figure 4.

Raincloud plots for jump-landing knee kinematics.³⁵ Knee flexion (+)/extension (−) at, A, initial contact, and, B, 50% of stance phase. Knee valgus (−)/varus (+) at, C, initial contact and, D, 50% of stance phase. Knee external (−)/internal (+) rotation at, E, initial contact and, F, 50% of stance phase.

Figure 5.

Raincloud plots for jump-landing hip kinematics.³⁵ Hip flexion (−)/extension (+) at, A, initial contact, and, B, 50% of stance phase. Hip adduction (−)/abduction (+) at, C, initial contact, and, D, 50% of stance phase. Hip external (−)/internal (+) rotation at, E, initial contact, and, F, 50% of stance phase.

Table 2.

Percentile Ranges for Knee Kinematics

Percentilea	Flexion		Valgus (−)/Varus (+)		External (−)/Internal (+) Rotation
	Initial Contact, °	50% Stance, °	Initial Contact, °	50% Stance, °	Initial Contact, °	50% Stance, °
Male cadets
10th	10.7	62.3	−5.5	−16.0	−9.3	−7.0
25th	15.1	70.8	−2.0	−9.8	−5.0	−0.3
50th	20.3	80.5	1.9	−3.2	−0.1	6.4
75th	24.9	90.3	5.7	3.8	4.5	13.3
90th	29.9	100.2	9.3	9.5	9.1	19.4
Female cadets
10th	9.3	60.2	−7.4	−18.1	−13.6	−11.4
25th	13.7	68.1	−4.0	−12.4	−9.3	−5.3
50th	18.2	76.2	−0.4	−6.2	−4.2	1.4
75th	23.0	85.1	3.3	−0.1	1.2	8.0
90th	27.7	94.0	6.6	5.2	5.7	13.4

^a The 50th percentile is the median; the 25th and 75th percentiles are the lower and upper bounds of the interquartile range.

Table 3.

Percentile Ranges for Hip Kinematics

Percentilea	Flexion		Adduction (−)/Abduction (+)		External (−)/Internal (+) Rotation
	Initial Contact, °	50% Stance, °	Initial Contact, °	50% Stance, °	Initial Contact, °	50% Stance, °
Male cadets
10th	17.2	38.6	−18.4	−18.4	−13.5	−12.2
25th	23.3	51.5	−14.8	−12.6	−8.7	−6.4
50th	30.5	65.8	−10.8	−6.5	−3.2	0.3
75th	38.0	80.8	−6.5	−0.9	2.0	7.2
90th	44.9	93.2	−1.7	4.4	6.9	13.8
Female cadets
10th	17.6	36.8	−17.8	−16.0	−12.6	−11.3
25th	23.4	47.3	−14.1	−10.6	−7.5	−5.7
50th	30.1	60.5	−9.6	−4.8	−2.4	0.3
75th	36.9	74.7	−4.9	0.8	2.2	6.2
90th	43.6	87.6	−0.8	5.5	6.8	11.8

^a The 50th percentile is the median; the 25th and 75th percentiles are the lower and upper bounds of the interquartile range.

The resultant comparisons for the MD at IC (Table 4) demonstrated that female cadets exhibited less knee flexion (MD = −1.77°, P < .001, d = 0.24). In addition, female participants displayed greater knee valgus (MD = −2.23°, P < .001, d = 0.39) and knee external rotation (MD = −3.91°, P < .001, d = 0.53). However, the effect sizes for hip adduction (d = −0.16), hip flexion (d = 0.05), and hip rotation (d = −0.08) were trivial, indicating minimal differences between male and female cadets in these variables at IC.

Table 4.

Jump-Landing Kinematics, Mean Differences by Sex

	Male Cadets		Female Cadets		Mean Difference	P Valuea	Cohen d Effect Size (95% CI)
	Mean ± SD	95% CI	Mean ± SD	95% CI
Knee flexion (+)/extension (−), °
Initial contact	20.23 ± 7.58	19.97, 20.49	18.46 ± 7.31	18.15, 18.78	−1.77	<.001b	0.24 (0.18, 0.29)
50% stance	80.89 ± 15.01	80.37, 81.40	76.79 ± 13.51	76.21, 77.38	−4.09	<.001b	0.28 (0.23, 0.34)
Knee varus (+)/valgus (−), °
Initial contact	1.82 ± 5.83	1.62, 2.02	−0.41 ± 5.53	−0.65, −0.17	−2.23	<.001b	0.39 (0.33, 0.45)
50% stance	−3.06 ± 9.95	−3.40, −2.72	−6.3 ± 9.12	−6.69, −5.91	−3.24	<.001b	0.34 (0.28, 0.39)
Knee internal (+)/external (−) rotation, °
Initial contact	−0.13 ± 7.22	−0.38, 0.12	−4.04 ± 7.55	−4.37, −3.72	−3.91	<.001b	0.53 (0.48, 0.59)
50% stance	6.41 ± 10.51	6.05, 6.77	1.37 ± 9.88	0.94, 1.79	−5.04	<.001b	0.49 (0.43, 0.55)
Hip flexion (+)/extension (−), °
Initial contact	30.89 ± 10.92	30.52, 31.27	30.37 ± 10.22	29.93, 30.81	−0.52	<.001b	0.05 (−0.01, 0.10)
50% stance	66.02 ± 20.82	65.30, 66.73	61.39 ± 19.73	60.54, 62.25	−4.62	<.001b	0.23 (0.17, 0.28)
Hip adduction (+)/abduction (−), °
Initial contact	−10.39 ± 6.62	−10.62, −10.17	−9.34 ± 6.69	−9.63, −9.05	1.06	<.001b	−0.16 (−0.21, −0.10)
50% stance	−6.79 ± 8.94	−7.11, −6.50	−5.05 ± 8.48	−5.42, −4.69	1.75	<.001b	−0.20 (−0.25, −0.14)
Hip internal (+)/external (−) rotation, °
Initial contact	−3.32 ± 8.03	−3.61, −3.04	−2.71 ± 7.74	−3.03, −2.36	0.62	.01	−0.08 (−0.13, −0.02)
50% stance	0.61 ± 10.13	0.26, 0.96	0.34 ± 9.15	−0.05, 0.74	−0.27	.33	0.03 (−0.03, 0.08)

^a Calculated using independent t tests.

^b Indicates difference (P < .01).

The same comparisons at 50% of the stance phase revealed that female cadets exhibited less knee flexion (MD = −4.09°, P < .001, d = 0.28) and hip flexion (MD = −4.62°, P < .001, d = 0.23; Table 4). In addition, female participants displayed larger knee valgus (MD = −3.24°, P < .001, d = 0.34), knee external rotation (MD = −5.04°, P < .001, d = 0.49), and hip adduction (MD = 1.75°, P < .001, d = −0.20). Notably, no disparities were identified for hip rotation (MD = −0.27°, P = .33, d = 0.03).

DISCUSSION

Female and male athletes often exhibit distinct movement patterns, underscoring the necessity for sex-specific criteria within the military cadet population.^18,23–25 The purpose of this study was to address this need by pursuing 2 main objectives: establish normative ranges for LE kinematics related to sex and explore the inherent differences in kinematics during the bilateral jump-landing task between female and male cadets. Our investigation yielded a comprehensive normative dataset that identified sex-related variations in joint angles. These variations were seen at IC for knee-flexion, knee-valgus, and knee–external-rotation angles, as well as at 50% of stance for knee-flexion, hip-flexion, knee-valgus, knee–external-rotation, and hip-adduction angles. These observations align with those reported in previous literature, including findings by Teyhen et al, who found sex differences in other movement tasks and emphasized the importance of using sex-specific normative data for accurate assessment and intervention strategies within military populations.³⁶

Our findings, derived from the largest cohort to date (N = 5308 participants), reinforce the presence of sex differences in LE joint kinematics during the jump-landing task. These differences are particularly pronounced at IC, in which female cadets exhibited less knee flexion along with greater knee valgus and knee external rotation compared with their male counterparts. This trend continued for the peak angles at 50% of stance, with female cadets displaying less knee and hip flexion and greater knee valgus, knee external rotation, and hip adduction. These findings support our hypothesis that female cadets would exhibit greater frontal- and transverse-plane kinematics than their male counterparts. Notably, although statistically significant, these differences may not be clinically meaningful given the predominance of small effect sizes (d range, 0.05–0.39), with knee external rotation alone demonstrating a moderate effect at both IC and 50% of stance (d = 0.53 and 0.49, respectively).

This study builds on previous work in which researchers tried to establish normative ranges and cross-sectional differences between males and females for jump-landing tasks.^17,23 The jump-landing task used in this study has the same setup as that of the Landing Error Scoring System and is regularly used in the research, clinical, and performance settings.²⁶ This work takes on added importance as 3D motion analysis gains momentum beyond academic biomechanical laboratories, driven in part by accessible, portable, markerless motion-capture solutions.³⁷ These solutions make the establishment of normative 3D kinematic ranges within populations of young adults, such as military service academy cadets, increasingly pivotal.

Previous researchers have highlighted the importance of assessing and addressing LE biomechanics in athletes for injury prevention and performance enhancement.³⁸ Our study adds to the existing literature by providing normative reference ranges for hip- and knee-joint angles during the jump-landing task, which can be used to help identify individuals displaying aberrant movement patterns. Similar to our study, Fox et al, in a systematic review, examined LE kinematic data from jump-landing tasks in healthy female athletes (range, 40–199 athletes) within a similar age group.¹⁷ Notably, their findings largely echo ours, aside from disparities in the transverse plane that may stem from variances in data-collection techniques.¹⁷

One objective of our study was to ensure transparency of the data. Therefore, we have included raincloud plots (Figures 4 and 5) and tables that display percentile ranges (Tables 3 and 4) to allow for the examination of the full spectrum of normal observations. Our study does not establish causality or indicate that extremes of normal kinematics are directly associated with a higher risk of injury. However, by quantifying jump-landing kinematics in individuals, we aimed to provide practitioners and clinicians with valuable data that encompass the full range of what is considered “normal” human movement. This information, coupled with clinical judgment and risk factors identified in prospective studies, empowers practitioners to effectively assess and address jump-landing kinematics in athletes and patients.^{2,3,12,13,21,26} Our study does not aim to determine optimal kinematics; instead, it presents a representation of both average and extreme movement patterns exhibited by military cadets.

Sex-related differences in LE kinematics during jump-landing tasks have been implicated as potential predictors of injury risk and performance outcomes in athletes.^18–21 Previous researchers have shown that females exhibit larger knee-valgus and smaller knee-flexion angles during landing tasks compared with males, which is consistent with our findings.^{17,18,23–25} In the context of these findings, it is noteworthy that Cronström et al reported similar MDs in knee valgus at IC (1.02°) and at peak value (3.66°) in their systematic review.²³ These increased frontal- and transverse-plane motions at the hip and knee have been identified as factors contributing to the risk of ACL injuries.^18–21 However, our sex-related differences had mostly small effect sizes, and the predictive potential of these movements in ACL injury remains contentious. Recent prospective investigations have cast doubt on the direct predictability of ACL injury solely based on altered jump-landing movement profiles, particularly jump-landing knee valgus, even in large and varied populations.^39–41 In a systematic review and meta-analysis, Cronström et al further elucidated that knee-abduction kinematics and kinetics during weight-bearing activities may not consistently predict initial ACL injury.¹⁹ The complex nature of predicting first-time acute injuries persists, and although the ability of jump-landing kinematics to predict initial ACL injury is debated, growing evidence suggests that altered kinematics and coordination persist after injury and may serve as indicators for reinjury.^20,42–44

Moreover, our study extends beyond ACL injuries. Although the bulk of extant literature centers on ACL injuries, other injury types merit exploration. Notably, our findings carry implications for identifying altered movement profiles for various injuries, such as bone-stress injury,^12,14 patellofemoral pain,^13,16 and patellar tendinopathy.⁴⁵ This broader perspective underscores the multifaceted effect of jump-landing kinematics on different injury types and emphasizes the importance of considering a comprehensive range of potential injury outcomes when interpreting and applying these findings.

Lastly, our study establishes normative ranges for discrete variables of interest during the jump-landing task that may be useful for mass movement screenings.^12–16,36 Although female cadets exhibited different hip and knee kinematics from those of their male counterparts, the extent of these differences observed in both our study and similar research does not seem substantial.^23–25 Simply labeling a movement pattern as potentially harmful solely based on direct comparisons between sexes may not be appropriate. Instead, we propose looking at sex-specific values based on the percentile rankings and classifying movement patterns as potentially harmful when the value is below the 25th or above the 75th percentile (Tables 2 and 3). Our findings can help us understand potentially harmful movement. The following is a specific scenario in which this can be applied. Previous researchers have indicated that stiffer knee flexion during jump-landing tasks is associated with future ACL injury.^2,21 A female athlete presenting with <13° of knee flexion at IC, <68° of knee flexion at 50% of stance, and <47° of hip flexion at 50% of stance (<25th percentile; Tables 2 and 3) may be categorized as exhibiting a “stiff” and “erect” type of potentially harmful movement pattern, which has been associated with ACL injury risk.^3,21,46 Comparable assessments could be extended to the other joints, planes, and time points. By leveraging these normative ranges, practitioners and researchers can devise targeted interventions to modify movement patterns, which may reduce injury risk and enhance performance for both male and female athletes.^38,47 To enhance the scope of these findings, we recommend future studies be done to examine other domains, including kinetics at discrete time points, mechanics throughout the full landing time series,⁴⁸ joint and limb coordination, and measures of linear and nonlinear variability of these movement tasks.^49,50

Our study had limitations. This cross-sectional investigation was aimed at establishing normative ranges and comparing discrete values at specific time points between sexes. A strength of our approach was that these discrete values are commonly assessed in the literature as well as clinically and are relatively easy to extract. A limitation was that it did not capture the full picture of movement across the time series of the landing phase of the jump-landing task.⁴⁸ Although time intensive and requiring greater computational demand, future research could be done to explore establishing normative ranges for the full time series of the relevant task, which is outside the scope of this paper. In addition, we investigated the joint kinematics of only the dominant side for each participant, so the presentation of limb symmetry was not possible. A limitation associated with the Flock of Birds system was the necessity of small cables attached to the sensors on the cadets during the jump-landing task. Despite our efforts to minimize interference from the cables, their presence could have introduced constraints, potentially altering natural movement patterns.

Further research is necessary to investigate whether a causal relationship exists between extreme jump-landing kinematics (either restricted/stiff or excessive motion) and the risk of initial injury, reinjury, or performance outcomes in real-world scenarios. Promising evidence suggests that jump-landing tasks may aid in guiding decisions regarding return to sport or duty and identifying at-risk populations for secondary injuries.^5,20 Future studies should be done to explore this potential utility in depth, drawing on previous work that has demonstrated its effectiveness^5,7 and its relevance in identifying vulnerable groups susceptible to secondary injuries.^1,9,51

More work is needed to examine normative ranges for jump-landing task mechanics for specific sport specialization and of different age groups. Additional avenues for investigation include the following: (1) validating newer low-cost approaches to assessing 3D motion capture that could be readily available for practitioners; (2) determining how jump-landing kinematics change at different time points after various injury types; and (3) determining jump-landing movement patterns as predictive or moderating factors, or both, for secondary or recurrent LE injury risk.³⁷

CONCLUSIONS

We aimed to establish normative hip and knee kinematic values for key time points in the jump-landing task, using the largest representative sample of physically active individuals to date. Although female cadets demonstrated distinctive hip and knee kinematics compared with their male counterparts, the magnitude of these differences did not appear to be large. Thus, the movement patterns of either sex should not be considered potentially harmful for simple comparison. Rather, potentially harmful movement patterns could be better determined by using percentile ranges. These insights carry important implications for injury prevention and performance enhancement, as they not only identify deviations in movement patterns but also lay the groundwork for tailored intervention strategies.