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. Author manuscript; available in PMC: 2026 Jan 6.
Published in final edited form as: Med Sci Sports Exerc. 2025 Jun 16;57(11):2460–2467. doi: 10.1249/MSS.0000000000003787

Relationship Between Changes in Physical Activity and Knee Health 18 months after ACL Reconstruction

David M Werner 1, Yvonne M Golightly 2, Michael D Rosenthal 3, Balarinivasa Sajja 4, Chris Wichman 5, Melissa Manzer 6, Matt Tao 7, Elizabeth Wellsandt 8,*
PMCID: PMC12767282  NIHMSID: NIHMS2119162  PMID: 40523235

Abstract

Purpose:

This study aimed to determine the relationship between changes in daily physical activity (PA) and knee health 18 months after anterior cruciate ligament reconstruction (ACLR). Knee health was defined using structural (quantitative magnetic resonance imaging [MRI]) and functional (patient-reported and objectively-measured knee function) constructs.

Methods:

Eighteen individuals (83.3% female, 19.7±5.6 years old, BMI 23.9±3.7 kg/m2) completed testing. Daily steps over one week and structural cartilage health, measured using a waist-worn accelerometer (Actigraph wGT3X-BT) and T2 relaxation time on MRI, respectively, were collected six and 18 months after ACLR. Eighteen months after ACLR patient-reported and objectively measured knee health were assessed using the International Knee Documentation Committee Subjective Knee Form (IKDC) and isometric quadriceps strength, respectively. A linear regression model was used to test the relationship between the change in PA from six to 18 months after ACLR and the percent change in T2 relaxation time of four cartilage regions (lateral and medial femoral and tibial cartilage) from six to 18 months after ACLR. A Fisher’s Exact test assessed the relationship between change in PA (increase/decrease) between six and 18 months after ACLR and adequate/inadequate knee function using patient-reported (IKDC) and objectively measured (quadriceps strength) knee function 18 months after ACLR.

Results:

Participants averaged 7547.3±2439.7 daily steps six months after ACLR and 7504.9±3736.3 daily steps 18 months after ACLR. There was no association between change in PA and structural knee health (p=0.069) or knee function (p=0.638).

Conclusions:

Average daily steps did not change from six to 18 months after ACLR. PA from six to 18 months after ACLR was not associated with knee health outcomes at 18 months.

Keywords: cartilage, disability, knee function, walking

Introduction

Worldwide, the incidence of anterior cruciate ligament (ACL) injuries with subsequent surgical ACL reconstructions (ACLR) has increased by up to 143% over the last ten years.(1–5) After undergoing an ACLR, individuals are at increased risk of developing knee osteoarthritis (OA). A definition of OA that includes structural signs and patient symptoms (6, 7) allows a more comprehensive understanding of physiologic changes and the patient’s perception during OA development and progression, facilitates better-designed interventions, and promotes better patient outcomes. This more comprehensive definition of OA can be applied to knee health after ACLR, promoting a holistic view of knee health after ACLR. Structural OA, referred to as the disease of OA, describes the physical and physiological changes that occur locally within the joint and globally within the patient and typically requires more advanced testing (e.g. measure of cartilage and bone structure via imaging or bloodwork). Regarding structural knee health after ACLR, up to 82% of individuals have structural changes related to knee OA on radiographs within 15 years of ACL injury and ACLR.(8, 9) Symptomatic OA, referred to as the illness of OA, describes the patient’s experience of OA and typically requires clinical examination (e.g., measure of pain, function, joint stiffness, quality of life). (7) As early as six months after ACL injury and subsequent ACLR, up to 54% of individuals report symptoms of reduced knee function that could indicate knee OA risk.(10, 11) An important factor related to knee health after ACLR is quadriceps strength, as worse quadriceps strength has been linked to greater rates of clinical OA at 5 years after ACLR.(12)

Physical activity (PA) participation is connected to the progression of functional limitations related to OA in individuals with and at risk for OA.(13–16) In individuals with an ACL injury and subsequent ACLR, there is a relationship between PA, as measured by daily steps, and structural knee health, specifically articular cartilage health. This relationship is hypothesized due to articular cartilage requiring cyclic loading to promote a healthy extracellular environment. (17–19) In one study, of individuals within one month of ACL injury an increase in daily steps was associated with worse articular cartilage health as measured by compositionally-MRI assessed T2 relaxation time (an indicator of extracellular water content).(20) However, other studies have shown the opposite relationship. In males, between six and 12 months after ACLR reduced daily steps were associated with reduced cartilage health, as assessed by compositionally-MRI assessed T1rho time (an indicator of proteoglycan content),(21) and, at least 10 months after ACLR reduced daily steps were associated with reduced cartilage health as assessed by serum levels of cartilage oligomeric matrix protein (an indicator of cartilage degradation).(22) There is also a potential relationship between PA and patient-reported and objectively measured assessments of knee health in individuals from three months to five years after ACLR. (23–26) For example, after ACLR, individuals who spend more time in moderate to vigorous PA report greater quality of life. (25) While individual studies have investigated either structural or functional signs of knee health after ACLR, there is limited evidence regarding the combination of the structural and functional signs of knee health in the assessment of individuals after ACLR. To better understand the entire recovery process, a comprehensive approach to knee health that combines structural and functional assessments of knee health is needed.

As there appears to be a relationship between PA and overall knee health, the purpose of this study was to determine the relationship between changes in objectively measured PA with both structural and functional signs of knee health in individuals after ACLR. Structural signs of knee health were assessed using changes in compositional MRI, specifically T2 relaxation time from six to 18 months after ACLR. (27) Functional signs of knee health were measured 18 months after ACLR using the International Knee Documentation Committee Subjective Knee Form 2000 (IKDC), one of the most frequently used patient-reported outcomes measures after ACLR(28), as well as isometric quadriceps strength, a commonly used objective assessment of knee function.(29–32). We hypothesized that an increase in PA from six to 18 months after ACLR would be associated with less structural change in cartilage and fewer individuals with symptoms of poor knee health 18 months after ACLR.

Methods

Participants

This study analyzed individuals from a larger prospective cohort study who were available for follow-up at 18 months after ACLR. The original study followed 40 individuals from the time of ACL injury through six months after surgery. Of the original 40 individuals, 11 were unavailable at 18 months for follow-up. Eight of the 11 did not consent to be followed past six months, two moved out of state, and one withdrew from the study prior to collecting six month data. Of the remaining 29 potential participants from the original study, 18 had complete data for the present study and were included in this analysis. Individuals were included in the original cohort study if they were between the ages of 15 and 35 and could enroll within one month of their ACL injury but before undergoing an ACLR. Individuals were excluded if they had previous injury or surgery to either knee, concomitant and symptomatic grade III tear to other knee ligaments, anticipated meniscectomy by treating orthopedic surgeon, chondral lesions or degenerative cartilage changes noted on pre-operative MRI, or open growth plates altering the ACLR technique. Additionally, individuals were excluded if they had a history of inflammatory disease, were immunocompromised, had chronic use of nonsteroidal anti-inflammatory medications, underwent a cortisone injection to their knee within the prior three months, were pregnant or had any contraindications to MRI. All participants provided written informed consent as approved by the Institutional Review Board at the University of Nebraska Medical Center (UNMC).

Objectively Assessed Physical Activity

Individuals wore a waist-worn accelerometer (Actigraph wGT3X-BT) for one week of waking hours at six and 18 months after ACLR. This monitor is a valid and reliable device to objectively assess PA. (33–35) A valid week of wear required at least ten hours of wear for at least four of the seven days. Nonwear time was defined as any period of at least 90 consecutive minutes with no activity counts. (36) Data were downloaded and processed in ActiLife software (ActiLife v6.13.4, ActiGraph Corp Pensacola, FL) to calculate daily step counts and daily minutes of moderate-to-vigorous physical activity (MVPA). Activity level is assigned by the ActiLife software using acceleration counts, with higher counts equating to greater intensity. Acceleration counts were totaled for every one-minute interval, which subsequently allowed for categorizing PA into one of four categories: sedentary (0–99 counts/minute), light (100–2019 counts/minute), moderate (2020–5998 counts/minute), or vigorous (≥5999 counts/minute). (37) Daily steps and daily minutes of MVPA were measured at six and 18 months. Change in daily step counts and daily MVPA was calculated from six to 18 months, such that a positive value indicated an increase in average PA, and a negative value indicated a reduction in average PA.

Structural Measure of Knee Health

T2 relaxation times measured from MRI were used to evaluate the structural signs of OA. T2 relaxation time is a marker of the water content and organization of the cartilage extracellular matrix.(38) A longer T2 relaxation time indicates higher concentrations of water and a more disorganized extracellular matrix, suggesting worse cartilage health. (39)

MRI data of the injured limb were collected and processed at six and 18 months after ACLR, as previously described. (20) Briefly, MRI data were collected using a 3-Tesla Phillips Ingenia MRI scanner using a 16-channel transmit/receive knee coil (Phillips North America Corporation). Each MRI was performed between 4:15 pm and 5:45 pm to account for the impact of daily activity on quantitative MRI readings.(40) Participants were positioned in supine with their knee minimally flexed and neutrally rotated. A spin-echo sequence with multiple echoes was acquired with the following parameters: Repetition Time = 2700 msec; 10 echoes with the echo times at 10 msec increments (10, 20, 30,…100); Field of View: 120 × 120 mm; Acquisition Matrix = 252 × 250; Slice Thickness = 3.0 mm; Slice Gap = 0.5 mm; Range of Slices = 23–31; Pixel Size = 0.3125 × 0.3125; Echo Train Length = 10; Number of Averages = 1. An additional fat-suppressed proton density-weighted spin-echo sequence in axial, coronal, and sagittal orientations, and a sagittal T1-weighted spin-echo were also included.

T2 relaxation time was computed by fitting the multi-echo MRI data to the signal modal equation: (Si) = S0 exp(−TEi / T2) where S0 is the signal at echo time = 0 ms and TEi is the ith echo time]. This data fitting was realized using Levenberg-Marquardt nonlinear least squares algorithm at each pixel to generate T2 maps.(41)

Medial and lateral tibiofemoral compartments of each knee were identified using the center of the intercondylar notch for the femur and the tibia, followed by manual segmentation using ITK-Snap software (Figure 1).(42) The weightbearing portions of the femur and tibia were defined as the regions within the boundaries of the menisci. The four cartilage regions of interest for this study were the medial and lateral weightbearing portions of the tibial (MTC and LTC, respectively) and femoral cartilage (MFC and LFC, respectively). Percent change of the average T2 relaxation time from six to 18 months in each of these four regions was the variable of interest.(43)

Figure 1:

Figure 1:

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MRI of a participant demonstrating: a) pixel by pixel T2 relaxation map, and b) articular cartilage regions of interest delineated by location of meniscus (black triangles). Red brackets indicate weight bearing region.

Functional Knee Health

Patient-Reported Knee Symptoms and Function

Participants completed the IKDC at 18 months after ACLR to measure patient-perceived symptoms and function. Study data were collected and managed using REDCap electronic data capture tools hosted at UNMC. REDCap (Research Electronic Data Capture) is a secure, web-based application designed to support data capture for research studies. REDCap at UNMC is supported by Research IT Office funded by Vice Chancellor for Research (VCR). This publication’s contents are the sole responsibility of the authors and do not necessarily represent the official views of the VCR and NIH. (44, 45) The IKDC is a valid and reliable assessment of knee-related pain, symptoms, and function after ACLR.(46–48) The IKDC uses a scale of 0–100%, with higher values representing better knee function. Various cutoff scores for the IKDC have been proposed to help identify adequate patient-reported knee function.(31, 49–52) Because our participants completed the IKDC at a time further from ACLR (18 months) than many previous studies, the highest cutoff score recommendation of 90% was selected for our study to define adequate patient-reported knee function.(49) Individuals who scored under 90% at 18 months after ACLR were categorized as having inadequate patient-reported knee function, while those at or above 90% were categorized as having adequate patient-reported knee function after ACLR.

Objectively Assessed Knee Function

To objectively assess knee function at 18 months after ACLR, isometric quadriceps strength testing at 90 degrees of knee flexion was performed using an electromechanical dynamometer (Biodex System 4 Pro, Biodex Medical Systems, Shirley, NY). After a standardized warmup of treadmill walking and bodyweight exercises, participants were secured with straps to ensure their trunk was upright, their hips and test knee were flexed to 90 degrees, and the pad of the dynamometer was secured two inches above their lateral malleolus.(53) After three warm-up trials at 50%, 75%, and 100% maximal effort, participants completed three five-second maximal trials with at least 60 seconds of rest between each trial on the uninvolved limb first, followed by the involved limb. Participants were instructed to kick out against the shin pad “as hard and fast as possible,” with verbal encouragement throughout the length of each effort provided by the tester. Using the maximum value during any of the three trials in each limb, a limb symmetry index was calculated using the following formula: [(involved/uninvolved)] x 100%). A commonly recommended cutoff for isometric quadriceps strength limb symmetry is 90%.(50) Therefore, 90% was used as the threshold to define adequate quadriceps strength. Individuals below 90% were identified as having inadequate objectively measured knee function, while those at or above 90% were considered to have adequate objectively measured knee function after ACLR.

Analytical Approach

All statistics were completed using SPSS version 29.0 (IBM, IBM Corp., Armonk, NY, USA). For continuous data, means, standard deviations, and 95% confidence intervals were calculated. For nominal data, frequencies and proportions were calculated. A priori α was set to 0.05 for all statistical tests.

Structural Measure of Knee Health

Linear regression models were used to test the relationship between change in PA (mean daily step counts, mean minutes of daily MVPA) from six to 18 months and the percentage change in cartilage T2 relaxation times from six to 18 months after ACLR in each of the four weightbearing knee cartilage regions of interest (LTC, LFC, MTC, MFC), while controlling for PA level at six months. PA levels at six months were controlled for in models to account for each individual participants’ capacity for changing PA over time since participants with a lower PA level at six months after ACLR would have a higher capacity to increase their PA. A Benjamini-Hochberg False-Discovery Rate(54) correction was applied within each comparison to reduce the likelihood of Type I errors. There was a high correlation between six-month daily steps and daily MVPA, as well as six- to 18-month changes in daily steps and MVPA (six-months: r = 0.866, p<0.001; change from six- to 18-months: 0.888, p <0.001). Therefore, the results of daily step counts are presented below, and the results of the MVPA analyses are presented in Appendix 1.

Functional Knee Health

Participants with an IKDC score ≥ 90% and isometric quadriceps strength limb symmetry ≥ 90% at 18 months after ACLR were categorized as having adequate overall knee function (n = 8). Any participant that scored below 90% on either IKDC or isometric quadriceps strength limb symmetry was categorized as having inadequate overall knee function (n = 10). Participants were also separated into two categories of PA change: those who increased their steps per day from six to 18 months (n = 8) and those who decreased (n = 10). A Fisher’s Exact test was used to assess the relationship between change in step counts (increased/decreased) and knee function (adequate/inadequate). As the outcome variable of knee function is dichotomous (adequate/inadequate), a logistic regression model could be used to assess the impact of change in PA on the odds of adequate knee function; however, due to our small sample size in comparison to recommended sample sizes for logistic regression,(55) a logistic regression model was not used in the primary analysis. Instead, it was used to explore associations between change in daily steps from six to 18 months after ACLR with adequate knee function at 18 months, after controlling for daily steps at six months after surgery. The relationship between MVPA and knee function was not assessed, as 50% (9/18) of the individuals had a change in MVPA less than 15 minutes per day, limiting the creation of meaningfully different PA groups. Post hoc comparisons of participant characteristics and PA levels between the adequate and inadequate knee function status groups were conducted using a Mann-Whitney U test for continuous data and Fisher’s Exact tests for proportional comparisons. The nonparametric, Mann-Whitney U test was used due to the small sample size.

Results

Demographic data for the participants analyzed are presented in Table 1. There was an average decrease of 98±2563 steps per day (95% Confidence Interval of change: −1373 to 1176 steps per day) (Figure 2). In participants who increased their PA from six to 18 months, there was an average increase of 2098.4 ± 1411.9 steps per day, while those who decreased their PA from six to 18 months had an average decrease of 1754.9 ± 1724.6 steps per day. The effect size (Cohen’s d) between groups was calculated to be large at 2.42. In comparing participants who completed all data collections to those who dropped out, a higher proportion of female participants completed all data collections compared to male participants (p=0.010). Additionally, those who completed all follow up testing reported lower function on the IKDC 6 months after ACLR (73.6 ± 8.5 for completed vs 80.8 ± 11.1 for not completed, p = 0.049). There were no differences in BMI, quadriceps strength six months after ACLR, and PA six months after ACLR between those who were included and excluded in this analysis (p>0.13 for all).

Table 1:

Participant demographic and clinical information.

Frequency or Mean (SD) 95% CI
Female:Male 15:3 -
Age (years) 19.7 (5.6) 16.9 – 22.5
BMI (kg/m2) 23.9 (3.7) 22.0 – 25.7
Concomitant meniscus repair (Yes:No) 9:9 -
Graft type (HS:PT:QT) 2:12:4 -
Daily steps - 6 months after ACLR 7547.3 (2439.7) 6334.1 – 8760.5
Daily steps - 18 months after ACLR 7504.9 (3736.3) 5646.9 – 9363.0
Δ daily steps 6 to 18 months after ACLR −42.3 (2505.4) −1199.8 – 1115.1
Daily MVPA - 6 months after ACLR (minutes) 34.5 (20.6) 24.3 – 44.8
Daily MVPA -18 months after ACLR (minutes) 32.7 (30.0) 17.8 – 47.6
Δ MVPA 6 to 18 months after ACLR (minutes) −1.8 (23.8) −12.8 – 9.2
IKDC score – 18 months after ACLR 92.5 (6.9) 89.0 – 95.9
Quadriceps LSI– 18 months after ACLR 87.4 (14.8) 80.0 – 94.7
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Abbreviations: SD: standard deviation; CI: confidence interval BMI: body mass index; kg: kilogram; m: meter; ACLR: anterior cruciate ligament reconstruction; HS: Hamstring; PT: Patellar tendon; QT: Quadriceps tendon; Δ: change in; MVPA: moderate-to-vigorous physical activity; IKDC: International Knee Documentation Committee; LSI: Limb symmetry index

Figure 2:

Figure 2:

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Plot of the average daily steps at 6 and 18 months after anterior cruciate ligament reconstruction (ACLR) for each participant. The solid line indicates participants who increased daily steps over time while the dashed line indicates participants who decreased daily steps over time.

Structural Knee Health

The mean T2 relaxation time for each region at each time point and the mean percent change between time points are reported in Table 2. No region had a percent change in T2 relaxation time greater than 1.7% from six to 18 months after ACLR.

Table 2:

T2 relaxation times 6 and 18 months after ACLR (Mean ± Standard Deviation [95% Confidence Interval]).

6 months after ACLR (ms) 18 months after ACLR (ms) % Change
LFC 48.7 ± 2.5 [47.5 – 50.0] 49.1 ± 3.6 [47.3 – 50.9] 0.8 ± 4.2 [−1.3 – 2.9]
LTC 42.7 ± 2.5 [41.4 – 44.0] 43.4 ± 2.5 [42.1 – 44.7] 1.7 ± 4.9 [−0.7 – 4.1]
MFC 50.0 ± 2.8 [48.6 – 51.3] 49.4 ± 3.5 [47.7 – 51.2] −1.0 ± 5.8 [−3.8 – 1.9]
MTC 45.7 ± 2.5 [44.4 – 47.0] 45.7 ± 3.0 [44.3 – 47.2] 0.2 ± 5.4 [−2.5 – 2.8]
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Abbreviations: LFC: lateral femoral cartilage; LTC: lateral tibial cartilage; MFC: medial femoral cartilage; MTC: medial tibial cartilage; ACLR: anterior cruciate ligament reconstruction; ms: milliseconds, %, percentage.

The results of the multiple linear regressions are presented in Table 3. There was no significant relationship between change in daily steps with percent change in T2 relaxation time in any region from six to 18 months after ACLR, after accounting for daily steps at six months after ACLR.

Table 3:

Results of the linear regression models with daily steps at 6 months after ACLR and change in daily steps from 6 to 18 months after ACLR as independent variables and percent change in T2 relaxation time of the injured limb as the outcome of interest.

Cartilage Region R2 Adjusted R2 p Factor β p
Lateral Femoral Cartilage 0.081 −0.041 0.53
6-Month Daily Steps −0.132 0.604
Change in Daily Steps 0.272 0.294
Lateral Tibial Cartilage 0.238 0.137 0.13
6-Month Daily Steps 0.144 0.536
Change in Daily Steps 0.446 0.069
Medial Femoral Cartilage 0.281 0.185 0.084
6-Month Daily Steps 0.334 0.152
Change in Daily Steps 0.367 0.118
Medial Tibial Cartilage 0.036 −0.092 0.758
6-Month Daily Steps 0.079 0.763
Change in Daily Steps 0.163 0.535
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Abbreviations: ACLR: anterior cruciate ligament reconstruction.

Functional Knee Health

Frequency counts for increasing and decreasing daily steps as well as achieving or not achieving adequate knee function are presented in Table 4. There was no relationship between change in daily step counts and knee function status (p = 1.000). When comparing participants with adequate knee function to those with inadequate knee function at 18 months, there was a moderate effect size in daily steps six months after ACLR (Cohen’s d = 0.74). The exploratory logistic regression also demonstrated no relationship between change in daily steps from six to 18 months after ACLR with knee function status 18 months after ACLR, after adjusting for daily steps six months after ACLR. (Odds Ratio = 1.0 [95% Confidence Interval 0.999–1.000]; p = 0.638).

Table 4:

Frequencies of individuals with increased compared to decreased daily steps and adequate compared to inadequate knee function.

Adequate knee function (N=8) Inadequate knee function (N=10)
Increased Daily Steps 4 4
Decreased Daily Steps 4 6
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Post-hoc comparisons of participant characteristics and PA levels between adequate and inadequate knee function status groups are presented in Table 5. There were no differences between groups regarding age, sex, BMI meniscus repair status, or daily steps.

Table 5:

Group comparison data between individuals with and without adequate knee function.

Adequate knee function (N=8) Inadequate knee function (N=10) p
Age 18.0 (2.9) 21.1 (6.9) 0.46
Female:Male 7:1 8:2 1.00
BMI (kg/m2) 23.3 (3.4) 24.3 (4.0) 0.36
Meniscus Repair Status (Yes:No) 3:5 6:4 0.64
Daily Steps - 6 months after ACLR 8548.18 (3151.7) 6746.6 (1383.4) 0.20
Change in Daily Steps - 6 to 18 months after ACLR −234.8 (3301.6) 111.7 (1821.8) 1.00
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Abbreviations: BMI: body mass index; kg: kilogram; m: meter; ACLR: anterior cruciate ligament reconstruction

Discussion

The purpose of this study was to explore the relationship between daily PA and structural and functional signs of knee health. Contrary to our hypothesis, the findings of this study do not suggest a relationship between change in PA levels and T2 relaxation times in the weightbearing tibiofemoral cartilage from six to 18 months after ACLR. Additionally, there was no association between change in daily steps and knee function at 18 months after ACLR. Of note, there were minimal changes in average PA and T2 relaxation time from six to 18 months after ACLR.

The results of this study add to the data investigating the relationship between PA and structural knee health early after ACLR. In a group of 27 individuals within one month of ACL injury but before surgery, increased PA (as measured by daily steps) was associated with higher cartilage T2 relaxation time (worse cartilage health). (20) In 26 individuals six to 12 months after ACLR, male and female participants demonstrated differing relationships between PA metrics and cartilage health via compositional MRI.(21) In female individuals after ACLR, a greater number of daily steps was associated with lower T1rho relaxation times, a metric of higher proteoglycan density, in the cartilage of the medial femoral condyle. In contrast, males demonstrated greater T1rho relaxation times were associated with higher daily steps counts after ACLR. (21) When assessing MVPA in this same study, higher levels of daily MVPA were associated with greater cartilage T1rho relaxation times, indicating lower proteoglycan density and worse cartilage health.(21) While these previous studies identified either a direct or inverse relationship between increasing PA and compositional MRI findings, the current study did not find a relationship. This is potentially due to our limited sample size or the methodology of reporting composition cartilage outcomes. While previous studies have utilized a single data collection and interlimb ratio assessment (15, 16), our study used a longitudinal assessment of the injured limb in isolation to directly assess change within the injured knee. The ability to assess for potential longitudinal change in PA and compositional MRI outcomes allows understanding of more specific changes that occur within the injured limb during recovery. Further studies with larger sample sizes are needed to confirm the relationship between longitudinal changes in PA and compositional MRI.

The results of this study align with other comparisons of quantitative PA and knee function after ACL injury. Similar to the current findings, in a cohort of 128 individuals two years after non-operatively managed ACL injury, PA (measured with daily steps and minutes of MVPA) was not associated with patient-reported function on the IKDC.(56) In 51 female athletes one year after ACLR, there was no relationship between daily minutes of MVPA and second injury risk.(57) While it appeared that those who had adequate function at 18 months after ACLR had greater PA six months after ACLR, the change in PA throughout the following 12 months did not impact knee function. It has been hypothesized that increasing quadriceps strength could improve PA based on the relationship between preoperative quadriceps strength and return to sport after ACLR.(58) However, our findings do not support a relationship between quadriceps strength and daily PA. Previous research demonstrates that current levels of PA do not relate to patient-reported function.(25) Combined with the current evidence that changing PA was not associated with changes in knee health, increasing PA in isolation may not directly result in improved overall knee health.

Categorizing participants as having adequate or inadequate knee function based on a combination of patient-reported (IKDC) and objectively measured (quadriceps strength) knee function allowed a more comprehensive measure of overall knee function rather than assessing each individually. To the authors’ knowledge, an outcome of overall knee health combining patient-reported knee function with objectively measured knee function has not previously been completed. This novel outcome provides a more holistic approach to knee health and could provide important information regarding recovery. The sample size of the current study prevented the creation of four distinct groups based on IKDC scores and quad strength; thus, participants were only separated into one of two groups. Individuals were categorized as having inadequate knee function if they did not achieve 90% on the IKDC and/or 90% LSI on quadriceps strength testing. However, no participants who had >90% LSI on their quadriceps strength also reported <90% on the IKDC. Of the individuals who were categorized as having inadequate knee function, 40% reported >90% on the IKDC but failed to reach a 90% LSI during quadriceps strength testing. (Appendix Table II) The knee function outcomes in the current cohort align with previous studies, thus promoting generalizability of our findings. Mean quadriceps strength in the current cohort was 86.6%, with 55.6% of individuals falling below 90% LSI. The mean LSI reported in the current study is similar to values reported during isokinetic testing one year after ACLR,(59) two years after ACLR,(60, 61) and three years after ACLR.(62) Lastly, the mean IKDC score in the current study of 93.1 ± 6.7% is similar to previous reports of individuals 12 to 18 months after ACLR (all > 90%)(63) and higher than reports of individuals two years after ACLR (61.9 – 89%)(64) and after ACL injury without surgery (77.2%).(56)

Limitations exist when interpreting these results. The small sample size with a greater percentage of female participants limits the potential generalizability, as the current cohort may not represent the population of individuals who undergo an ACLR. To better understand the relationship between patient-reported and objectively measured knee function with PA, a larger study that allows for the differentiation between both IKDC and quadriceps LSI <90% and those with <90% on only one of the measurements may be helpful. An additional benefit of a larger sample size would be the ability to better separate individuals into groups based on change in PA. For example, some participants demonstrated relatively small increases or decreases in PA from 6 to 18 months after ACLR. However, we were unable to create three distinct groups for analysis: those that increased, those that decreased, and those that had little to no change in PA. A larger study, using three distinct groups, may be better able to detect a relationship between PA and overall knee health. As noted previously, the current cohort who followed up 18 months after ACLR was more heavily female and had a higher level of patient-reported function six months after ACLR, limiting generalizability.

Conclusion

The current study did not identify a relationship between PA and knee health outcomes within the first 18 months after ACLR. Average PA remained relatively steady from six to 18 months after ACLR. The impact of not increasing PA during a period in recovery where return to sport and prior activities is expected is not fully understood and warrants further exploration.

Supplementary Material

Appendix 1
Appendix II

Acknowledgments

The results of this study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. The results of this study do not constitute endorsement by the American College of Sports Medicine. Funding for this work was provided by the National Institutes of Health (NIH) (1R21AR075254), the Rheumatology Research Foundation Investigator Award, a New Investigator Fellowship Training Initiative from the Foundation for Physical Therapy, and the University of Nebraska Medical Center. EW’s efforts in this study were also partially supported by National Institute of General Medical Sciences (U54 GM115458), which funds the Great Plains IDeA-CTR Network. The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIH.

Contributor Information

David M. Werner, Office of Graduate Studies, University of Nebraska Medical Center, NE, USA; Physical Therapy Program, Department of Health and Rehabilitation Sciences, College of Allied Health Professions, University of Nebraska Medical Center, NE, USA

Yvonne M. Golightly, College of Allied Health Professions, University of Nebraska Medical Center, Omaha, NE, USA; Physical Therapy Program, College of Allied Health Professions, University of Nebraska Medical Center, Omaha, NE, USA; Department of Epidemiology, University of Nebraska Medical Center, Omaha, NE, USA

Michael D. Rosenthal, Physical Therapy Program, Department of Health and Rehabilitation Sciences, College of Allied Health Professions, University of Nebraska Medical Center, NE, USA

Balarinivasa Sajja, Department of Radiology, College of Medicine, University of Nebraska Medical Center, NE, USA.

Chris Wichman, Department of Biostatistics, College of Public Health, University of Nebraska Medical Center, NE, USA.

Melissa Manzer, Private Practice, Omaha, NE.

Matt Tao, Physical Therapy Program, Department of Health and Rehabilitation Sciences, College of Allied Health Professions, University of Nebraska Medical Center, NE, USA; Department of Orthopaedic Surgery and Rehabilitation, College of Medicine, University of Nebraska Medical Center, NE, USA.

Elizabeth Wellsandt, Physical Therapy Program, Department of Health and Rehabilitation Sciences, College of Allied Health Professions, University of Nebraska Medical Center, NE, USA; Department of Orthopaedic Surgery and Rehabilitation, College of Medicine, University of Nebraska Medical Center, NE, USA.

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Supplementary Materials

Appendix 1
NIHMS2119162-supplement-Appendix_1.docx (17.4KB, docx)
Appendix II
NIHMS2119162-supplement-Appendix_II.docx (18.6KB, docx)

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