The most predominantly applied methods for the assessment and monitoring of the quadriceps and hamstrings strength after anterior cruciate ligament reconstruction (ACLR) are isometric (IMT) and, particularly, isokinetic (IKT) strength tests, based on sustained contractions of the tested muscle (17,20,32). Although the application of IKT could be considered as a standard method for the assessment of muscle function after ACLR, the use of IMT has also been valuable. That has been the case particularly when only single transducers with simple custom-made devices are available instead of the relatively expensive isokinetic dynamometers (18).
A strength test based on alternating consecutive maximal contractions (ACMC) performed under isometric conditions has been recently proposed to overcome some of the shortcomings of the standard IMT (3–5,18). Probably, the most important one originates from the differences in the neural activation pattern between the rapid and sustained maximum contractions (9,24,27). Namely, IKT and IMT may not capture the neural activation pattern typical for rapid exertion of force that could be critical for functional tasks that provide limited time for exerting relatively high muscle force (such as in explosive and rapid cyclic movements), or require transient consecutive actions of antagonistic muscle groups (walking and running). As a result, the assessment of the abilities to exert a sustained maximum force (as assessed by IKT and IMT) and to exert it rapidly could require separate methods of evaluation (2,31). In addition, the exertion of a sustained contraction that provides maximum force could be painful or inappropriate for some individuals such as injured/recovering persons (22,35).
Regardless of the applied test, along with the absolute strength measures (usually peak torques [PT]), strength imbalance ratios have been used to monitor rehabilitation progress and identify possible risk factors for ACL reinjuries (8,13,26). In particular, limb symmetry index (LSI) reveals the strength imbalance between contralateral legs, whereas the Hamstrings-to-Quadriceps ratio (HQ ratio) describes the strength imbalance of antagonistic muscle groups. Among others, these imbalances have been taken into account in the decision-making processes regarding whether the athlete is ready to return to routine athletic training and competition (7,11,15).
Both PT and the indices of strength imbalance need to be both reliable and valid to be used either for monitoring rehabilitation or as a screening test of muscle function. Previous reports have suggested “high” to “very high” relative and absolute reliability of both isokinetic and isometric PT of quadriceps and hamstrings (1,13,21,32,35). However, only few studies have examined the reliability of HQ ratio and particularly of LSI, reporting low-to-moderate values (13,32). Although both PT and strength imbalance ratio measures have been widely used to monitor rehabilitation of muscle function, there is an apparent lack of information regarding their longitudinal construct validity. A construct validation process is usually based on construct that the involved leg's muscle function changes after the surgery, and later on during the rehabilitation process, while the muscle function of uninvolved leg remains unchanged (28). Further information regarding this type of validity of strength measures is apparently needed to adequately track clinically important changes or to accurately assess the recovery progress, and to appropriately use the same measures in future research on clinical populations.
To address the aforementioned issues, this study was designed with the main aim to explore the longitudinal construct validity of the IKT, IMT, and ACMC when used to monitor the muscle function recovery after the ACLR. The secondary aim was to explore the concurrent validity of the ACMC with respect to IKT and compare it with the validity of IMT. Regarding the outcome variables, in addition to the directly recorded PT of the knee muscles, the validity of both the HQ ratio and LSI was also evaluated. The results were expected to provide important data regarding the evaluated strength tests, and to motivate further development of ACMC either as an alternative or complementary strength test to IMT when used to assess the muscle function in ACLR individuals or, possibly, other clinical populations.
Experimental Approach to the Problem
An experimental longitudinal study was designed to test quadriceps and hamstrings through the standard IKT, IMT, and isometric ACMC of 2 antagonistic muscles. Because the ACMC has previously been evaluated on healthy and physically active participants (3–5), there is an apparent need to evaluate the properties of the same test when applied on clinical populations, such as on athletes with an ACL injury. Although various strength tests have been proposed as outcome measures after ACLR, limited reports of their measurement properties exist. Specifically, both PT and the indices of strength imbalance need to be both reliable and valid to be used either for monitoring rehabilitation or as a screening test of muscle function. To assess the longitudinal validity of variables obtained from the applied tests, comparisons between the consecutive sessions were made. To assess the concurrent validity of ACMC, its variables were related to IKT and, afterwards, compared with the validity of IMT.
The sample size estimate was based on the concurrent validity of ACMC observed in previous studies (4,5). According to standard guidelines (6) with power of 0.8 and an alpha level of 0.05 [calculated by G*Power 3.1 free software; (10)], the sample size was between 8 and 13 for the concurrent validity (4). Conservatively, 23 male athletes with an ACL injury were recruited in the study, but 3 of them were later lost to follow up (Figure 1). A post hoc analysis of sensitivity for the used sample size with the power of 0.8 and an alpha of 0.05 revealed the effect size of 0.5. The subjects were soccer (12), handball (5), and judo (3) competitors (age, 24.2 ± 5.1 years; body mass, 84.0 ± 11.1 kg; height, 180.3 ± 4.0 cm; body mass index, 25.1 ± 3.5). They were recruited through the Clinic for Orthopedic Surgery and Traumatology, between September 2010 and March 2012. The inclusion criteria were first ACL injury; no other knee ligaments injured; no history of concurrent fractures, osteoarthritis, or hereditary and neuromuscular diseases, participation in competitive sports at national level or higher. In all subjects, the ACL reconstruction procedure was performed by a board certified orthopaedic surgeon, using the bone-patellar-bone tendon autograft. After the surgery, the subjects were allocated to a standard postoperative rehabilitation program for athletes. All subjects received a complete explanation regarding the purpose, procedures, and possible risks of the study. The institutional review board approved the project and appropriately informed consents have been gained from subjects, and their rights were protected.
All measurements were performed within a university research laboratory, using a Kin-Com AP125 isokinetic dynamometer (KinCom, Kinetic Communicator; Chattecx Corp., Chattanooga, TN, USA). The design of the study is presented in the Figure 1.
Measurements were taken through 3 separate sessions: preoperatively (i.e., within 7 days pre-ACLR—session 1), 4 months (session 2), and 6 months post-ACLR (session 3). Each session consisted of 2 experimental days, separated by 48 hours of rest. Isokinetic tests were conducted in one experimental day, whereas the IMT and ACMC were conducted in the other experimental day. The order of the experimental days was randomized.
Before muscle strength testing, each subject was given a 5-minute warm-up period on a stationary bicycle, followed by passive stretching exercises (15 seconds per muscle group) focused upon the quads, hamstrings, hip adductors, and calf muscles. Thereafter, the subject was fixed to the testing apparatus with the tight straps used to fix his pelvis, thigh, and malleoli. The axis of rotation of the dynamometer was aligned with the lateral femoral epicondyle.
The same test leader supervised all tests. A detailed explanation and qualified demonstration was provided before each muscle strength test, and standardized verbal encouragement was consistently used. Real-time visual feedback regarding the current force was shown at a computer screen positioned in front of the subject.
Torque measurement was performed first at a low speed of 60° 1.05 rad·s−1 (IKT60) and, after a 2-minute rest, at 180° 3.14 rad·s−1 (IKT180). Each participant exerted 5 cycles of maximal voluntary repetitions of alternating knee extension and flexion. The range of motion during the knee extension/flexion was set to 80° (i.e., from 10 to 90° of flexion).
Standard isometric test was conducted on the knee extensors and flexors separately. The subjects were instructed “to rapidly exert the maximum force” against the dynamometer lever attached to the lower leg and to retain it for 3–4 seconds (35). Quadriceps and hamstrings PT were measured at the knee angle of 45° (18).
Alternating consecutive maximal contractions were performed at the knee angle of 45° (18). The participants were instructed “to consecutively exert the alternating maximum contractions of the knee extensors and flexors as strong and as quickly as possible” where the ACMC frequency should be considered as self-selected. Specifically, the participants were asked to exert self-paced maximum contractions consecutively in 2 opposite directions. In addition to allowing for testing 2 antagonistic muscles within a single trial, the advantage of the ACMC test could be its close correspondence to a number of everyday and, in particular, high performance movements. Namely, although without allowing for the use of the stretch-shortening cycle because of the static conditions, ACMC is inevitably based on the consecutive maximum activation of the antagonistic muscle groups. The trial duration covered 5 full periods of ACMC force.
Both in IKT60 and IKT180, and in the following tests (IMT and ACMC), one practice trial preceded 2 experimental trials with 1 minute of rest between them. The uninvolved leg was always tested first. None of the participants reported pain in either of the legs during the testing.
A custom-made Lab View application was used for data acquisition and processing of the muscle strength tests and for the online visual feedback on a computer screen. The force-time curves were recorded at a rate of 500·s−1 and low-pass filtered (10 Hz) using a fourth-order (zero-phase lag) Butterworth filter (18). The peak forces were multiplied by the individual lever arm length to calculate the PT. The peak values observed from the middle 3 cycles of the IKT and ACMC force profiles provided averaged PT for quadriceps and hamstrings, respectively. Peak torques of the quadriceps (PTQ) and hamstring (PTH) muscles observed from IKT, IMT, and ACMC served for the calculation of HQ ratios as PTH/PTQ. The LSI between PT of the uninvolved and involved leg was separately calculated from PTQ and PTH:
where the score represents PT of the involved leg, expressed as a percentage of PT of the uninvolved leg (19).
Descriptive statistics were initially calculated for the selected variables; data were reported as mean (SD). To assess the longitudinal construct validity of PT, HQ ratio, and LSI, mean values of 3 sessions were compared using repeated measures analysis of variance (ANOVA). When a significant effect was found, Bonferroni post hoc test was applied to detect the systematic bias among the sessions. The level of statistical significance was set to α = 0.05. For the estimation of accuracy of the individual scores obtained from consecutive sessions, a coefficient of variation was used (12), which has been typically considered good if below 15% (33). The intraclass correlation coefficients (ICC) were calculated as a measure of the degree to which individuals maintained their position in a sample with repeated measures (12). Intraclass correlation coefficients were considered as being high (≥0.8), moderate (0.6 < ICC < 0.8), or low (<0.6). Pearson's correlation coefficients (r) were used to assess the relationship between the applied tests. Concurrent validity of the ACMC and IMT (predictor variables) with respect to IKT (common dependent variable) was assessed by the Meng test for correlated correlation coefficients (see Meng at al. (23) for details). Data were analyzed using SPSS 20.0 software (SPSS Inc, Chicago, IL, USA) and Office Excel 2010 (Microsoft Corporation, Redmond, WA, USA).
Typical force profiles obtained from the applied muscle strength tests are shown in Figure 2. As expected, the maxima of the forces obtained from the concentric contractions performed at the lower (IKT60) and particularly the higher angular velocity (IKT180) are lower than the maxima of both isometric tests (i.e., IMT and ACMC).
Longitudinal Construct Validity
The indices of longitudinal construct validity of PT obtained from preoperative and postoperative sessions are depicted in Table 1. Regarding the quadriceps and hamstrings PT of the uninvolved leg, ANOVA revealed no differences among the sessions in any of the applied tests. However, a significant effect of session was detected in all strength tests regarding the quadriceps of the involved leg. The values observed in session 2 were lower than those observed in sessions 1 and 3. ACMC seems to reveal particularly high relative differences among the consecutive sessions. The effect of session on the hamstrings PT was found only in ACMC because of lower values recorded in session 2 than in session 3. Coefficient of variation was lower in the uninvolved (all below 13.5%) as compared with the involved leg (11.7–22.1%). Intraclass correlation coefficient was generally moderate-to-high for both muscles of the uninvolved leg, and between low (in IMT and ACMC) and high (in IKT) for the quadriceps of involved leg.
The indices of longitudinal construct validity of the HQ ratios and LSI are reported in Tables 2 and 3, respectively. In general, both variables only revealed significant differences among the consecutive sessions for the quadriceps of the involved leg. Specifically regarding the HQ ratios, CV was somewhat lower in the uninvolved (10.4–15.5%) than in the involved leg (14.1–25.2%). The ICC was generally low, with the exceptions of the IMT and ACMC data obtained from the uninvolved leg, where it was moderate. Regarding the LSI, ANOVA revealed significant effect of session for all quadriceps data, but not for hamstrings. The CV was generally high, whereas the ICC was, on average, low-to-moderate both for quadriceps and hamstrings LSI.
The concurrent validity of both the IMT and ACMC were assessed with respect to IKT. In general, the correlations between the corresponding PT obtained both between the IMT and IKT and between the ACMC and IKT were moderate-to-high (Table 4). When the same correlation coefficients observed separately for the IMT and ACMC were compared by means of the Meng test, the difference seemed to be nonsignificant (Z < 1.57; p > 0.05).
Regarding the concurrent validity of the HQ ratios, the correlation coefficients observed both between the IMT and IKT, and between the ACMC and IKT seemed to be, on average, low for the uninvolved leg and moderate to mainly significant for the involved leg (Table 5). The Meng test revealed no significant differences between the corresponding correlation coefficients observed from the IMT and ACMC (Z < 1.58; p > 0.05), with the only exception of the HQ ratio of the involved leg recorded in session 1 (Z = 2.89; p ≤ 0.05). Regarding the concurrent validity of quadriceps' and hamstrings' LSI, both the IMT and ACMC revealed moderate and significant correlations with IKT. The Meng test once again revealed no significant differences between the corresponding correlation coefficients except for the hamstrings LSI recorded in session 1.
The main aims of this study were to explore longitudinal construct validity of strength measures obtained from the routinely used IKT, standard IMT test, and the recently introduced test of isometric ACMC, recorded before surgery and during post-ACLR rehabilitation. The obtained results indicate that both isometric tests could possess a similar level of the longitudinal construct validity as the routinely applied IKT. In addition, the concurrent validity of ACMC with respect to IKT could be similar to the same concurrent validity of IMT.
The recorded changes in muscle function associated with the applied treatments generally seemed to be in line with our construct for change. The significant effect of session and a higher CV and a lower ICC of the strength measures obtained from the involved leg support the presumed construct. It therefore provides the evidence of longitudinal construct validity of the applied methods for strength assessment. As expected, the PT data obtained from IKT and IMT revealed significant time-related changes in the involved quadriceps, but neither in the involved hamstrings nor in the muscles of the uninvolved leg. Similar changes were also detected by the ACMC test, both in PT of the involved quadriceps and the corresponding LSI, but also in PT of the involved hamstrings and the HQ ratio, which could suggest that the ACMC may be as sensitive as either the IMT or IKT. Although the between sessions differences in HQ ratio were significant only at the lower isokinetic velocity of IKT and in ACMC, the overall percent change was still large enough to indicate that HQ ratio could capture the expected changes over time in the strength imbalance of the antagonistic muscles.
Percent change between the sessions, CV, and ICC of the recorded strength measures were also reported as a part of the longitudinal construct validity of the applied tests. In general, the comparison of scores over the 3 sessions indicated that substantial changes took place on the involved limb from the first to second and from the second to third test sessions. Similar changes were not observed in the contralateral limb, suggesting that the functional status of the uninvolved leg remained stable over the 6-month period. Note that some authors suggest that neuromuscular dysfunction and quadriceps strength loss after ACL injury affect the uninvolved side as well and, therefore, the uninvolved leg should be used as a control variable with caution when evaluating strength deficits after ACL injury (8,15). However, our data show virtually no changes in the uninvolved leg over the entire tested time interval. Similarly, the indices of longitudinal construct validity suggest that PT and HQ ratios of the uninvolved leg were relatively stable across the sessions, unlike the quadriceps' PT and corresponding HQ ratios obtained from the involved leg. However, although the corresponding CV and ICC indicate both the within-individual and between-session variability of strength imbalance ratios to be relatively high in the involved leg, the same measures could still be suitable for detecting prominent changes between the sessions that could be affected by the applied rehabilitation procedures.
We were not able to find published estimates of the longitudinal construct validity of isokinetic and isometric strength measures obtained from ACLR participants. However, note that our results obtained from IMT and IKT, and from the ACMC test, indicate a significant reduction in the quadriceps strength after the surgery are in line with the previous studies (14,19,25,26).
Although the isokinetic testing has been the most frequently applied method for the assessment and monitoring the quadriceps and hamstrings strength after ACLR, there is an apparent lack of data regarding the evaluation of isometric strength measures. The outcomes of the IMTs applied in this study (i.e., IMT and ACMC) were generally similar to those previously reported for isokinetic measures obtained from both healthy and ACL participants. In particular, previous studies suggest that the absolute measures (such as PT) observed from various isokinetic tests could be more reliable and stable than the derived measures, such as HQ ratio or LSI (13,32,33). However, these studies investigated strength measures obtained from healthy and physically active individuals within a single testing session. Ross et al. (30) investigated the reliability of PT and LSI of quadriceps in ACLR participants and reported a high reliability of the PT and LSI, but the IKTs were conducted between 12 and 72 months post-ACLR. The reliability was not directly calculated in one of the first studies that assessed HQ ratios in ACLR population (15). However, the author found that the HQ ratios were highly variable across the participants (range, 31–80%), even in the uninvolved leg. Finally, note that both the HQ ratios and the LSI are derived from several individual PT's. Because each PT has its own limited reliability and its variance is nonnegligible, a relatively high variability and low reliability of HQ ratios and LSI (as compared with individual PT) should not be viewed as a surprise.
Our findings generally suggest that the obtained indices of longitudinal construct validity of IMT and ACMC were comparable with the routinely used IKT. This could be of importance in clinical settings because isokinetic dynamometers could be replaced with relatively inexpensive single force transducer and custom built devices (4,5). Moreover, although the properties of ACMC proved to be comparable with the IMT, ACMC could still retain some important methodological advantages, such as a brief and simple procedure for testing 2 antagonistic muscles, while exposing the muscle and joint tissues to transient forces (5,34).
The validity of inferences based on isokinetic and isometric measures has been supported by a number of studies rendering IKT use highly valuable whenever muscle strength needs to be assessed (7,15,29). Therefore, the relationship between the strength indices obtained from ACMC and those obtained from routinely used IKT could be interpreted as an index of concurrent validity of ACMC. The same indices of concurrent validity obtained from ACMC were also compared with those observed from standard IMT.
In general, the results revealed a moderate-to-high concurrent validity of PT of both ACMC and IMT, whereas the same validity of HQ ratios and LSI were somewhat lower. The obtained relationships between the isometric and isokinetic PT were in line with the results of previous studies (16,20). A similar level of the concurrent validity of ACMC with respect to IMT was recently recorded in physically active participants (4). Regarding LSI, our results are similar to Reinking at al. (29), who found a moderate relationship among the isokinetic and isometric LSI, which was explained by the variability in percentage deficits between the strength tests. However, there is a lack of published data describing the relationship between the isometric and isokinetic HQ ratios, which seems to be low-to-moderate in this study. Here, one could speculate that the isometric outcome measures (such as in IMT and ACMC) are knee angle–dependant, whereas IKT tests muscles within a considerable range of motion. However, we believe that similar to their construct validity and variability, the concurrent validity of HQ ratios and LSI could be low on average (as compared with individual PT) because of the method in which these variables were calculated. Namely, the summed-up variance of several PT used for calculation of individual HQ ratios and LSI reduces the relationships between their values obtained from different tests. Nevertheless, in addition to the longitudinal construct validity, the observed concurrent validity also suggests that not only the standard IMT, but also the ACMC could be considered for routine strength testing of ACL and, possibly, other clinical populations.
Among the limitations of our study could be that our patient group was composed of a relatively small sample size (n = 20) and of exclusively male professional athletes. Namely, different results could have been found in other populations, such as female athletes, sedentary individuals, or individuals participating in different rehabilitation programs. Note also that only the participants with the bone-patellar-bone tendon graft were evaluated, but not the hamstrings tendon graft replacement. Future studies should address the aforementioned limitations, explore the external validity of the ACMC test regarding some ACL-specific functional performance tests, and evaluate the same tests on other clinical populations.
In general, the data suggest that IMTs could replace standard IKTs of leg strength that require isokinetic devices. Moreover, taking into account some shortcomings of the standard tests of muscle strength based on sustained maximal actions, a part of the observed findings suggest that the tests based on alternating consecutive maximal contractions could be considered as either a complementary or alternative tests to often employed isometric tests. Of particular importance could be that the ACMC test revealed not only the longitudinal construct validity comparable with IKT and IMT, but also a moderate-to-high concurrent validity with respect to the routinely applied IKT that could be comparable with the same concurrent validity of IMT. Taking also into account the potential advantages of ACMC over IMT, such as a brief procedure for testing 2 antagonist muscles together, the findings suggest that ACMC could be a particularly promising method for routine testing of the neuromuscular function after the ACL reconstruction.
Supported in part by NIH grant (AR06065) and grants from the Serbian Ministry of Education, Science and Technological Development (175037 and 175012). The authors disclose professional relationships with companies or manufacturers who will benefit from the results of this study. The results of this study do not constitute endorsements of the product by the authors or the National Strength and Conditioning Association.
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Keywords:Copyright © 2014 by the National Strength & Conditioning Association.
anterior cruciate ligament injury; validity; peak torque; limb symmetry index; hamstrings-to-quadriceps ratio