Background
Knee osteoarthritis is a leading cause of functional limitation in older adults [
1,
2]. Functional limitation is manifest as reductions in both self-reported difficulty performing activities of daily living and in objectively quantified mobility [
3‐
5]. Identifying modifiable factors that contribute to pain, difficulty performing activities of daily living, and mobility is necessary to inform efficient therapeutic regimens that effectively reduce functional limitation in individuals with knee osteoarthritis.
Performing functional tasks requires sufficient lower extremity muscle strength [
6]. Individuals with symptomatic knee osteoarthritis demonstrate deficits in quadriceps and hamstrings strength when compared to healthy matched controls [
7‐
10]. It has been suggested that lower quadriceps and hamstrings strength contribute to a variety of clinical features, including poorer patient-reported function [
11,
12], worse physical performance [
13‐
15] and disease progression [
16]. The relationship between lower extremity muscle strength and clinical features may be confounded, however by radiographic disease severity [
17] in addition to demographic characteristics such as sex [
18‐
20] and body mass index (BMI) [
18]. As muscle strength is modifiable, understanding the associations between quadriceps and hamstrings strength and a broad set of clinical features could help to determine efficacious treatment targets.
Previous studies assessing the associations between quadriceps and hamstrings muscle strength and clinical features of knee osteoarthritis have quantified muscle strength using instrumentation only available in research laboratories [
13‐
15,
21,
22], precluding their applicability in the clinical setting. Hand-held dynamometers are small, portable devices that allow for the measurement of muscle strength in clinical settings. Strong, positive associations (i.e., Pearson correlation coefficient range = 0.72–0.85) between the assessment of quadriceps and hamstrings muscle strength using a hand-held dynamometer and an isokinetic dynamometer have been reported [
23]. Additionally, hand-held dynamometers demonstrate excellent inter-rater and inter-session reliability [
24]. Limited research demonstrates hip abductor strength, assessed using a hand-held dynamometer associates with physical function in patients with knee osteoarthritis [
25]. Therefore, our purpose was to evaluate the associations among quadriceps and hamstrings strength measured with a hand-held dynamometer and 1) structural damage quantified via radiograph and MRI, 2) patient-reported pain, symptoms and difficulty performing activities of daily living quantified via the Knee Injury and Osteoarthritis Outcomes Score (KOOS), and 3) performance of mobility tasks quantified via the Timed Up and Go (TUG) test in individuals with symptomatic meniscal tear and osteoarthritis. We hypothesized that less quadriceps and hamstrings strength is associated with greater structural damage (i.e. radiographic severity and depth and size of cartilage damage), greater patient-reported limitations, and poorer mobility.
Discussion
In this cross-sectional study of individuals with symptomatic meniscal tear and knee osteoarthritis, we observed that greater quadriceps and hamstrings strength were significantly associated with less patient-reported pain and difficulty completing activities of daily living, and better objectively measured mobility after accounting for age, sex, BMI and structural damage. The difference in pain and difficulty performing activities of daily living between the strongest and weakest quartiles of quadriceps and hamstrings strength exceeded the minimal clinically important difference of 8 points for the KOOS [
32]. While our cross-sectional results will need to be tested in longitudinal studies, they suggest that increasing quadriceps and hamstrings muscle strength may be beneficial for reducing pain and difficulty performing activities of daily living, and improving mobility in individuals with symptomatic meniscal tear and knee osteoarthritis.
The quadriceps are critical for generating joint actions and attenuating loading during functional tasks such as walking and rising from a chair [
6]. In our study, we determined that quadriceps strength was associated with both subjective and objective clinical features of symptomatic knee osteoarthritis. In general, our results agree with previous studies using laboratory-specific instrumentation to quantify quadriceps muscle strength, as greater strength was associated with better functional task performance and less self-reported disability in individuals with knee osteoarthritis [
13‐
15,
21,
22]. Conversely, other studies have determined weak correlation coefficients between lower extremity muscle strength and pain [
34,
35]. Contrasting results between our study and previous investigations are likely due to the evaluation of lower extremity muscle strength during a concentric, bilateral leg extension task in one study [
35] or the assessment of knee pain using a visual analog scale [
34]. We assessed open-chain quadriceps strength using a hand-held device, which allowed us to isolate the quadriceps muscle on the index limb. Our results suggest that quadriceps strength measured in a clinical setting using a hand-held dynamometer is associated with various clinical features that may contribute to functional limitation in individuals with symptomatic meniscal tear and knee osteoarthritis.
Similar to quadriceps strength, we determined that greater hamstrings strength is associated with less pain, less difficulty performing activities of daily living and greater mobility in individuals with symptomatic meniscal tear and knee osteoarthritis. The hamstrings provide dynamic joint stability during walking [
6], and greater patient-reported joint stability has been associated with less knee pain [
36]. Our results build upon previous work suggesting that hamstrings strength contributes to both subjective [
19] and objective [
14,
15] clinical features of knee osteoarthritis. Previous studies have determined greater hamstrings strength is associated with less pain and better performance on stair climb and sit to stand assessments [
14,
15,
19]. Our results suggest that hamstrings strength contributes to multiple activities of daily living as greater hamstrings strength associated with time to complete the TUG test, which comprises both balance and mobility, and KOOS ADL scores, which assesses the difficulty experienced while completing a variety of daily activities ranging from sitting to ascending and descending stairs. Our results suggest that multiple lower extremity muscles contribute to clinical features of knee osteoarthritis, and intervention strategies aiming to improve both quadriceps and hamstrings strength may be beneficial.
Classifying our participants into quartiles of quadriceps and hamstrings strength allows us to determine the clinical relevance of quadriceps and hamstrings muscle strength regarding subjective and objective clinical features of knee osteoarthritis. In our cohort, the difference in mean KOOS pain and KOOS ADL scores between the strongest and weakest quartiles of quadriceps and hamstrings muscle strength when adjusted for structural damage and demographic characteristics ranged from 13 to 15 points (Figs.
1 and
2), which represents a clinically meaningful difference in KOOS scores [
32]. While we cannot determine how changes in quadriceps and hamstrings muscle strength are associated with changes in clinical features, a previous study has determined that a small reduction in quadriceps and hamstrings muscle strength of approximately 6% was associated with a clinically relevant decline in physical function quantified via the WOMAC [
12]. Similarly, individuals in the weakest quartiles of quadriceps and hamstrings muscle strength in our study took on average 2 to 3 s longer to complete the TUG test compared to those in the strongest quartiles, which is greater than the minimum detectible difference of 1.14 s [
33]. The TUG test assesses balance and self-selected walking speed, and a greater time to complete the TUG test has been associated with fall risk [
37]. Additionally, individuals with slower walking speed are also less likely to meet physical activity guidelines [
4]. Individuals with or at risk of developing knee osteoarthritis who take more steps per day have a lower risk of developing functional limitation over 2 years compared to those who take fewer steps per day [
3]. Future research is needed to determine how quadriceps and hamstrings muscle strength may contribute to additional features of knee osteoarthritis, such as decreased physical activity, and how improving physical activity may be beneficial for patients with symptomatic meniscal tear and knee OA.
In recent years, multiple clinical trials have demonstrated that arthroscopic partial meniscectomy offers little additional improvement in pain when compared to non-operative interventions for the treatment of symptomatic meniscal tear [
26,
38‐
40]. As the management of symptomatic meniscal tear and knee osteoarthritis transitions to conservative approaches, our results highlight the importance of appropriately strengthening the quadriceps and hamstrings musculature. Previous work demonstrated that a 12-week neuromuscular and strength exercise program consisting of squats, lunges, leg press and hamstring curl exercises improved quadriceps and hamstring muscle strength in patients with meniscal tear [
41]. Additionally, high intensity strength training and high velocity power training have been shown to be effective at increasing quadriceps and hamstrings muscle strength in individuals with knee osteoarthritis [
42]. Utilizing high intensity strength training and power training may be beneficial for improving lower extremity muscle strength in patients with symptomatic meniscal tear and knee osteoarthritis, and therefore may improve pain and physical function. However, gains in muscle strength following training programs may be limited by underlying deficits in voluntary activation [
22,
43]. While improving quadriceps and hamstrings muscle strength is likely beneficial for improving clinical features of knee osteoarthritis, rehabilitation programs may need to treat underlying neural factors (i.e. arthrogenic muscle inhibition) that limit the ability to fully restore muscle strength. Future research is needed to determine the most efficacious therapeutic interventions for improving lower extremity muscle strength in patients with symptomatic meniscal tear and knee osteoarthritis.
While this study improves our understanding of the association between quadriceps and hamstrings muscle strength and subjective and objective clinical features of knee osteoarthritis, there are limitations that should be addressed to inform future research. As our study is cross-sectional in nature, we are unable to determine the causal relationship between quadriceps and hamstrings muscle strength and clinical features of knee osteoarthritis. We also cannot determine how other lower extremity muscles, including the gluteus medius and gluteus maximus, may have contributed to KOOS scores and time to complete the TUG in our study. We limited our inclusion criteria to individuals with mild to moderate knee osteoarthritis based upon K-L grade, therefore it is unknown how our results translate into the larger population of individuals who are at risk of developing knee osteoarthritis or those with end-stage disease. Additionally, we did not determine the presence of bilateral osteoarthritis in this cohort, therefore it remains unknown if the presence of bilateral osteoarthritis may influence our results. We used a hand-held dynamometer to assess quadriceps and hamstrings muscle strength due to its cost-effectiveness, portability and ease of use, rather than an isokinetic dynamometer. However, strong, positive associations (i.e., Pearson correlation coefficient range = 0.72–0.85) between the assessment of quadriceps and hamstrings muscle strength using a hand-held dynamometer and an isokinetic dynamometer have been reported [
23]. We also used inelastic straps to stabilize the dynamometer and maintain joint positioning during the assessments, which has been demonstrated to improve the reliability of these measures [
30]. We chose to normalized peak force in newtons to body mass to account for differences in body size. The lever arm length was not available in this cohort; therefore, we are unable to determine joint torque. Previous research, however has demonstrated that taking the lever arm length into account for the purpose of normalizing muscle strength does not influence results when compared to normalizing peak force to body mass [
19]. Lastly, there are additional factors that may influence quadriceps and hamstrings muscle strength, including neuromuscular activation, muscle co-contraction, and joint range of motion, and we are unable to discern how additional unmeasured factors may contribute to quadriceps and hamstrings muscle strength.