quinta-feira, 16 de julho de 2015

Knee Flexor Strength

Knee Flexor Strength and Endurance Profiles After Ipsilateral Hamstring Tendons Anterior Cruciate Ligament Reconstruction

Giampietro L. Vairo, PhD, ATCcorrespondence
Athletic Training and Sports Medicine Research Laboratory, Department of Kinesiology, College of Health and Human Development, The Pennsylvania State University, University Park, PA
Sports Medicine Clinical Research Agenda, Penn State Hershey Bone and Joint Institute – State College, State College, PA
Published Online: October 10, 201






Abstract

Objectives

To profile knee flexor strength and endurance responses to ipsilateral hamstring tendon autograft anterior cruciate ligament reconstruction in patients, and to describe knee flexor strength and endurance as predictors of subjective outcomes. The hypothesis was that the involved leg would demonstrate deficits compared with uninvolved and matched legs.

Design

Retrospective cohort.

Setting

Controlled laboratory.

Participants

Women (n=15; mean age, 20.47±1.96y; height, 1.69±.08m; weight, 68.51±12.64kg), who were a mean ± SD of 25.93±11.25 months postsurgery, were matched to 15 sex-matched controls (mean age, 20.93±1.22y; height, 1.65±.06m; weight, 66.52±10.69kg).

Interventions

Not applicable.

Main Outcome Measures

Concentric peak flexor moment over the entire isokinetic joint range of motion, peak flexor moment at 105° of joint flexion, and knee flexor total work, normalized to body mass, and subjective questionnaire scores for the involved leg.

Results

Significantly lesser peak flexor moment over the entire isokinetic joint range of motion (P=.034) and total work (P=.048) existed for the involved leg (.959±.186Nm/kg; 21.933±5.881J/kg) compared with the matched leg (1.108±.134Nm/kg; 27.431±6.499J/kg). Significantly lesser peak flexor moment at 105° of joint flexion (P=.002) existed between the involved (.221±.116Nm/kg) and uninvolved (.40±.234Nm/kg) and matched (.475±.183Nm/kg) legs. Significantly greater strength deficits (P≤.001) existed at peak flexor moment at 105° of joint flexion compared with peak flexor moment over the entire isokinetic joint range of motion for side (53.83%±38.8%; 9.87%±10.77%) and group (77.61%±44.14%; 18.09%±11.73%) differences. Peak flexor moment at 105° of joint flexion was a significant predictor of subjective pain (P=.007), symptoms (P=.006), function (P=.011), and sports (P=.022) outcomes.

Conclusions

Knee flexor strength and endurance deficits suggest susceptibility to reinjury, and strength in a deep joint angle predicts subjective outcomes.

Keywords:

Athletic performance, General surgery, Joints, Muscles, Rehabilitation

List of abbreviations:

ACL (anterior cruciate ligament), ANOVA (analysis of variance), CI (confidence interval), HT (hamstring tendon), KOOS(Knee Injury and Osteoarthritis Outcome Score), ROM (range of motion)
Recent reports1, 2, 3 have communicated an increased use of the ipsilateral hamstring tendon (HT) autograft as a primary tissue for anterior cruciate ligament (ACL) reconstruction, with some authors4 advocating that it be considered the criterion standard. Despite this, apprehension to ipsilateral HT harvest remains because of clinical concern for hindering the agonistic function of the associated musculature to the native ACL or intra-articular autograft.5 The hamstrings are suggested to significantly contribute to maintaining knee stability.6, 7, 8 Thus, improving knee flexor strength and endurance is important for improving functional performance after ACL injury and reconstruction.9 Consequently, knee flexor strength10, 11, 12 and endurance13 deficits have been reported in associated physically active patients. This has been clinically indicated by isokinetic measures of peak flexor moment over the entire isokinetic joint range of motion and knee flexor total work at slow and fast angular velocities, respectively. Such decrements have been revealed with unilateral pre- and postoperative13 and bilateral postsurgical11, 12 assessments approximately 2 years after ACL reconstruction. A greater extent of deficiencies has also been observed when comparing the involved leg with a healthy matched control.11, 12
Conversely, it has been proposed that most14 prior research studies have demonstrated that related strength6, 15, 16, 17, 18and endurance6 recover to satisfactory levels approximately 2 years after surgery, as determined from unilateral pre- and postoperative15 and bilateral postsurgical6, 16, 17, 18 measures. These results, however, may be limited by the conventional seated testing method used in these studies. This technique may not replicate orientation of the lower extremity in a functional position representative of standing tasks and evaluates muscular strength and endurance over a limited 90°19to 100°20 range of motion (ROM). This yields less clinically applicable measures of such isokinetic indices,21 considering the knee may undergo a functional ROM of approximately 110° with squatting and stair-climbing22 or 118° during sprinting activities.23 Subsequently, the seated testing position lengthens the hamstrings to yield a mechanical advantage for generating a knee flexor moment.24, 25 Hence, a large proportion of corresponding experiments may underrepresent the incidence of resultant postoperative knee flexor strength and endurance insufficiencies. Thus, various authors19, 21, 26have attempted to address this by conducting isokinetic measures in similar patients using a prone position, which better approximates orientation of the lower extremity during functional tasks when standing.24, 25
Despite such assessment modifications, related studies consist of considerable limitations that include measuring absolute rather than relative indices,19, 21, 26 such as that normalized to body mass, and evaluating measures over a restricted ROM.19 Absolute isokinetic indices yield greater interparticipant variability, which lessens the reliability of such measures and limits the validity of comparative outcomes among related literature.27, 28 Furthermore, assessing isokinetic strength and endurance through constrained joint ROM diminishes the applicability of captured data to functional tasks, such as athletic endeavors.19, 29 Therefore, the primary aim of this research study was to profile isokinetic concentric knee flexor strength and endurance, normalized to body mass, responses to ipsilateral HT autograft ACL reconstruction in physically active patients approximately 2 years after surgery using a prone position over an ROM representative of functional athletic tasks. It was hypothesized that the involved leg would display knee flexor strength and endurance deficits compared with the contralateral uninvolved leg19, 26 and healthy matched control leg,12 and that greater deficiencies would be revealed when compared with the matched leg.12 It was also hypothesized that knee flexor strength deficiencies would be greater at a deep joint angle, defined as 105° of flexion.30 A secondary aim was to describe knee flexor strength and endurance as predictors of self-reported outcomes for the involved leg.

Methods


Participants

The sample size was based on the primary aim of this study and was determined using the 50°/s isokinetic concentric peak flexor moment over the entire isokinetic joint range of motion data of Hiemstra et al.12 For a 1-way analysis of variance (ANOVA) with an alpha of .05, a minimum of 13 patients and 13 controls was sufficient to detect statistically significant differences in peak flexor moment over the entire isokinetic joint range of motion among the involved, uninvolved, and matched legs with 80% power.
Correspondence was distributed to 27 patients that met the qualifications for potential enrollment via primary analysis of medical records. Seventeen patients expressed interest in potentially participating; 2 of these participants were excluded. One man was excluded because he had fractured his contralateral fibula approximately a year prior, and 1 woman was excluded for not being physically active. Subsequently, 15 physically active women 18 to 35 years of age who underwent arthroscopic ipsilateral semitendinosus and gracilis tendon autograft reconstruction within 3 months of ACL injury and 12 to 36 months prior to this study were consecutively enrolled.6 Patients possessed no other ligamentous sprains greater than grade I or meniscal lesions requiring greater than one third debridement in the involved knee and followed a standardized prescribed postoperative physical rehabilitation protocol. The basis of rehabilitation was specific to the HT procedure, which intended to restore knee ROM, muscular strength and endurance, and joint functionality. The protocol was supervised by a licensed athletic trainer or physical therapist 2 to 3 times per week for 12 weeks and included supplemental home exercises. After this, patients were prescribed a standardized 12-week program that was completed independently. A gradual return to full participation in preinjury activities was permitted at 6 to 9 months postsurgery on surgeon approval. Patients self-reported no history of traumatic injury to the hip or ankle of the involved leg and also had no account of traumatic injury to the contralateral leg or spine. Fifteen healthy control participants matched by sex, approximate age, height, mass, reciprocal ponderal index, and physical activity level self-reported no history of traumatic injury to either leg or the spine. Reciprocal ponderal index is an anthropometric measure providing a global index for body composition and reflects the quotient of height divided by the cubed root of the mass. Physically active was delineated as participants being in compliance with the United States Centers for Disease Control and Prevention exercise guidelines for adults31 over a 6-month period before enrolling in this study and gauged with the reliable32 and valid33 Minnesota Leisure Time Physical Activity Questionnaire.
Prior to enrollment, all participants underwent a standard orthopedic clinical evaluation by a licensed athletic trainer. Bilateral comparisons were used for this examination. No signs of knee joint effusion, edema, hamstring tendinopathy, or Baker cyst were observed. Manual capsuloligamentous assessments were determined to be normative to nearly normative per the International Knee Documentation Committee Knee Evaluation Form.34 Each qualified participant completed an institutional review board approved, written informed consent form before data collection. The objective measures were collected with participants wearing an athletic top and shorts while barefoot.

Experiment protocol

Two orthopedic sports medicine surgeons from a university medical center bone and joint institute referred interested patients to me as the principal investigator. I recruited matched participants from an affiliated university campus. Patients completed a single data collection session. Matched participants also completed a single session for testing 1 leg corresponding to a patient's involved leg based on dominance or nondominance.6 Leg dominance was determined by asking participants to identify which leg they would prefer to kick a soccer ball with for distance and accuracy.35 Data collection took place in a controlled research laboratory setting, and all testing was entirely conducted by me, a licensed athletic training clinical scientist with over a decade of professional practice experience. Each participant serially completed a self-reported physical activity level scale and knee outcome questionnaire followed by the strength and endurance assessments. The sequence for testing a patient's involved and uninvolved leg was randomized by generating random permutations with Minitab 15a to prevent an order effect.

Tegner physical activity level scale

After completing the written informed consent form, participants also completed the reliable, valid, and responsive36Tegner physical activity level scale to appraise the current physical activity level for facilitating the matching of patient and control groups.37
Group means, SDs, and percent differences were calculated for the demographic, anthropometric, and current physical activity level data (table 1).
Table 1Participant demographic and anthropometric measures
Group ComparisonsPatientsControlsDifference (%)P (95% CI)
No. of participants (women)1515NANA
Time since surgery (mo)25.93±11.25NANANA
Tegner level (current)6.80±1.526.13±1.0610.31.175 (−0.314 to 1.647)
Age (y)20.47±1.9620.93±1.222.25.440 (−1.688 to 0.755)
Height (m)1.69±0.081.65±0.062.46.121 (−0.012 to 0.094)
Mass (kg)68.51±12.6466.52±10.692.95.645 (−0.012 to 0.094)
RPI (cm/kg1/3)41.52±3.1040.84±2.201.65.494 (−1.331 to 2.693)
NOTE. Values are mean ± SD or as otherwise indicated. P<.05 denotes statistical significance.
Abbreviations: NA, not applicable; RPI, reciprocal ponderal index.

Self-reported knee outcome questionnaire

After the Tegner scale, patients completed the reliable,38 valid,38 and responsive39 Knee Injury and Osteoarthritis Outcome Score (KOOS) survey for the involved leg. The KOOS comprises 5 subscales: pain, symptoms other than pain (symptoms), function in activities of daily living (function), function in sports and recreation (sports), and quality of life (quality).

Knee flexor strength and endurance

Isokinetic concentric knee flexor strength and endurance, normalized to body mass, were evaluated with the calibrated Biodex System 2.b Peak flexor moment over the entire isokinetic joint range of motion and peak flexor moment at 105° of joint flexion were measured at 60°/s to gauge strength.6, 30 Knee flexor total work was measured at 240°/s as an index of endurance.6, 13, 40 Strength was assessed first followed by endurance to limit the effect of fatigue on endurance.6
Using a prone position, the participant's tested leg was positioned with the base of the patella approximately 3cm from the edge of the system's chair. The participant's trunk and tested leg were then secured with the use of belt straps crossing over the pelvic girdle and midthigh to limit displacement. The most prominently palpated aspect of the respective lateral femoral epicondyle was used to align the knee joint and resistance adaptor axes of rotation. The lever arm was orientated so the resistance pad was in line with the apex of the medial malleolus and secured to the shank with a self-grip strap. After this, gravity correction was completed. Finally, the cushion parameter for the ROM ends was adjusted to the dynamometer's lowest setting to limit the effect of limb deceleration on reciprocal movements.41
The ROM for testing was 120°, with 0° serving as a reference for knee extension, which also represented the starting position for the test. Participants were instructed to keep their upper extremity, trunk, and contralateral leg relaxed during the strength and endurance trials. This was accomplished by asking participants to fold their hands over the headrest so as not to grasp the system chair, not to hyperextend their back or not to kick the contralateral leg. If I noted noncompliance with these instructions, the participants were prompted accordingly, but such feedback was limited to 1 cue. Furthermore, participants were also asked to maximally extend the ankle for the leg being tested while concomitantly performing the reciprocal knee flexion-extension repetitions in an effort to minimize the effect of the gastrocnemius on knee flexor measures. The participants were instructed to observe the system's computer monitor in an attempt to visualize attainment and maintenance of maximal output.6
A gradient submaximal to maximal warm-up was incorporated to familiarize participants with the testing protocol.42 The warm-up trials consisted of 3 sets where the participant exerted a particular self-perceived percentage of maximal effort over 3 repetitions of reciprocal knee flexion-extension. Specifically, participants were asked to exert 50%, 75%, and 100% of self-perceived maximal effort for the first, second, and third warm-up trial sets accordingly. Warm-up trials were separated by a 2-minute rest interval. After the third warm-up trial, participants were given a 2-minute rest period before initiating the test trial, which has been suggested to minimize muscle fatigue.43 The test trial comprised 1 set where the participant was asked to exert 100% of self-perceived effort for 3 repetitions of knee flexion-extension.6, 40 Peak flexor moment over the entire isokinetic joint range of motion was assessed as the single greatest repetition value among the 3 test repetitions over the entire 120° ROM (fig 1).41 Similarly, peak flexor moment at 105° of joint flexion was garnered in this fashion as a means to evaluate knee flexor strength at a deep joint angle (see fig 1).19, 30

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Fig 1

Isokinetic knee flexor strength and endurance measurement technique: 120° ROM for measuring peak flexor moment over the entire isokinetic joint range of motion and knee flexor total work (A) and peak flexor moment at 105° of joint flexion representing knee flexor strength in a deep joint angle (B).
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The validity of isokinetic dynamometry has been recognized with ACL reconstructed patients,44 and reliability of the Biodex System 2 for assessing concentric peak flexor moment over the entire isokinetic joint range of motion at 60°/s has been established.40, 45 Arden et al30 also reported reliability of the Biodex System at 60°/s for calculating concentric peak flexor moment at 105° of joint flexion to be acceptable. Test-retest reliability (intraclass correlation coefficient model 3,k) for the concentric knee flexor peak flexor moment over the entire isokinetic joint range of motion and peak flexor moment at 105° of joint flexion measurement techniques was .882 (95% confidence interval [CI], .648–.960; standard error of measurement=.05Nm/kg) and .695 (95% CI, .093–.898; standard error of measurement=.10Nm/kg), respectively, as determined by pilot testing.
After the strength measure, participants were given a 2-minute rest period before the endurance assessment. Participants then underwent 1 warm-up trial for the purpose of familiarization with a faster angular velocity.6 The warm-up trial consisted of completing 5 reciprocal knee flexion-extension repetitions through the 120° ROM starting from 0°, signifying knee extension.6 Four of the warm-up repetitions were at a participant self-selected effort, and 1 was at maximal effort.46 A 2-minute rest interval followed the warm-up trial before the test trial, which consisted of completing as many maximal effort repetitions as possible in a 45-second time period.6, 40 The flexion total work during this 45-second period was used as a measure of knee flexor endurance.6, 40 Patients were allotted 5 minutes of rest between testing each leg.
The Biodex System 2 is reliable when measuring knee flexor total work at 240°/s,45 and the endurance protocol used in this study was previously determined to be reliable.40 Test-retest reliability (intraclass correlation coefficient model 3,k) for the knee flexor total work measurement technique was .898 (95% CI, .695–.966; standard error of measurement=2.07J/kg), as determined by pilot testing.

Statistical analyses

Minitab 15 was used to analyze the data. Separate 2-tailed, pooled, 2-sample t tests were calculated to determine that no statistically significant differences in demographics or anthropometrics existed between patient and control groups.
Means and SDs were calculated for the main dependent variables of interest, which included peak flexor moment over the entire isokinetic joint range of motion, peak flexor moment at 105° of joint flexion, and total work. These calculations were sorted by leg condition (involved, uninvolved, matched), which represented the independent variable. In addition to these descriptive statistics, percent differences and effects sizes47 were calculated between legs.
A 1-way ANOVA was computed to test the hypotheses that the involved leg would demonstrate knee flexor strength and endurance deficits compared with the uninvolved and matched legs, and that greater related deficits would exist when compared with the matched leg. An a priori probability of P<.05 was the statistical significance threshold for the ANOVA. As appropriate, Tukey post hoc analysis assessed pairwise comparisons with a 95% simultaneous CI used to determine statistical significance accordingly.
The hypothesis that the involved leg would display greater knee flexor strength deficits in a deep joint angle was examined with a paired t test, which compared the mean percent differences between peak flexor moment at 105° of joint flexion and peak flexor moment over the entire isokinetic joint range of motion for both bilateral and group comparisons. Multiple linear regression assessed peak flexor moment over the entire isokinetic joint range of motion, peak flexor moment at 105° of joint flexion, and knee flexor total work as predictors of KOOS subscale scores for the involved leg. Multicollinearity among predictor variables was measured by way of variance inflation factors, and Mallow Cp was used to fit the best subset regression model accordingly. A P value <.05 denoted statistical significance a priori for the t tests and regression analyses.

Results

No statistically significant differences in demographics or anthropometrics existed between patient and control groups (see table 1).
Statistically significant differences in peak flexor moment over the entire isokinetic joint range of motion (P=.034) and knee flexor total work (P=.048) were found among legs. Specifically, the involved leg demonstrated lesser peak flexor moment over the entire isokinetic joint range of motion and knee flexor total work compared with the matched leg (table 2). Statistically significant differences in peak flexor moment at 105° of joint flexion (P=.002) also existed among legs; explicitly, the involved leg displayed lesser peak flexor moment at 105° of joint flexion compared with the uninvolved and matched legs (see table 2). No other significant differences were noted among related knee flexor strength and endurance comparisons.
Table 2Knee flexor strength and endurance measures
Strength
Leg ConditionPFMROM95% SCI(upper, lower)Cohen d
(Nm/kg)Difference (%)
Involved0.959±0.186NANANA
vs uninvolved1.012±0.1315.318(−0.083, 0.188)0.344
vs matched1.108±0.13414.385(0.013, 0.284)0.976
Uninvolved vs matchedNA9.085(−0.039, 0.232)0.632
Table 2Knee flexor strength and endurance measures
Leg ConditionPFM10595% SCI (upper, lower)Cohen d
(Nm/kg)Difference (%)
Involved0.221±0.116NANANA
vs uninvolved0.400±0.23457.78(0.014, 0.179)0.961
vs matched0.475±0.18373.19(0.089, 0.420)1.366
Uninvolved vs matchedNA17.23(−0.090, 0.241)0.405
Table 2Knee flexor strength and endurance measures
Endurance
Leg ConditionKnee Flexor Total Work95% SCI (upper, lower)Cohen d
(J/kg)Difference (%)
Involved21.933±5.881NANANA
vs uninvolved25.557±5.56115.262(−1.699, 8.947)0.606
vs matched27.431±6.49922.276(0.175, 10.821)0.908
Uninvolved vs matchedNA7.073(−3.449, 7.197)0.302
NOTE. Values are mean ± SD or as otherwise indicated.
Abbreviations: NA, not applicable; PFM105, peak flexor moment at 105° of joint flexion; PFMROM, peak flexor moment over the entire isokinetic joint range of motion; SCI, simultaneous confidence interval.
∗Statistical significance.
The involved leg demonstrated a significantly greater knee flexor strength deficit (P < 0.001) at peak flexor moment at 105° of joint flexion (53.83%±38.8%) compared with peak flexor moment over the entire isokinetic joint range of motion (9.87%±10.77%) for side comparisons. The involved leg also demonstrated a significantly greater knee flexor strength deficit (P≤.001) at peak flexor moment at 105° of joint flexion (77.61%±44.14%) compared with peak flexor moment over the entire isokinetic joint range of motion (18.09%±11.73%) for group comparisons.
A profile of the patient's KOOS subscale group mean scores was plotted as descriptive data (fig 2). Moderate multicollinearity was detected between peak flexor moment over the entire isokinetic joint range of motion and knee flexor total work for all regression models, as evidenced by associated variance inflation factors that were >2.8. Based on the variance inflation factors and Mallow Cp, knee flexor total work was removed from the multiple linear regression models to yield the best subset of predictor variables for each KOOS subscale score. Subsequently, peak flexor moment at 105° of joint flexion was found to be a statistically significant predictor for pain (fig 3A), symptoms (fig 3B), function (fig 3C), and sports (fig 3D). In contrast, peak flexor moment over the entire isokinetic joint range of motion was not a significant predictor for self-reported outcomes (table 3).

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Fig 2

KOOS subscale scores for the involved leg. Normalized score (%) = 100 − (total patient subscale score × 100) / maximum possible subscale score. Error bars represent SD of the group mean.
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Fig 3

Linear relation between peak flexor moment at 105° of joint flexion and KOOS subscale scores: pain (A), symptoms other than pain (B), function with activities of daily living (C), and function with sports and recreational activities (D) for the involved leg. *Statistical significance (P<.05).
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Table 3Regression equations of knee flexor strength and self-reported outcomes for the involved leg
KOOS SubscaleModelMallow CpPredictors
Pain90.1−8.42 PFMROM + 45.2 PFM1052.2PFMROM (r2=1.4%, P=.352); PFM105 (r2=43.1%, P=.007)
Symptoms93.2−24.5 PFMROM + 67.0 PFM1059.9PFMROM (r2=1.1%, P=.077); PFM105 (r2=31.9%, P=.006)
Function95.8−4.03 PFMROM + 22.5 PFM1052.2PFMROM (r2=1.5%, P=.401); PFM105 (r2=40.4%, P=.011)
Sport86.0−15.7 PFMROM + 62.2 PFM1052.0PFMROM (r2=0.1%, P=.307); PFM105 (r2=30.7%, P=.022)
Quality75.5−8.40 PFMROM + 53.0 PFM1052.2PFMROM (r2=0.9%, P=.691); PFM105 (r2=17.1%, P=.136)
Abbreviations: PFM105, peak flexor moment at 105° of joint flexion; PFMROM, peak flexor moment over the entire isokinetic joint range of motion.
∗Statistical significance (P<.05).

Discussion

The results did not support the hypothesis that the involved leg would display significantly lesser peak flexor moment over the entire isokinetic joint range of motion and knee flexor total work than the uninvolved leg. Discrepancies between this and prior similar experiments may be partially explained by differences in the reporting of and ROM for isokinetic indices and specific patient demographics. However, the results did support the hypotheses that peak flexor moment over the entire isokinetic joint range of motion and knee flexor total work deficits for the involved leg would be greater when compared with a matched leg and that these patients would also display greater insufficiencies at peak flexor moment at 105° of joint flexion than peak flexor moment over the entire isokinetic joint range of motion.
Not normalizing isokinetic data limits the extent of related bilateral comparisons28, 48 and comparisons between patients and healthy controls48, 49 because of anthropometric differences; however, normalization to body mass accounts for this,50which facilitates comparisons within and among populations over time or with treatments.51 Therefore, the normalization of knee flexor strength and endurance may have attenuated bilateral significance in this study. This is supported by the post hoc analysis conducted by Kramer et al,26 which examined concentric knee flexor strength of the involved leg relative to the uninvolved leg as a percent ratio. The calculation demonstrated peak flexor moment over the entire isokinetic joint range of motion of the involved leg to be 96% of the uninvolved leg, indicating a small bilateral strength difference regardless of statistical significance.26 Subsequently, a percent ratio within 10% of the uninvolved leg is not considered a substantial clinical deficit post-ACL injury or reconstruction.52, 53 Coincidentally, the percent ratio of Kramer26complements the related small bilateral effect size and percent difference in this study.
Another distinction between this study and similar investigations relates to the associated isokinetic ROM. Kim et al19noted decrements in peak flexor moment over the entire isokinetic joint range of motion because the knee was tested from only 60° to 120° of flexion in patients who underwent ipsilateral semitendinosus and gracilis tendon autograft ACL reconstruction. Thus, a lack of significant bilateral difference in this study may be because peak flexor moment over the entire isokinetic joint range of motion occurred at different joint angles with respective testing methods. Consequently, peak flexor moment over the entire isokinetic joint range of motion occurs between 15° and 30° of flexion using the prone position over a 120° ROM in healthy participants,54 and post hoc analysis for the angle of peak flexor moment over the entire isokinetic joint range of motion in this study indicated that the data approximated this range and did not statistically differ among legs (involved: 30.2°±4.3°, uninvolved: 33.9°±11.2°, matched: 28.8°±7.1°; P=.216). Conversely, Kim19 did not provide the angle of the peak flexor moment over the entire isokinetic joint range of motion data for their results, which prevents a direct comparison. However, the related 60° to 120° knee flexor strength outcome is similar to the peak flexor moment at 105° of joint flexion measure of this study, and appraisals for the bilateral percent differences of peak flexor moment over the entire isokinetic joint range of motion and peak flexor moment at 105° of joint flexion yielded discrepancies of approximately 5% as opposed to >50%, respectively. This observation suggests that knee flexor strength is less affected at shallow compared with deep joint angles in these patients, which potentially underpins the bilateral peak flexor moment over the entire isokinetic joint range of motion results of this study.
Instead, the bilateral peak flexor moment over the entire isokinetic joint range of motion findings of this study complement those of Elmlinger et al,21 which noted no related significant differences with implementation of a prone position testing technique in ipsilateral semitendinosus and gracilis ACL reconstructed patients. Similarly, post hoc analysis of strength data presented by Elmlinger21 indicates a bilateral small effect size (.145). Surgical technique difference represents the main disparity between this investigation and the Elmlinger study21 and that of Kramer.26 Interestingly, concomitant harvest of the gracilis tendon has been reported to yield a greater extent of knee flexor strength deficit compared with harvesting the semitendinosus alone in related patients.20, 30, 55 However, use of the ligament augmentation device in ACL reconstructed patients has been reported to cause recurrent effusion and joint synovitis56 that can negatively affect knee muscle strength.57 Consequently, Kramer26 did not specify if joint effusion was controlled, which may influence these conflicting outcomes.
Consonant with the bilateral peak flexor moment over the entire isokinetic joint range of motion outcomes, the involved leg demonstrated lesser knee flexor endurance compared with the uninvolved leg; however, the difference was not significant. A lack of significance is in agreement with prior findings for a similar patient population using analogous measurement parameters6; however, Vairo et al6 instituted a conventional seated position. Conversely, these results contrast those of Tow et al,13 which reported significant bilateral differences. Although Tow13 used a comparable isokinetic protocol for measuring knee flexor endurance, it was not clarified whether testing was completed in a seated or prone position, which limits related comparisons. Another distinguishing feature between this study and the Tow et al study13pertains to patient demographics. Most patients enrolled in the Tow study13 underwent partial meniscectomy or meniscal repair; however, Tow13 failed to describe thresholds for concomitant injury as patient inclusion or exclusion criteria. Consequently, excising or debriding greater than one third of the meniscus has been reported to yield neural impairment that negatively affects knee muscular function,58 which may influence the discrepancy between these studies.
An additional cause for the specific bilateral peak flexor moment over the entire isokinetic joint range of motion and knee flexor total work observations of this study represents that the knee flexor strength and endurance of the involved leg were successfully established to the level of the uninvolved leg, which reflects the knee symmetry model of ACL reconstruction rehabilitation.59 In fact, a formal rehabilitation protocol that facilitates equivalence between legs in associated patients has been suggested as a basis for the lack of significant bilateral peak flexor moment over the entire isokinetic joint range of motion and knee flexor total work differences approximately 2 years after surgery.6 Fittingly, the patients of this study completed a relatively standardized rehabilitation protocol that used the contralateral leg as a reference for related benchmarks.
Although the results of this study did not support the hypothesis specific to bilateral comparisons, there is evidence to accept the hypothesis that deficits of greater magnitude would be found when comparing the involved leg with a healthy matched control. Accordingly, all analyses between the involved and matched legs were found to be statistically significant with corresponding large effect sizes. These findings complement the similar measures of one author group12but contrast those of another6 for a similar patient population. Hiemstra12 described such a phenomenon with knee flexor strength measures in ipsilateral HT autograft ACL reconstructed patients and attributed such findings to diminished central neural drive to the knee flexors of the uninvolved leg. Consequently, such an occurrence, focal to the knee flexors, has been documented to ensue in ACL reconstructed patients, irrespective of harvesting the ipsilateral HT autograft.19 Thus, standard bilateral comparisons as a means to gauge knee flexor strength and endurance may be of limited clinical utility in ACL reconstructed patients and warrants attention because it pertains to the efficacy of rehabilitative interventions that entail establishing aptitude of the injured leg with the uninjured leg.12 Based on these observations, bilateral equivalence of postoperative knee flexor strength and endurance may be more appropriately viewed as a rehabilitative goal, not an endpoint.12 Therefore, targeting knee flexor strength and endurance patient outcomes that are comparable with healthy matched controls may be more important for preventing reconstructive failures and further joint injury and heightening levels of physical performance.12 This approach would subsequently call for developing more advanced rehabilitative methods and assessment techniques that address this potential issue. Other factors that potentially influence the observation for greater deficiencies noted for comparisons with a matched control may stem from overall patient deconditioning and insufficient reconditioning after ACL reconstruction.12 The patients' being generally weaker than the matched controls, predisposing them to knee injury, is another possibility; however, these latter assertions are difficult to determine retrospectively and require well-designed prospective studies.
The fact that greater knee flexor strength and endurance deficits for comparisons with a matched leg in this study conflict with those of Vairo6 may be linked to differences in matching participants. Although these experiments similarly matched participants per demographics and anthropometrics, this study consisted of more astringent criteria for being considered physically active. Vairo6 proposed generic guidelines for defining what was physically active, which were not as demanding as standards31 used in this study, and omitted use of valid clinical tools for quantifying activity level to appropriately match patients and healthy controls. Not controlling for activity level with related comparisons represents a considerable limitation because the intensity and frequency may substantially vary among populations. Hence, the activity level of patients and healthy controls was matched in this experiment to avoid biased comparisons in an effort to yield results that are more appropriately specified and clinically applicable.
The significant bilateral and group decrements in peak flexor moment at 105° of joint flexion when compared with peak flexor moment over the entire isokinetic joint range of motion support the hypothesis that greater knee flexor strength deficits would be found in a deep joint angle. This finding also complements accounts14, 55 of isometric knee flexor strength insufficiencies in ipsilateral HT autograft ACL reconstructed patients when the joint is at or surpasses 70° of flexion, as evaluated in the prone position. When assessing knee flexor strength in a prone position, the neutral hip orientation decreases the length of the hamstrings, which lessens the musculature's capacity to generate joint flexion with concentric contraction.24, 25, 55 Thus, the hamstrings length-tension relation associated with this testing method coupled with the fact that knee flexor capacity decreases as the joint flexion angle increases54 because of the musculature approaching active insufficiency, potentially underpin this finding. Correspondingly, prior authors14 examining patients who have undergone this mode of ACL reconstruction reported a significant knee isometric flexor strength deficit when the involved leg was compared with the uninvolved leg while the joint was positioned at 100° of flexion, despite bilateral isokinetic peak flexor moment over the entire isokinetic joint range of motion differences being insignificant. The bilateral peak flexor moment over the entire isokinetic joint range of motion and peak flexor moment at 105° of joint flexion comparative outcome of this study complements this specific prior reported observation. Therefore, based on the results of this and similar studies, it may appear that current associated rehabilitative protocols do not equate knee flexor strength in deeper joint angles of the involved leg with the contralateral leg or healthy controls.
Regression analyses revealed significant positive linear associations between peak flexor moment at 105° of joint flexion and the KOOS pain, symptoms, function, and sports subscale scores. This signifies that patients with greater concentric knee flexor strength in a deeper joint angle experienced improved subjective accounts of pain, symptoms other than pain, function with activities of daily living, and function with sports or recreational activities for the involved leg. This investigation represents the first study, to my knowledge, to establish isokinetic knee flexor strength as a predictor of self-reported clinical outcomes in ACL reconstructed patients, and specifically those undergoing ipsilateral HT autograft harvest. Previous studies examining such linear relations have focused predominantly on exploring correlations between peak flexor moment over the entire isokinetic joint range of motion and self-reported outcomes using various knee questionnaires, with most reporting insignificant findings in different ACL reconstructed patients.44 In a similar fashion, the results of this investigation established that peak flexor moment over the entire isokinetic joint range of motion was not a significant predictor for such outcomes.
Major disparities in comparisons between this study and those having previously explored associations between objective knee flexor strength and subjective outcomes include the specific isokinetic parameters and questionnaires used. Accordingly, none of these prior related experiments used similar testing protocols or the KOOS. Thus, these particular variables may influence such results and deserve attention. However, this study and similar studies did share a commonality of examining peak flexor moment over the entire isokinetic joint range of motion as an index of knee flexor strength. Therefore, limitations may exist when relying strictly on peak flexor moment over the entire isokinetic joint range of motion as a predictor of self-reported knee outcomes because such responses may be angle specific in ACL reconstructed populations. Consequently, Bryant et al60 discovered a significant positive correlation between average knee extensor moment and the Cincinnati Knee Rating System when the joint was in greater angles of extension in bone-patellar tendon-bone ACL reconstructed patients. In a similar fashion, this investigation found that peak flexor moment at 105° of joint flexion accounted for approximately 30% to 43% of the variance in KOOS subscale scores, excluding quality of life in related patients. Therefore, exclusive reliance on peak flexor moment over the entire isokinetic joint range of motion as a sole measure of knee flexor strength for predicting self-reported outcomes may be of limited clinical utility in ACL reconstruction patients. This discovery has considerable clinical implications given that patients undergoing ipsilateral HT autograft harvest typically exhibit significant knee flexor strength deficits in deep joint flexion angles with the involved leg. Hence, improving knee flexor strength in deep joint flexion angles may yield more favorable subjective clinical outcomes in associated patients. Coincidentally, a lack of association between knee flexor strength and quality of life in this study mimics the study by Hiemstra,12 which noted no significant correlation between peak flexor moment over the entire isokinetic joint range of motion and the ACL Quality of Life Measurement Questionnaire in related patients. Hence, definitively determining the influence of isokinetic indices on self-reported quality of life after ACL reconstruction in patients merits further attention.

Study limitations

The generalizability for the results is limited because enrollment included only young women, indicating a sex bias. This was not a specific aim of the experiment but may reflect the more common use of this surgical technique in women to prevent knee extensor mechanism trauma and corresponding donor-site morbidities. All the patients were white, perhaps because of the demographic area, which is also a limitation. The results of this experiment are sample-specific and may only be generalized to the population being investigated. Not accounting for leg dominance as a factor in the data analysis represents another limitation. These data being collected retrospectively and respective examiner bias also limit the application of related findings. Furthermore, the sample size was too small to address the effects of physician variability on the results. This theme also applies to the different athletic trainers and physical therapists treating these patients, regardless of a standardized rehabilitation protocol. Another limitation stems from the fact that these results are contraction-type specific. Future studies are needed to address these limitations.

Conclusions

The results suggest that the involved leg does not display knee flexor strength and endurance deficits when compared with the contralateral uninvolved leg. The exact cause for this is currently unknown but may reflect the efficacy of contemporary treatment for establishing bilateral balance. However, the involved leg's insufficiency for knee flexor strength in a deep joint angle suggests an apparent related asymmetry of large magnitude. Furthermore, the large knee flexor strength and endurance deficiencies revealed with comparisons with a matched control may suggest the susceptibility for reinjuring the ACL reconstructed knee. The findings also propose that related knee flexor strength at a deep joint angle is a significant predictor of subjective outcomes in associated patients. Clinical consideration should be given to improving knee flexor strength and endurance in deep joint angles as a means to improve functional performance and prevent reinjury and improve self-reported outcomes in related patients.

Suppliers

  • a.
    Minitab Inc, 1829 Pine Hall Rd, State College, PA 16801.
  • b.
    Biodex Medical Systems Inc, 20 Ramsey Rd, Shirley, NY 11967.

Acknowledgments

I thank Wayne J. Sebastianelli and Paul S. Sherbondy for referring patients to this investigation and Penn State Hershey Bone and Joint Institute – State College for administrative assistance. I also thank Sayers John Miller, PhD, PT, ATC, William E. Buckley, PhD, MBA, ATC, John H. Challis, PhD, and Samuel W. Monismith, PhD, for their preliminary appraisal of this experiment. I appreciate the support of Brooke Ficara, Patrick Gut, Andrew Slifko, and Joseph Leone as research assistants.

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