Assessing Readiness: The Hidden Dangers of Drop Jump Tests in Injury Recovery

By Mike Croskery and Heather Grandy, BEng, MASc,

Article Summary: This study revealed greater limb asymmetry in an anterior cruciate ligament repair (ACLR) athlete during single-leg drop jumps at 30 cm heights, emphasizing the potential value of proper drop heights for Return to Play assessments (RTP). Differences in Reactive Strength Ratio (RSR), jump height, and pelvis kinematics suggest impaired reactive strength and proprioception on the ACLR side. These findings highlight the need for comprehensive and ongoing monitoring of limb function post-ACL reconstruction. Pelvis kinematics emerged as a promising metric for evaluating asymmetry, offering a simpler and more accessible alternative to full marker-based motion capture. However, further validation involving larger sample sizes and higher drop heights should occur before incorporating pelvis kinematics into standard RTP protocols. Future research should explore the feasibility of using pelvis kinematics in combination with IMUs to identify asymmetries and refine RTP strategies. This approach could enhance rehabilitation outcomes and minimize re-injury risk, ultimately supporting athletes' safe and effective return to sport.

Introduction

The transition to return to sports can be tricky when it comes to getting back into sports after an injury, particularly an ACL tear. Drop jumps are one standard method to assess an athlete's readiness. But did you know that not all drop heights are equal? Using a height that is too low can lead to a false sense of security, making it seem like an athlete is ready to return to the field when they may not be.

Recently, Heather Grandy, BEng, MASc, and I delved deep into this issue through a case study involving a triple jumper recovering from a severe ACL tear and reconstruction. Initially cleared to return to competition, our comprehensive analysis uncovered a significant imbalance that hinted at underlying issues.

The Triple Jump Challenge

The triple jump is not just any event—it's a technically demanding discipline that requires a unique combination of strength, speed, and precision across its three phases: the hop, step, and jump (1,2). For athletes in this sport, reactive strength is vital, especially during the explosive take-off that defines the event (3).

However, the high forces at play can also place substantial stress on the knee joint, particularly on the anterior cruciate ligament (ACL). It's concerning to note that ACL injuries are on the rise, especially among young female athletes in Canada, posing serious challenges to their rehabilitation and return-to-play (RTP) protocols (10,16).

Re-evaluating RTP Criteria

Traditionally, RTP criteria have leaned heavily on limb symmetry indexes (LSI) and reactive strength index (RSI) assessments to determine whether an athlete is ready to play again (13,15). While these methods serve a purpose, they may not always pinpoint athletes at risk of re-injury (14). This is where innovation and further exploration become crucial.

In our study, we decided to investigate an alternative approach: analyzing pelvis kinematics. By understanding how the hips move during drop jumps, we might discover a more nuanced picture of an athlete’s readiness. This analysis could simplify assessments and shed light on existing force and power production asymmetries.

The Power of Motion Capture

To explore this possibility, we employed advanced motion capture technology to measure pelvis kinematics during drop jumps (see Figure 1). We aimed to see if these metrics could provide additional insights for ACL-reconstructed athletes. If we could identify performance differences through these measurements, we might also explore using an Inertial Measurement Unit (IMU) placed on the pelvis. This cost-effective tool can effectively track pelvic angles and help identify movement pattern asymmetries during rehabilitation (6,7).

Participant Jumps: An Overview of the Trials

In this case study, our jumper engaged in three trials of single-leg drop jumps from a 15 and 30 cm height onto a force platform to assess their performance. A force platform is like a sensitive electronic scale that can calculate and record when and how hard you hit the ground (or ground reaction forces, see Figure 1).  The aim was to minimize ground contact time while maximizing vertical height.

What Did We Measure?

We measured peak force, impulse, ground contact time, flight time, and frontal plane (side to side) pelvic tilt angle during three trials of single-leg drop jumps for both legs from a 15 cm and 30 cm height onto a force platform.

Figure 1: Red lines represent landing and take-off from vertical force measures (black).

We then calculated the Reactive Strength Ratio (RSR), which compares contact time to flight time. The RSR assesses an athlete's ability to quickly transition from landing to takeoff during plyometric movements. Additionally, we determined jump height based on flight time using Moir’s method (8), which calculates jump height using the following formula: Jump Height = g (t /2)² where g = 9.81 m/s², t = time in the air.

How Did the Drop Heights Affect Performance?

Looking at the data visually, it was clear that the pelvis angles showed a distinct difference between the two sides (see Figures 2 and 3). The right side (the ACLR side) had lower angles and was much more variable, especially at the 30cm height. This variability suggests some instability or inconsistency in movement on that side when the body attempted to brace the pelvis to add more stability. The left and right sides were relatively similar regarding force, pelvis position, and velocity while jumping. However, the left side generally showed slightly higher values, indicating it might function better. Additionally, the overall jump height was lower (Table 3), and the RSR was higher on the right side (Table 1), again indicating a significant difference between the two limbs when jumping from the 30cm platform. Remember that a higher RSR value represents lower reactive strength, as RSR is the ratio between the foot's contact time on the ground and flight time.

Figure 2: 15cm drop height comparison between left (red) and right (green) limbs. The line displays the mean across three trials and the band represents the associated standard deviation.

Figure 3: 30cm drop height comparison between left (red) and right (green) limbs. The line displays the mean across three trials, and the band represents the associated standard deviation.

Reactive Strength Ratios (RSR)
Metric	15cm Platform Height	30cm Platform Height
Mean RSR – Left	0.832 ± 0.047	0.768 ± 0.031
Mean RSR – Right	0.936 ± 0.106	0.929 ± 0.037
R/L Difference	+ 12.5%	+ 21.0%

Table 1: Reactive Strength Ratios for left and right (ACLR) sides at 15cm and 30cm platform heights. The table shows the mean +/- the standard deviation. Lower RSR signifies higher reactive strength.

	Impulse
Metric	15cm Platform Height	30cm Platform Height
Overall Impulse – Left	429.463 ± 7.737	470.128 ± 6.568
Overall Impulse – Right	417.322 ± 10.304	451.600 ± 7.961
R/L Difference	-2.8%	-3.9%

Table 2: Overall impulse for left and right (ACLR) sides at 15cm and 30cm platform heights. The table shows the mean +/- the standard deviation. A higher impulse signifies better performance.

Jump Height
Metric	15cm Platform Height	30cm Platform Height
Jump Height – Left	10.46 ± 0.41	12.44 ± 0.93
Jump Height – Right	9.02 ± 0.98	9.53 ± 0.49
R/L Difference	13.8%	23.4 %

Table 3: Jump height for left and right (ACLR) sides at 15cm and 30cm platform heights. The table shows the mean +/- the standard deviation. A higher jump height signifies better performance.

When athletes return from injuries, especially ACL (anterior cruciate ligament) tears, they must check if they are ready to jump back into competing. One of the tools often used in this process is the Limb Symmetry Index (LSI). Generally, an LSI score above 90% is a good sign. In comparison, scores below 85% suggest some imbalance between the injured leg and the healthy one (9). However, there's a bit of a debate around this measurement. Since it only compares the affected leg with the uninjured one, it doesn't consider any loss of muscle conditioning that might happen in the unaffected leg during recovery.

In this case, our athlete found that their right leg didn't clear the LSI threshold when tested with a 30cm drop jump, showing more than a 15% difference between the two legs for RSR and jump height. It's hard to say if this was because of the ACL surgery or if the imbalance was there even before the injury. Research has shown that athletes who have had ACL reconstruction often struggle with equalizing reactive strength when compared to those who haven't had the injury (11). This difference in performance could likely be related to the surgery itself.

During the testing, the athlete displayed lower jump heights and a higher RSR on their ACL-reconstructed side, suggesting they faced some challenges compared to their uninjured side. These patterns align with other studies and can be helpful indicators of how well someone is recovering post-ACL surgery (4).

One interesting observation was the variable performance between the left and right sides during 30cm drop jumps. The side that had undergone surgery was much more inconsistent. This lack of control could be tied to the ACL's critical role in sensing movement and the knee position. Unfortunately, the reconstruction can negatively affect this proprioception after an ACL tear, mainly due to the loss of specific receptors in the torn ligament (4,11). This decreased ability to translate the body and knee position in space could explain the greater variation in the athlete's movements on that side.

In our study, a 30cm height was necessary for testing, as it showed more noticeable differences in RSR between the legs. Reactive strength explains how well your muscles respond to landings with powerful counter-movements. It also relates to how stiff the muscles are and how well the muscles and tendons work together in the lower leg. Studies suggest these factors become especially relevant when jumping from heights above 30cm (5,12). Therefore, testing at this height could be key in understanding an athlete's limb symmetry as they prepare to return to play, helping us explain why there might be less asymmetry at lower jump heights.

In a nutshell, understanding LSI and RSR is important for athletes recovering from ACL injuries. When using drop tests to assess performance, the height makes a difference, so practitioners should choose wisely. A height that is too low may not show significant differences, whereas a height that is too high may increase the risk of injury. Correctly performed, these assessments not only help add to the picture to determine if they're ready to return to competition, but they also highlight the complexities involved in recovering from such challenging injuries. Monitoring pelvis angles with an IMU could be another way to check the readiness of an athlete to return to the game.

However, keep in mind that this study also has several limitations. Since we only looked at one athlete, the study design limits the generalizability of the results to a broader population. We also chose only two consecutive, relatively conservative drop heights to minimize the risk of injury during the competitive season. We also allowed a standard arm swing during the jumps, which may have affected the ground reaction forces and overall results.

Conclusion: A New Perspective on RTP

As we move forward, let's keep challenging the traditional methods of assessing athlete readiness. By combining advanced technology with innovative thinking, we can better understand the complexities of recovery. The goal? To ensure that athletes return to their sports ready and safe, minimizing the risk of re-injury and promoting long-term performance.

Remember, when it comes to drop jumps, pay attention to those heights; they might be the key to ensuring a smoother and safer return to the field!

References

1. Cissik, JM. Strength and Conditioning for the Triple Jumper. Strength Cond J 35: 56–62, 2013.Available from: https://journals.lww.com/00126548-201310000-00010
2. Čoh, M, Matjačić, Z, Peharec, S, et al. Kinematic, Dynamic and EMG Analysis of Drop Jumps in Female Elite Triple Jump Athletes. Coll Antropol 39 Suppl 1: 159, 2015.
3. Čoh, M and Žvan, M. Technique Model of the Triple Jump for Women. Sport Science, International Scientific Journal of Kinesiology 9: 8–13, 2016.
4. Fleming, JD, Ritzmann, R, and Centner, C. Effect of an Anterior Cruciate Ligament Rupture on Knee Proprioception Within 2 Years After Conservative and Operative Treatment: A Systematic Review with Meta-Analysis. Sports medicine (Auckland) 52: 1091–1102, 2022.
5. Kipp, K, Kiely, MT, Giordanelli, MD, Malloy, PJ, and Geiser, CF. Biomechanical Determinants of the Reactive Strength Index During Drop Jumps. Int J Sports Physiol Perform 13: 44–49, 2018.
6. Kumar, S, Singh, J, Pradhan, P, Kumar, S, and Thapa, RK. Validity and Reliability of an Inertial Measurement Unit (BTS G-Walk) for Measurement of Countermovement Jump Height: A pilot-study. J Anthr Sport Phys Educ 7, 2023.
7. Mobbs, RJ, Perring, J, Raj, SM, et al. Gait metrics analysis utilizing single-point inertial measurement units: a systematic review. Mhealth. 8, 2022.
8. Moir, GL. Three Different Methods of Calculating Vertical Jump Height from Force Platform Data in Men and Women. Meas Phys Educ Exerc Sci 12: 207–218, 2008.Available from: http://www.tandfonline.com/doi/abs/10.1080/10913670802349766
9. Ohji, S, Aizawa, J, Hirohata, K, et al. Single-leg hop can result in higher limb symmetry index than isokinetic strength and single-leg vertical jump following anterior cruciate ligament reconstruction. Knee 29: 160–166, 2021.
10. Paudel, YR, Sommerfeldt, M, and Voaklander, D. Increasing incidence of anterior cruciate ligament reconstruction: a 17-year population-based study. Knee Surg Sports Traumatol Arthrosc 31: 248–255, 2023.
11. Read, PJ, Davies, WT, Bishop, C, et al. Residual deficits in reactive strength indicate incomplete restoration of athletic qualities following anterior cruciate ligament reconstruction in professional soccer players. J Athl Train , 2020.
12. Sousa, F, Ishikawa, M, Vilas-Boas, JP, and Komi, P V. Intensity- and muscle-specific fascicle behavior during human drop jumps. Journal of applied physiology (1985) 102: 382–389, 2007.
13. Di Stasi, SL, Logerstedt, D, Gardinier, ES, and Snyder-Mackler, L. Gait Patterns Differ Between ACL-Reconstructed Athletes Who Pass Return-to-Sport Criteria and Those Who Fail. Am J Sports Med 41: 1310–1318, 2013.
14. Welling, W, Benjaminse, A, Lemmink, K, and Gokeler, A. Passing return to sports tests after ACL reconstruction is associated with greater likelihood for return to sport but fail to identify second injury risk. Knee 27: 949–957, 2020.
15. Welling, W, Benjaminse, A, Seil, R, Lemmink, K, and Gokeler, A. Altered movement during single leg hop test after ACL reconstruction: implications to incorporate 2-D video movement analysis for hop tests. Knee Surg Sports Traumatol Arthrosc 26: 3012–3019, 2018.
16. Zhang, Y, McCammon, J, Martin, RK, et al. Epidemiological Trends of Anterior Cruciate Ligament Reconstruction in a Canadian Province. Clinical journal of sport medicine 30: e207–e213, 2020.

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