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Associations between Distance and Loading Symmetry during Return to Sport Hop Testing

PEEBLES, ALEXANDER T.1; RENNER, KRISTEN E.1; MILLER, THOMAS K.2; MOSKAL, JOSEPH T.2; QUEEN, ROBIN M.1,2

Author Information

1Kevin P. Granata Biomechanics Lab, Bisaomedical Engineering and Mechanics, Virginia Tech, Blacksburg, VA;

2Department of Orthopaedic Surgery, Virginia Tech Carilion School of Medicine, Roanoke, VA

Address for correspondence: Robin M. Queen, Ph.D., F.A.C.S.M., 495 Old Turner St., 230 Norris Hall, Blacksburg, VA 24061; E-mail: [email protected].

Submitted for publication May 2018.

Accepted for publication October 2018.

Medicine & Science in Sports & Exercise: April 2019 - Volume 51 - Issue 4 - p 624-629
doi: 10.1249/MSS.0000000000001830
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Abstract

The annual numbers of anterior cruciate ligament (ACL) injuries and surgical reconstructions (ACLR) have continued to increase in recent years despite a large amount of research focusing on understanding and preventing injury (1–3). In 2014/2015, there were 160 ACL injuries per 100,000 females under the age of 25 yr in Australia (2). The goal of ACLR and the 6–12 months of intense physical therapy that follow is to restore knee stability and neuromuscular control to allow patients to safely return to sport (4,5). Currently, only two thirds of patients with an ACLR successfully return to their preinjury level of sports participation (6). In patients that do successfully return to sports, around 30%–35% will sustain a secondary ACL injury to either the graft or the contralateral ACL (7,8). Asymmetrical movement mechanics have been observed in ACLR patients during walking (9), running (10), jumping (11,12), hopping (13–15), and cutting (16). Movement asymmetries have been associated with secondary injury risk when these patients return to sports (17,18). Therefore, one focus of postoperative physical therapy is to improve surgical limb function and to reduce asymmetry before releasing a patient to return to sport (4,5).

Along with joint laxity, range of motion, strength, proprioception, and self-reported outcome measures, hop testing is widely used to determine readiness to return to sport (4,5,19). Hop testing is specifically used to evaluate lower extremity symmetry in ACLR patients throughout rehabilitation and is often used in return to sport assessments (4,5,19). Hop tests compare the independent ability of the surgical limb to generate explosive power relative to the nonsurgical limb using the limb symmetry index (LSI). For most studies, the LSI is calculated as the ratio of hop distance between the surgical and the nonsurgical limbs (4,17). Hop tests are clinically important as they have a strong association with self-reported knee function (20,21). More recently, ACLR patients achieving an LSI of >85% on the single-leg hop test have been shown to be more likely to return to competitive sport relative to those who do not achieve this benchmark (22). In addition, clinical return to sport test batteries that include hop testing have been shown to be predictive of secondary ACL injuries (17,23,24). Hop tests are ideal for clinical settings as they are easy to administer, not time consuming, and do not require expensive instruments as they are measured in time or distance. However, there is an abundance of evidence that patients with clinically normal hop distance symmetry values have significant asymmetries in their unilateral landing mechanics during hop testing (14,25–27), suggesting that quantifying mechanics during hop testing may improve their diagnostic ability.

ACLR patients have reduced peak knee flexion and range of motion (13,14,26), peak knee extension moments (13,26,28), and knee power absorption (14) during unilateral surgical limb landings, representing a compensatory and potentially maladaptive landing strategy. Furthermore, the LSI of single hop distance is not significantly associated with the LSI of peak hip, knee, and ankle angles or moments during the landing phase of the single hop in ACLR patients (27). These results demonstrate that individuals who have highly symmetric hop distances do not necessarily have symmetric landing strategies. Although it is unknown whether these altered features of unilateral landing mechanics are associated with secondary ACL injuries, asymmetrical limb loading during the bilateral drop vertical jump test is associated with an increased risk for a secondary ACL injury to either the surgical or contralateral limb (18). Loading symmetry during hop testing has not been extensively studied; however, it may offer unique insight into hop landing mechanics. High peak impact force has been associated with low knee flexion angle (29,30) and angular velocity at initial contact (31), high proximal tibia anterior shear forces (31,32), and low trunk flexion (33). As stiff landings, with low knee flexion and high peak impact forces, have been prospectively linked to an increased risk for primary ACL injuries (34,35), soft landings are important to reduce injury risk. It has been suggested that clinicians work on soft unilateral landings with their patients, striving toward generating peak impact forces that are below three times their body weight and which are symmetric (5). Low peak knee flexion symmetry during single-limb drop landings when returning to sport is linked to more knee pain and worse patient-reported quality of life 2 yr after being released to sport (36). Therefore, subtle biomechanical asymmetries during hop testing may be pertinent to address clinically before releasing patients to return to sport.

Loading asymmetry may be a relevant and clinically feasible measure of movement quality during hop testing. The loadsol® (Novel Electronics, St. Paul, MN) is a single-sensor force sensing insole that is lightweight, wireless, relatively inexpensive, and easy to use (Fig. 1). Furthermore, the loadsol® device has been shown to measure peak impact force, loading rate, and impulse, as well as the symmetry of each of these kinetic measures, during single hop and stop jump landings with excellent validity and between-day repeatability (Peebles et al., unpublished data). The first purpose of this study was to compare hop distance symmetry and loading symmetry between ACLR athletes at the time of return to sport and healthy uninjured recreational athletes. We hypothesize that both hop distance and loading symmetry would be worse in ACLR patients when compared with healthy controls. The second purpose of this study was to determine the association between hop distance symmetry and loading symmetry. Based on previous results that hop distance symmetry is not significantly associated with joint kinematic or kinetic symmetry during hop testing in ACLR patients (27), we hypothesize that there will not be a significant relationship between hop distance symmetry and loading symmetry.

F1
FIGURE 1:
Loadsol® device worn inside the shoe and data that were collected wirelessly via Bluetooth.

METHODS

Twenty-five patients recovering from primary unilateral ACLR and 30 healthy recreational athletes participated in the present study (Table 1). All ACLR patients were over the age of 14, had to have a closed growth plate confirmed by the treating surgeon, and followed a similar rehabilitation program designed to allow them to return to sports that involve cutting and jumping. All testing for this study was completed at 1.52 ± 2.23 wk after being cleared by their treating orthopedic surgeon to return to sports. All healthy control participants were recreationally active for at least 30 min three times per week, had no history of a major lower extremity injury, had not had a minor lower extremity injury in the previous 2 months, and did not have a preexisting condition that limited physical activity. Although healthy control participants and ACLR patients were matched as close as possible, a perfect group match was not obtained. However, previous literature has demonstrated that hop distance symmetry is not affected by gender or activity level (37). All participants signed university institutional review board approved informed consent before participating in the study.

T1
TABLE 1:
Demographics of healthy control (HC) and ACL reconstruction (ACLR) participants.

Each participant was fitted with a pair of athletic socks and neutral running shoes (Air Pegasus, Nike Inc, Beaverton, Oregon) and an appropriately sized pair of single-sensor insoles (loadsol®, Novel Electronics). Subjects were asked to walk around for 5 min to both become comfortable wearing the insoles and shoes and to warm the insoles up to body temperature. The sensors were then calibrated according to the manufacturer’s guidelines. The loadsol® sensors measured bilateral plantar force at 100 Hz throughout the testing protocol.

Each participant was asked to complete three single-leg hop tests, 1) the single hop, 2) the triple hop, and 3) the crossover hop test (38,39). Each of these hop tests were completed on both limbs independently. During the single hop test, participants were instructed to hop as far as possible while taking off and landing on the same foot. For the triple hop test, participants hopped three consecutive times without pausing between hops. Similarly, for the crossover hop test, participants hopped three consecutive times without pausing; however, they had to laterally crossover a 6-inch wide strip while still hopping forward. Participants crossed the strip toward the nonhopping leg on the first and third hop and toward the hopping leg on the second hop (38,39). For all three tests, participants were required to stick the final landing, defined as maintaining balance for 2 s without touching the ground with the contralateral leg or either hand. Participants were allowed to practice each test, and failed tests were repeated until successfully completed. Each hop test was performed twice on each limb for the ACLR patients and three times on each limb for the healthy control subjects. Hop distance was measured to the nearest centimeter on each test. The testing protocol was completed on the nonsurgical limb first for the ACLR patients and on the nondominant limb first for the healthy controls based on the protocol described by Reid et al. (38). The single hop was always performed first, triple hop second, and crossover hop last. Breaks of 5 min were provided between hop testing conditions to prevent the onset of muscle fatigue.

Peak impact force, loading rate, and impulse were calculated using the loadsol® data for the final landing of each trial (third landing for triple and crossover hop). Initial contact was defined as the first point when the force exceeded 50 N. Peak impact force was calculated as the maximum force in the first 200 ms after initial contact, loading rate was taken as the peak force divided by the time it took to reach the peak force after initial contact (12), and impulse was defined as the area under the force–time profile for the first 200 ms after initial contact (12). Each outcome measure (distance, peak impact force, loading rate, and impulse) was averaged for each individual hop test and limb for each subject, then an LSI was calculated as the ratio between the surgical (nondominant) limb and the nonsurgical (dominant) limb (13,21).

All demographics were compared between groups using an independent t-test except for gender, which was compared using a chi-square test. The effect size of t-tests was determined using Cohen’s d, and effect size of the chi-square test was determined using Cramer’s V, both using previously established methods (40,41). As Shapiro–Wilk tests for normality revealed that many outcome measures were nonnormally distributed, nonparametric comparisons were used for analysis. Mann–Whitney U tests were used to compare outcome measures (LSI of hop distance, peak impact force, loading rate, and impulse) between the ACLR group and the healthy controls. Effect size of Mann–Whitney U tests were calculated using Cohen’s r (40,41). Effect sizes of 0.2, 0.5, and 0.8 were considered small, moderate, and large, respectfully (40,41). In addition, Spearman rank correlations were used to compare hop distance symmetry with each hop loading symmetry measures individually in the ACLR group and in the healthy control group. Correlations between 0 and 0.2 were considered negligible, 0.21 and 0.35 weak, 0.36 and 0.67 moderate, 0.68 and 0.90 strong, and 0.91 and 1.00 very strong (42). Significance was set at 0.05, and all statistical analysis was performed using the Statistical Package for the Social Sciences (version 24, SPSS Inc., Chicago, IL).

RESULTS

Compared with the healthy control participants, the ACLR group was younger (P < 0.001) and had a higher score on the Marx Scale (P < 0.001), indicating that they were more physically active (Table 1). There was no significant difference between groups for gender, height, or weight. Compared with the healthy control group, the ACLR patients had lower hop distance LSI values and loading LSI values with small-to-moderate effect sizes (Table 2). Both distance and loading LSI values for the ACLR group were lower than that of healthy controls and lower than 100%, indicating that hop distance and limb loading were lower on the surgical limb than that of the nonsurgical limb.

T2
TABLE 2:
Hop distance LSI and loading LSI compared between healthy controls (HC) and ACLR patients using Mann–Whitney U tests.

Many significant associations between hop distance symmetry and loading symmetry were identified in the ACLR group (Table 3). There was a moderate to strong relationship between impulse symmetry and hop distance symmetry for each test, and between peak impact force symmetry and hop distance symmetry on the single and crossover hop tests. There was also a moderate relationship between loading rate symmetry and hop distance symmetry during the crossover hop test. For the healthy control group, a moderate relationship between hop distance symmetry and both peak impact force and impulse symmetry was observed on the single hop test and between hop distance symmetry and peak impact force, loading rate, and impulse symmetry on the crossover hop test. No other significant relationships between hop distance and loading symmetry were observed in either the ACLR or control group.

T3
TABLE 3:
Spearman rank correlations between hop distance LSI (left) and loading LSI (top) on the single hop (SH), triple hop (TH), and crossover hop (CH) in both the healthy control (HC) and the ACLR groups.

DISCUSSION

The first purpose of this study was to compare hop distance symmetry and loading symmetry between ACLR athletes at the time of return to sport and healthy uninjured recreational athletes. Our first hypothesis was supported as both hop distance and loading symmetry would be worse in ACLR patients when compared with healthy controls. This finding supports previous literature that has demonstrated asymmetrical hop distances and unilateral landing mechanics in patients recovering from ACLR (14,26,28,38). The second purpose of this study was to determine the association between hop distance symmetry and loading symmetry. Our second hypothesis was only partially supported. Although many comparisons between distance and loading symmetry were weak and nonsignificant, we did find many moderate-to-strong relationships, particularly in the ACLR group. Although these results demonstrate that although there is a relationship between distance and loading symmetry during hop testing, distance and loading symmetry provide different information and should be used simultaneously during return to sport hop testing.

Patients with ACLR were more asymmetric than healthy uninjured participants in both hop distance and loading during the landing portion of the hop tests, indicating they hopped shorter distances and had lower loading variables when hopping on their surgical limb. Specifically, hop distance symmetry was reduced in the ACLR group by 12% on the single hop test, 21% on the triple hop test, and 17% on the crossover hop test relative to healthy controls (all P < 0.001 with small-to-moderate effect sizes). In addition, the average hop distance LSI on the three hop tests was well below suggested clinical cutoffs (85% [4] or 90% [43]) for when patients should return to sport. This indicates that many of the ACLR patients included in the present study may not have been ready to fully return to sport based on previously suggested return to sports criteria (4,43). To our knowledge, this is the first study that identifies significant loading asymmetry during the landing phase of common return to sport hop tests. These findings are in agreement with previous work, which found ACLR patients landed softer on their surgical limb relative to their nonsurgical limb when hopping a prescribed distance of 75% body height (28). Previous work has demonstrated that ACLR patients have reduced knee flexion when landing on their surgical limb (13,14,26), which is typically associated with stiffer landings in healthy control populations (29,30). However, ACLR patients compensate for reduced knee flexion and extension moments by increasing ankle extension moments and power absorption and increasing trunk range of motion (13,14,28,36). Although it appears these compensatory strategies may lead to softer landings based on the results of the present study, they have also been prospectively linked to an increased risk for having worse subjective knee function and pain on the Knee Injury and Osteoarthritis Outcome Scale (36). Compensatory kinetic asymmetries have also been repeatedly observed during bilateral landing in patients with ACLR and have been linked to an increased risk for secondary ACL injuries (18,44–46). These findings collectively demonstrate significant deficits in surgical limb neuromuscular control present during landing in ACLR athletes, which is important to address clinically before releasing patients to return to sport.

The second purpose of this study was to determine the association between hop distance symmetry and loading symmetry. Generally, we found that there appears to be a relationship between hop distance and loading symmetry, particularly in the ACLR patients. This finding intuitively makes sense, as longer hop distances are expected to generate more kinetic energy that needs to be dissipated at ground contact. However, there were many nonsignificant correlations, and the significant correlations were weak to moderate (0.3–0.7) (Table 3). These results demonstrate that while hop distance and loading symmetry are associated, loading symmetry does provide different information than distance symmetry. Therefore, it is likely that assessing loading symmetry would help quantify landing mechanics symmetry and assist in clinical decisions about readiness to return to sport. This is in agreement with a previous study which found that hop distance LSI was not associated with the LSI of any lower extremity kinematic or kinetic measures when landing from a single hop (27). These results demonstrate that symmetrical hop distances do not necessarily indicate symmetrical landing mechanics. Future work should focus on understanding the association between hop distance, loading, and lower extremity mechanics during hop testing in ACLR patients and in healthy controls. This would provide us with a more complete understanding of what loading symmetry during single-limb landings is reflective of so that these functional deficits can be addressed clinically before releasing patients for return to sport. Although future work is clearly needed to fully understand and demonstrate the clinical importance of measuring loading symmetry during hop testing, the present study is an important first step in providing clinicians with feasible methods for objectively quantifying movement quality. In addition, loading symmetry should be evaluated during hop testing in ACLR patients who achieved >90% hop distance LSI values to determine whether the loadsol® can identify alterations in movement mechanics in individuals who would have otherwise been determined to have achieved normal surgical limb functional performance.

The significant asymmetrical landing kinetics identified in the present study were measured using a lightweight, portable, wireless, inexpensive, and easy-to-use in-shoe force sensing insole. Previous studies investigating asymmetrical landing patterns in patients with ACLR have used motion capture technology and embedded force plates for measuring kinematics and kinetics. These assessments are difficult to implement clinically as they are expensive, take up a lot of space, and require a lot of time to set up, collect, and process data. For these reasons, the loadsol® may be an easy and affordable method for quantifying movement quality during outpatient physical therapy. Other authors have also aimed to address this issue of clinical feasibility by using digital cameras for quantifying kinematics. Using a digital camera, Welling et al. (25) identified significant differences in knee flexion angle at initial contact between the surgical and the nonsurgical limbs of ACLR patients during the single hop test. However, this method for quantifying kinematics needs to be validated against the gold standard motion capture technology. Simultaneous measurements of landing kinetics and kinematics using loadsol® sensors and digital cameras may be an ideal method for quantifying landing quality in a clinical environment and should be investigated in future research.

A primary limitation in the present study was that ACLR group and healthy control groups differed in age and activity level (Table 1), which could have influenced the present findings. However, previous work has indicated that hop testing symmetry is not different based on gender and activity level in a healthy control population (37). It is not currently known if age and maturation status effects hop testing symmetry, which could be determined in future studies.

In conclusion, both hop distance symmetry and hop loading symmetry is reduced during the single hop, triple hop, and crossover hop in ACLR patients who have been cleared by their treating orthopedic surgeon to return to sports. These results are consistent with previous findings and demonstrate significant deficits in surgical limb neuromuscular control in ACLR patients relative to uninjured participants. In addition, the present study found significant, yet moderate, associations between hop distance symmetry and hop landing force symmetry. Although further work is needed to validate the utility, the use of the loadsol® sensors during return to sport functional testing may improve the diagnostic ability to identify neuromuscular control deficits in ACLR patients before releasing them to return to sport. These results provide an important first step into using the loadsol® device in a clinical environment to quantify movement quality during hop testing in hopes of enhancing the diagnostic ability of these tests.

Funding for portions of this study came from DonJoy Orthopaedics. In addition, the authors thank Michael Diersing for taking the photo included in the manuscript and all of the study participants for their willingness to participate. The results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. In addition, the results of the present study do not constitute endorsement by the American College of Sports Medicine.

Conflict of Interest: The authors declare no conflict of interest.

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Keywords:

PRESSURE INSOLES; SYMMETRY; RETURN TO SPORT; LANDING MECHANICS

Copyright © 2019 by the American College of Sports Medicine