Identification and Predictors of Age-Relevant and Activity-Relevant Hop Test Targets in Young Athletes After Anterior Cruciate Ligament Reconstruction

Anterior cruciate ligament (ACL) injuries are one of the most common sport-related knee injuries among young athletes participating in cutting and pivoting sports. For those who wish to resume high-level activities, ACL reconstruction (ACLR) is often recommended to restore functional joint stability. However, despite restoration of anatomic stability, outcomes varied widely in this young patient population. Although 84% of individuals expected to return to preinjury activities post-ACLR, only 48% of those younger than 25 years old returned to cutting and pivoting sports.^1,2 Among young athletes who did participate in sports post-ACLR, 20% to 30% sustained a second ACL injury,^3–5 with the highest risk early in the first 12 months of return to sport (RTS).⁴ These high rates of second ACL injuries in this young population equated to a 30 to 40 times greater risk of an ACL injury than in young athletes without a history of ACL injury.⁵ The low RTS rate and high second-injury rate experienced by a substantial portion of young athletes post-ACLR indicate the need for improved RTS guidelines.

Functional performance measures are often used in a clinical setting to inform readiness for RTS, as they represent clinically feasible measures for assessing performance in many settings. A battery of single-leg hop tests—including the single hop for distance, triple hop for distance, crossover hop for distance, and timed 6-m hop—have been established as practical strategies for evaluating functional performance in young athletes post-ACLR.⁶ Limb-to-limb differences in performance are typically expressed as a limb symmetry index (LSI; ratio of the involved and uninvolved limb performance × 100%), with LSI values of ≥90% being recommended as the “passing” criterion cutoff value.^6–8 However, a recent study⁹ indicated that achieving LSI values of ≥90% on single-leg hop tests did not predict longitudinal outcomes of knee function. Further, LSI scores frequently overestimated knee function post-ACLR, which may have been related to diminished performance of the uninvolved limb.^10,11 The shortcomings of current single-leg hop LSI recommendations create the opportunity to provide greater specificity and clinical relevance for interpretation of hop test data. Benchmarking the hop test performance of young athletes post-ACLR against age-relevant and activity-relevant normative performance values for individual-limb performance and symmetry between limbs could overcome the shortcoming of relying only on traditional 90% LSI to inform decision making. In particular, the clinical use of benchmarked values for individual-limb performance on the hop tests may better represent the status of functional performance recovery of the involved limb with the potential to better inform RTS readiness in this vulnerable population.

In this work, our primary objective was to identify age-relevant and activity-relevant single-leg hop test target values from athletes without a history of ACL injury for individual-limb performance and symmetry between limbs, as well as to evaluate the proportion of young athletes post-ACLR at the time of RTS clearance who met the newly derived target values. We hypothesized that a moderate to high proportion of the ACLR group would achieve the newly derived individual-limb performance and symmetry between limbs target values and that a lower proportion of the ACLR group would meet the newly derived symmetry between limbs target values compared with the traditional 90% LSI criterion.

Single-leg hop tests are often used in conjunction with patient-reported outcome measures and assessment of thigh muscle strength to guide rehabilitation decision making, including clearance for sport participation. Several groups^12–14 reported positive associations among knee-related function, quadriceps muscle strength, and functional performance at various time points post-ACLR but also indicated that high-level performance on 1 measure did not reflect similar high-level performance on other measures. In particular, quadriceps femoris strength deficits were often apparent even when performance on functional performance measures, such as single-leg hop tests, was good.^10,15–19 To further understand these associations in the context of the newly derived hop test target values, our secondary objective was to determine clinical measures associated with meeting the new target values at the time of RTS post-ACLR. We hypothesized that better patient-reported function and quadriceps strength in young individuals post-ACLR would be associated with meeting the newly derived target values for hop tests. The determination of age-relevant and activity-relevant normative values, along with additional information about clinical factors associated with achieving the benchmarked values, may provide a more complete picture of limb recovery to inform rehabilitation interventions and progression.

METHODS

Statistical and Data Analysis

All statistical analyses were performed using SPSS (version 27; IBM Corp; α = .050). Means, standard deviations, and frequencies were calculated to describe sample characteristics. Demographic data were compared between the ACLR and control groups using independent 2-sample t tests (continuous data) and Pearson χ² tests (categorical data).

Derivation of Single-Leg Hop Test Target Values

Control group single-legged–hop test data were summarized using descriptive statistics. Target values were defined as the lower bound of a 95% CI for each hop test from the control data.²⁵ Single-leg hop test target values were defined for surgical-limb performance (normalized by height; normalized target values) and for LSI values (LSI target values) for each hop test (Table 1).

Table 1 Hop Test Data From the Control Group for Normalized Limb and LSI Values

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Proportion Meeting Target Values

To meet the primary objective of the study, we determined the proportions of the ACLR group who achieved the normalized target values (based on performance of their surgical limb) and LSI target values at the time of testing (within 4 weeks of RTS clearance) for each single-leg hop test and cumulatively for all single-leg hop tests (ie, meeting all normalized target values or meeting all LSI target values). We also determined the proportion of the ACLR group who met the traditional 90% LSI criteria for each hop test and across all hop tests. We used McNemar tests to compare the proportions of the ACLR group who achieved the newly derived LSI target values against the proportions of the ACLR group who met the traditional 90% LSI criteria for each hop test.

Predictors of Meeting Target Values in the ACLR Group

We used separate 2-sample t tests (continuous data) or Pearson χ² tests (categorical data) to evaluate differences between participants in the ACLR group who met or did not meet all normalized target values and all LSI target values for injury variables (presence of meniscal injury: yes/no; time from ACLR to testing: months; quadriceps strength variables: peak torque [Nm/kg]; quadriceps femoris strength LSI > 90%: yes/no) and KOOS subscales. For each comparison (normalized target value groups and LSI target-value groups), variables with a P value <.1 were put forth as predictors into logistic regression analyses. To meet our secondary objective, separate multivariate logistic regression analyses were conducted to determine predictors of achieving all normalized target values and all LSI target values. The accepted rule of thumb for sufficient power in logistic regression models was 10 events per predictor variable.²⁶

RESULTS

Demographic Data

The ACLR group (n = 159) and control group (n = 47) did not differ in age, sex distribution, height, or distribution of level of sport participation (Table 2). The weight of the ACLR group was higher than that of the control group (P = .019; Table 2).

Table 2 Demographic Data for the Cohort^a

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Proportions of the ACLR Group Meeting Single-Leg Hop Target Values

Target values, derived from the control group data, were calculated for normalized target values and LSI target values for each single-leg hop test (Table 1). Low proportions of the ACLR group (18%–30%) met the normalized target values with their surgical limb on individual hop tests, and only 12% of the ACLR group met the normalized target values for all hop tests (Figure 1). For symmetry between limbs, moderate proportions of the ACLR group (43%–69%) met the LSI target values on individual hop tests and a small proportion of the ACLR group (25%) met the LSI target values for all hop tests (Figure 2). The proportion of the ACLR group that achieved the newly derived LSI target values was lower than the proportion of the ACLR group that achieved the traditional 90% LSI values on all hop tests (single hop = 43% versus 79%, respectively; triple hop = 48% versus 82%, respectively; crossover hop = 50% versus 79%, respectively; timed hop = 69% versus 84%, respectively; all hops = 25% versus 60%, respectively; all P values < .001).

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Predictors of Meeting All Single-Leg Hop Tests Normalized or LSI Target Values

In the ACLR group, injury, quadriceps strength, and KOOS data for the group that attained all normalized target values (n = 19) and the group that did not attain all normalized target values (n = 140) are shown in Table 3. The group that met all normalized target values had a higher normalized surgical-limb quadriceps peak torque compared with the group that did not meet all normalized target values (Table 3). Predictors for the logistic regression analysis included surgical-limb and uninvolved-limb normalized quadriceps femoris peak torque to predict achievement of all normalized hop test target values. From the logistic regression analyses, we found that higher surgical-limb normalized quadriceps femoris peak torque was associated with greater odds of meeting all normalized target values. Specifically, for every 1 Nm/kg increase in quadriceps peak torque in the involved limb, the odds of meeting all normalized target values were approximately 4.1 times greater (odds ratio = 4.10 [1.10, 15.24], P = .035).

Table 3 Variables of Interest for Those Post-Anterior Cruciate Ligament Reconstruction Who Met and Those Who Did Not Meet All Normalized Target Values^a

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In the ACLR group, injury, knee-related function, and quadriceps strength data for the group that met all LSI target values (n = 40) and the group that did not meet all LSI target values (n = 119) are shown in Table 4. The group that achieved all LSI target values demonstrated higher (better) KOOS-Pain, Symptoms, Sports, and QOL subscale scores, as well as a higher surgical-limb normalized quadriceps peak torque and a greater proportion with quadriceps femoris LSI ≥ 90% than those who did not attain all LSI target values (Table 4). Potential predictors of meeting all LSI target values in the logistic regression analysis, including KOOS-Pain, Symptoms, Sports, and QOL subscale scores; surgical-limb normalized quadriceps peak torque; and having quadriceps femoris strength LSI ≥ 90% (yes/no), were entered into the model. From the logistic regression analyses, we observed a trend that those who had quadriceps femoris strength LSI ≥ 90% had approximately 2.4 times greater odds of meeting all LSI target values (odds ratio = 2.44 [0.97, 6.14]; P = .058).

Table 4 Variables of Interest for Those Post-Anterior Cruciate Ligament Reconstruction Who Met and Those Who Did Not Meet All LSI Target Values^a

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DISCUSSION

Our primary objective in this study was to identify age-relevant and activity-relevant single-leg hop test target values in athletes without a history of ACL injury for individual-limb performance and between-limbs symmetry, as well as to evaluate the proportion of young athletes post-ACLR who met the newly derived target values at the time of RTS clearance. The most important findings were that the newly derived normalized hop test target values and LSI hop test target values were more stringent than those commonly used to inform RTS clearance and that few young athletes post-ACLR met or exceeded these target values at the time of RTS clearance. For individual single-leg hop tests, only 18% to 30% of young athletes at the time of RTS clearance post-ACLR achieved the target values for surgical-limb performance (normalized target values). Further, only 12% met the normalized target values for all 4 hop tests. Hop test performance is often evaluated using an LSI to assess performance symmetry between limbs with recommendations for RTS clearance of LSI > 90%.^6–8 All of the newly derived age-relevant and activity-relevant hop test target values for symmetry (LSI target values) were >90% (94%–96%). On individual single-leg hop tests, 43% to 69% of young athletes attained the LSI target values at the time of RTS clearance post-ACLR, but only 25% attained the LSI target values on all hop tests. In line with our hypothesis, a smaller proportion of the ACLR group achieved the newly derived LSI target values compared with the traditional LSI > 90% criterion for all hop tests.

Our secondary objective was to determine clinical measures associated with meeting the newly derived target values post-ACLR. Our hypothesis was partially supported in that a higher surgical-limb quadriceps femoris peak torque at the time of RTS was associated with meeting all normalized target values in the surgical limb. Benchmarking the hop test performance of young athletes post-ACLR against these newly derived age-relevant and activity-relevant normative surgical-limb performance values, obtained from an uninjured control group, may better represent the status of functional performance recovery in the surgical limb with the potential to better inform RTS readiness in this young, active patient population.

Functional performance measures are often used clinically to track rehabilitation progress and inform readiness for RTS post-ACLR.^10,27–29 Clinical milestones related to functional performance are frequently evaluated with a battery of single-leg hop tests, with symmetry between limbs in hop test performance (LSI value) commonly reported as the criterion to “pass” in order to return to sport participation (LSI > 90%).^6–8 Measuring hop test performance with LSI values is a practical clinical strategy; however, the authors⁹ of a recent systematic review reported only a fair positive association between hop test performance (LSI > 90%) and successful RTS participation (at 12 months). Several sets of researchers^10,30–32 studies characterized further shortcomings of the current single-leg hop LSI recommendations, including the use of the uninvolved limb as the reference standard, which may have led to an overestimation of involved limb functional performance post-ACLR and erroneous interpretation. Indeed, among young individuals post-ACLR, a high level of symmetry between limbs in movement patterns or functional performance was often attained by reduced loading or performance, respectively, of the uninvolved limb.^10,31 In our analysis, 79% to 84% of the ACLR group met the traditional 90% LSI values on the individual hop tests, while only 43% to 69% met the newly derived hop test LSI target values (target values of 94% to 96%). This demonstrates that the traditional 90% criterion for performance symmetry between limbs during functional performance is likely too low; however, using the LSI target values alone does not address the potential shortcomings of using measures of symmetry to evaluate functional performance. As such, we derived age-relevant and activity-relevant normalized-limb performance target values (normalized target values) for the surgical limb.

Only 18% to 30% of the ACLR group was able to meet the normalized limb performance target values for the surgical limb, indicating that a majority of this ACLR cohort, who were cleared for sport participation, lacked the functional performance capacity of their age-similar and activity-similar peers. Recent work³³ in young uninjured athletes (age = 6–18 years old) provided normative values for many common functional performance tests, including some of the single-leg hop tests we used (single hop, crossover hop, 6-m timed hop). The average age of participants in their study (11.7 years)³³ was younger than that of our uninjured cohort (17.0 years). Although height was a significant predictor of all performances on the hop tests, their results were reported as raw values (cm and seconds),³³ limiting direct comparison with our normalized data. Differences in age between the cohorts (current analysis = 17.0 ± 2.3 years versus previous study = 11.7 ± 3.6 years ³³) and height (current analysis = 167.5 ± 9.3 cm versus previous study = 59.3 ± 8.1cm³³) likely explained the higher raw values in our uninjured group compared with those of Magill et al³³ (single hop = 169.8 ± 24.1 cm versus 102.1 ± 29.9 cm, crossover hop = 454.2 ± 86.2 cm versus 287.9 ± 106.2 cm, 6-m timed hop = 1.9 ± 0.3 seconds versus 2.7 ± 0.8 seconds, respectively). The normalized target values reported in our study represented activity-relevant normalized (by height) limb performance values for adolescents and young adults who regularly participated in cutting and pivoting sports.

We noted that a higher involved-limb quadriceps femoris peak torque and quadriceps femoris strength LSI > 90% showed a trend toward being factors associated with meeting the newly derived normalized and LSI hop test target values, respectively. Surgical-limb quadriceps femoris strength was lower in the groups that did not achieve the normalized or LSI hop test target values compared with the groups that achieved the normalized or LSI hop test target values. Quadriceps femoris strength deficits remain a ubiquitous impairment throughout the rehabilitation plan of care post-ACLR.^{13,14,23,34–36} These deficits also persisted after rehabilitation, with a substantial portion of young athletes demonstrating quadriceps strength deficits at the time of medical clearance for sports participation^15,24,35 and beyond.^13,23,34 Our findings were consistent with previous work showing that quadriceps strength deficits contributed to worse functional performance^13,34,35 and altered movement patterns.^14,23,34,35 Optimizing involved limb quadriceps femoris strength and performance may be an important rehabilitation target throughout the plan of care to maximize the odds of attaining age-relevant and activity-relevant functional performance benchmarks in the surgical limb in preparation for RTS participation.

We evaluated athletes who were within 4 weeks of medical clearance for full sport participation. We did not control the rehabilitation program or the decision for release to RTS. Many investigators have recommended RTS criteria of LSI values ≥ 90% for quadriceps strength and functional performance measures, as well as a high score (eg, 90/100) on patient-reported measures of function. However, a substantial portion of this cohort displayed values lower than those currently recommended, as has been previously shown.¹⁵ This result continues to highlight the need for objective measures to inform readiness for sport participation or continued interventions. For evaluation of strength and functional performance, using age-relevant and activity-relevant values for benchmarking limb performance, rather than LSI, may better inform the determination of RTS readiness of young individuals post-ACLR, although further examination is needed. Unknown is whether these benchmarks would predict second ACL injury, which was beyond the scope of this study. This study filled a critical gap in the literature by establishing identifying age-relevant and activity-relevant normative data for single-leg hop tests, a common assessment of functional performance used for RTS decision making post-ACLR.

CONCLUSIONS

Age-relevant and activity-relevant hop test target values, derived from young and active uninjured individuals, were more stringent than those used traditionally to inform RTS decisions post-ACLR (ie, LSI ≥ 90%).^6–8 Factors associated with meeting the age-relevant and activity-relevant hop test target values included greater normalized surgical-limb quadriceps femoris strength and a quadriceps femoris strength LSI ≥ 90%. Only a small to moderate proportion of young, active individuals post-ACLR at the time of RTS clearance met the age-relevant and activity-relevant hop test target values, which is considerably lower than the proportion who meet the traditional criterion of LSI ≥ 90%. The possible shortcomings of symmetry between limbs data in evaluating the functional capacity of the involved limb create the opportunity to benchmark hop test performance against these newly derived age-relevant and activity-relevant normative values to potentially better inform the determination of RTS readiness in this young patient population.