Resistance exercise training to improve post‐operative rehabilitation in knee arthroplasty patients: A narrative review

Abstract

Knee osteoarthritis is associated with deficits in muscle strength, muscle mass, and physical functioning. These muscle‐related deficits are acutely exacerbated following total knee arthroplasty (TKA) and persist long after surgery, despite the application of standardized rehabilitation programs that include physical/functional training. Resistance exercise training (RET) has been shown to be a highly effective strategy to improve muscle‐related outcomes in healthy as well as clinical populations. However, the use of RET in traditional rehabilitation programs after TKA is limited. In this narrative review, we provide an updated view on whether adding RET to the standard rehabilitation (SR) in the recovery period (up to 1 year) after TKA leads to greater improvements in muscle‐related outcomes when compared to SR alone. Overall, research findings clearly indicate that both muscle strength and muscle mass can be improved to a greater extent with RET‐based rehabilitation compared to SR. Additionally, measures of physical functioning that rely on quadriceps strength and balance (e.g., stair climbing, chair standing, etc.) also appear to benefit more from a RET‐based program compared to SR, especially in patients with low levels of physical functioning. Importantly though, for RET to be optimally effective, it should be performed at 70%–80% of the one‐repetition maximum, with 3–4 sets per exercise, with a minimum of 3 times per week for 8 weeks. Based upon this narrative review, we recommend that such high‐intensity progressive RET should be incorporated into standard programs during rehabilitation after TKA.

Schematic summary of this narrative review. Osteoarthritis is associated with a decline in muscle strength, mass, and function (1). After total knee replacement a further decline in muscle strength, mass, and function is observed (2). Although standard rehabilitation results in a partial or (sometimes) full recovery in these outcomes (3a), rehabilitation involving high‐intensity resistance exercise training leads to greater improvements in muscle strength, muscle mass, and physical functioning compared to standard rehabilitation (3b).

Highlights

• Following standard rehabilitation (SR) protocols, patients recovering from total knee replacement still have deficits in muscle strength, muscle mass, and physical functioning compared to their healthy counterparts.

• Rehabilitation involving high‐intensity resistance exercise training (RET) following total knee replacement leads to greater improvements in muscle strength, muscle mass, and physical functioning compared to SR.

• We recommend incorporating high intensity RET in the SR following total knee arthroplasty.

1. INTRODUCTION

Osteoarthritis is a degenerative joint disease that is highly prevalent among older adults. Characterized by a degradation of articular cartilage, osteoarthritic joints undergo increased friction during movement, ultimately leading to inflammation, pain, and mobility impairments. One of the most commonly affected joints is the knee (Lawrence et al., 2008), with an estimated worldwide prevalence of 22.9% in adults aged 40 years and over (Cui et al., 2020). As the knee is essential in everyday locomotion, patients with knee osteoarthritis display pronounced deficits in physical functioning, muscle strength, and muscle mass when compared to healthy counterparts (Alnahdi et al., 2012).

Total knee arthroplasty (TKA) is the ultimate surgical intervention used for end‐stage knee osteoarthritis patients who do not respond well to other, less invasive treatment methods. Despite surgically induced improvements in pain, durability, and mobility in the long‐term, TKA acutely exacerbates the deficits in physical functioning, strength, and muscle mass in the early postoperative period (Judd et al., 2012; Mizner et al., 2005). Although the acute decreases in muscular fitness can be counteracted with the help of physical rehabilitation, deficits in muscle mass, strength, and function still persist for months and even years after TKA when compared to the non‐operated leg, as well as compared to healthy individuals (Bade et al., 2010; Silva et al., 2003; Valtonen et al., 2009). These muscle deficits put TKA patients at an increased risk of sarcopenia (Lovett et al., 2021), metabolic disorders (Atlantis et al., 2009), falls and bone fractures (Balogun et al., 2017). Furthermore, low muscle mass is associated with an increased risk of prosthetic infection after TKA (Babu et al., 2019). Hence, postoperative care needs to be carefully designed to not merely target functional improvements, but also restore overall muscular fitness, preferably aiming to surpass preoperative levels.

Despite the ever‐growing incidence of TKA surgeries, there currently exists no consensus on the optimal rehabilitation strategy. Typically, traditional rehabilitation approaches apply low‐intensity, functional exercises, aiming to improve functional outcomes (i.e., joint range of motion, balance, and performance of daily activities), with much less focus on muscle hypertrophy and explicitly building muscle strength (Bandholm et al., 2012). Therefore, it has been recommended that higher intensity resistance exercises be incorporated into TKA rehabilitation regimes (Bade et al., 2012; Bandholm et al., 2012).

Resistance exercise training (RET) is a well‐established exercise modality that has been shown to effectively increase muscle mass and strength and improve physical functioning in a variety of young and older populations (Aamann et al., 2020; Mertz et al., 2021; Tieland et al., 2012). However, studies evaluating the efficacy of RET as a rehabilitation tool for TKA patients are scarce and inconsistent. The apparent variation in efficacy of different RET programs is likely attributed to differences in methodology, quality, and outcome parameters, making it difficult to draw definite conclusions (Chen et al., 2021; Liu et al., 2021; Papalia et al., 2020; Skoffer et al., 2015). In accordance, two systematic reviews with meta‐analyses published in 2021 reported opposing conclusions regarding the efficacy of progressive resistance training compared to standard rehabilitation (SR) for improving muscle outcomes after TKA (Chen et al., 2021; Liu et al., 2021). More specifically, Liu et al. (2021) concluded that RET may lead to improved muscle strength and physical performance after TKA compared to SR, while Chen et al. (2021) argued that progressive RET does not differ from SR for these outcomes. This discrepancy is likely due to the large heterogeneity in protocols used in the included studies, as well as the limited overlap of studies included in these reviews. As such, there is still no consensus on the additional benefits of RET as a rehabilitation strategy after TKA.

In this narrative review we provide an updated view on whether adding RET to the SR after TKA leads to greater improvements in physical functioning and increases in muscle strength and muscle mass when compared to SR alone, thereby critically evaluating the strengths and weaknesses of the reported study designs. A literature search was performed for all trials published until May 2022 that has compared a traditional rehabilitation approach (e.g., the hospital's standard postoperative care or a similar function‐based training) with a specific progressive RET‐based protocol, within the first year after TKA surgery. Outcome measures included at least one of the following: physical functioning tests, muscle strength or power, and muscle mass or size.

2. RESISTANCE EXERCISE TRAINING AND MUSCLE STRENGTH

Quadriceps muscle strength is associated with the clinical success of rehabilitation after TKA, with low strength being predictive of poor long‐term patient outcomes (Zeni et al., 2010). This is because a minimum level of strength, particularly of the leg muscles, is required to execute everyday activities, such as walking, climbing stairs, and standing up from a chair. TKA patients not only have low muscle strength due to osteoarthritis, but they also experience a further substantial loss of leg muscle strength in the early postoperative period due to factors such as hospitalization and disuse (Bade et al., 2010). Therefore, a key focus of rehabilitation should be to reverse the postoperative strength deficits, preferably improving muscle strength up to levels comparable to healthy individuals.

Several studies have investigated the efficacy of RET‐based rehabilitation to enhance muscle strength in TKA patients. Husby et al. (2018) compared the effects of a progressive RET intervention on lower‐body muscle strength compared to SR. TKA patients randomized into the intervention group (n = 16) performed 3 supervised sessions of RET per week for 8 weeks, starting 8 days after TKA. During each session, the patients exercised at a high‐intensity (80%–90% of the one‐repetition maximum, 1RM) on standard weightlifting machines (leg press and leg extension). The SR group (n = 18) followed the hospital's standard recovery protocol, comprising of physiotherapy sessions and instructions for non‐weight‐bearing exercises to perform at home. The authors reported a substantial decline in muscle strength 1 week after TKA in both groups, followed by an increase in muscle strength 8 weeks later. However, whereas patients increased leg extension strength by ∼43% above presurgery levels following RET, patients in the SR group were still ∼33% lower in strength compared to preoperative values. Moreover, the strength gains in the RET group persisted up to the 12‐month follow‐up, at which point the gain in muscle strength from presurgery was still greater compared to the SR group. The findings from this well‐controlled study suggest that RET is not only more effective at recovering lost strength after TKA, but that it also has the potential to induce long‐term strength gains beyond preoperative levels.

Such a long‐term benefit was also reported by Petterson et al. (2009). First, they showed that a relatively short (i.e., 6 weeks, ∼17 total sessions) progressive, high‐intensity (10RM) RET program with or without additional neuromuscular electrical stimulation (NMES) is accompanied by substantial improvements in muscle strength and physical functioning at 3 and 12 months post‐surgery, with no added effect of NMES. Next, they compared the 12‐month follow‐up outcomes of 41 of the patients who followed the RET protocol to a cohort of patients (n = 41) who only underwent the SR procedures. Despite receiving less therapy sessions (17 vs. 23), patients who received RET‐based rehabilitation had higher quadriceps muscle strength and better functional performance (timed up‐and‐go test, stair climbing, and 6‐min walk test) than the SR cohort 12 months after TKA. Moreover, a greater percentage of patients who underwent RET had “normal” muscle strength (i.e., within the range of a healthy reference group) when compared to the cohort that did not receive RET (Pozzi et al., 2020). Although some selection bias toward more “motivated” patients cannot be ruled out in this analysis, these findings strongly suggest that incorporating progressive RET in the rehabilitation of TKA patients leads to greater and long‐lasting improvements in muscle strength, approaching the levels observed in healthy individuals.

Using a 24‐week exercise program, Hsu et al. (2019) also showed greater increases in muscle strength and functional performance following RET compared to SR. However, although measures of hip strength and physical functioning still appeared greater in the RET group 12 weeks after the intervention, statistical significance was no longer shown. Of note, the program was started at a later time point (12 weeks post‐TKA) and the sample was relatively small (n = 14/15 per group), making it difficult to decipher whether there simply were no long‐term benefits, or whether other factors may explain the discrepancies with previous work (Husby et al., 2018; Petterson et al., 2009; Pozzi et al., 2020).

In contrast to the previous studies, others were not able to show any additional benefits of incorporating progressive RET into post‐TKA rehabilitation. For example, Jakobsen et al. (2014) observed no differences in either muscle strength or physical performance immediately after a 7‐week progressive RET program, nor at the 6‐month post‐TKA time point. At first glance, the RET protocol used in this study appears quite similar to that used by Husby et al. (2018), as both included 7–8 weeks of RET on leg press and leg extension machines, starting 1 week after surgery. However, the RET program was much less intense in Jakobsen's study (Jakobsen et al., 2014) compared to Husby's study (Husby et al., 2018), comprising only 2 versus 3 RET sessions per week, 2 versus 4 sets on each machine, and a load‐intensity of 12‐8RM versus 6RM, respectively. As such, the specific characteristics of the exercise program in terms of frequency, total volume, and exercise intensity, likely represent key factors for inducing benefits of RET over SR in post‐TKA strength recovery. Indeed, other studies in which no benefits were observed in TKA patients following RET compared to SR may have had suboptimal RET protocols in terms of training volume, as too few (≤12) RET sessions were performed (Bily et al., 2016; Jørgensen et al., 2016). Likewise, the reported lack of differences between groups in strength outcomes in a study aiming to compare low versus high‐intensity RET during recovery from TKA may be explained by the marginal differences in the intensity at which the trainings were actually performed (Bade et al., 2017).

Taken together, the available evidence suggests that the recovery of muscle strength after TKA can be enhanced by incorporating progressive RET to the postoperative rehabilitation program. However, as with any exercise program, the actual volume of work performed throughout the study (intensity × repetitions × sets × duration) will determine the exercise response (Mitchell et al., 2012; Ralston et al., 2017). Specifically for muscle strength, it appears that the preferred characteristics of a RET program for TKA patients includes exercising at 70%–80% 1RM, with 3–4 sets of each exercise, three times per week for a minimum of 8 weeks to reach the appropriate training stimulus. Importantly, proper supervision throughout such an intense program is essential to ensure adherence and effectively maximize muscle strength gains (Lacroix et al., 2017).

3. RESISTANCE EXERCISE TRAINING AND MUSCLE MASS

Apart from the profound reduction in muscle strength, patients with knee osteoarthritis tend to have deficits in muscle size, which is associated with poor physical performance and health in older adults (Janssen et al., 2002). These muscle deficits are observed not only when compared to healthy counterparts, but also when comparing the involved with the uninvolved leg, and are at least partly due to decreases in physical activity and (joint) loading caused by joint pain and dysfunction. TKA surgery results in a short period of further physical inactivity, which likely exacerbates the muscle size deficits (Kouw et al., 2019). Accordingly, muscle mass deficits can persist even 9 months after TKA (Valtonen et al., 2009). Yet, skeletal muscle hypertrophy is not generally a focus point in post‐TKA rehabilitation programs. Although it is well established that RET is the most effective exercise modality to induce skeletal muscle hypertrophy in both healthy and more compromised populations (Holwerda et al., 2018; Tieland et al., 2012; Verdijk et al., 2009), it remains uncertain whether RET is superior to SR for muscle hypertrophy after TKA.

To the authors' knowledge, only three studies have investigated the effects of RET on muscle hypertrophy after TKA (Liao et al., 2020; Tsukada et al., 2020; Valtonen et al., 2010). Tsukada et al. (2020) investigated the effects of a 12‐week rehabilitation program that consisted of RET combined with electrical stimulation on several muscle outcomes compared to SR. Whereas the circumference of the operated thigh did not change from pre‐operation following RET, there was a significant decline in thigh circumference in the SR group. The authors concluded that RET combined with electrical stimulation has a protective effect against muscle atrophy. Although thigh circumference is not a direct measure of the muscles (i.e., it includes fat, skin, etc.), their conclusion was supported by findings from Liao et al. (2020). Here, the effects of a 12‐week elastic‐band RET program were determined by examining appendicular lean mass (ALM), representing a more direct measure of muscle mass than thigh circumference. While the SR group showed a decline in ALM (−0.31 kg) compared to pre‐surgery, patients in the RET group increased ALM (+0.5 kg) following the intervention. The authors additionally determined the proportion of patients in each group that exhibited “low muscle mass,” characterized by having an ALM index below 6.12 kg/m². Whereas the proportion of patients with “low muscle mass” declined from 60% to 20% following RET, the proportion of low muscle mass patients increased in the SR group, from 53% to 68% (Liao et al., 2020). Finally, similar results were shown in a study in which TKA patients following a 12‐week aquatic RET program had greater increases in thigh muscle cross‐sectional area as assessed with computed tomography scanning (∼5%) when compared to the SR group (∼2%) (Valtonen et al., 2010). Although more studies are needed using robust measures of muscle mass/size (e.g., MRI, CT, or Ultrasound scanning), the current evidence clearly suggests that RET is preferred over SR for not only preventing muscle atrophy, but even inducing muscle hypertrophy after TKA.

4. RESISTANCE EXERCISE TRAINING AND PHYSICAL FUNCTIONING

Compared to healthy individuals, TKA patients experience limitations in physical functioning, including an impaired ability to walk, stand up from a chair, and negotiate stairs. Accordingly, improving physical functioning is generally the primary objective in SR programs following TKA surgery. Although it has been shown that RET can improve measures of physical functioning in older and more compromised populations (Churchward‐Venne et al., 2015; Tieland et al., 2012), it remains unknown whether RET‐based rehabilitation after TKA can further enhance improvements in physical functioning when compared to SR programs, which are generally targeted at functional abilities.

All of the previously mentioned studies (Bade et al., 2017; Bily et al., 2016; Hsu et al., 2019; Husby et al., 2018; Jakobsen et al., 2014; Jørgensen et al., 2016; Liao et al., 2020; Petterson et al., 2009; Pozzi et al., 2020; Tsukada et al., 2020; Valtonen et al., 2010) have also assessed at least one measure of physical functioning. Unsurprisingly, the studies that reported no additional benefit of RET compared to SR for increasing muscle strength also observed no differences between groups for the improvement in physical functioning (Bade et al., 2017; Bily et al., 2016; Jakobsen et al., 2014; Jørgensen et al., 2016). This is likely because improvements in muscle function are at least partly attributable to increases in muscle strength. On the other hand, most of the studies that did show greater strength increases in TKA patients following RET compared to SR also showed greater improvements in at least one measure of physical functioning in the RET group (Hsu et al., 2019; Liao et al., 2020; Pozzi et al., 2020; Tsukada et al., 2020; Valtonen et al., 2010). Only Husby et al. (2018) did not observe greater improvements in physical functioning following RET versus SR, despite utilizing a very high‐intensity RET protocol and reporting exceptionally large muscle strength gains compared to other studies. This may be explained by the fact that this particular study only assessed a single physical functioning outcome, namely the six‐minute walking test (6MWT). In general, others that have assessed 6MWT performance in TKA patients also observed no benefit of RET over SR (Piva et al., 2017; Pozzi et al., 2020). In fact, only Hsu et al. (2019) were able to detect an improvement in the distance covered during the 6MWT in favor of the RET versus SR group. Of note, the patients in Hsu's study achieved a 6MWT distance of less than 400 m by the end of the study (RET: 395 m, SR: 342 m). In contrast, patients in the other studies achieved 6MWT distances of ∼500 m (Husby et al., 2018; Piva et al., 2017; Pozzi et al., 2020), which falls within the normal range of healthy adults of 55–75 years old (Camarri et al., 2006). As such, it is likely that improvements in physical functioning following RET compared to SR, as measured by the 6MWT, can only be detected in more physically compromised patients, as they have more to gain from the intervention. Aside from that, one can speculate that the 6MWT does not capture the full extent of physical disability in TKA patients as it only assesses the ability to walk on flat ground, while TKA patients may experience greater difficulty to perform movements that are more dependent on quadriceps strength and balance (Fernandes et al., 2018; Stan et al., 2013; Valtonen et al., 2009).

Chair stand tests and stair climbing tests are commonly employed in geriatric and compromised populations (Kulkarni et al., 2022; Widmer et al., 2022). As sufficient quadriceps strength and balance are necessary to complete these activities, it is unsurprising that TKA patients perform worse on these tests compared to healthy individuals (Valtonen et al., 2009). Although not reaching statistical significance, a study by Piva et al. (2017) showed markedly greater improvements in TKA patients following a partially supervised, 6‐month RET program compared to SR in both stair climbing time (−24 vs. +1%; p = 0.054) as well as a timed chair stand test (+15 vs. −5%; p = 0.149). In accordance, others have shown greater improvements in stair climbing and chair rising ability following RET compared to SR in TKA patients (Liao et al., 2020; Pozzi et al., 2020; Tsukada et al., 2020), with only one relatively small study showing no improvements (Hsu et al., 2019).

With regard to assessing balance, the single‐leg‐stance test is commonly performed. In the studies from both Liao et al. (2020) and Piva et al. (2017), TKA patients who had followed the RET program performed better on the single‐leg‐stance test compared to SR. In accordance, RET has been associated with improvements in balance ability in older adults (Šarabon et al., 2020). On the other hand, less consistent findings were reported for the timed‐up‐and‐go test, a measure that demands both balance and chair standing ability (Hsu et al., 2019; Liao et al., 2020; Pozzi et al., 2020; Tsukada et al., 2020; Vuorenmaa et al., 2014). Liao et al. (2020) showed a statistically significant improvement in the timed‐up‐and‐go test in the RET versus SR group. Such a RET‐induced benefit was supported by observational data from Pozzi et al. (2020), who reported better timed‐up‐and‐go performance at 12 months post‐TKA surgery in patients who participated in RET versus those who did not. In contrast, others have not been able to confirm the superiority of RET versus SR for improving timed‐up‐and‐go test performance. This inconsistency may be explained by high physical functioning (i.e., within the normal range for their age group) at baseline for this outcome (Vuorenmaa et al., 2014), great inter‐individual variability at baseline (Tsukada et al., 2020), and/or a small sample size (Hsu et al., 2019; Pozzi et al., 2020). In agreement, the study in which the greatest improvements in timed‐up‐and‐go performance were observed analyzed a relatively large number of patients (n = 55) and also reported the worst performance on this outcome at baseline (Liao et al., 2020). Moreover, as already described for the muscle strength outcomes, several differences exist in the various study designs, especially in terms of the exact RET protocol adopted, making it difficult to compare between studies.

To provide a clearer overview of the main methodological differences between the studies included in this review, as well as the main study outcomes, all studies are summarized in Table 1. Interestingly, those studies that found no differences between groups for the improvements in physical functioning all performed the RET trainings at a low intensity and/or low total training volume (Bade et al., 2017; Bily et al., 2016; Jakobsen et al., 2014; Jørgensen et al., 2016). In fact, from the summary data in Table 1, it is obvious that training near 80% of 1RM, and/or training three times per week, and/or implementing 3–4 sets per exercise session, and/or training for an extended time period after TKA, may all play a decisive role in a training program's efficacy to improve physical functioning post TKA.

TABLE 1.

Summary table of included studies, describing study and intervention details, outcomes, and conclusions.

Ref	Intervention	Outcomes	Conclusions
Husby et al. (2018)	SR (n = 18, 63 y) Physiotherapy recommended; non‐weight bearing; home exercises RET (n = 16, 61 y) Machines; unilateral (operated leg) When: 8 d post‐TKA, 8 wk, 3x/wk Volume: 4 sets × 5 reps per exercise Intensity: 5–‐6 RM	Changes from pre‐TKA in RET versus SR Strength, 1RM leg press: +37% versus −12%, 10 wk post‐TKA +37% versus +12%, 12 mth post‐TKA Strength, 1RM leg extension: +43% versus −33%, 10 wk post‐TKA +80% versus +13%, 12 mth post‐TKA Function, 6MWT: +15% versus +6% (NS), 10 wk post‐TKA +27% versus +22% (NS), 12 mth post‐TKA	Strength: RET > SR Function: RET = SR
Petterson et al. (2009)	SR (n = 41, 66 y) Separate cohort (not randomized) Physiotherapy; 23 sessions RET (n = 41, 65 y) Bodyweight + added weights; (added NMES for some) When: 3–4 wk post‐TKA, 6 wk, 3x/wk Volume: 3 sets × 10 reps per exercise Intensity: 10 RM	Comparison of RET versus SR, 12 mth post‐TKA Strength, Isometric MVC (ext.): 21% higher in RET versus SR group Function, TUG: 24% faster in RET versus SR group Function , SCT: 44% faster in RET versus SR group Function , 6MWT: 15% farther in RET versus SR group	Strength: RET > SR Function: RET > SR
Pozzi et al. (2020)	Same groups as Petterson et al. (2009); added healthy reference group SR (n = 40, 66 y) RET (n = 165, 65 y) Reference (n = 88, 65 y)	Proportion of RET versus SR with normal* results, 12 mth post‐TKA Strength, Isometric MVC (ext.): 18% versus 5% Function, TUG: 33% versus 20% (NS)# Function , SCT: 34% versus 18% Function , 6MWT: 23% versus 15% (NS)	Strength: RET > SR Function: RET > SR
Hsu et al. (2019)	Females only SR (n = 15, 70 y) Non‐weight bearing; home exercises RET (n = 14, 72 years) Machines When: 3 mth post‐TKA, 24 wk, 3x/wk Volume: 3 sets × 12 reps per exercise Intensity: 80% 1RM	Changes from pre‐training in RET versus SR Strength, Isokinetic MVC (ext.): +41% versus +9%, post‐training, +61% versus +56% (NS), 12 wk post‐training Strength, Isokinetic MVC (flex.): +22% versus +7%, post‐training, +27% versus +17% (NS), 12 wk post‐training Function, 6MWT: +17% versus +5%, post‐training, +15% versus +4% (NS)#, 12 wk post‐training Function, 8‐ft Up‐and‐Go: −14% versus −11% (NS), post‐training, −10% versus −6% (NS), 12 wk post‐training Function, CST: +26% versus +13% (NS), post‐training, +19% versus +10% (NS), 12 wk post‐training	Strength: RET > SR Function: RET > SR
Jakobsen et al. (2014)	SR (n = 37, 63 y) Home exercises + supervised function‐focused physiotherapy RET (n = 35, 66 y) Home exercises + machines When: 7 d post‐TKA, 7 wk, 2x/wk Volume: 2 sets × 10 reps per exercise Intensity: From 10RM to 8RM	Changes from pre‐TKA in RET versus SR Strength, Isometric MVC (ext.): −27% versus −40% (NS), 8 wk post‐TKA −11% versus −19% (NS), 26 wk post‐TKA Strength, Isometric MVC (flex.): −14% versus −10% (NS), 8 wk post‐TKA −4% versus +4% (NS), 26 wk post‐TKA Strength, Leg press power: −0% versus −22% (NS), 8 wk post‐TKA +18% versus +2% (NS), 26 wk post‐TKA Function, 6MWT: −4% versus −2% (NS), 8 wk post‐TKA +7% versus +8% (NS), 26 wk post‐TKA	Strength: RET = SR Function: RET = SR
Jørgensen et al. (2016)	SR (n = 25, 65 y) Home exercises RET (n = 24, 64 y) SR + machines When: 1 wk post‐TKA, 8 wk, 2x/wk Volume: 4 sets × 8 reps per exercise Intensity: 8RM	Changes from pre‐TKA in RET versus SR Strength, Leg extension power: +27% versus +3% (NS)#, 10 wk post‐TKA +25% versus +23% (NS), 12 mth post‐TKA Function, Gait speed: +2% versus +3% (NS)#, 10 wk post‐TKA, +15% versus +12% (NS), 12 mth post‐TKA Function, 6MWT: +15% versus +6% (NS), 10 weeks post‐TKA, +15% versus +20% (NS), 12 mth post‐TKA	Strength: RET = SR Function: RET = SR
Bily et al. (2016)	SR (n = 29, 68 y) Function‐focused supervised physiotherapy with low load progressive resistance exercises RET (n = 26, 65 y) Isokinetic leg press training with vibration; unilateral When: 6 wk post‐TKA, 6 wk, 2x/wk Volume: 120 s per leg per training	Changes from pre‐training in RET versus SR Strength, Isometric MVC (ext.): +25% versus +29% (NS) Strength, Isometric MVC (leg press): +22% versus +36% (NS) Function, TUG: −14% versus −18% (NS) Function, SCT: −24% versus −27% (NS)	Strength: RET = SR Function: RET = SR
Bade et al. (2017)	SR (n = 77, 64 y) Supervised functional exercises with slower progression to weight‐bearing exercises; no machines RET (n = 77, 63 y) Progression from body‐weight to machines (individual basis) When: 4 d post‐TKA, 11 wk, ∼2x/wk Volume: 2 sets × 8 reps per exercise Intensity: 8RM	Changes from pre‐TKA in RET versus SR Strength, Isometric MVC (ext.): +3% versus −7% (NS), 3 mth post‐TKA +20% versus +15% (NS), 12 mth post‐TKA Strength, Isometric MVC (flex.): +4% versus +1% (NS), 3 mth post‐TKA +15% versus +13% (NS), 12 mth post‐TKA Function, SCT: −24% versus −17% (NS), 3 mth post‐TKA −34% versus −28% (NS), 12 mth post‐TKA Function, TUG: −16% versus −10% (NS)#, 3 mth post‐TKA −18% versus −16% (NS), 12 mth post‐TKA Function, 6MWT: +9% versus +3% (NS), 3 mth post‐TKA +17% versus +10% (NS), 12 mth post‐TKA	Strength: RET = SR Function: RET = SR
Liao et al. (2020)	Females only SR (n = 27, 70 y) Function‐focused supervised physiotherapy 2x/wk, no RET RET (n = 28, 72 y) Elastic bands (including leg press and leg curl) When: 1 mth post‐TKA, 12 wk, 2x/wk Volume: 3 sets × 10–20 reps per exercise Intensity: 13–15 RPE on 15‐point borg scale (∼65%–80% 1RM)	Changes at 4 mth post‐TKA in RET vs SR Function, TUG: −33% versus −14% compared to pre‐TKA −38% versus −17% compared to pre‐training Function, Gait speed: +51% versus +13% compared to pre‐TKA +92% versus +53% compared to pre‐training Function, CST: +72% versus +30% compared to pre‐TKA +91% versus +54% compared to pre‐training Function, SLS: +42% versus +9% compared to pre‐TKA +97% versus +52% compared to pre‐training Mass, ALM: +3% versus −2% compared to pre‐TKA Mass, % low muscle mass: 20% versus 67%	Function: RET > SR Mass: RET > SR
Tsukada et al. (2020)	Females only SR (n = 20, 74 y) Supervised rehabilitation 5 d/wk (2x/d); progressive resistance exercises (body weight and/or with added weights) Volume: 2–3 sets × 10 reps RET (n = 20, 73 y) SR + greater resistance through antagonistic activation (via NMES) When: 1 d post‐TKA, 12 wk, 3x/wk Volume: 10 sets × 10 reps (3 s contractions) Intensity: 80% max tolerable NMES intensity	Changes from pre‐TKA in RET versus SR Strength, Isometric MVC (ext.): −6% versus −28% (NS), 6 wk post‐TKA +18% versus −1% (NS), 12 wk post‐TKA Strength, Isometric MVC (flex.): +6% versus −9% (NS), 6 wk post‐TKA +30% versus +19%, 12 wk post‐TKA Function, 10 m walk time: −18% versus −10%, 6 wk post‐TKA −27% versus −27% (NS), 12 wk post‐TKA Function, TUG: −27% versus −10% (NS), 6 wk post‐TKA −35% versus −24% (NS), 12 wk post‐TKA Function, SCT: −5% versus +22%, 6 wk post‐TKA −22% versus −7% (NS), 12 wk post‐TKA Mass, Thigh circumference: +0% versus −4% (NS), 6 wk post‐TKA −1% versus −4% (NS), 12 wk post‐TKA	Strength: RET > SR Function: RET > SR Mass: RET > SR
Valtonen et al. (2010)	SR (n = 21, 66 y) No intervention RET (n = 25, 66 y) Aquatic training with resistance boots When: 10 mth post‐TKA, 12 wk, 2x/wk Volume: 2–4 sets × 12–30 reps Intensity: 14–17 RPE on 15‐point Borg scale	Changes from pre‐ to post‐training in RET versus SR Strength, Isokinetic MVC (ext.): +29% versus −0% Strength, Isokinetic MVC (flex.): +36% versus +2% Function, Max gait speed: +3% versus +2% (NS) Function, Gait speed: +8% versus −1% Function, SCT: −14% versus +1% Mass, Thigh muscle CSA: +5% versus +2%	Strength: RET > SR Function: RET > SR Mass: RET > SR
Piva et al. (2017)	SR (n = 20, 68 y) Machines (supervised, 12 sessions) + home exercises Volume: 2 sets × 20 reps per exercise, Intensity: 40%–50% 1RM RET (n = 21, 68 y) Machines (supervised, 12 sessions); + home exercises; functional tasks; physical activity/diet promotion When: 3–6 mth post‐TKA, 13 wk Volume: 2 sets × 8 reps per exercise Intensity: 60%–80% 1RM	Changes from pre‐ to post‐training in RET versus SR Function, SCT: −24% versus +1% (NS)# Function, CST: −15% versus +5% (NS) Function, SLS: +14% versus −10% Function, 6MWT: +7% versus +6% (NS) Function, Gait speed: +10% versus +5% (NS)	Function: RET > SR
Vuorenmaa et al. (2014)	SR (n = 55, 69 years) Home exercises; no guidance after baseline (2 mth post‐TKA) RET (n = 53, 69 y) Unsupervised, home exercises; body weight + dumbbells; guidance at 0, 1 and 4 mth into training When: 2 mth post‐TKA, 12 mth, 3x/wk Volume: 2–3 sets × 10–20 reps per exercise	Changes from pre‐ to post‐training in RET versus SR Strength, Isometric MVC (ext.): +83% versus +89% (NS) Strength, Isometric MVC (flex.): +43% versus +24% Function, Max gait speed: +24% versus +13% Function, TUG: −17% versus −14% (NS)	Strength: RET > SR Function: RET > SR

Note: Percentage change was retrieved directly from the referred papers; or indirectly calculated based on reported absolute values, Sample size (n) represents the number of patients included in the first post‐intervention analyses.

Abbreviations: #, trend for significance (i.e., 0.05 ≤ p ≤ 0.10); 1RM, one‐repetition maximum; 6MWT, six‐minute walking test; ALM, appendicular lean mass; CST, chair stand test; NMES, neuromuscular electrical stimulation; NS, not statistically significantly different; RET, resistance exercise training group; RPE, rating of perceived exertion; SLS, single‐leg stance test; SCT, stair climb test; SR, standard rehabilitation group; TKA, total knee arthroplasty; TUG, timed‐up‐and‐go test; *“normal”, above lower bound of the 95% CI of reference.

Altogether, current literature suggests that adding high‐intensity RET to the post‐TKA recovery program has the potential to improve physical functioning outcomes to a greater extent than SR alone, much in line with the findings for muscle strength and muscle mass, as schematically visualized in Figure 1. Of note, the benefits for physical functioning appear more robust, especially for those patients who exhibit relatively low functional performance. Importantly, the collection of functional tests chosen to measure these improvements must be well considered and cover the full spectrum of patient‐specific, TKA‐related mobility limitations, as these may be related to balance, strength, endurance, or a combination of these. Hence, future research should attempt to differentiate physical functioning tests and provide further insight into what extent specific physical functioning‐related benefits are induced by RET.

FIGURE 1.

Graphical representation of the course of muscle outcomes (muscle strength, muscle mass, physical functioning) over time during healthy aging (black line), and following knee replacement (X) with standard rehabilitation (gray line) or a resistance exercise training‐focused rehabilitation program (green line).

5. CONSIDERATIONS WHEN IMPLEMENTING RESISTANCE EXERCISE TRAINING

Despite the apparent efficacy of RET for TKA patients, several obstacles may be present throughout the post‐surgery rehabilitation period that could limit the feasibility of applying high‐intensity loads to the muscles. For example, pain and swelling around the knee joint as well as fear for injuries may render high‐intensity RET intolerable for some patients, especially early after surgery. In these cases, rather than delaying the start of RET, modifications to the RET program should be implemented, given the importance of mitigating the sharp declines in muscle strength, mass, and physical functioning that occur early postoperatively (Judd et al., 2012; Mizner et al., 2005). The most straightforward modification to increase tolerability of RET is to reduce the load while increasing the number of repetitions performed in each set. Although low‐load RET is suboptimal in terms of increasing muscle strength compared to high‐load training, muscle hypertrophy seems to be load‐independent when all sets are taken to failure (Mitchell et al., 2012). Alternatively, low‐load RET with blood flow restriction (BFR) has been suggested as a training modality suitable for rehabilitation in physically compromised populations. For example, although low‐load BFR training appears less effective for increasing strength in clinical populations than high‐load training alone, it is more effective than low‐load training alone (Hughes et al., 2017). This suggests that low‐load BFR training may serve as an effective option in the early post‐operative period, that is, for TKA patients who are not yet able to successfully perform high‐intensity RET. However, specific research is needed on BFR training in TKA patients, especially as concerns of rhabdomyolysis have been reported with this training modality (Iversen et al., 2010; Tabata et al., 2016). Finally, NMES could represent another strategy for optimizing muscular fitness in the first weeks following TKA surgery. Indeed, there is evidence that early‐initiated NMES can be an effective strategy to prevent muscle atrophy and losses in functional performance, muscle strength, and voluntary muscle activation in the early post‐operative period, when the decline in muscle‐related outcomes is most severe (Mizner et al., 2005; Paravlic et al., 2022). However, since NMES does not reflect the functional movements performed during daily living activities, and provides a weaker stimulatory effect on muscle, it should only be performed until the patient is able to actively perform resistance exercise.

Other than physical limitations, practical obstacles also exist with regard to applying RET as a post‐TKA rehabilitation tool. For example, providing RET as an additional program, as was done in Husby's study (Husby et al., 2018), means that patients need to free up extra time to attend the RET sessions. This may make 3‐times weekly RET programs unfeasible for patients, as they already have to attend physiotherapy sessions several times per week. Therefore, we strongly advocate the incorporation of progressive high‐intensity RET by physiotherapists as part of the SR regime. It is important to note that not all physiotherapy practices have access to exercise machines. Although the current evidence base is lacking, the elastic band‐based RET has been shown to be successful in improving muscle mass and physical functioning in Liao's study (Liao et al., 2020). Hence, elastic bands may be a valuable alternative to traditional exercise machines when the latter is not an option. However, one of the main challenges in incorporating progressive high‐intensity RET into the SR regime may be convincing physiotherapists to change standard practices; that is, from a mainly functional performance‐based focus to a more holistic approach that includes muscle hypertrophy and strength goals. Hence, dissemination of scientific evidence‐based approaches in the practical field is extremely important. Clearly, adopting a change in the treatment strategy is a time‐consuming and difficult process that is only feasible if the entire network involved in the care surrounding TKA patients is informed and convinced (Damschroder et al., 2009). This includes patients themselves, their families, patient organizations, surgeons, physicians, nursing staff, physiotherapists, and insurance companies. Only when all involved are “on board,” and the due attention is maintained for individualized patient progression throughout the rehabilitation phase, a true change in treatment strategies can be implemented, ensuring long‐term compliance and effectiveness (Damschroder et al., 2009; Drew et al., 2019). Hence, future work should also address the practical incorporation of high‐intensity RET into current healthcare systems, taking into account the needs of the individual patient, the resources of the physiotherapist practices, as well as the broader network involved in the care for TKA patients.

6. CONCLUSION

The current body of evidence clearly shows that patient outcomes in terms of muscle strength and muscle mass can be greatly improved when high‐intensity, progressive RET is applied during rehabilitation following TKA. Additionally, RET‐based rehabilitation protocols induce greater improvements in certain aspects of physical functioning compared to SR practices. In particular, activities that rely on quadriceps strength and balance appear to benefit most from RET, with those patients starting off at low levels of physical functioning likely showing the greatest improvements. Although future work is needed to optimize RET‐based rehabilitation, the current findings show that high‐intensity resistance training protocols (70%–80%1RM) performed 3 times per week for at least 8 weeks (preferably longer) and starting early after TKA, may yield the most improvements during recovery from TKA, including long‐term benefits up to at least a year after surgery. We strongly recommend incorporating high‐intensity RET into the SR protocol for TKA patients. To circumvent any limitations in the implementation of true high‐intensity/high loads, the application of low‐load to failure, low‐load with BFR, or NMES could be considered, especially in the first 4 weeks after surgery.

CONFLICT OF INTEREST STATEMENT

The authors report no potential conflict of interest.