Background
Anterior cruciate ligament (ACL) injury often negatively affects knee function not only in the short term but also in the long term despite rehabilitative interventions [
1]. Treatment involves either physiotherapy alone or in conjunction with additional reconstructive surgery, with conflicting evidence as to the best approach and a lack of informed guidance for individual tailoring. Regardless of treatment strategy, there is an elevated risk for re-injury/secondary injury during the subsequent years [
2], as well as for longer term problems such as knee osteoarthritis (OA) [
1]. Studies investigating the very long-term (> 20 years) effects of ACL injury on knee function are, however, scarce.
Clinically evaluating knee function following ACL injury often includes, for example, thigh muscle strength, knee range of motion, and jumping/hopping ability. The injured leg is commonly compared to the contralateral non-injured leg using the Limb Symmetry Index (LSI), where achieving
> 90% of the outcome measures is a return-to-sport criteria [
3]. However, the LSI has been shown to overestimate post-ACL injury quadriceps strength and hop ability [
4,
5]. To complement the LSI, assessing movement quality during functional tests may reveal movement patterns that potentially predispose this group to further knee-related issues [
6]. Reliable functional tests are therefore needed that can discriminate such patterns from asymptomatic knees while providing suitable loading for different populations, including those of an older age for longer term assessment.
The One-leg rise (OLR) test, involving standing and sitting from a stool with only 1 foot on the ground, has been applied in clinics and research to assess knee function. Thorstensson et al., (2004) found that chronic knee pain sufferers unable to perform 20 repetitions of the OLR were more likely to develop radiographic knee OA 5 years later [
7]. The OLR was also more sensitive than gait at identifying changes in peak adduction moment following an exercise programme among the same cohort [
8]. A one-leg test may be particularly advantageous when assessing ACL-injured individuals, who have been shown to reduce loading of the injured leg during double-leg squats [
9]. In fact, worse knee confidence on average 9 years post-ACL reconstruction has been shown to be associated with poorer performance of the OLR [
10]. Moreover, at 5–10 years post-ACL reconstruction, worse performance of the OLR has also been associated with greater tibiofemoral OA severity [
11]. The OLR may thus be a relevant test of lower limb function among ACL-injured persons where both performance regarding the number of repetitions achieved and knee kinematics are of interest.
The potential added value of knee kinematics during the OLR would facilitate assessments of knee joint stability, defined here in accordance with Riemann and Lephart [
12] as the ability to remain or promptly return to proper alignment, something that is believed to be a major contributing factor to long-term post-ACL injury knee problems such as OA [
13]. Indeed, greater knee abduction of the injured leg compared to the non-injured leg during a one-leg half squat has been seen among non-operated ACL-injured males and females [
14]. Greater knee abduction was also observed for the injured leg of non-operated ACL-injured persons compared to controls during tests such as a mini-squat, one-leg half squat and rising from half-kneeling [
15]. Additionally, mediolateral knee control, as assessed by measures of knee position in the frontal plane, has been shown to be worse among ACL-injured persons compared to controls during a one-leg hop for distance [
16]. Among ACL-injured males, poorer mediolateral knee control during a drop jump was associated with worse knee proprioception [
17]. Thus, measures of mediolateral knee control during the OLR may provide additional valuable information regarding knee function among ACL-injured persons. However, a necessary first step before studying OLR knee kinematics to interpret knee function, is to assess the within-session reliability firstly among individuals with asymptomatic knees and secondly among the population of interest, something which we believe has not been done before.
Our aims in this study were to 1) assess the discriminative ability of OLR performance and knee kinematic outcome measures among ACL-injured persons, treated with and without surgical reconstruction, in the very long term after injury between the injured and non-injured legs and to controls without knee complaints, and 2) assess the within-session reliability of knee kinematics during execution of the OLR among asymptomatic individuals and ACL-injured cohorts. We hypothesised that both ACL-injured groups would show worse knee function and stability of the injured leg compared to the non-dominant leg of controls and to their non-injured contralateral leg, as characterised by significantly fewer OLR repetitions and greater knee abduction/adduction range of motion. We further hypothesised that the knee kinematics would show high within-session reliability.
Discussion
ACL-injured persons treated solely with physiotherapy performed significantly fewer OLR repetitions than age- and sex-matched persons with asymptomatic knees when using their injured and non-dominant leg respectively, albeit with a small effect size. The distribution of cumulative repetitions for ACL-injured/CTRL non-dominant leg comparisons revealed that 59% of ACL
PT were unable to achieve the 20-repetition cut-off for predicting knee OA development stated by Thorstensson et al., (2004) compared to 33% ACL
R and 36% CTRL, although these differences were not statistically significant. ACL
PT also displayed significantly greater knee abduction of medium effect sizes than both ACL
R and CTRL during the
Rise and
Down phases of the OLR. Despite this, our findings showed inconsistent differences, particularly of knee kinematics, when comparing the ACL-injured groups to CTRL. This contradicted our previous research which found negative outcomes for the same ACL groups when compared to CTRL with regard to reduced control of single-limb stance [
29], lower self-reported knee function and hop/jump capacity [
18], and reduced knee muscle strength [
30], as well as altered movement patterns during hop tests [
20,
31,
32]. Thus the knee kinematics during the OLR, as performed and analysed in our study, did not discriminate certain existing disparities in knee movement control in the very long term after ACL injury.
Nevertheless, there was greater maximal knee abduction among ACL
PT compared to ACL
R and CTRL, although the differences were rather small but still significantly different. These differences in knee abduction align with a previous study of the same groups during landings from one-leg hops [
20], although the clinical relevance in relation to detectable change remains to be determined. This finding is however further supported by Zhang and colleagues [
33] who found greater knee abduction among ACL-deficient persons on average 5 years after injury compared to controls at heel contact during gait. Trulsson et al., [
15] observed a greater medial position of the knee relative to the foot among non-operated ACL-injured persons compared to controls when performing a battery of tests including a mini-squat. That said, a more medial position of the knee would not necessarily result in knee abduction, which is more specifically defined by rotation of the shank relative to the thigh. The greater knee abduction for the non-injured leg of ACL
R in our study compared to their injured leg during the
Down phase indicates even bilateral effects of the ACL injury. Indeed, reduced balance during a single-leg stance for both legs was previously seen for our ACL
R and ACL
PT groups [
29]. Culvenor et al., [
34] also reported reduced postural control 12 months post-ACL
R for both legs when performing single-leg squats. One possible explanation for these bilateral effects may be neuroplastic changes following ACL injury, of which there is growing evidence [
35,
36].
Advantages of the OLR include its convenience due to the lack of required equipment or space. The consistent stool height (0.48 m) used in our study is similar to that which is encountered daily and enhances ecological validity. The movement itself resembles the everyday task of standing and sitting which can provide a relevant evaluation of an individual’s independence while isolating performance between legs. However, despite requiring more muscular effort than two-legged closed kinetic chain exercises, the OLR has been shown not to produce greater strains on the ACL than such tasks and can be considered appropriate for ACL-injured persons who can perform, for example, a traditional two-legged squat [
37]. Further, the relative simplicity of the OLR compared to, e.g. a one-leg hop for distance, improves feasibility among populations of different ages and conditions. Nevertheless, the OLR requires adequate lower limb strength and endurance, coordination, balance and proprioceptive ability, factors which deteriorate across the life span. The OLR thus encompasses a number of important outcome variables for assessment of movement control. Furthermore, within-session reliability of our knee kinematic variables was excellent for all groups and legs, thus indicating that the observed movement patterns of these groups are consistent during repetitions 2–11 of the OLR and that averaged values are likely representative of each individual. This was also supported by the lack of systematic bias seen in Bland-Altman plots for these variables. Our proposal for assessing mediolateral knee control based on knee movement units revealed neither between- nor within-group differences for our comparisons in the present task. A similar movement control measure of the knee denominated fluency, defined as the number of times the velocity of the knee position in the coronal plane crossed zero when averaged per second, has however revealed worse mediolateral knee control among ACL-injured persons compared to controls during a one-leg hop for distance [
16]. It is thus possible that our measure of knee movement units may discriminate movement control disparities in other more demanding tests and among populations with more severe pathologies and warrants further investigation.
Limitations of our study include the maximum 50 repetitions, applied to reduce fatigue effects on between-leg comparisons as well as the extreme delayed onset muscle soreness evident during pilot testing with no maximum. Statistically this created a ceiling effect and results would likely have been different without this maximum considering that 34 of 106 participants completed 50 repetitions on at least one leg and that CTRL accounted for 16 of those. Further, up to 229 repetitions were achieved in a previous study of chronic knee pain sufferers of similar age [
7]. Additionally, the LSI was not an appropriate measure due to the maximum repetition limit and for those unable to perform a repetition on at least one leg. The determination of leg dominance, used to provide the most stringent comparison to controls by comparing the hypothesised less-competent and more competent legs separately between groups, i.e. ACL-injured vs. CTRL non-dominant and vice versa, was made according to which leg the participants preferred to kick a ball. However, recent evidence shows that certain healthy individuals change leg preference depending on the task involved [
38], which may also be true for the OLR and for some injured persons. Thus, whether or not our between-group analysis resulted in the most stringent comparisons regarding injury side and dominance remains unclear. Our cross-sectional study design with long-term follow-up means that treatment strategies for ACL injuries have evolved since our participants were injured. Thus, our specific results may not be relevant for all ACL-injured persons. Other confounding factors across the two decades since injury such as, e.g. physical activity level, are also likely to have affected the outcome measures. We used 10% of the maximum/minimum hip joint centre velocity as a threshold level for setting the start/stop events of the OLR phases. Due to the lack of previous research investigating OLR kinematics, this decision was based on our own testing of various threshold levels across a number of participants and repetitions. Although we deemed this threshold level to be more appropriate than the alternatives that we tested, it is possible that choosing another threshold level may have changed the outcome of the results and thus further research is required to establish the most appropriate method. Further, there are common technical limitations to three-dimensional analyses, such as visibility of markers (hip and foot markers were often obscured when participants leaned forwards and due to the stool, respectively) or soft tissue artefacts which we tried to minimize using cluster markers and placement on solid anatomical landmarks [
39]. The use of maximum values for kinematic variables is also sensitive to such artefacts and thus as well as data filtering, thorough manual checks were performed on movement profiles and data values in an attempt to ensure representative data.
Our study is the first to evaluate reliability of knee kinematics during execution of the OLR as well as implement the test to compare between legs of ACL-injured persons in the very long term after injury and to controls with asymptomatic knees. In future, adjustments to our protocol may help to improve the standardisation and discriminative ability of the OLR, which may lead to more successful application within research and clinics. Removing the maximum repetition limit, for example, appears feasible and should benefit interpretation. However, this may take a rather long time for completion, depending on the patient’s functional state, which may make it less feasible for application in clinical settings. Further, standardisation regarding performance speed, e.g. using a metronome, may be considered. Randomisation of leg order in research studies would also help to avoid potential fatigue bias. The addition of kinetic data to enable analysis of body centre of pressure and joint moments is likely to provide valuable biomechanical information. Although advanced three-dimensional analysis was used in this study, if specific key movement control outcome variables can be identified, the use of simpler and less expensive video and software solutions may add value to clinical implementation of the OLR. Further reliability analysis should establish the minimum number of OLR repetitions required to provide reliable knee kinematic data, fatigue effects and additional pathological groups.
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