The drop vertical jump (DVJ) is a task routinely used to assess athletic injury risk and performance potential. Uncontrolled frontal plane knee motion during the DVJ, for example, is a risk factor for anterior cruciate ligament tears (6,16) and patellofemoral pain syndrome (10,27). Rate of vertical ground reaction force (GRF) development (2), peak whole-body power output (18,28), and the reactive strength index (1) during the DVJ are used to assess athletic performance potential and to make training recommendations. Recent studies have investigated methodological questions pertaining to footwear worn (13), knee bracing (25), and signal processing and analytical techniques (17,26) during DVJ assessments. However, the impact of different verbal instructions on the interpretation of such assessments has yet to be examined directly.
There is evidence to suggest that different verbal instructions could acutely affect DVJ performance outcomes (28,11,30). For example, when given specific instructions to maximize their reactive strength index (e.g., “jump as high as possible while minimizing ground contact time [CT]”), athletes do indeed decrease their CT, but their maximum jump height also decreases (30). Maximum jump height also decreases when instructions are used to direct the focus away from external task objectives (e.g., “concentrate on … reaching as high as possible during the jumps”) towards characteristics of body movements (e.g., “concentrate on the tips of … fingers … during the jumps”) (29). As a consequence, if athletes are given specific instructions to “minimize CT” or “triple-extend” during DVJ assessments, it is conceivable that judgments about their athletic performance potential could be misconstrued.
It is also conceivable that athletes could be mislabeled as being at low or high risk of sustaining a future injury based on how they are instructed during DVJ assessments. When instructed to “land with your knees over your toes,” for example, peak knee flexion angle increases (23) and the peak vertical GRF decreases (19). Athletes who execute the DVJ in this way would be more likely to be categorized as being at lower risk of sustaining lower extremity injuries than would athletes who land more “stiffly” during the DVJ (19,20,23). These findings suggest that when basing assessments of injury risk on DVJ biomechanics, assessors should consider how different verbal instructions could affect their interpretations and associated intervention strategies.
The purpose of this study was to examine effects of 3 different verbal instructions on a battery of spatiotemporal, body segment, and joint kinematic and kinetic variables commonly used in DVJ assessments of athletic performance potential and injury risk. It was hypothesized that these variables would change with different instructions, and that judgments about athletic performance potential and injury risk could be affected as a result.
Experimental Approach to the Problem
In a 1-hour testing session, bodily motion and GRF data were collected while athletes performed DVJs after being provided with different verbal instructions. Instructions were based on coaching cues commonly used to maximize jump height, minimize ground CT, or to stabilize specific interjoint coordination patterns (i.e., “triple-extend” the hips, knees, and ankles). Dependent variables included a battery of spatiotemporal, body segment, and joint kinematic and kinetic quantities that have been linked with athletic performance potential and/or lower extremity injury risk. Using a within-subject experimental design, dependent variables were statistically compared across the conditions.
Ten men (age 20.8 ± 1.4 years; height 1.87 ± 0.07 m; mass 82.5 ± 10.8 kg) and 10 women (age 20.8 ± 2.0 years; height 1.76 ± 0.07 m; mass 67.6 ± 7.1 kg) from local varsity and club volleyball (6 men; 4 women), basketball (4 men; 1 woman), figure skating (3 women), and track and field (2 women) teams volunteered to participate. At the time of testing, subjects provided informed written consent and indicated that they had been free of musculoskeletal pain for at least 6 months, answering “No” to all questions on the Physical Activity Readiness Questionnaire (PAR-Q). All recruitment materials, subject information and consent forms, and experimental procedures received approval from the University of Toronto's Office of Research Ethics.
Subjects first performed a 10-minute lower body warm-up consisting of dynamic stretching, running drills, multidirectional broad jumps, and plyometric ankle hopping. Participants were then given an opportunity to perform a maximum of 5 practice DVJs while being provided only with enough instructions to ensure that the task could be completed as desired (e.g., the athletes demonstrated that they could consistently jump straight-up between the first and second landing). The specific instructions provided during practice trials were as follows: “while maintaining hands on hips and with whichever foot feels most comfortable, step-off the box, land on 2 feet, and jump straight-up with maximum effort.” The height of the box was 30 cm, and it was positioned beside an in-ground force plate (BP600900; Advanced Mechanical Technology, Inc., Watertown, MA, USA) onto which subjects were to land.
On completion of the warm-up and practice trials, retroreflective markers were secured to major body segments using tape and/or tensor bandages (Figure 1). All subjects wore spandex shorts and athletic shoes, and women also wore a sports bra. Once markers were applied, subjects were asked to stand “quietly” (in anatomical position) on the force plate for 2–3 seconds while marker positions were recorded synchronously with the GRF using an 8-camera optoelectronic motion capture system (Oqus 1; Qualisys AB, Gothenburg, Sweden). After the collection of this static calibration trial, a subset of the markers was removed (i.e., calibration-only markers), and subjects performed the DVJ trials as described below.
Subjects were asked to exert “maximal effort” when performing 3 sets of 5 DVJ trials but were also provided with different verbal instructions before each set of jumps. In one set, an emphasis was placed on achieving maximum jump height by instructing subjects to “focus on jumping as high as you can” (HT condition). In a second set, an emphasis was placed on minimizing ground CT by instructing athletes to “focus on getting-off the ground as fast as you can after landing” (CT condition). In a third set, an emphasis was placed on the interjoint coordination pattern to be used (i.e., lower extremity triple-extension) by instructing athletes to “focus on simultaneously extending your hips, knees, and ankles when jumping” (EX condition). These instructions were read directly from a script to ensure the instructions provided for each condition were consistent across subjects. Subjects were given at least 15 seconds of rest between each trial and a minimum of 3 minutes of rest between each set. Individual trials were rejected and repeated if the subject failed to land on the force plate or if markers were occluded. The order in which sets were performed was randomized across subjects.
Marker trajectories and GRFs were collected in all trials. Force plate data were temporally synchronized with marker position data through a ±10 V 16-bit analog-to-digital conversion system (Analog Acquisition Interface Unit; Qualisys AB), and spatial synchronization between the force plate and marker position data was achieved using an MTD-2 CalTester Rod (Motion Lab Systems, Inc., Baton Rouge, LA, USA) with CalTesterPlus software (C-Motion, Inc., Germantown, MD, USA). Sampling rates for marker position and force plate data were 200 and 1000 Hz, respectively. All data acquisition systems were controlled with Qualisys Track Manager software (QTM; Qualisys AB).
Data Processing and Analyses
A linked-segment model (LSM) of the body was generated offline using Visual3D software (C-Motion, Inc.). Briefly, marker position data recorded in the static calibration trial were used to construct and establish spatial relationships between 2 coordinate systems affixed to each segment (i.e., “technical” and “anatomical” systems). Segment-fixed anatomical coordinate systems were created based on the standard algorithms embedded within Visual3D, whereby markers on the medial and lateral aspects of major body articulations are used to calculate the end point positions and longitudinally, anteriorly, and laterally directed axes of each body segment. Segment-fixed technical coordinate systems were derived based on the relative positions of at least 3 markers that were noncollinearly arranged on segments. All segment-fixed coordinate systems were orthonormalized, and transformation matrices were derived between segment-fixed coordinate systems based on their relative positions and orientations in the static calibration trial. With this information, positions and orientations of body segments were optimally reconstructed based on the trajectories of tracking markers (5), and the relative motion between adjacent segments (i.e., normalized “joint” angles) were calculated in a clinically interpretable way (7). A thorough description of the general LSM-building procedures implemented in Visual3D can be found elsewhere (14).
Three-dimensional motion of the lower extremity joints and trunk segment were calculated during all DVJ trials, but with the exception of frontal plane knee angles, only the sagittal plane components of lower extremity joint and trunk segment angles were analyzed. To aid in interpreting kinematic quantities, hip and knee joint flexion, knee joint “valgus,” ankle dorsiflexion, and trunk inclination were defined as positive values, and right- and left-side hip and knee flexion/extension and ankle dorsiflexion/plantarflexion were averaged (i.e., one time series was generated to describe the sagittal plane motion of the hips, knees, and ankles). Lower extremity joint angles were defined as changes from their alignment during the static calibration trial (i.e., angles of the hips, knees, and ankles in upright standing were defined as zero degrees).
In addition to quantifying body segment and joint angles, the trajectory of the whole-body (LSM) center-of-mass (CoM) was also calculated throughout all DVJ trials. This was achieved in Visual3D through the segmental method, whereby the instantaneous weighted average of all modeled segment CoM positions is considered representative of the whole-body CoM position. Segment inertial properties were estimated through the default procedures in Visual3D (i.e., using anatomical landmark locations and whole-body mass as inputs into geometric models (15) and regression equations (9)).
Before being used to generate the LSM, marker and force plate data were low-pass filtered (zero-lag, fourth-order, Butterworth) at an effective cutoff frequency of 15 Hz to maintain dynamic consistency between the LSM kinematics and kinetics (3).
Using data from the LSM and force plate, 3 sets of “events” were created in Visual3D using customizable detection algorithms embedded within the software. These events were used to distinguish different phases of the DVJ and to derive/extract a battery of spatiotemporal, body segment, and joint kinematic- and kinetic-dependent variables for inclusion in statistical analyses. Ground contact and flight phases were demarcated in each DVJ trial by first creating events at the following instances (in succession): first time, the vertical GRF magnitude exceeded a 5 N threshold after stepping-off the box (first ground contact); when the vertical GRF magnitude next fell below a 5 N threshold (takeoff); and second time, the vertical GRF magnitude exceeded a 5 N threshold (second ground contact).
Events were then created when the vertical position of the whole-body CoM reached a minimum value (between first ground contact and takeoff) and maximum value (between takeoff and second ground contact). Together with information obtained from the first set of events, this second set of events was used to distinguish between the eccentric (“landing”) and concentric (“jumping”) portions of the ground contact phase and to calculate jump height (i.e., vertical distance between whole-body CoM position at takeoff and at its maximum height during the flight phase) and the reactive strength index (jump height divided by ground CT).
Finally, events were created between the first ground contact and takeoff when the vertical GRF magnitude reached a maximum value and at 25 and 50 milliseconds after the minimum whole-body CoM position was identified. After the creation of this third set of events, the following dependent variables were calculated: rate of vertical GRF development (peak vertical GRF magnitude divided by time between first ground contact and the peak vertical GRF event), vertical stiffness (peak vertical GRF magnitude divided by distance between the position of the whole-body CoM), net vertical impulse (integral of net vertical GRF time series during the ground contact phase [i.e., bodyweight was subtracted from the vertical GRF before integrating]), and vertical whole-body power output (vertical GRF magnitude multiplied by vertical velocity of the whole-body CoM). From the LSM kinematic and vertical power time histories, peak values were extracted together with values at first ground contact, when the peak vertical GRF was registered and when the vertical whole-body CoM was at a minimum (during the ground contact phase). Vertical whole-body power outputs were also extracted 25 and 50 milliseconds from the start of the “jump” (i.e., beginning of the concentric portion of the ground contact phase).
Spatiotemporal and discrete kinematic and kinetic variables were compared between CT, EX, and HT conditions using general linear model analyses of variance (ANOVAs) with 1 within-participant factor (condition). Alpha levels were set a priori at 0.05; p values less than 0.05 were considered to be statistically significant. If statistically significant differences were detected in the ANOVAs, post hoc analyses were performed using the Tukey's studentized range test. All statistical analyses were conducted using SAS system software (version 9.3.1, TS1M2; SAS Institute Inc., Cary, NC, USA).
Almost all variables that were statistically analyzed were significantly different between the CT, EX, and HT conditions. However, post hoc tests revealed that there were no statistically significant differences between the EX and HT conditions for any of the variables examined, indicating that it was the instruction to “minimize CT” (CT condition) that was responsible for the differences detected. Qualitatively, these findings were supported by the observation that DVJ technique was different in the CT condition than it was in the EX and HT conditions. More specifically, in the CT condition, athletes maintained a more vertically oriented trunk and used less sagittal hip, knee, and ankle joint range-of-motion throughout most of the ground contact phase of the DVJ (Figure 2); these differences were more pronounced at the “open” end of the kinetic chain (i.e., trunk segment) than they were at the “closed” end (i.e., feet). Associated with these qualitatively observed between-condition differences in body segment and joint kinematic time histories were numerous quantitative (statistically significant) differences in spatiotemporal and discrete kinematic and kinetic variables as presented below.
Between the CT, EX, and HT conditions, statistically significant differences were detected in flight time (F = 14.6, p < 0.0001), jump height (F = 17.8, p < 0.0001), ground CT (F = 53.1, p < 0.0001), and reactive strength index (F = 24.3, p < 0.0001) (Figure 3). Significant between-condition differences were also found in the duration of the eccentric (F = 47.9, p < 0.0001) and concentric (F = 55.0, p < 0.0001) portion of the ground contact phase; concentric and eccentric portions were of shorter durations in the CT condition (concentric = 153 ± 42.1; eccentric = 118 ± 37.5 milliseconds) than they were in the EX (concentric = 224 ± 56.1; eccentric = 183 ± 53.6 milliseconds) and HT (concentric = 223 ± 65.4; eccentric = 182 ± 61.6 milliseconds) conditions (p ≤ 0.05).
Discrete Body Segment and Joint Kinematic Variables
As expected based on visual inspection of the body segment and joint kinematic time histories (Figure 2), there were a number of statistically significant between-condition differences in the sagittal plane angles of the trunk (Table 1), hips (Table 2), knees (Table 3), and ankles (Table 4). Generally, there were significantly less sagittal plane trunk, hip, knee, and ankle motion exhibited in the CT condition than in the EX or HT conditions (p ≤ 0.05).
Peak frontal plane knee angles were significantly different between the conditions (right side: F = 16.6, p < 0.0001; left side: F = 10.3, p = 0.0003); less frontal plane knee motion was exhibited in the CT condition than in the EX and HT conditions (p ≤ 0.05, Figure 4). At the instant when peak vertical GRF was recorded, the frontal plane knee angles were significantly different between the CT, EX, and HT conditions (right side: F = 17.3, p < 0.0001; left side: F = 16.0, p < 0.0001). In the CT condition, the mean frontal plane angles of the right and left knees were negative (“varus”) when the peak GRF was recorded but were positive (“valgus”) in the EX and HT conditions (p ≤ 0.05, Figure 4).
Discrete Kinetic Variables
As summarized in Figure 5, statistically significant between-condition differences resulted in peak vertical GRF magnitudes (F = 35.3, p < 0.0001), rates of GRF development (F = 12.0, p < 0.0001), net vertical impulses (F = 10.8, p = 0.0002), and vertical stiffnesses (F = 42.2, p < 0.0001). Peak vertical GRF magnitudes, rates of GRF development, and stiffnesses were significantly larger in the CT condition than in the EX and HT conditions (p ≤ 0.05), whereas net vertical impulses were significantly smaller in the CT condition than in the EX and HT conditions (p ≤ 0.05).
Significantly larger peak vertical whole-body power outputs were produced during the CT condition than in the EX and HT conditions, as were the vertical whole-body power outputs produced at 25 and 50 milliseconds of the concentric portion of the ground contact phase (Table 5).
There were times when individual athletes exhibited responses that differed that of the group (aggregated) data. In Figure 6, for example, the reactions of 2 athletes to different verbal instructions are compared. It can be seen that different verbal instructions were required to expose characteristics in these athletes that have been linked with reduced performance potential and increased injury risk. The fact that changes in DVJ biomechanics were elicited using different verbal instructions was consistent with the findings presented above based on aggregated data. However, the examples presented in Figure 6 demonstrate that the direction of changes could vary between individual athletes.
In support of the hypothesis, DVJ biomechanics were acutely affected by the verbal instructions provided. More specifically, when instructed to minimize ground CT, athletes in this study landed more “stiffly” and achieved lower maximum jump heights than they did when instructed to simultaneously triple-extend the hips, knees, and ankles when jumping (EX) or to jump as high as possible (HT). Between the HT and EX conditions, there were no statistically significant differences in any of the spatiotemporal, body segment, and joint kinematic or kinetic variables analyzed. It is important to note, however, that there were practically meaningful interindividual differences in responses to different instructions. Given that previous studies have shown that augmented feedback can alter DVJ biomechanics immediately (11,12,30) and over time (11,22), the general finding of this study (i.e., verbal instructions acutely alter DVJ biomechanics) was not unexpected. As discussed below, however, the results do demonstrate that DVJ assessments of athletic performance potential and injury risk can be influenced by how performers are verbally instructed to execute the task.
In DVJ, assessments of athletic performance potential, jump height (JMP), reactive strength index (RSI), peak whole-body power output (PWR), and rate of ground reaction force development (RFD) are variables commonly measured or derived to make judgments (1,2,18,28). Results of this study indicate that all of these variables can be influenced by the verbal instructions provided to performers during such assessments. It is especially important to emphasize that conflicting interpretations regarding athletic performance potential and/or injury risk could result based on the verbal instructions provided and variables examined. For example, although some assessors may view increases in RSI, PWR, and RFD during the CT condition as positive (from a performance potential standpoint), the finding that acute performance outcomes were negatively affected could be interpreted conversely (i.e., JMP decreased, as reported previously (30)). Moreover, when using verbal instructions to produce increases in RSI, PWR, and RFD during DVJ assessments, movement strategies that have been linked with lower extremity injury risk (e.g., “stiff” landing) could be (re-)enforced through repeated exposures (i.e., through practice) and/or because athletes are motivated to specifically “train for the test.”
Several studies have found that athletes who land more “stiffly” during the DVJ are more likely to sustain a noncontact ACL injury during sport participation than are those who land more “softly” (4,16). Moreover, athletes who exhibit uncontrolled frontal plane knee motions during the ground contact phase of the DVJ are also at risk of sustaining such injuries (4,16,24). When participants in this study were instructed to minimize CT during the DVJ, they landed with a more vertically oriented trunk, exhibited less sagittal plane hip, knee, and ankle joint range-of-motion, and produced higher magnitude and rate vertical ground impact forces; all of these variables are associated with “stiff” landing. Greater frontal plane knee motions were exhibited when athletes in this study were instructed to jump as high as possible (HT) or to simultaneously triple-extend their lower extremity joints when jumping (EX). Although it is unlikely that assessors would explicitly instruct athletes to “minimize ground CT” or “triple-extend” during DVJ injury risk assessments, it is certainly plausible that instructions pertaining to jumping speed or technique, respectively, could be perceived similarly by athletes.
Although the group-level effects of verbal instructions on DVJ kinematics and kinetics were statistically significant in this study, it is important to highlight that there were practically (clinically) meaningful differences between the responses exhibited by individual athletes (Figure 6). Given that movement behavior is affected by a complex interaction of task, environmental, and personal (psychological and physical) factors (8), it is not surprising that there were interindividual variations in responses to verbal instructions. However, it does highlight some interesting theoretical and practical challenges associated with making judgments about athletic performance potential and injury risk based on simplified biomechanical analyses of complex whole-body movements. Most notably, it must be appreciated that movement behavior can vary in response to rather subtle variations in task and environmental conditions. Therefore, to enhance the specificity and sensitivity of movement assessments, it could be argued that a variety of verbal instructions, feedback, and task and environmental perturbations should be used. In this way, it may increase the likelihood that performance-limiting and injury-promoting personal “traits” (i.e., persistent movement characteristics) can be more readily distinguished from normal (functional) fluctuations in “states” (i.e., movement variations required to achieve stable performance outcomes under changing task, environmental, and individual conditions).
There are several study limitations and assumptions that must be weighed when interpreting the results. First, given that only 1 force plate was available to the investigators, it was not possible to simultaneously quantify bilateral lower extremity joint reaction kinetics through inverse dynamics analyses. This limited the ability to examine if other lower extremity injury risk variables commonly surveilled during DVJ assessments (e.g., knee abduction moments (21) are affected by different verbal instructions). Second, it cannot be assured that the results of this study are generalizable to athletes who have physical characteristics (e.g., ages, anthropometrics, etc.) or abilities (e.g., muscular strength, flexibility, etc.) that differ markedly from those who participated. Third, it is possible that the impact of verbal instructions on DVJ biomechanics would fade over time, as athletes who are frequently assessed (e.g., to monitor training adaptations) could exhibit more stable patterns of movement coordination and control. Finally, it is possible that the results of this study would differ if an external task goal (i.e., overhead target) had been provided to the athletes, as previous research indicates that DVJ performance outcomes (i.e., jump height) and biomechanics (i.e., joint kinematics and kinetics) are influenced by the presence of an overhead target (12).
Study limitations and assumptions notwithstanding, it can be concluded that when conducting DVJ assessments of athletic performance potential and injury risk, it is advisable to control and/or explicitly document how performers are verbally instructed to ensure that athletes are accurately and reliably characterized.
There are 3 important take-home messages from this study. First, the results suggest that verbal instructions should be controlled and/or clearly documented when using the DVJ to assess athletic performance potential and injury risk. Second, it could be argued that including a number of different verbal instructions during DVJ assessments of athletic performance potential and injury risk could improve the sensitivity and specificity of such assessments. Finally, practitioners who devise performance enhancement and injury prevention strategies based on DVJ assessments are urged to consider that using instructions to acutely enhance performance outcomes (i.e., training to “ace” the assessment) could lead to potentially undesirable chronic adaptations (e.g., engraining potentially risky movement behaviors) unless effective movement-oriented feedback is also provided.
Supported by the Government of Ontario's Ministry of Tourism, Culture and Sport through the Research Program in Applied Sport Sciences (RPASS). The authors would also like to thank Christopher Chapman (Canadian Sport Institute, Ontario, Canada) and Adrian Lightowler (University of Toronto) for their input on study design.
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Keywords:Copyright © 2015 by the National Strength & Conditioning Association.
movement screen; ACL; injury prevention; athletic development; motor behavior