Pain Early After Anterior Cruciate Ligament Reconstruction is Associated With 6-Month Loading Mechanics During Running

Abstract

Background:

Anterior cruciate ligament reconstruction (ACLR) results in persistent altered knee biomechanics, but contributing factors such as pain or patient function, leading to the altered loading, are unknown.

Hypothesis:

Individuals with worse self-reported pain after ACLR would have poorer biomechanics during running, and poor loading mechanics would be present in the ACLR limb compared with contralateral and control limbs.

Study Design:

Cohort pilot study.

Level of Evidence:

Level 3.

Methods:

A total of 20 patients after ACLR (age, 18.4 ± 2.7 years; height, 1.7 ± 0.1 m; mass, 84.2 ± 19.4 kg) completed visual analog scale and Knee Injury and Osteoarthritis Outcomes Score (KOOS) at 1 and 6 months postsurgery. At 6 months postsurgery, patients underwent biomechanical testing during running. A total of 20 control individuals also completed running biomechanical analyses. Associations between patient outcomes and biomechanics were conducted, and differences in running biomechanics between groups were analyzed.

Results:

KOOS pain score 1 month after surgery was associated with peak ACLR knee abduction moment (R² = 0.35;P = 0.01). At 6-months, KOOS sport score was related to peak abduction moment in the ACLR limb (R² = 0.23; P = 0.05). For change scores, the improvement in pain scores related to ACLR limb peak knee abduction moment (R² = 0.55; P = 0.001). The ACLR limb had lower knee excursion, extension moments, and ground-reaction forces compared with the uninvolved and control limb. The uninvolved limb also had higher ground-reaction forces compared with the ACLR limb and control limb.

Conclusion:

These results suggest that patient-reported outcomes 1 and 6 months after surgery are associated with running mechanics 6 months after ACLR. Further, the underloading present in the ACLR limb and overloading in the uninvolved limb indicates greater need for running rehabilitation after ACLR.

Clinical Relevance:

Understanding pain and how it may be linked to movement dysfunction is important for improving long-term outcomes.

Anterior cruciate ligament (ACL) injury commonly results in altered knee biomechanics that lead to posttraumatic knee osteoarthritis (PTOA) present in approximately one-half of patients within 15 years of reconstruction.^3,24,26 The onset of PTOA has been linked to several post-ACL injury and ACL reconstruction (ACLR) detriments, including altered loading patterns, ¹¹ and reduced patient-reported function. ¹⁶ After surgery, patients have presented with less knee range of motion and underloading on the ACLR limb compared with the contralateral limb,^27,28,31,38 and have been linked to altered knee function, ³¹ knee cartilage thinning, ²⁹ and biochemical markers for PTOA measured via magnetic resonance imaging.^32,37 As such, altered knee mechanics post-ACLR have been linked to an increased risk of secondary knee injury, and the development of PTOA.^6,11,24

A return to running after ACLR is a necessary step in rehabilitation, especially for those returning to sport,^1,27 and requires different muscular requirements and loading profiles compared with gait, in part due to the absence of a doubl-limb support phase. ²⁵ Aberrant running mechanics have been established in athletes as early as 4 months after ACLR,^21,33 and continue to be apparent for years after surgery.^10,28 After ACLR, during running, athletes are reported to have decreased knee flexion angles and sagittal plane knee moments during stance phase compared with their contralateral limb,^{14,17,19,28,33} and also to control individuals.^14,21,28,33 Although it is possible that, while the ACLR limb is under loading compared with both the contralateral limb and a control limb, the contralateral limb may not be similar to control running mechanics (ie, running biomechanics in those without ACLR). Thus, further investigation into the comparison of the ACLR limb and uninvolved contralateral limb running mechanics, to control limb running mechanics, is necessary for adequately understanding how we can improve running rehabilitation.

Further, self-perceived knee function has been identified as a limiting factor to ACLR rehabilitation, and patients who present with lower self-perceived knee function are shown to be at a greater risk for secondary injuries and PTOA.^22,23,41 Research has identified that patients after ACLR who have increased levels of pain during everyday activities are more likely to have decreased hop performance 1 year after surgery. ¹³ In addition, increased levels of pain before surgery have been associated with altered knee mechanics during gait 6 months after surgery, ⁵ but little information exists on how pain contributes to running mechanics later in rehabilitation. If we can address patient function earlier in rehabilitation, we may be able to contribute to improve the cascade of negative biomechanical function and PTOA.

While altered mechanics after surgery are believed to be contributors to greater declines in knee joint health later in life, there is little understanding how patient function and pain early after surgery (<2 months) may contribute to altered mechanics later in rehabilitation. Further, repetitive underloading on the ACLR limb, such as during running, may be a contributing factor to poor knee joint health, and those at higher risks may be able to be identified early in the rehabilitation timeline. Therefore, the primary purpose of this study was to determine whether patient-reported outcomes (assessed 1 and 6 months after surgery) after ACLR were associated with running mechanics, and secondarily to determine how loading during running in the ACLR limb is altered compared with the uninvolved contralateral limb and a control limb. We first hypothesized that patients with worse self-reported pain after ACLR would have altered loading mechanics during running 6 months after surgery. We also hypothesized that patients who have undergone ACLR will exhibit underloading on their ACLR limb compared with their uninvolved limb and a control limb.

Methods

Research Design

This pilot study is a secondary analysis cohort study of a randomized clinical trial (registered clinical trial NCT03429140). The evidence level II, University of Kentucky institutional review board-approved double-blind randomized controlled pilot study was developed to understand the effects of intraarticular hyaluronic acid administered 1 week post-ACLR. Patients were enrolled before surgery and data from the 1-and 6-month postoperative timepoints were analyzed in this study. There were no differences in patient-reported outcomes or knee biomechanics at either time point between the treatment groups, ⁷ and treatment group is controlled for in this analysis.

Subjects and Rehabilitation

A total of 24 patients between the ages of 14 and 33 years who suffered an ACL tear during sport participation and presented within 11 days of injury were enrolled prospectively in this study. In our final pool of patients, 10 received a sodium hyaluronate injection and 10 received a placebo. Patients were excluded if they had previous surgery on either knee, open growth plates, posterior cruciate ligament tear, collateral ligament sprain grade >2, presence of an inflammatory disease, or other concomitant injury beyond a meniscal tear. Patients were recruited from 1 surgical site between 2017 and 2018 and were prescribed a standard ACLR rehabilitation protocol including range of motion exercises, quadriceps strengthening, and functional training. Patients typically began functional training 3 to 4 months postoperatively and started a jogging progression between 4 and 6 months postoperatively. All patients followed standard of care rehabilitation guidelines from their orthopaedic surgeon as directed by a clinician (physical therapist or athletic trainer). Further, a representative sample of 20 control participants were recruited to collect running mechanics data for comparison. Control individuals were included if they were at minimum recreationally active (participation in recreational sport or exercise, including participation in a run, walk, jog program for at least a year), ¹⁵ had never undergone surgery on their lower extremity, and had not sustained a lower extremity injury in the previous 12 months.

Patient-Reported Outcomes

At 1 month after ACLR, patients completed a visual analog scale (VAS) for knee pain and Knee Injury and Osteoarthritis Outcomes Score (KOOS) for function. To measure VAS, patients were presented with a 100-mm line and asked to make a mark on the line that indicated their pain on a scale from 0 to 10, with 10 being the worst pain.^9,13 Patients completed the KOOS questionnaire with scores for each of the 5 sections including knee symptoms, pain, quality of life, activities of daily living (ADL), and sport and recreation (SR). KOOS subscale scores range from 0 to 100, with 100 being considered a healthy score. ³⁴

Biomechanics

At 6 months after ACLR, study participants underwent a biomechanical analysis during running, using a 14-camera 3-dimensional (3-D) motion capture system (Vicon) and 2 floor-mounted force plates (Bertec). All patients in this study had been cleared for running by their rehabilitation specialists. Participants were instructed to look straight ahead and run overground, along a 16 m runway, over the top of the 2 force plates. After participants completed a self-directed 5-minute warm-up, they completed 2 practice runs so researchers could adjust their speed, as they were required to keep their speed between 2.7 and 3.5 m/s. ¹⁸ If, during a trial, the participant ran faster or slower than this speed range, the trial was repeated. Run trials were also considered good trials when the participant had a complete foot strike on 1 force plate, as participants were instructed to look straight ahead while running to prevent an alteration in stride or mechanics. Three good trials were recorded.

Lower extremity segments were defined and tracked using 14-mm markers placed at bilateral iliac crests, anterior superior iliac spines, greater trochanters, medial and lateral femoral epicondyles, medial and lateral malleoli, proximal and distal heel, midfoot distal to the lateral malleoli, and first and fifth metatarsals. Tracking clusters, created by rigid thermoplastic with 4 markers affixed, were placed over the pelvis at the posterior superior iliac spine and bilaterally on the lateral aspect of the thighs and shanks. ¹⁸ Lower limb joint rotations were defined based on the coordinates of the reflective markers and rotations were calculated using the Cardan rotation sequence. Kinematic data were sampled at 200 Hz and marker trajectories were filtered using a fourth-order low-pass Butterworth filter with a cutoff of 12 Hz. Raw vertical ground-reaction forces (VGRF) were sampled at 1200 Hz and filtered using a fourth-order low-pass Butterworth filter with a cutoff of 12 Hz. Biomechanical data were analyzed using Visual3D (C-Motion). Joint kinetics were calculated using inverse dynamics methods and normalized to body mass by height; moments were recorded as internal joint moments. ³⁸ Involved limb knee excursion, peak VGRF, peak knee extension moments (KEM), and peak knee abduction moments (KAM) were identified during the stance phase of running in each trial, and averaged across the trials. Knee excursion was calculated as the difference between peak knee flexion angle during stance phase and the knee flexion angle at initial ground contact (when ground-reaction forces exceed 20 N).

Statistical Analysis

Comparisons in demographic variables (age, height, mass) between groups were assessed and are presented in Table 1. Change scores from 1-month patient-reported outcomes to 6-month patient-reported outcomes were calculated as follows: change score = 6-month score - 1-month score. Normality of all patient-reported outcomes (1-month, 6-month, and change scores) and biomechanical variables were assessed utilizing Shaprio-Wilks tests. Variables with a non-normal distribution utilized nonparametric tests (Spearman correlation coefficient) where appropriate.

Table 1.

Demographic and 1-month and 6-month patient-reported outcomes and predictor variables for patients who had undergone ACLR, and demographic information for control individuals, presented as mean (SD) ^a

Variable	ACLR	Control
Age, y	18.4 (2.7)	19.8 (3.6)
Sex	11 Females 9 Males	14 Females 6 Males
Height, m	1.7 (0.1)	1.7 (0.1)
Mass, kg	84.2 (19.4)	76.7 (9.9)
Graft type	16 BPTB 4 HAM
Meniscal injuries	16 meniscal tears
Meniscal surgery	12 meniscal repairs 4 meniscectomies
Time since surgery, mo	1.1 (0.3)
1-mo VAS	29.5 (26.7)
1-mo KOOS scores
Symptoms	63.7 (16.0)
Pain	72.1 (19.3)
ADL	76.3 (16.5)
QOL	43.1 (24.5)
SR	42.9 (39.1)
6-mo VAS	8.22 (12.8)
6-mo KOOS scores
Symptoms	86.9 (10.9)
Pain	92.7 (9.6)
ADL	98.8 (4.5)
QOL	71.8 (14.7_
SR	82.8 (16.0)

^aACLR, anterior cruciate ligament reconstruction; ADL, activities of daily living; BPTB, bone-patellar tendon-bone; HAM, hamstring tendon; KOOS, Knee Injury and Osteoarthritis Outcomes Score; QOL, quality of life; SR, sport and recreation; VAS, visual analog scale.

Patient-reported outcomes after surgery that were related to 6-month running biomechanics variables (P < 0.05) were included in the regression analyses. Specifically, univariate linear regressions were used to determine if related variables collected at 1 and 6 months after surgery, and change scores of patient-reported outcomes were associated with 6-month biomechanical variables, including involved limb knee excursion, peak VGRF, peak KAM, and peak KEM during running. Due to the high level of collinearity between VAS pain and KOOS pain levels, and between individual KOOS scores, the patient-reported outcome at each time point with the strongest relationship to the running biomechanics variable was selected for the model.

To further understand loading mechanics during running, paired t tests were used to compare the ACLR limb with the uninvolved contralateral limb, while independent t tests were used to compare the ACLR limb with the control group, as well as the uninvolved contralateral limb with the control limb. A Bonferroni-corrected α value (to account for a total of 10 comparisons) of P < 0.005 was used for all comparisons, using SPSS (SPSS 22, IBM Corporation).

Results

Of the 24 patients with ACLR who were enrolled in the study, 1 screen failed, 1 withdrew immediately after surgery, and 2 were lost to follow-up between the 1- to 6-month timepoint. The remaining 20 patients (83% follow-up, Table 1) completed testing at the 6-month timepoint (Table 2). Patients who had undergone ACLR ran at a speed of 3.12 ± 0.16 m/s. A total of 20 patients were enrolled in the control group (Table 1). The control group used for comparison ran at a speed of 3.12 ±0.10 m/s. There were no differences in age, height, or mass between groups.

Table 2.

Six-month predicted variables presented as mean (±SD) ^a

Variable	ACLR Limb	Uninvolved Limb	Control Limb
Time since surgery, mo	6.1 (0.2)
Knee excursion, deg	20.58 (4.69)*,**	27.87 (4.30)	28.81 (3.25)
Peak KEM, Nm/kg.m	1.19 (0.74)*,**	2.04 (0.68)	1.83 (0.42)
Peak KAM, Nm/kg	-0.42 (0.21)	-0.49 (0.43)	-0.45 (0.26)
Peak VGRF, × BW	2.22 (0.20)*,**	2.72 (0.33)***	2.41 (0.17)

^aBW, body weight; KAM, knee abduction moments; KEM, knee extension moments; VGRF, vertical ground-reaction forces.

^*Significant difference between the ACLR limb and the uninvolved limb; **significant difference between the ACLR limb and the control limb; ***significant difference between the uninvolved limb and the control limb. Significance determined by P < 0.05.

Patient-Reported Outcomes Associated With Biomechanics

The 1-month VAS pain scores (r = -0.522; P = 0.03), KOOS pain scores (r = 0.574; P = 0.02), and KOOS ADL (r = 0.542; P = 0.03) scores were related to peak KAM on the ACLR limb during running. The overall model that included KOOS pain was significantly associated with peak ACLR KAM (Figure 1;R² = 0.35; F (1,18) = 8.14; β = 0.07; P = 0.01). No other 1-month patient-reported outcomes were associated with 6-month running biomechanics, including VGRF, KEM, or knee excursion in the ACLR or uninvolved limb. At the 6-month time, KOOS sport was related to 6-month peak KAM in the ACLR limb (r = 0.618, P = 0.008).The univariate regression model including KOOS sport was significantly associated with peak ACLR KAM during running (Figure 2; R² = 0.23; F(1,18) = 4.71; β = 0.007; P = 0.05). There were no other 6-month patient-reported outcomes related to running biomechanics in either the ACLR or uninvolved limb.

Figure 1.

Predicted over observed KAM values (Nm/kg.m) during the stance phase of running, in which the model includes KOOS pain scores measures 1-month postsurgery. KAM, knee abduction moment; KOOS, Knee Injury and Osteoarthritis Outcomes Score.

Figure 2.

Predicted over observed KAM values (Nm/kg.m) during the stance phase of running in which the model includes 6-month KOOS SR scores. KAM, knee abduction moment; KOOS, Knee Injury and Osteoarthritis Outcomes Score; SR, sport and recreation.

When assessing change scores in patient-reported outcomes, the improvement in VAS score (r = 0.726; P = 0.001) and KOOS ADL (r = -0.557; P = 0.02) from 1- to 6-months post-ACLR were related to ACLR limb peak KAM. A model including the change in VAS pain score was significantly associated with ACLR KAM (Figure 3; R² = 0.55; F(1,17) = 18.69; β = 0.01; P = 0.001). There were no relationships between any other change scores for patient-reported outcomes to 6-month VGRF, KEM, or knee excursion in the ACLR limb.

Figure 3.

Predicted over observed KAM values (Nm/kg.m) during the stance phase of running, in which the model includes the change of the VAS from 1-month postsurgery to 6-months postsurgery. KAM, knee abduction moment; VAS, visual analog scale.

Limb Differences During Running

In the ACLR limb, there was less knee excursion (Figure 4b; P = 0.0001), smaller KEM (Figure 4a; P = 0.0001), and lower VGRF (Figure 4d; P = 0.0001) during the stance phase of running compared with the uninvolved, contralateral limb. When compared with a control limb, the ACLR limb presented with less knee excursion (Figure 4b; P = 0.0001), lower KEM (Figure 4a; P = 0.003), and lower VGRF (Figure 4d; P = 0.005) during the stance phase of running. In addition, the uninvolved contralateral limb had significantly greater VGRF (P = 0.002) when compared with a control limb. Knee excursion and KEM were not different between the uninvolved limb and the control limb. There were also no differences between any limbs in the KAM (Figure 4c).

Figure 4.

Ensemble averages for (a) sagittal plane knee moment, (b) knee flexion angle, (c) frontal plane knee moment, and (d) VGRF during the stance phase of running. Solid black lines, ACLR limb; dashed black lines, contralateral limb, solid gray lines, control limb. ACL, anterior cruciate ligament; ACLR, anterior cruciate ligament reconstruction; BW, body weight; VGRF, vertical ground-reaction force.

Discussion

The primary purpose of this study was to determine whether patient-reported outcomes and pain after surgery were related to 6-month postoperative running mechanics. Overall, our primary hypothesis was partially supported as pain 1 month after surgery was related to frontal plane knee loading during running, though not necessarily providing the direction of the relationship that we expected. Interestingly, higher levels of pain 1 month after surgery were related to high KAM during running 6 months after surgery. Further, 6 months after ACLR, those with better KOOS SR scores were also more likely to have a smaller KAM during running. As for the change in patient-reported outcomes, those with a greater decrease in pain from 1-month postsurgery to 6-months postsurgery were more likely to present with increased KAM during running. Interestingly, pain 6 months after surgery was not related to KAM. While these associations of pain to running mechanics were not as expected, they posit interesting theories related to the underloading of knee biomechanics after ACLR.

KOOS pain measured 1 month after surgery had the strongest association with running mechanics 6 months after surgery. In previous cross-sectional analyses, Perraton et al ¹⁴ reported that those who presented with inferior Cincinnati Knee Rating Scale scores 12 to 24 months after ACLR had smaller VGRF and knee flexion moments during running. Similarly, in our longitudinal analyses, we identified that those who presented with higher levels of pain early after surgery, later presented with lower frontal plane knee moments 6 months after ACLR, but lacked a relationship to VGRF. While controlling pain early after surgery is important for a successful rehabilitation, appropriate knee loading to preserve cartilage is also important. It may be possible that those who are experiencing less pain may be doing so exactly because they are loading the knee less. Although this may explain the relationship between pain and knee loading, it does not necessarily provide a positive outcome for patient mechanics. This is also somewhat supported by the relationship we identify in the greater decrease of pain scores from 1 to 6 months postsurgery is predicting a significant amount of variance of greater peak KAM. While addressing the underlying mechanisms of pain is beyond the scope of this study, further understanding on whether prolonged pain may inhibit functionality during rehabilitation, or appropriate cartilage health, due to altered loading profiles may be warranted. In addition, the relationship between the decrease in pain from 1 month after surgery to 6 months after surgery to frontal plane knee loading provides an interesting addition to the literature. While it looks to be driven by 1 individual, even with the removal of this participant the model was still significant with R² = 0.30 (P = 0.02). Thus, as a greater decrease in pain throughout rehabilitation was able to explain 55% of the variance of frontal plane knee loading during running 6 months after surgery, it may be possible for patients to overcome loading deficits due to the improvement in overall pain and function throughout rehabilitation. The biopsychosocial responses to ACLR, including pain and fear avoidance, have explained functional deficits after rehabilitation and should continue to be incorporated in rehabilitation to improve long-term outcomes.^20,35,39

In our secondary analysis of this study, we determined that running mechanics in the ACLR limb 6 months after surgery are altered compared with the contralateral limb and compared with control individuals. We identified that the ACLR limb presented with less knee flexion excursion, lower KEM, and lower peak VGRF during running, compared with the uninvolved contralateral limb and the control limb. The lower KEM during running we found in the ACLR limb has been previously related to reduced quadriceps strength and quadriceps avoidance strategies,^8,17,28 which are common in an ACLR population. Further, our study agrees with previous studies reporting reduced knee flexion excursion and lower sagittal plane knee moments in the ACLR limb compared with the uninvolved limb during running.^27,28 Contralateral limb ACL injuries are common after an initial ACLR,^40,41 and the large asymmetry in KEM between the contralateral limb and the ACLR limb may be a contributing factor, and is similar to other patients who have sustained contralateral ACL injuries. ³⁰ This underloading of the ACLR limb has been commonly linked to tissue alterations in the knee in which greater knee flexion excursion during walking was associated with thicker medial and lateral femoral cartilage. ²⁹ Further, Shimizu et al ³⁷ identified that smaller sagittal plane knee moments during landing were associated with greater femoral and tibial T1ρ markers, indicating that this underloading of the ACLR limb is likely indicative of a greater risk for future PTOA.

There are apparent loading differences between the ACLR limb, the uninvolved limb, and a control limb, as the ACLR limb had significantly lower peak KEM compared with both the uninvolved limb and the control limb. In addition, both the uninvolved and control limbs exhibit similar knee excursion measures during the stance phase of running, while the ACLR limb is moving through considerably less knee flexion excursion. The uninvolved contralateral limb presented with greater VGRF than the control participants, though both exhibit similar knee ranges of motion. In addition, while patients after ACLR may be underloading their involved limb, they may also be overloading their contralateral limb, indicating that compensation strategies are detrimental to both limbs. As there is no defined healthy level of loading during running, the defined differences between limbs and groups allow only for theoretical ideas of underloading and overloading, especially as the comparison of VGRF between the ACLR limb and the contralateral uninvolved limb are inconsistent throughout research. ²⁷ In our case, VGRF was greatest for the uninvolved limb (Table 2; 2.72 × body weight [BW]) after by the control limb (2.41 × BW) with the lowest VGRF being demonstrated by the ACLR limb (2.22 × BW). Although there is interestingly no difference between the contralateral limb and the control limb in peak KEM and knee excursion, it may be possible that the knee flexion angle during walking is more so driving the changes in KEM during running than the VGRF.

There is a continuing need to address the altered running mechanics of both the ACLR and contralateral limbs, and determining new clinical strategies to improve aberrant running mechanics is critical for ACLR rehabilitation. Sigward et al ³⁸ identified that limb asymmetries in knee loading during walking present 1 month after reconstruction persist during running 4 months postreconstruction, and such limb asymmetries early after surgery relate to limb asymmetries later in rehabilitation. Thus, we believe that those who are under loading during running are also likely under loading during walking on a daily basis. During walking, greater frontal plane moments have been identified as significant predictors of knee osteoarthritis progression^2,36; although this relationship between knee mechanics and PTOA has not been defined during running, we theorize that they may be related. In conjunction, if we can identify such loading alterations early in the running rehabilitation process, we may be able to target different clinical therapies to lessen the amount of time an person spends with poor loading patterns. There are likely many contributors to the aberrant running mechanics we found in this study, such as quadriceps weakness and function,^4,19,28 though the mechanisms of altered biomechanics are not always straightforward. Extensive biopsychosocial factors were not included in these analyses, though this information would provide an additional layer to the recovery process beyond more accustomed patient-reported outcomes and there is benefit to assess biopsychosocial factors regarding ACLR injury and their relationship to running biomechanics.

This research has major limitations as a pilot study. The sample of this study was very small but was an important first step in identifying a relationship between early postoperative pain and function on running mechanics. Further, while all participants consistently engaged in running as a part of their rehabilitation and sport, assessing their habitual knee loading is not relevant to those who are not classified as runners. In addition, there are several factors when conducting a running analysis in patients after ACLR that were not controlled for in this study (due to the small sample size to avoid overfitting) including meniscal pathology, foot strike patterns, footwear, and speed. Controlling for meniscal injury, assessing a standard running speed, and nonstandardized footwear can lead to fluctuations and alterations in mechanics, and additional studies accounting for such variables would be beneficial. We also recognize that long-term follow-ups for this population are the standard. Even if sagittal plane knee moments and ground-reaction forces increase beyond what is recorded at the 6-month time point, when a person exhibits an altered loading profile, it can be detrimental to their cartilage health, ¹² thus addressing factors that may contribute to aberrant running mechanics as early as possible in the rehabilitation process is imperative to knee joint health after ACLR.

In conclusion, 1-month pain and the change in pain scores were significantly associated running mechanics 6 months after surgery. Those with higher levels of pain early after surgery and had greater reductions in pain from 1 to 6 months after surgery presented with greater KAM during the stance phase of running. Identifying early measures (whether it be pain or knee loading mechanics) that may influence later levels of function is critical to aiding rehabilitation post-ACLR to achieve successful outcomes more consistently. In addition, while less loading is associated with lower levels of pain, it is possible that this decrease in knee loading is contributing to a hindrance of achieving a desired symmetrical level of loading postsurgery.