Utility of Kinetic and Kinematic Jumping and Landing Variables as Predictors of Injury Risk: A Systematic Review

Jump-landing assessments provide a means to quantify an individual’s ability to attenuate ground reaction forces, generate lower limb explosive power and maintain joint alignment. In order to identify risk factors that can be targeted through appropriate training interventions, it is necessary to establish which (scalar) objective kinetic, kinematic, and performance measures are most associated with lower-extremity injury. Online searches of MEDLINE, SCOPUS, EBSCOHost, SPORTDiscus and PubMed databases were completed for all articles published before March 2020 in accordance with PRISMA guidelines. 40 articles investigating nine jump-landing assessments were included in this review. The 79% of studies using drop jump (n = 14) observed an association with future injury, while only 8% of countermovement jump studies (n = 13) observed an association with injury risk. The 57% of studies using unilateral assessments found associations with risk of injury (n = 14). Studies using performance measures (jump height/distance) as outcome measure were only associated with injury risk in 30% of cases. However, those using kinetic and/or kinematic analyses (knee abduction moment, knee valgus angle, knee separation distance, peak ground reaction force) found associations with injury in 89% of studies. The landing element of jump-landing assessments appears to be superior for identifying individuals at greater risk of injury; likely due to a closer representation of the injury mechanism. Consequently, jump-landing assessments that involve attenuation of impact forces such as the drop jump appear most suited for this purpose but should involve assessment of frontal plane knee motion and ground reaction forces.


Introduction
Lower extremity injuries account for a large proportion of time loss injuries in sports people and professional service men and women, with the knee and ankle being the primary site in many of these cases [45,76,80]. Knee and ankle injuries primarily occur within passive tissue such as ligament and tendon, which in part may be due to altered neuromuscular control resulting in undesirable magnitudes of force being transmitted to these structures rather than attenuated by the muscle-tendon unit [11]. A large proportion of these injuries have non-contact mechanisms, which are predominantly a result of modifiable risk factors that can be corrected through appropriate training [45,65,76].
A systems-based approach to injury prevention which highlights the need to identify mechanisms of injury and risk factors for injury in order to tailor training interventions towards addressing these risk factors has been previously proposed [13,72,81]. Through highlighting the mechanism of injury, practitioners can design specific movement challenges for athletes to complete to assess an individual's level of risk. Cadaver simulations and match video analysis studies illustrate that stiff landings with aberrant movement strategies such as frontal plane medial knee displacement place excessive strain on passive joint restraints such as anterior cruciate ligament leading to tissue failure [7,49,86]. Recent commentary has suggested that using screening assessments to predict future injury is not a viable approach to support injury prevention strategies [6]. However, analysis of the competency of future injured athletes in executing a movement challenge can be used to establish performance traits of high-risk individuals. To this end, since evidence highlights landing as a particularly common mechanism of injury [41,42,49,86], numerous jump-landing assessments have been designed and investigated with respect to their ability to detect individuals at greater risk of injury [21,39,68,75].
For a test to be useful, it needs to have good sensitivity in accurately identifying those at greater risk of injury versus those who are not, i.e. it needs to maximise the true positives while minimizing the number of false positives. Previous research has suggested performance markers can identify injury risk from a hop for distance test [36], a standing broad jump [78] and a countermovement jump [84], while others have shown that kinetic and kinematic markers during rebound jump tasks can be significant predictors of injury [39,54,83].
These assessments range in complexity from simply measuring jump heights and distances during these tasks to full-body, 3-dimensional motion analysis and consequently have a variety of cost and time implications.
Existing systematic reviews assessing the utility of jumplanding assessments have been limited in their scope so as to only address subjective assessments for a single type of injury such as anterior cruciate injury (ACL) rupture [27], have solely focused upon kinematic variables that can be assessed in a clinical setting [1] or have assessed the utility of tests in distinguishing between injured people after the fact [37]. None of these reviews required assessment methods to be validated by prospective cohort studies. Retrospective injury surveillance is undermined by the tendency for recall bias and misreporting [30]. Additionally, studies using a design whereby previously injured participants are screened to identify differences with individuals who have suffered no injury are flawed because it is not known whether any kinematic differences that present are a result of, or the cause of the injury.
Simple assessments of jumping performance (e.g. height or distance) have provided conflicting findings in terms of their association with injury risk. Gabbett and Domrow [31] reported that rugby league players with a reduced countermovement jump height were at greater risk of injury than those with better vertical jumping ability. In contrast, Visnes et al. [84] found that greater vertical jumping ability increased the likelihood of overuse knee injury. Similarly, Hewett et al. [39] found that knee abduction moment during a drop jump landing could predict ACL injury with 78% sensitivity and 73% specificity. However, Krosshaug et al. [50,51] found no significant relationship between knee abduction moment or medial knee displacement and risk of a first ACL injury. Though ACL injuries are one of the most severe timeloss lower extremity injuries, they are also relatively infrequent [45]. Practitioners would be well advised to screen for other more common, albeit less severe, knee and ankle injuries too since their mechanism of injury has also been linked to aberrant landings [10,24,46,56,85].
Several jump-landing screening protocols have been utilised to identify risk factors for lower limb injury in athletes. The body of literature has investigated numerous jumping tasks and measured a breadth of variables to create a considerable body of knowledge. In order to progress this area of research, a consensus is required on the most valid screening protocols and measures that are currently available and to highlight appropriate avenues for future research. The breadth of jumping tasks, variables, methods of measurement and populations that have been studied have created controversy and inconsistency that makes synthesis of the findings challenging for a practitioner. Therefore, a study that systematically reviews and critically analyzes the currently available prospective evidence regarding the utility of kinetic and kinematic jumping and landing variables as injury risk factors in athletic populations seems to be warranted in order to shed light regarding this issue.

Literature Search
This review was conducted in accordance with the PRISMA guidelines [59] and registered in the PROSPERO register of systematic reviews (CRD42020169776). A literature search was performed to retrieve articles using jump-landing assessments to prospectively identify biomechanical risk factors for lower extremity injury in athletes. The following search terms were entered into PubMed, MEDLINE, EBSCOHost, SPORTDiscus and SCOPUS online databases on 13th March 2020 without any publishing date restrictions: "injur*", "jump*" and "prospective". All terms were searched for in the title and/or abstract of articles. The search outcomes were subjected to a preliminary screening of their title and abstract to remove duplicates, clearly irrelevant studies, non-English language publications and studies utilising non-human participants.

Literature Selection
The full text of the remaining articles were reviewed for final inclusion in the review by the first author based upon the following inclusion criteria: (1) a full text of an article was available, excluding abstract only articles and conference presentations; (2) a unilateral or bilateral baseline jump-landing assessment must have been performed and assessed in isolation for any relationship to injury risk; studies using jump assessments to contribute to a composite score for a testing battery and that did not report statistics for the jump assessment alone were excluded; (3) prospective injury surveillance monitoring the frequency of at least one lower-extremity injury must have been conducted; (4) participants were required to be free of injury at the time of the study commencing with no known history of the injury of interest prior to engaging in the study; (5) only objective assessment tools to measure jump-landing performance were permitted, with any subjective assessments involving an assessor rating of performance being excluded; (6) studies needed to present statistics indicating injury risk. Studies were included if they reported hazard ratios, risk ratios or odds ratios with sensitivity and specificity also reported in cases where it was provided. Where studies did not report any of these ratios, they were excluded unless they provided sufficient information (specificity, sensitivity, numbers of injured and uninjured participants, cut-off value) to calculate an odds ratio ± 95% confidence interval using the method of Altman [2].

Methodological Quality Assessment
All included studies were reviewed for methodological quality and risk of assessment using a modified version of the Cochrane Group on Screening Diagnostic Test Methodology quality assessment tool as previously utilised by Fox et al. [27]. Two reviewers, including the first author screened the included studies against the 11 criteria that were previously deemed most relevant to prospective cohort studies. In the event of disagreement, discussion with a third reviewer was conducted and a decision reached by vote.

Data Extraction and Analysis
The remaining articles were screened and tabulated to collate the following information: 1. Which jump-landing assessment was deployed in the study? 2. Did analysis of the test involve kinetic and/or kinematic methods and what equipment and sampling rate was used? 3. Which variables were assessed and reported? 4. What was the age group of the study cohort? 5. What was the sample size of the study cohort? 6. What was the duration of follow-up injury surveillance? 7. Which injuries were investigated? 8. How many injuries were reported during the study? 9. Key findings relating to injury risk identification 10. Was a link between performance in the test and injury reported (yes/no)? 11. Were predictive statistics provided or sufficient information to calculate this post hoc?

Search Findings and Study Selection
The online database searches returned 999 results. A summary of the review process and screening strategy is shown in Fig. 1. Duplicates within each database's results were removed to leave 798 papers. The preliminary search of the titles and abstracts removed 719 articles due to the irrelevance of the study, language of publication, and use of nonhuman participants. The full texts of the remaining 79 studies were screened for eligibility; a further 45 were removed due to insufficient statistical information (n = 13), use of a subjective jump screen (n = 5), absence of any baseline jump assessment (n = 10), no prospective injury surveillance conducted (n = 11), or implementation of a training study (n = 5). The reference lists of the remaining 34 studies that met the inclusion criteria were screened for relevant articles that had been missed by the online database search. This yielded a further 12 studies of which 6 were excluded for insufficient statistical information leaving a final list of 40 studies to be included in the systematic review ( Table 1). The 40 articles revealed that nine jump-landing assessments had been investigated in prospective cohort studies to determine injury risk factors. The prospective injury surveillance period following initial baseline screening varied from eight weeks to six years, using sample sizes ranging from 43 up to 3893. There was a bias towards studies investigating injuries in younger populations, with 29 of the 40 studies included using populations with a mean age of < 20 years. A relatively even split of studies examined female (n = 15) and male (n = 14) populations while 11 studies examined mixedsex samples. Most papers investigated a broad but loosely defined range of injuries (all injuries, n = 12; all lower extremity injuries, n = 9). Knee injuries were investigated by 13 studies (all knee, n = 1; patellofemoral pain syndrome, n = 5; non-contact ACL rupture, n = 7) while ankle injuries were only investigated in 4 studies. Muscular strains and medial tibial stress syndrome were the focus of one study each. Research studies were heavily biased towards acute injuries (n = 33) rather than chronic onset injuries (n = 7).

Study Methodology Screening Outcomes
All studies received a rating of five or greater from the methodological screening tool and the median and mode score was eight out of a possible 11. There was 97.7% agreement (κ = 0.948; P < 0.001) between the first two reviewers with nine instances of disagreement. There was 100% agreement for eight of the 11 criteria. Of the nine disagreements, four Training studies (5) No baseline jump assessment (10) No injury surveillance (11) Subjective jump assessment (5) Abstract only (1) Insufficient statistical analysis (19)  were resolved through discussion between reviewer one and reviewer two, whilst a third reviewer was required to resolve the remaining five instances of disagreement. No study was excluded from the review on the basis of the screening outcome. A summary of the methodological assessment for all studies included in the review is available in Supplementary  Table 1.

Screening Methods Identified
A mixture of vertical (drop jump, countermovement jump, squat jump, loaded countermovement jump, single leg drop jump, single-leg countermovement jump) and horizontal (standing broad jump, single-leg hop for distance, hop and stick) unilateral and bilateral screening assessments were identified utilising performance measures and/or kinetic and/ or kinematic analysis. The most frequently investigated jump landing assessments were the bilateral countermovement jump and drop jump (Fig. 2). From the bilateral assessments, drop jump had the greatest proportion of studies showing a relationship with injury (79%; 14 studies). There were fewer studies investigating the individual unilateral assessments (n = 14) though when pooled, unilateral assessments (singleleg drop jump, single-leg countermovement jump, single-leg hop for distance and hop and stick) were associated with injury risk in 57% of studies. Across the 40 studies, screening tests were examined 49 times owing to the fact that some studies utilised more than one assessment. Performance metrics (jump/hop height, jump/hop distance, reactive strength index) were outcome measures for 33 of these test investigations; only 9 of those 33 (30.3%) reported an association between the outcome of the test and risk of lower extremity injury. Within these 9 test investigations, the direction of causality between injury and performance was inconsistent with seven investigations showing poorer performance in a single leg-hop for distance [3,18,36], single-leg countermovement jump [38], drop jump [73] or standing broad jump [78,79] was associated with greater risk of injury and two showing greater performance in a single leg hop for distance [16] or countermovement jump [84] elevated injury risk. Tests utilising kinetic and/or kinematic assessment of movement were administered 18 times of which 16 found an association with injury (89%). Of the 24 test investigations demonstrating an association with injury risk, 50% utilised a drop jump. From test investigations identifying a significant association with injury risk, those using an assessment that did not involve a rebound jump found associations with landing forces rather than elements of the jump take-off [23,75,83].

Drop Jump
Drop jump was the most frequently investigated jump assessment. One study failed to report the drop height used [60] while the remaining studies all used a 30 cm (12 inch) drop height [14,15,35,39,44,51,53,54,61,64,66,67] and a single study used a drop height of 40 cm [73]. Boling et al. [14], Boling et al. [15] and Goetschius et al. [35] included the jump technique used in the Landing Error Scoring System (LESS) [68] which involves jumping forward 50% of standing height from the top of the box, but then used objective assessment techniques rather than using the subjective screening criteria of the LESS. All studies collected kinematic data and 11 of these 14 analysed the association of kinematic variables with risk of injury without the addition of kinetic data. Of these 11 studies, nine reported significant associations with injury risk [14,15,44,53,54,64,66,67,73] while two studies found no association with risk of injury [51,60]. Only seven of the 14 studies utilising drop included kinetic data in their assessment [14,35,39,51,53,54,61]. This was usually processed along with kinematic data to calculate joint moments; the only ground reaction force variable analysed by any drop jump study was peak vertical ground reaction force (Fig. 3). Studies using the drop jump as an injury screening tool have predominantly investigated the relationship of these variables with knee injury (patellofemoral pain n = 4 studies; ACL rupture n = 6 studies) while four studies examined the risk of suffering all lower extremity injuries [60,64,67,66].
Six of the seven drop jump studies gathering kinetic variables utilised 3D motion-analysis technology alongside ground reaction force data. Goetschius et al. [35] utilised two-dimensional video analysis in both the frontal and sagittal plane to predict knee abduction moment using an algorithm devised and validated by Myer et al. [62]. Peak knee abduction moment was analysed by five studies with two finding an association with risk of suffering a knee injury [39,61] and three failing to observe such a relationship [35,51,54]. Myer et al. [61] identified a knee abduction moment of > 15 N·m as the threshold that caused a two-fold increase in the risk of sustaining patellofemoral pain (OR = 2.34; 95% CI 1.07-4.65). Meanwhile, Hewett et al. [39] identified that a cut-off point of > 25.25 N·m could detect female athletes who would suffer an ACL rupture with 78% sensitivity and 73% specificity (OR = 9.38; 95% CI 1.89-46.58). Peak vertical ground reaction force was investigated by 3 studies, with Leppänen et al. [54] (HR = 1.26; 95% CI 1.09-1.45) and Boling et al. [14] (RR = 3.57; 95% CI 1.26-10.0) finding a significant association with risk of ACL rupture and patellofemoral pain syndrome respectively (Fig. 3). However, Krosshaug [51] observed no association between peak vertical ground reaction force and risk of ACL rupture. Boling et al. [14] observed a significant association between knee Three dimensional motion-analysis was employed by six of the 11 drop jump studies performing kinematic analysis [14,15,51,53,54,64] while three studies used frontal plane two-dimensional video analysis [44,66,67] and two utilised a force plate to derive jump height [60,73].
Analysis of associations between kinematic variables and risk of lower extremity injury were conducted by 11 of the 14 drop jump studies (Fig. 4). Variables that indicate frontal plane knee motion such as knee valgus angle, medial knee displacement, valgus displacement and normalized knee separation distance were analysed by seven  [44] showed that a frontal plane knee valgus displacement of greater than 10.6º increased the risk of patellofemoral pain syndrome 11-fold (OR = 11.60; 95% CI 2.93-45.89). Nilstad et al. [64] was the only study that observed that an increase in knee valgus angle decreased the risk of ankle injury (OR = 0.64, 95% CI 0.41-1.00). Neither Krosshaug et al. [51] or Leppanen et al. [54] observed a significant association between knee valgus at initial ground contact and the risk of suffering an ACL rupture. Peak knee flexion angle was investigated by three studies with two finding an association with risk of injury [14,54] and the remaining study seeing no association [51]. Leppanen et al. [54] observed an increase in risk of ACL injury for each 10º decrease in knee flexion during ground contact (HR =  Boling et al. [15] reported that females landing with less than 10° of hip abduction (OR = 1.86; 95% CI 1.06-3.26) were twice as likely to suffer patellofemoral pain while individuals landing with more than 10° of knee internal rotation were also more likely to suffer patellofemoral pain (OR = 1.71; 95% CI 1.08-2.73) than those with less than 10° of knee internal rotation. Leppanen et al. [53] was the only study to investigate hip flexion range of motion and found significant associations with risk of injury. A reduced range of motion during ground contact was associated with greater risk of ACL injury (HR = 1.64; 95% CI 1.01-2.62).

Other Bilateral Jump Assessments
The other bilateral assessments included countermovement jump, loaded countermovement jump and standing broad jump. All 12 studies that used a countermovement jump as a screening tool utilised jump height as the sole variable of interest and only a single study demonstrated a relationship with lower extremity injury risk [84]. Visnes et al. [84] reported a two-fold increase in risk of developing patellar tendonitis with each 1 cm increase in jump height above the asymptomatic group (OR = 2.09; 95% CI 1.03-4.25). All other studies investigating the relationship between countermovement jump height and risk of hamstring/groin strain [4], ankle sprain [5,77], all knee injuries [57], or lower extremity injury in general [25,29,31,48,55,70,82] failed to find a significant association (Fig. 5). Two studies investigated the relationship between squat jump height and lower-extremity injury risk with neither showing a relationship between lower extremity injury and jump height [4,29]. Two studies also used loaded countermovement jump protocols to assess injury risk [32,33]. Gabbett et al. [32] used incremental loading to determine peak power and found no relationship with future contact injury in rugby league players. Using a repeated jump protocol, Gastin et al. [33] found that shorter ground contact times (OR = 4.08; 95% CI 1.19-14.08) but not concentric or eccentric power was associated with increased risk of injury (Fig. 6). The only horizontal bilateral jump-land assessment identified by the search was a standing broad jump (Fig. 6). All five studies using a standing broad jump used either absolute jump distance or jump distance as a function of standing height as their outcome metric [16-18, 78, 79]. Of these five studies, only Taanila et al. [78] observed a relationship between absolute jump distance and risk of lower extremity musculoskeletal injury (HR = 1.6, 95% CI 1.2-2.0). Only one study investigating any of the bilateral jump assessments reported sensitivity and specificity. Brumitt et al. [18]  reported that a cut-off of value of < 80% of standing height could predict lower extremity injury with 58% sensitivity and 39% specificity (OR = 1.1; 95% CI 0.4-2.8).

Unilateral Jump Assessments
Four unilateral assessment tools were identified; two horizontal (single-leg hop for distance and hop and stick) and two vertical (single-leg drop jump and single-leg countermovement jump). Two studies investigated the relationship between landing variables in a hop and stick test. DuPrey et al. [23] found that backward hop time to stabilization was the only significant predictor of ACL injury risk in a multidirectional hop protocol (OR = 2.95; 95% CI 1.28-6.77) and a cut-off of > 1.19 s could identify individuals who would go on to sustain an ACL rupture with 89% sensitivity and 79% specificity. Read et al. [75] also used a hop and stick protocol whereby participants were required to hop 75% of their maximal single leg hop distance and then stabilise as rapidly as possible. The authors found no significant difference in peak vertical ground reaction force between participants who did and did not suffer a lower limb injury. The most widely investigated unilateral horizontal jump-land assessment was the single leg hop for distance. Of the 7 studies using this test, 3 observed a relationship with injury.
Brumitt et al. [16] showed that females with a single leg hop asymmetry of greater than 10% were 4.4 (95% CI 1.2-15.4) times more likely to suffer a foot or ankle injury. The authors also observed that males who could not hop more than 75% of their standing height were at significantly greater risk of thigh and knee injuries (OR = 3.3; 95% CI 1.1-10.0). In a follow up study, the same authors found no significant relationship between asymmetry and risk of thigh or knee injury and could only predict the injured athletes with 30% sensitivity and 76% specificity [18]. Goossens et al. [36] observed that participants who suffered a hamstring injury had a significantly shorter single hop distance and the risk of injury was 13% greater for every standard deviation decrease in hop length (OR = 1.13; 95% CI 1.03-1.23). The remaining four studies failed to observe any significant relationship between single leg hop distance or asymmetry between left and right leg and risk of lower extremity injury.
Two unilateral vertical assessment tools used in prospective cohort studies were identified; the single leg countermovement jump and single leg drop jump. Three studies applied a single-leg countermovement jump protocol prior to prospective injury surveillance [38,75,82]. Read et al. [75] observed that peak landing force asymmetry was a significant predictor of lower extremity injury in elite youth soccer players with a 10% increase in the risk of injury for  [38] observed that amateur soccer players had a ninefold increase in risk of suffering a non-contact ankle injury when lower limb power output was below 30 W/kg (OR = 9.20; 95% CI 1.13-75.09). Conversely, van Seters et al. [82] found no association between single-leg countermovement jump height and risk of lower extremity injury in dance students.
Two studies investigated the relationship between single-leg drop jump variables and the risk of lower extremity injury. Verrelst et al. [83] found a selection of 3D kinematic variables were significant predictors of injury. Thorax rotation in the transverse plane between first ground contact and maximal knee flexion was a significant predictor of exertional medial tibial pain (OR = 10.20; 95% CI 3.10-33.58) and a cut-off value of > 12.27° could identify the symptomatic group with 53% sensitivity and 89% specificity (Fig. 7). Similarly, thorax rotation during the concentric phase of the drop jump landing was significantly associated with greater risk of overuse injury (OR = 5.88; 95% CI 1. 77-19.52) and a cut-off point of > 13.24° predicted the injured participants with 39% sensitivity and 89% specificity. Hip internal rotation between ground contact and maximum knee flexion identified the injured participants with 76% sensitivity and 64% specificity when using a cut-off of > 8.93° (OR = 5.83; 95% CI 1.87-18.10). Furthermore, hip internal rotation during the concentric component of the ground contact phase was significantly associated with risk of injury (OR = 9.69; 95% CI 1.21-77.77); with a cut-off point of > 6.12° predictive of injury with 95% sensitivity and 32 specificity. Fransz et al. [28] analysed multiplanar time to stabilization in the second landing of a single leg drop jump and observed increased root mean square of mediolateral ground reaction force during the first 0.4 s after landing reduced risk of lateral ankle sprain by 40% (OR = 0.60; 95% CI 0.41-0.86). Further, increased mean resultant horizontal force 3.0-5.0 s after landing elevated injury risk by 57% (OR = 1.57; 95% CI 1.13-2.18).

Discussion
The aim of this review was to systematically summarise the existing body of literature relating to jump-landing assessments for the purposes of identifying risk factors for lower extremity injury in athletic populations. A consistent trend across all studies and jumping tasks was the poor association of performance outcome measures, such as jump height. Countermovement jump and drop jump were the most commonly investigated jump landing assessments for injury screening purposes with some kinetic and kinematic variables associated with a higher risk of injury in the latter. A variety of unilateral assessments were identified however each assessment was only used by a small number of studies.

Drop Jump
A drop jump requires large force attenuation and production within short timeframes (typically < 250 ms), underpinned by a number of strength parameters and neuromuscular recruitment strategies [69]. Dysfunction of these feedforward strategies are reported to be key lower extremity injury risk factors [74]. Acute traumatic injuries such as ACL rupture are reported to occur within the first 56 ms of ground contact [86]. Impact forces experienced during this time can exceed 2.5 × BW with an average loading rate of up to 60 BW/s [71]; feedforward strategies that prepare the neuromuscular system to attenuate force effectively could help to mitigate the risk of injury from such loading. Consequently, the drop jump provides a controlled environment to assess an athlete's capacity to tolerate high vertical loading whilst maintaining dynamic joint stability. A number of kinematic variables (knee internal rotation, valgus displacement, normalized knee separation distance, knee flexion angles) [14,15,44,54,66,67] (Fig. 4) and kinetic drop jump variables (knee abduction moment, knee flexion moment, peak vertical ground reaction force) [14,39,53,54,61] appear to be significantly associated with risk of lower-extremity injury.
In [35], used a clinic-based algorithm developed by Myer et al. [62] to predict knee abduction moment. The algorithm was shown to be ineffective at determining which athletes would go on to sustain an ACL rupture. However, it should be noted that the algorithm was developed based upon data from vertical drop jumps, but [35] utilised a drop jump technique that includes a horizontal element as is used in the LESS [68]. This subtle change in technique would have a substantial effect on the ground reaction forces, the kinetic chain and associated movement pattern as a result of the forward momentum of the body upon ground contact, which would likely invalidate a predictive tool based on data from a different movement pattern [20]. In [35], also had participants perform the assessment barefoot which in running gait studies has been demonstrated to change kinetics and kinematics and would not be representative of landing strategies used in gameplay when wearing footwear [22]. In [51], used a vertical drop jump and kinetic and kinematic analyses to screen 710 female soccer and handball players and following reanalysis of their data set found no relationship between any variable and risk of ACL injury [50]. Two of the three studies that did not observe any relationship with injury risk, focused on a single injury; ACL rupture [35,51]. The remaining study only used a performance measure as the predictor variable [60]. It is extremely unlikely that an athlete displaying aberrant landing mechanics is only at high-risk for an ACL injury but no other lower-extremity injuries. In [63], demonstrated that those at high risk of ACL injury might only represent the extreme end of a continuum and observed that a lower threshold for knee abduction moment was also predictive of those that went on to suffer from patellofemoral pain, an overuse injury. Consequently, a broader assessment of injury risk that encompasses acute and overuse injuries might be appropriate in future drop jump studies while non-standardization of protocols may lead to inconsistent results, and more standardized protocols should be adopted.

Other Bilateral Jump Assessments
Only a single study using a bilateral countermovement jump identified a relationship with injury risk [84]. Across the 15 studies identified for this review, the only variable investigated for a relationship to injury was jump height. In [84], observed that athletes with a greater vertical jump were more likely to develop patellar tendonitis. Both studies investigating the association between squat jump performance and injury risk used jump height as the sole outcome variable and neither observed an association with risk of injury.
Jump height as a risk factor for injury presents a paradox since jumping higher suggests greater lower limb strength and power which has frequently been shown to be advantageous in reducing injury incidence (see [52] for a review). However, it could be that greater jump height results in greater magnitudes of force and velocity to be attenuated upon landing which have both been shown to be injurious [40,71,74]. From this perspective, further investigation into landing variables utilising both kinetic and kinematic analysis and their associations with risk of lower extremity injury risk should be pursued. Recent research into countermovement jump performance has highlighted the importance in assessing the force application strategy that an individual deploys in order to achieve a given performance outcome [8,9,19,34,58]. These variables may provide greater insight into any potential relationship between countermovement jump performance and injury risk and subsequently further research is required in assessing these variables in prospective cohort studies.
Two studies conducted on samples of elite male Rugby League and Australian Football players used loaded countermovement jump protocols [32,33]. In [33], demonstrated that longer ground contact times were associated with a reduced risk of injury and reduced injury severity, while larger concentric power was also associated with reduced injury severity. These findings suggest that when external load is applied and forces placed on the system exceed the capacity of the kinetic chain to absorb force safely and utilise the stretch-shortening cycle, the ability to decelerate over a wider range of movement and longer time frames is a useful skill in order to reduce injury risk. Previous research from injury risk in drop jumps supports the notion that stiff landings can be injurious [54]. Conversely, from a performance perspective, stiffer landings with less compliance at the point of ground contact are essential in achieving high reactive strength scores [47]. Once again this highlights the limitations of assessing kinematic variables in isolation and emphasises that it is not necessarily that stiff landings are dangerous but rather how that stiffness is achieved which is important with respect to injury risk. In [32], also assessed the relationship between injury risk and loaded countermovement jump performance. Their study found no relationship between peak power and injury risk though it must be highlighted that the authors only addressed contact injuries that are also influenced by multiple external factors.
Standing broad jump was the only bilateral horizontal jump assessment returned by the search. Across the five studies investigating a relationship with injury, all five used jump distance as the only variable for analysis, with two studies observing a relationship with injury [78,79]. Both of these studies showed that military recruits were more likely to suffer a lower extremity injury during basic training if they had a standing broad jump distance of < 2 m. In contrast, the remaining three studies failed to observe a similar relationship, even when using different cut-off values [16][17][18]. As with countermovement jump height, the standing broad jump distance provided inconsistent relationships as an indicator of injury risk. Injuries to the lower extremity during horizontal movement tasks are more likely to occur during the landing phase rather than the propulsive phase of ground contact [49,53,54,86]. Consequently, it is likely that landing variables in the standing broad jump might offer more insight into injury risk than a surrogate indicator of take-off force. In light of these considerations, future research should investigate braking forces and 2D and 3D motion analysis of the lower limb during landing from a horizontal jump.

Unilateral Jump Assessments
Seven studies used the single leg hop for distance test of which three found a significant relationship with injury risk. In [36], showed that physical education students with a shorter single leg hop distance were at greater risk of hamstring strain. The single leg hop test requires participants to stick the landing and stabilize on the standing leg in order for a trial to be successful. Decelerating in a forward direction requires a large eccentric action by the quadriceps muscle group; concurrently, a counter moment must be produced by the hamstrings group to maintain knee joint stability. Individuals with weak hamstrings or large quadriceps-tohamstring strength imbalances might be incapable of producing sufficient force to counter large moments produced at the knee and subsequently hop distance must be shortened. In contrast, Brumitt et al. [16] found that an increased single leg hop distance increased risk of lower extremity injury in males. The remaining five studies investigating single leg hop for distance failed to observe a relationship between hop distance and risk of lower extremity injury [3,17,18,29,75]. These inconsistent trends serve to emphasise the limitations of solely using a performance outcome to identify high risk individuals. Brumitt et al. [16] observed that females with single leg hop distance asymmetry between legs of > 10% were at increased risk of lower extremity injury. No other study examined single leg hop distance asymmetry but asymmetry has been demonstrated to be an injury risk factor in a variety of biomotor tasks [12,26,43,74] and therefore warrants further investigation.
The hop and stick test is very similar to the single leg hop for distance; however, hop distance is constrained so the effort is submaximal. In [75], showed no relationship between peak vertical ground reaction force when landing and risk of non-contact lower extremity injury in male youth soccer players. In contrast, [23] found that time to stabilization when performing a backwards hop was a significant predictor of ACL injury risk. Due to only two studies meeting all inclusion criteria using this jump-landing assessment, and these studies investigated different variables in distinctly separate tasks, it is difficult to draw firm conclusions on the validity of these assessments for identifying individuals at high risk of injury.
Three studies investigated the relationship between single leg countermovement jump performance and lower extremity injury risk. In [82], assessed jump height and found no relationship with injury risk. In contrast, [38] showed that individuals with a relative peak power < 30 W/kg were 9.2 times more likely to incur a non-contact ankle injury than those above this threshold. This could potentially be due to the requirement for greater force attenuation as a result of greater jump height. Alternatively, this relationship could be a result of a bias in exposure as a result of the more powerful players experiencing more competitive match time. In [75], observed that vertical ground reaction force asymmetry was a significant predictor of lower extremity injury risk in elite male youth soccer players. Leg asymmetries have been suggested to preferentially load the stronger leg, which with repeated exposures may exceed its force tolerating capacity and result in injury [12]. Further research is required to determine what should be considered "normal" magnitudes of asymmetry in single leg countermovement jump landing forces.
In [83], found that trunk rotation and hip internal rotation during a single leg drop jump was associated with lower extremity overuse injury. These findings serve to emphasise the extra challenge of unilateral assessments, as the movement of body segments more distal from the ground have an influence on the stability of the lower limb as a result of the narrower base of support. Consequently, assessments using performance metrics will not likely detect these aberrant movement patterns and kinematic analysis of the whole body might be essential when using unilateral assessments to take account for control of the largest proportion of body mass; the trunk. In [28], was notably the only study to assess multi-planar ground reaction forces in a jump-landing assessment. They observed that greater medio-lateral forces immediately after ground contact reduced the risk of lateral ankle sprain in footballers. They speculated that the noninjured players made greater effort to stabilize in the frontal plane upon landing and this protected the ankle from injury. Single leg drop jump studies present interesting avenues for future research with promising prelimary results. However, with only two studies meeting the inclusion criteria for this review further research is required to fully understand the value of this assessment in determining an individual's risk of lower extremity injury.
The findings of the current review indicate that kinetic and kinematic variables collected during drop jump and unilateral assessments present the best opportunity for injury risk screening based upon the existing literature. However, further research is required to more clearly elucidate the influence of drop jump ground reaction force variables on the risk of suffering a lower extremity injury. All of the studies using unilateral assessments alongside kinetic or kinematic analyses that were included in the current review observed associations with injury risk, though the magnitude of these associations varied substantially. Due to the aforementioned limitations of performance metrics in predicting risk of injury, future research using unilateral jump performance and landing assessments should report limb asymmetries and analyse joint stability since these seem to be the primary advantages of unilateral assessments.
A limitation of the current review is the bias for authors to only report significant associations with injury. An inclusion criterion for the current study was the publication of predictive statistics (or sufficient data to calculate these post hoc). Many studies that were included in this review evaluated variables where there was no significant association with risk of injury but did not then report the associated statistics for these null findings. Finally, a meta-analysis of the findings of this review was not performed due to the variation in the reporting of the risk of injury across the 40 studies that were included. Studies reported a range of hazard ratios, risk ratios, and odds ratios and while these statistics are related they are not synonymous. Furthermore, in order to calculate a single standardised odds ratio for the purpose of a metaanalysis, the sensitivity, specificity and numbers of injured and uninjured participants are needed. Only six of the 40 studies included in this systematic review reported these statistical measures. It is recommended that future prospective injury screening studies report these statistics to allow meta-analysis of findings and extrapolation of conclusions beyond single study populations.
Practitioners are advised that performance measures of jumping and hopping height might provide valuable insights into performance improvement but offer little insight into an athlete's risk of injury. A broad range of assessment protocols are available for athletic trainers to screen their athlete's for lower limb injury risk but regardless of the jump assessment chosen, kinetic and kinematic variables measured during the landing phase of the test will provide the greatest insight of an athlete's risk of injury. The drop jump and unilateral assessments present the best opportunity for injury risk screening based upon the existing literature while the drop vertical jump remains the most valid evidence-based screening assessment for lower-extremity injury risk. Athletic trainers choosing to utilise a drop jump screening protocol should endeavour to measure a combination of frontal plane (knee valgus, valgus displacement, normalized knee separation distance) and sagittal plane (knee flexion at initial contact and peak knee angular displacement) kinematic variables and kinetic variables (knee abduction moment, peak vertical ground reaction force) to give a holistic view of an athlete's competency. Future research using unilateral jump performance and landing assessments should report limb asymmetries and analyse postural control since these seem to be the primary advantages of unilateral assessments. Researchers are advised to broaden the scope of injuries that are investigated in future research studies in the hope of identifying injury risk factors for other types of injury provided that the task presents some degree of similarity to the mechanism of injury.
Acknowledgements The authors have no financial or equipment disclosures to be made related to this systematic review.
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