Biomechanics and Performance of Single-Leg Vertical and Horizontal Hop in Adolescents Post-Anterior Cruciate Ligament Reconstruction

Abstract

Background/Objectives: Single-leg hops are used to determine return to sport after anterior cruciate ligament reconstruction (ACLR). Adult studies support the use of single-leg vertical hop (SLVH) due to higher power generation from knee extensors compared to single-leg horizontal hop (SLHH). Research in children is lacking. This study examines the differences between SLVH and SLHH in pediatric athletes post-ACLR. Methods: We retrospectively examined patients with ACLR who performed SLHH and SLVH on each limb while kinematics and kinetics were collected with a Vicon motion capture system. The limb symmetry index (LSI) for hop distance/height was used to classify the patients as asymmetric (LSI < 90%) or symmetric (LSI ≥ 90%). Biomechanics were compared between limbs and as a function of group using linear mixed models. Results: Among the 19 patients (15 female; age 16.3 years; 9.2 months post-surgery), approximately half were classified as asymmetric (10/19 = 53% for SLHH; 9/19 = 47% for SLVH). During SLHH, the symmetric patients’ uninjured limb produced less power and a shorter hop. During SLVH, the symmetric patients produced more power and hopped higher bilaterally. Regardless of symmetry, the reconstructed knee was offloaded (p ≤ 0.03) and contributed less to power absorption (p ≤ 0.02). Conclusions: SLVH height symmetry may be a better indicator of knee recovery than SHLH distance in pediatric athletes. However, knee offloading is common even when symmetry is achieved.

1. Introduction

It is clear from the current literature that there are a number of different criteria frequently used as part of return to sport (RTS) decision making after anterior ligament cruciate (ACL) injury and subsequent ACL reconstruction (ALCR) [1,2]. However, there is no clear consensus on the tests and measures that should be used, and current research has shown a large variability in clinical practice with little implementation of current research [3]. A scoping review by Burgi et al. found that time since surgery was the only RTS criteria used in 42% of studies, and that over 80% of studies allowed return to play before 9 months [1], despite research documenting increased risk with that return timeline, especially in the pediatric population [4,5]. In fact, the Internation Olympic Committee consensus statement advocates waiting at least 12 months post-op to allow pediatric athletes to RTS [6], and others advocate waiting two years [7]. In their survey of physical therapists, Korakakis et al. found that 28.3% of therapists evaluate knee strength based on hop capacity, despite research showing that hop distance is not indictive of knee strength [3,8] and is a poor indicator of RTS readiness [9]. Undoubtedly, further work needs to be completed to address the disconnect between current research and clinical practice. Studies are needed to establish measures that can be used in a clinical setting to improve translation into clinical practice and implementation in settings available to most patients.

Single-leg hop tasks are frequently used to assess performance in clinical populations [10,11]. These tasks are commonly performed as part of a battery of tests during rehabilitation for patients after ACLR to determine their RTS readiness, as they are performed on both the ACLR limb and contralateral limb separately, and hop distance is a commonly used RTS criteria [4,12]. The RTS decision is critical, especially for adolescents, as research has shown that reinjury rates of either the ipsilateral or contralateral limb are significantly higher in adolescents [13,14,15]. A systematic review by Barber-Westin et al. showed that one in five athletes younger than 20 years old sustained an additional injury to the ipsilateral or contralateral knee after returning to play after ACLR [15]. These injuries have been documented to impact pediatric and adolescent athletes’ quality of life, with previous researching finding a strong correlation between social and emotional quality of life domains and knee function after ACL injury [16].

When used for RTS decision making, unilateral hop tests often utilize a limb symmetry index (LSI) [17]. LSI is a measure comparing the surgical limb to the non-surgical limb, expressed as a percentage, with 100% being perfect symmetry between limbs. An LSI of greater than 90% is widely used as a clinical criteria for RTS decisions, indicating clinical limb symmetry (i.e., the surgical limb is within 10% of the non-surgical limb) [2,17,18]. However, prior research has shown that biomechanical asymmetries continue to exist when a hop distance or height symmetry of 90% is reached in both pediatric and adult populations [9,19]. In addition, these symmetry indexes can overestimate surgical limb performance due to deconditioning and/or neuromuscular adaptation of the non-surgical limb [20,21].

The single-leg horizontal hop (SLHH), measuring the distance a patient can hop forward on one limb, and single-leg vertical hop (SLVH), measuring the height a patient can hop upward on one limb, are common unilateral hop tests that are utilized clinically to determine RTS readiness [1]. While the SLHH is more commonly used, emerging literature in adult populations supports the usefulness of the SLVH to assess knee function in patients following a knee injury due to its increased reliance on power generation of the knee extensors compared to horizontal hop tests [22,23]. Research on the landing phase of the SLVH has shown roughly equal contributions from the hip, knee, and ankle, while the SLHH relies mostly on the knee. In addition, Kotsifaki et al. reported that the knee joint contributes 12.9% of the hop distance in the SLHH and 34.1% in the SLVH [23]. This finding indicates that the SLVH may be more effective for examining knee deficits, especially in a clinical setting where the height or distance of the hop can be measured but a full biomechanical analysis is not possible.

The differences between the SLVH and SLHH tasks have not been examined in the pediatric population. The purpose of our study was to examine the difference in intralimb contribution to takeoff and landing of the SLVH and SLHH in pediatric athletes post-ACLR to identify their utility in RTS testing.

2. Materials and Methods

2.1. Participants

This retrospective study examined data from patients 14 to 18 years old who had undergone primary unilateral ACLR and were seen in our Motion and Sports Analysis Laboratory between June 2023 and August 2024 for 3D biomechanical assessment. All patients had been cleared by their surgeon or physical therapist to begin cutting and jumping activities but were not yet cleared for return to sport at the time of testing. Patients were excluded if they had a history of other serious lower extremity injury or previous ACL injury, could not complete the SLVH or SLHH, or had missing motion analysis data during the takeoff or landing of either task. A total of 19 patients were included in the study (15 female; mean age 16.3, standard deviation (SD) 1.2 years; mean 9.2, SD 2.8 months post-surgery). These athletes participated in a variety of sports including soccer, volleyball, football, basketball, track and field, and lacrosse. The participants and parents provided signed assent and consent for their data to be used in research as approved by our institution’s Institutional Review Board (Children’s Hospital Los Angeles Institutional Review Board, Approval Code CHLA-14-00194).

2.2. Experimental Protocol

Data collection was performed by experienced pediatric physical therapists with specialized training in sport biomechanical assessment and motion analysis. Height, weight, and leg length measurements were taken, and the SLVH and SLHH were performed as part of a more extensive biomechanical testing protocol with a warmup and five other tasks performed in a standardized order, including anterior step down, drop jump, lateral shuffle, deceleration, and cutting. The SLHH was the 5th task and SLVH was the 7th task completed during testing. For the SLVH task, the participants were instructed to stand on one limb on a force plate and hop up as high as possible, reaching overhead with their preferred upper extremity to an overhead target, then landing on the same limb on the same force plate to keep the jump vertical. For the SLHH, the participants were instructed to stand on one limb and jump forward as far as possible, landing on the same limb. In order to obtain force plate measurements for the SLHH, takeoff and landing data were collected during separate trials; the participants first jumped out from the force plate to collect takeoff data and set a target distance, then they jumped from that starting point on to the force plate to collect landing data. For both tasks, the participants were instructed that the landing had to be stable, defined as maintaining their landing position for at least two seconds before placing their contralateral foot on the ground. Following two to three practice trials, three data collection trials were performed, and all trials with usable data (a minimum of two per subject) were included in the analysis. Trials were excluded if the landing was not fully in the force plate or the landing was not stable. The patients were given self-selected rest time between trials as needed.

Three-dimensional lower extremity motion analysis data were recorded during the SLVH and SLHH using a 10-camera motion capture system at 120 Hz (Nexus 2, Vicon Motion Systems Ltd., Oxford, UK) and analog force plates at 2400 Hz (AMTI OR6-5, Advanced Medical Terminology, Inc., Watertown, MA, USA). An experienced physical therapist placed markers on each participant’s trunk, pelvis, and lower extremities following a custom 6-degree-of-freedom (DOF) model [24]. The marker trajectories were filtered using a Woltring filter with a mean squared error of 10 mm², and the force plate data were filtered using a 16 Hz Butterworth filter.

2.3. Data Analysis

Kinematic and kinetic measures were calculated during the takeoff and landing phases of the SLVH and SLHH tasks. Takeoff was defined as the phase from peak knee flexion to foot off, and landing was defined as the phase from initial contact to peak knee flexion. Peak ankle dorsiflexion, knee flexion, and hip flexion angles and peak external ankle, knee, and hip flexion moments were evaluated during both the takeoff and landing phases of each task. For takeoff, power generation at the hip, knee, and ankle were calculated by integrating positive power values at each joint over time. These were summed to obtain the total power generation. The percent contribution from each joint was also calculated. Similarly, for landing, the power absorption at each joint was calculated by integrating negative power values over time. These were summed to obtain the total power absorption during landing. The percent contribution from each joint was also calculated.

Each biomechanical variable was averaged across all valid trials of each task and compared between ACLR and contralateral limbs using linear mixed models with a random effect to account for the repeated measures within each participant. A limb symmetry index (LSI) was calculated based on hop distance (for SLHH) or hop height (for SLVH) as follows: $LSI=\left.\left({\displaystyle \frac{ACLRlimbdistance}{Contralaterallimbdistance}}\right.\right)\times 100$. The SLVH height hopped was measured using the flags on the overhead target. The SLHH distance hopped was measured using the horizontal displacement of a marker on the athlete’s shoe. The LSI was compared to the perfectly symmetric value of 100% for each task using 2-sided t-tests. The patients were grouped for each task as symmetric (LSI ≥ 90%) or asymmetric (LSI < 90%) based on a 90% clinical LSI threshold for hop height or hop distance, and the proportion of patients with LSI < 90% was compared between tasks using Fisher’s exact test. Additional linear mixed model analysis was performed with symmetry group (LSI < 90% or LSI ≥ 90%) and limb (ACLR or contralateral) main effects and interaction, including the participant random effect. Analysis was performed using Stata/IC 14.2 (StataCorp LLC, College Station, TX, USA), with p ≤ 0.05 considered statistically significant.

3. Results

3.1. Performance and LSI

Both horizontal hop distance and vertical hop height were significantly shorter for the ACLR limbs compared with the contralateral limbs (p ≤ 0.01) (

Table 1. Comparison of ACLR and contralateral sides during takeoff of SLHH and SLVH.

Takeoff	SLHH			SLVH
	Contralateral	ACLR	p	Contralateral	ACLR	p
Hop distance or height (leg lengths)	1.41 (0.1)	1.28 (0.1)	0.005	0.30 (0.02)	0.26 (0.02)	0.01
Kinematics
Max ankle dorsiflexion [deg]	29.9 (1.3)	26.7 (1.3)	0.01	24.7 (1.4)	20.6 (1.4)	0.001
Max knee flexion [deg]	60.2 (2.4)	51.2 (2.4)	<0.001	63.3 (2.1)	54.8 (2.1)	<0.001
Max hip flexion [deg]	61.2 (2.7)	56.2 (2.7)	0.02	61.0 (2.6)	58.3 (2.6)	0.18
Moments
Max ankle dorsiflexion moment [N·m/kg]	3.1 (0.3)	3.0 (0.3)	0.41	2.0 (0.1)	2.0 (0.1)	0.93
Max knee flexion moment [N·m/kg]	2.0 (0.2)	1.2 (0.2)	0.001	2.1 (0.2)	1.5 (0.2)	<0.001
Max hip flexion moment [N·m/kg]	2.8 (0.4)	2.8 (0.4)	0.73	2.0 (0.2)	2.1 (0.2)	0.20
Power
Total generation [W/kg]	3.2 (0.2)	2.5 (0.2)	0.001	3.0 (0.2)	2.5 (0.2)	0.002
% ankle	46.3 (1.8)	48.3 (1.8)	0.15	39.5 (2.7)	42.6 (2.7)	0.04
% knee	24.6 (1.7)	15.6 (1.7)	0.001	36.9 (1.9)	27.9 (1.9)	0.001
% hip	29.0 (2.2)	36.0 (2.2)	0.005	23.5 (2.5)	29.4 (2.5)	0.02

Results are presented as model predicted mean (SE). deg = degree.

). The LSI averaged 90.8% (95% CI: [85.2, 96.3]) for the horizontal hop and 87.2% (95% CI: [77.3, 97.1]) for the vertical hop, both significantly lower than the perfectly symmetric value of 100% (p ≤ 0.01). Approximately half of the patients had LSI values below 90% during both tasks (10/19 = 53% for SLHH and 9/19 = 47% for SLVH). There was no significant difference in the proportion of patients with LSI values below 90% between tasks (p = 1.00).

In SLHH, symmetry tends to be achieved by hopping slightly farther on the ACLR side but also shorter on the contralateral side (Figure 1A). In contrast, during SLVH, symmetry is achieved with better performance (greater hop height) on both sides (p ≤ 0.02) (Figure 1B). During both tasks, hop performance (distance or height) differed significantly between ACLR and contralateral limbs in asymmetric patients with LSI < 90% (p < 0.001) but not in symmetric patients with LSI ≥ 90% (p ≥ 0.47) (

Table 2. Comparison of ACLR and contralateral sides during SLHH stratified by LSI.

SLHH	LSI < 90%			LSI ≥ 90%
	Contralateral	ACLR	p	Contralateral	ACLR	p
Hop Distance (leg lengths)	1.5 (0.1)	1.2 (0.1)	<0.001	1.3 (0.1)	1.3 (0.1)	0.89
TAKEOFF
Max ankle dorsiflexion [deg]	30.8 (1.8)	25.8 (1.8)	0.001	28.9 (1.9)	27.6 (1.9)	0.41
Max knee flexion [deg]	62.0 (3.3)	50.8 (3.3)	<0.001	58.2 (3.5)	51.6 (3.5)	0.03
Max hip flexion [deg]	64.8 (3.7)	59.2 (3.7)	0.04	57.1 (3.9)	52.9 (3.9)	0.16
Max ankle dorsiflexion moment [N·m/kg]	3.1 (0.5)	3.0 (0.5)	0.09	3.0 (0.5)	3.1 (0.5)	0.62
Max knee flexion moment [N·m/kg]	2.1 (0.3)	1.1 (0.3)	0.001	2.0 (0.3)	1.3 (0.3)	0.03
Max hip flexion moment [N·m/kg]	3.0 (0.6)	3.1 (0.6)	0.84	2.5 (0.7)	2.5 (0.7)	0.79
Total power generation [W/kg]	3.4 (0.3)	2.5 (0.3)	<0.001	2.9 (0.4)	2.5 (0.4)	0.09
Power generation, % ankle	44.2 (2.5)	46.7 (2.5)	0.18	48.7 (2.7)	50.2 (2.7)	0.46
Power generation, % knee	24.5 (2.3)	13.0 (2.3)	<0.001	24.7 (2.4)	18.5 (2.4)	0.047
Power generation, % hip	31.3 (2.8)	40.3 (2.8)	0.003	26.5 (3.0)	31.3 (3.0)	0.14
LANDING
Max ankle dorsiflexion [deg]	14.5 (2.5)	10.9 (2.5)	0.04	19.1 (2.6)	13.9 (2.6)	0.006
Max knee flexion [deg]	68.8 (3.7)	55.8 (3.7)	<0.001	73.2 (3.9)	63.9 (3.9)	0.001
Max hip flexion [deg]	76.5 (4.2)	71.7 (4.2)	0.13	70.9 (4.4)	68.1 (4.4)	0.41
Max ankle dorsiflexion moment [N·m/kg]	1.0 (0.1)	1.1 (0.1)	0.28	0.9 (0.9)	1.1 (0.9)	0.06
Max knee flexion moment [N·m/kg]	2.6 (0.2)	1.8 (0.2)	<0.001	2.7 (0.2)	2.1 (0.2)	<0.001
Max hip flexion moment [N·m/kg]	3.4 (0.4)	3.0 (0.4)	0.05	2.5 (0.4)	2.6 (0.4)	0.63
Total power absorption [W/kg]	2.8 (0.3)	1.8 (0.3)	<0.001	2.8 (0.3)	2.3 (0.3)	0.007
Power absorption, % ankle	12.0 (1.6)	20.1 (1.6)	<0.001	12.5 (1.7)	17.4 (1.7)	0.007
Power absorption, % knee	57.8 (3.2)	43.8 (3.2)	<0.001	64.9 (3.3)	51.3 (3.3)	<0.001
Power absorption, % hip	30.1 (3.0)	36.1 (3.0)	0.03	22.5 (3.2)	31.4 (3.2)	0.002

Results are presented as model predicted mean (SE). deg = degree.

and

Table 3. Comparison of ACLR and contralateral sides during SLVH stratified by LSI.

SLVH	LSI < 90%			LSI ≥ 90%
	Contralateral	ACLR	p	Contralateral	ACLR	p
Hop Height (leg lengths)	0.25 (0.03)	0.17 (0.03)	<0.001	0.34 (0.02)	0.34 (0.02)	0.47
TAKEOFF
Max ankle dorsiflexion [deg]	23.3 (2.1)	19.4 (2.1)	0.01	25.8 (2.0)	21.7 (2.0)	0.006
Max knee flexion [deg]	61.3 (3.1)	52.1 (3.1)	0.001	65.1 (2.9)	57.2 (2.9)	0.004
Max hip flexion [deg]	61.5 (3.9)	61.5 (3.7)	1.00	60.5 (3.7)	55.5 (3.7)	0.047
Max ankle dorsiflexion moment [N·m/kg]	2.0 (0.2)	1.9 (0.2)	0.20	2.0 (0.2)	2.1 (0.2)	0.18
Max knee flexion moment [N·m/kg]	2.0 (0.2)	1.3 (0.2)	<0.001	2.2 (0.2)	1.8 (0.2)	0.002
Max hip flexion moment [N·m/kg]	1.9 (0.3)	2.1 (0.3)	0.049	2.1 (0.3)	2.1 (0.3)	0.98
Total power generation [W/kg]	2.7 (0.3)	2.2 (0.3)	0.03	3.3 (0.3)	2.8 (0.3)	0.006
Power generation, % ankle	42.1 (4.0)	44.5 (4.0)	0.25	37.2 (3.8)	40.9 (3.8)	0.06
Power generation, % knee	35.5 (2.4)	21.5 (2.4)	<0.001	38.2 (2.2)	33.7 (2.2)	0.13
Power generation, % hip	22.4 (3.5)	33.9 (3.5)	<0.001	24.6 (3.3)	25.4 (3.3)	0.77
LANDING
Max ankle dorsiflexion [deg]	17.5 (1.6)	11.7 (1.6)	<0.001	24.3 (1.5)	19.2 (1.5)	<0.001
Max knee flexion [deg]	45.7 (4.3)	32.7 (4.3)	<0.001	60.0 (4.1)	52.2 (4.1)	<0.001
Max hip flexion [deg]	42.9 (5.5)	44.0 (5.5)	0.75	53.2 (5.2)	51.1 (5.2)	0.51
Max ankle dorsiflexion moment [N·m/kg]	1.9 (0.2)	1.8 (0.2)	0.41	2.2 (0.1)	2.3 (0.1)	0.39
Max knee flexion moment [N·m/kg]	1.9 (0.2)	0.9 (0.2)	<0.001	2.5 (0.2)	2.0 (0.2)	0.001
Max hip flexion moment [N·m/kg]	1.3 (0.2)	1.2 (0.2)	0.79	1.6 (0.2)	1.5 (0.2)	0.45
Total power absorption [W/kg]	1.7 (0.3)	1.2 (0.3)	<0.001	3.0 (0.3)	2.4 (0.3)	<0.001
Power absorption, % ankle	46.4 (5.8)	58.8 (5.8)	0.01	42.1 (5.5)	49.7 (5.5)	0.10
Power absorption, % knee	37.9 (3.3)	19.1 (3.3)	<0.001	42.2 (3.2)	33.6 (3.2)	0.02
Power absorption, % hip	15.7 (4.4)	22.1 (4.4)	0.10	15.7 (4.2)	16.6 (4.2)	0.79

Results are presented as model predicted mean (SE). deg = degree.

).

3.2. Takeoff

These patterns in performance reflect the trends seen in total power generation during takeoff. Overall, the total power generation was significantly lower for the ACLR limbs during both tasks (p ≤ 0.002) (Table 1). During SLHH, the symmetric patients generally produced slightly more power than the asymmetric patients on the reconstructed limb, but they also tended to produce less power than the asymmetric patients on the contralateral limb (Figure 1C). During SLVH, the symmetric patients tended to produce more power than the asymmetric patients on both limbs (Figure 1D), though power generation differed significantly between limbs in both the asymmetric and symmetric patients (p ≤ 0.03) (

Table 4. Comparison of ACLR and contralateral sides during landing of SLHH and SLVH.

Landing	SLHH			SLVH
	Contralateral	ACLR	p	Contralateral	ACLR	p
Kinematics
Max ankle dorsiflexion (deg)	16.7 (1.8)	12.3 (1.8)	0.003	21.1 (1.4)	15.7 (1.4)	<0.001
Max knee flexion (deg)	70.9 (2.7)	59.6 (2.7)	<0.001	53.2 (3.5)	42.9 (3.5)	<0.001
Max hip flexion (deg)	73.9 (3.0)	70.0 (3.0)	0.11	48.4 (3.8)	47.8 (3.8)	0.79
Moments
Max ankle dorsiflexion moment (N·m/kg)	4.5 (3.7)	4.9 (3.7)	0.16	9.6 (6.7)	7.9 (6.7)	0.34
Max knee flexion moment (N·m/kg)	10.8 (6.8)	7.1 (6.8)	0.23	9.6 (6.1)	6.0 (6.1)	0.23
Max hip flexion moment (N·m/kg)	9.9 (7.4)	10.7 (7.4)	0.44	7.5 (6.3)	7.9 (6.3)	0.47
Power
Total absorption (W/kg)	11.8 (8.7)	10.4 (8.7)	0.04	10.4 (7.7)	9.1 (7.7)	0.09
% ankle	12.2 (1.2)	18.8 (1.2)	<0.001	44.0 (4.0)	53.9 (4.0)	0.008
% knee	61.3 (2.4)	47.4 (2.4)	<0.001	40.1 (2.6)	26.7 (2.6)	<0.001
% hip	26.5 (2.3)	33.9 (2.3)	0.001	15.9 (3.0)	19.3 (3.0)	<0.001

Results are presented as model predicted mean (SE).

).

The distribution of power generation differs between the SLHH and SLVH tasks. SLHH relies more on the hip and ankle and less on the knee, while SLVH relies more on the knee and less on the hip (Figure 2A,B). Asymmetric patients generated power less through the knee and more through the hip on the ACLR limb during both tasks (p ≤ 0.003) (Table 2 and Table 3). The symmetric patients showed a smaller difference between limbs, with a significant difference only for the knee during SLHH (p = 0.047).

In terms of kinematics, ACLR limbs exhibited significantly less flexion for all joints during SLHH in the asymmetric patients (p ≤ 0.04) and less flexion for the knee in the symmetric patients (p = 0.03) (Table 2). During SLVH, flexion was lower on the ACLR side for all joints in the symmetric patients (p < 0.05) and for the knee and ankle in the asymmetric patients (p ≤ 0.001) (Table 3). Peak knee moments were lower on the ACLR side during both tasks, regardless of performance symmetry (p ≤ 0.03) (Table 2 and Table 3). The moments did not differ at the hip or ankle in either group during either task.

3.3. Landing

During landing, flexion was lower for the knee and ankle in the ACLR limbs relative to the contralateral side during both tasks (p ≤ 0.003) (Table 4). This occurred regardless of whether hop distance and hop height were symmetric or asymmetric (p ≤ 0.04) (Table 2 and Table 3). Peak knee moments and total power absorption were also significantly lower on the ACLR side in both groups during both tasks (p ≤ 0.007). Power absorption relied more on the knee, followed by the hip, during SLHH and more on the ankle, followed by the knee, during SLVH (Figure 3). The knee’s contribution to power absorption was reduced on the ACLR side during both tasks, regardless of hop distance symmetry (p ≤ 0.02).

4. Discussion

There are currently no data for adolescent performance of the SLVH, and our paper is the first to examine this task in this age group. Consistent with previous adult literature [23], our subjects had decreased performance on their ACLR limb on both the SLVH and SLHH tasks, hopping a shorter height and distance on the reconstructed side (Table 1). Despite mean LSI values of 90.8% for the SLHH and 87.2% for the SLVH, which are close to the 90% cutoff many clinicians use, approximately half of our patients had LSI below 90% on each test. As evaluations were performed 6–15 months post-ACLR and before clearance to return to sport, hop distance and height symmetry would be expected to continue to improve with additional rehabilitation.

Comparing tasks, a recent study has shown that 9 months after ACLR, athletes achieved 97% symmetry in SLHH distance but only 83% symmetry in SLVH height, suggesting that vertical hop height symmetry may be more difficult to achieve and therefore better represent limb function [25]. The results of the current study and our previous research [9] suggest that this may be because hop distance symmetry in SLHH is often achieved by hopping a shorter distance on the uninjured side, making it easier to appear symmetric and meet the 90% threshold (Figure 1A). In contrast, during SLVH, symmetry is achieved with better performance (greater hop height) on both sides (Figure 1D), suggesting that SLVH provides a better measure of performance. Power generation followed similar patterns, with the symmetric patients producing more power than the asymmetric patients bilaterally during SLVH but less power on the contralateral side during SLHH (Figure 1C,D).

In addition, the takeoff phase, which determines performance, is more dependent on the knee in SLVH, potentially making SLVH a better indicator of recovery of knee function. The literature in adult populations showed that the SLVH test has an increased reliance on power generation of the knee extensors compared to horizontal hop tests [22,23]. Our results indicate that these findings also apply to adolescent athletes after ACLR. We found that in asymmetric patients, power on the ACLR side was generated less from the knee and more from the hip during both the SLHH and SLVH, with a similar pattern observed in symmetric patients during SLHH. However, during SLVH, the symmetric patients demonstrated a more similar distribution of power generation between sides. Thus, our patients showed biomechanical offloading of their involved knee during takeoff of the SLHH, despite achieving symmetrical hopping distance. This offloading during takeoff was not present in the patients who reached a symmetrical hop height in the SLVH. This indicates that symmetry of the SLVH may be a more accurate indicator of knee function status post-ACLR.

During landing, the subjects had decreased total power absorption on their ACLR compared to the contralateral limb when performing asymmetrically or symmetrically in both tasks. Decreased power absorption has been a well-documented factor associated with an increased risk for ACL injury [26,27]. These findings highlight that the SLVH can be used to assess both single-limb performance and shock absorption during landing in a pediatric population as part of ACLR rehabilitation.

Previous research on the SLVH in collegiate athletes with a previous ACLR showed poor correlation with other single-leg horizontal hop tests, indicating that the vertical hop can provide distinctive information for decision making [28,29]. Our findings support that these hop tests provide distinct information even when symmetry is achieved on both, highlighting the complex decision making involved with creating a battery of RTS tests.

Our results also showed that a lower percentage of power was absorbed by the reconstructed knees compared with the contralateral knees during both tasks, shifting the demand to the hip and ankle musculature. This could indicate further protective offloading of the surgical knee, which is seen clinically and has been documented in previous research, highlighting the importance of training the patient to appropriately load the reconstructed knee in rehabilitation following ACLR [30,31]. This intralimb offloading, even in patients with symmetric performance, also emphasizes the need for careful selection of RTS criteria in the clinical setting, as it is difficult to capture kinetic deficits without a full biomechanical assessment. Dynamic knee valgus and suboptimal engagement of hip musculature have been suggested as likely contributors to ACL injury risk [32]. When a full biomechanical assessment with kinematics, kinetics, and electromyography is not available, however, SLVH and SLHH may still be useful tests, since hop height and distance can be measured in a typical clinical setting.

One limitation of our study was that we did not have isokinetic strength data available for this project. This would be a helpful addition for future studies, as research has shown that symmetry in hop distances is reached sooner in rehabilitation than symmetry of isokinetic strength [8,33]. However, isokinetic strength testing is not available for many non-elite clinical populations, highlighting the need to examine other variables for RTS readiness that can be tested easily in a clinical environment. In order to have kinetic data for both the takeoff and landing phases of the SLHH, the phases were taken for separate trials. Though the trials utilized the same patient-selected distance, having a target for the second jump may elicit greater attention to landing. LSI also has limitations as a metric, since the non-injured limb may also undergo changes due to deconditioning and neuromuscular adaptation [21]. Additional limitations are that the time post-ACLR was not able to be standardized, the task order was not randomized, there was an uneven distribution of male and female athletes, and we had a relatively small sample size. We also did not have an age-matched control group available for this project. This would be a helpful addition for future research in the pediatric population. While this project primarily focused on sagittal plane kinematics and kinetics during single-leg jump tasks, future research should include frontal and transverse plane biomechanics, strength contributions, and neurological adaptations to ligamentous injury during these tasks.

5. Conclusions

In summary, our results indicate that the SLVH may be a useful measure to assess single-limb performance in a pediatric population as part of ACLR rehabilitation. Patient performance (height and distance hopped) on both the SLVH and SLHH tests can be easily assessed in a clinical setting, and symmetry can be examined using LSI. However, symmetry of SLVH height may be a better indicator of knee recovery than symmetry of SLHH distance. Ideally, a full biomechanical assessment would be performed to identify offloading of the knee during both the takeoff and landing phases of the hop tests, since this is common even when hop height and distance are symmetrical between limbs. These differences are important to consider during rehabilitation of this patient population to ensure safe RTS, decrease their risk of future injury, and optimize their performance.