Osteoarthritis (OA) is a synovial joint disease that affects 12–15% of people aged 25–74, and its prevalence increases with age, with more than 70% of people aged 65 and older showing radiographic signs [1]. OA causes swelling, limitation of joint function, pain, and stiffness, and affects mental health, sleep, work participation, and mortality [2]. Increased ligament stiffness, altered muscle activation patterns, and decreased muscle strength can adversely affect joint kinematics and lead to degenerative changes in cartilage [3]. Pain is the most common symptom of OA, and the fear of using the knee joint due to pain leads to the weakening of muscle strength and a decrease in quality of life [4]. In addition, functional decline, such as reduced mobility and limitations in activities of daily living, leads to difficulties in social participation, increased social isolation, and economic burden [5].

Pain in OA is exacerbated by various factors, including increased joint loading and systemic inflammation associated with obesity. For overweight or obese individuals, reducing body weight by 7.5% or more has been shown to lower pain severity and decrease the risk of joint replacement surgery [6]. The increasing number of replacement surgeries for arthritis of the hip and knee joints highlights the need for effective non-surgical treatments for OA [7]. Osteoarthritis Research Society International (OARSI) recommends strengthening, cardio, balance training, and neuromuscular exercise programs as core recommendations for non-surgical management of knee and hip OA [8]. Among these, in terms of muscle strength, if the muscle strength of the extensor and flexor muscles of the knee joint is low [9], the risk of OA worsening increases, and it was found that knee OA patients had weaker quadriceps muscle strength than healthy adults [10]. In addition, patients with hip OA were found to have weaker hip abductor and knee flexor muscle strength than healthy adults [11]. Therefore, low muscle strength may be a cause of OA symptoms, and restoring muscle strength may reduce the risk of OA [12].

The effects of supervised progressive resistance training on function, pain, and quality of life in hip OA [13], the effects of resistance training on gait velocity and knee adduction moment in knee OA [14], and the effects of resistance training on pain, stiffness, and physical function [15] have been investigated. Recently, a study was conducted to compare the effects of resistance exercise on hip and knee OA according to the type of comparison group and to analyze whether improvements in muscle strength were associated with improvements in pain and physical function [16].

In this review, both pain and function showed significant effects compared to other groups, but muscle strength showed no effect compared to the usual care group. Regarding the effect of resistance training on OA, Li et al. [15] reported in a meta-analysis that resistance training was effective in improving pain and function compared to the control group. In addition, resistance training has been shown to increase strength and function and decrease pain in older adults with OA [49] and has benefits in increasing knee extensor strength [50,51]. The quadriceps muscle provides stability to the knee joint together with other lower extremity muscles and ligaments, and weakening of the quadriceps muscle increases the load on the passive components of the knee joint, increasing joint stiffness [52]. Increasing quadriceps muscle strength through resistance training reduces the risk of cartilage loss and space narrowing of the tibiofemoral joint [52]. Therefore, it is thought that increased muscle strength through resistance training helps improve pain and function by increasing load absorption and stability of the knee joint [53]. Another reason may be that patients felt their pain was improved on their own due to the placebo effect of resistance training. Contextual effects in chronic pain disorders may be regression to the mean or placebo effects [54]. Messier et al. [37] explained the lack of difference in pain improvement between the high-intensity resistance training group and the control group as the placebo effect. Because the functions and pain measures collected in this review were all patient-reported outcomes, the observed effects cannot be ruled out as a placebo effect.

In addition, Marriott et al. [16] reported that there were differences in the effects on pain and function depending on the comparison group, with the greatest effect compared to no intervention and no effect compared to other exercise or non-exercise (or combined) interventions. In this review, function appeared to be most effective in the no intervention group compared to other comparison groups, but there were no significant differences between groups in pain and strength. The reason for this difference in effect is that this study collected all outcomes, whereas the previous study collected only one outcome when there were two or more outcomes in a study. In addition, this study excluded cases where resistance or strength training was included in the comparison group, but previous studies compared cases by including resistance training. Therefore, it appears that the difference in effect is due to differences in the selection criteria for the comparison group and outcome, and the number of subjects included in the review.

In this review, pain and muscle strength showed effects at all intervention durations, but function showed no effect at durations less than 4 weeks. Marriott et al. [55] reported that resistance training for 3 to 6 months was most effective for pain and physical function than for 12 months or less than 3 months. In contrast, Turner et al. [56] reported that the largest effect sizes for improvements in pain and function occurred at 24 resistance training sessions and 8 to 12 weeks. These conflicting results appear to be mainly due to the fact that OA is influenced by several factors, including pain sensitivity, psychological distress, BMI, muscle strength, inflammation, obesity, and gender [57]. In addition, most of the measures included in this review were conducted over 8 weeks, and there are few studies that measured for 4 weeks or less, so there may be differences in effectiveness.

In this review, the effects of joint-specific resistance training were shown to be effective for both knee OA and hip OA. In the study by Marriott et al. [16], resistance training was also effective in improving pain and function in both hip and knee OA when compared to conservative intervention. However, this review included only two studies out of 27 that focused on hip OA, so caution is needed when interpreting the results. A review by Zacharias et al. [51] also collected studies on knee and hip OA and found that only one of 40 studies focused on hip OA. This suggests that further research on hip OA should be conducted, as there is a paucity of studies focusing on hip OA.

In particular, this review found no heterogeneity in the moderate effect on strength improvements, despite the fact that the studies included all resistance training methods, including aquatic resistance, concentric or eccentric resistance, and fast or slow velocity contractions. In the case of resistance training, the cost of exercise equipment or participation in related programs can be significant, and adjusting the intensity of training to the patient’s condition can increase the patient’s performance and confidence in exercise [3]. This allows patients to choose a resistance training method based on their preference or economic considerations to improve strength. In addition, increases in lower extremity muscle strength are expected to have a positive effect on pain and function [16,50]. Resistance exercise can alleviate OA symptoms by increasing muscle strength, improving articular surface loading, and rebalancing the activation patterns of leg muscles [3]. These findings suggest that resistance training is necessary for OA patients. However, the effect sizes of resistance training in this review were generally moderate. A recent network meta-analysis targeting OA showed that various exercises, including aquatic exercise, yoga, and cycling, in addition to resistance training, were effective [58]. The OARIS guidelines also recommend several types of exercise rather than just one type of exercise [8]. Therefore, there is a need to find more effective combined exercise interventions when used in conjunction with resistance training for OA.

Limitations of this review include the exclusion of surgical patients, which may have led to more positive results, as patients with more severe symptoms were not included in the study. In addition, this review did not include an analysis of the relationship between improvement in outcome and various factors such as age, sex, BMI, and disease severity. For example, women have weaker quadriceps muscles than men and are more prone to reduced knee joint protection [52]. Inflammation plays an important role in the development and progression of OA, and obesity disrupts immune homeostasis and induces inflammation [59]. Because the factors related to OA are extremely diverse, future research needs to be conducted that considers the relationship between various factors and improvement in outcome. In terms of methodological limitations, most of the included studies were not blinded to patients and therapists, and more than half of the studies did not perform random allocation and intention-to-treat analysis, which may lead to bias and overestimation in the results.