2.1 Study design and participants

This was a prospective, longitudinal single-centre study conducted at the Department of Physiotherapy and Department of Orthopedics, Kasturba Hospital, Manipal, Karnataka, India. Ethical approval was obtained from the Institutional Ethics Committee (IEC) of Kasturba Hospital, Manipal (IEC1-20–2022). The study protocol was registered in the Clinical Trial Registry – India (CTRI/2022/07/043642). The study was conducted in accordance with the World Medical Association (Declaration of Helsinki) - Code of Ethics. Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guidelines were followed.²²

The inclusion criteria for the knee OA group were patients aged 50 to 80 years of either sex with diagnosed primary severe (Kellgren - Lawrence Grade 4) knee OA who were scheduled for TKA. The inclusion criteria for the control group were age-, sex-, and BMI-matched healthy individuals who did not have any current lower extremity orthopaedic injuries or neurological, immunological, inflammatory, or cardiovascular diseases. Exclusion criteria were as follows: Individuals who had history of neurological impairments affecting lower limb function, or the presence of immunological or inflammatory conditions, history of major medical, cardiac, or vascular events within the past year, and those who were not willing to participate in the study.

2.2 Instrumentation and procedure

The participants were screened according to the eligibility criteria. The demographic information of the participants, including age (in years), sex, height (in cm), weight (in kg), and body mass index (BMI) (in kg/m²), was collected. Participants were informed of the study procedures, and written informed consent was obtained. The outcomes were assessed by a musculoskeletal physical therapist with clinical experience in knee assessment and rehabilitation. The outcome measures were recorded preoperatively and at 6 weeks, 3 months, 6 months, and 12 months postoperatively.

2.3 Intervention program

All the participants underwent a standard postoperative TKA rehabilitation program post-TKA. Stage 1 exercises were administered from day 1 to week 2 in the early function phase following TKA. Along with the standard rehabilitation program, specific exercises targeting VMO muscle activation, such as isometrics of the quadriceps, terminal knee extension short arc, straight leg raise, knee extension in sitting (90°-0), and the McConnell method of VMO contraction in high sitting, were included and were performed three times per day. Stage 2 exercises were administered in the progressive function phase from 3 to 6 weeks following TKA. Strengthening exercises with cuff weights ranging from 0.5 to 1 kg, which included terminal knee extension short-arc, straight leg raising, knee extension in sitting (90°-0°), and VMO contraction in high sitting using the McConnell method, were done twice a day, with 3 sets of 20 repetitions each. The closed-chain activity of VMO contraction in standing was done twice a day. Stage 3 exercises were administered in the advanced function phase from 7 to 12 weeks following TKA. The strengthening exercises progressed with cuffed weights of 2 kg or 60 % of 1 RM were done twice a day, with 3 sets of 30 repetitions. The progressive stages included other general exercises, mobility-based and stretching exercises, functional training, balance training, and aerobic conditioning ^14,23–25.

2.4 Muscle strength assessment

The strength of the quadriceps and hamstring muscles was assessed using the Chatillon DMG Series handheld dynamometer (Fig. 1). Standardized testing procedures were followed in accordance with established guidelines.^26,27 Handheld dynamometry has demonstrated moderate-to-good validity and excellent reliability for assessing most muscle groups,^27,28 with intraclass correlation coefficients (ICCs) ranging from 0.932 to 0.984 ²⁶. In addition to the quadriceps and hamstrings, other lower-limb muscle groups were screened to provide a comprehensive overview of lower extremity muscle function. Quadriceps and hamstring strength evaluations were performed using the calibrated dynamometer under standardized conditions. For quadriceps testing, participants were seated with the knee flexed to 90° and asked to perform a maximal isometric knee extension against the resistance pad. For hamstring testing, participants were positioned prone with the knee flexed to 30° and instructed to perform an isometric knee flexion contraction. Each test consisted of three maximal voluntary contractions lasting 5 s each, with a 30-s rest interval between trials to minimize fatigue. The mean of the three trials, expressed in pounds (lb), was used for statistical analysis.^27,28

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Fig. 1 Chatillon DMG Series handheld dynamometer for the evaluation of muscle strength.

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2.5 Muscle flexibility assessment

Muscle length assessment was performed following the standardized procedures described by Kendall and Kendall (1949) to ensure consistency and clinical accuracy. The evaluation focused on key lower-limb muscle groups, primarily the quadriceps and hamstrings, which play critical roles in knee joint mobility and stability. Each muscle group was assessed for flexibility and potential tightness using clinically accepted positioning and techniques under controlled conditions.²⁹ Hamstring flexibility was assessed using the passive knee extension test and quadriceps flexibility was assessed using the modified Thomas test, within the participant's comfort limits.²⁹

2.6 Statistical analysis

Statistical analyses were performed using Jamovi (version 2.6.23). The distribution of continuous variables was assessed for normality using the Kolmogorov–Smirnov test. All outcome variables were found to be normally distributed. Continuous data are presented as means and standard deviations, while categorical variables are presented as frequencies and percentages.

Baseline demographic and clinical characteristics were summarized descriptively. Comparisons between individuals with knee OAand healthy controls were performed using independent-samples t-tests for continuous variables and chi-square tests for categorical variables, as appropriate. Longitudinal changes in quadriceps and hamstring muscle strength and flexibility across the five assessment time points (preoperative, 6 weeks, 3 months, 6 months, and 12 months postoperatively) were evaluated using repeated-measures analysis of variance (ANOVA). This approach was selected to examine within-subject temporal changes over predefined postoperative follow-up intervals.

Given the longitudinal study design, attrition occurred across follow-up visits. A dropout analysis was conducted by comparing baseline demographic and clinical characteristics (age, sex, body mass index, quadriceps strength, hamstring strength, and muscle flexibility) between participants who completed the 12-month follow-up and those who were lost to follow-up. As no statistically significant baseline differences were observed, missing data were considered to be random. Repeated-measures ANOVA was therefore performed using available-case data at each time point. When a significant main effect of time was detected, post-hoc pairwise comparisons with Bonferroni correction were applied to control for multiple comparisons. All statistical tests were two-tailed, and the level of statistical significance was set at p ≤ 0.05.

3.1 Participant details and demographic information

A total of 134 individuals were assessed for eligibility, after which 77 individuals were recruited based on the predefined sample size. Follow-up was completed at 6 weeks (n = 77), 3 months (n = 77), 6 months (n = 54), and 12 months (n = 26). Dropouts primarily resulted from logistical constraints and non-compliance with follow-up visits. Baseline demographic and clinical characteristics did not differ significantly between participants who completed the 12-month follow-up and those lost to follow-up, indicating that attrition was unlikely to bias the outcomes.

The demographic and anthropometric characteristics of the participants were noted (Table 1). The occupations of the individuals were homemakers (n = 28, 36.36%), skilled agricultural, forestry and fishery workers (n = 19, 24.8%), elementary occupations (n = 10, 12.98%), support workers (n = 9, 11.68%), service and sales workers (n = 5, 6.49%), managers and professionals (n = 3, 3.89%), and shopkeepers (n = 3, 3.89%). The comorbidities reported were diabetes (n = 29, 37.66%), hypertension (n = 12, 15.58%), obesity (n = 8, 10.38%), pulmonary diseases (n = 7, 9.09%), cardiovascular history (n = 6, 7.79%), and thyroid disorders (n = 6, 7.79%).

3.2 Quadriceps and hamstring muscle strength after TKA

Preoperatively, quadriceps and hamstring muscle strengths were significantly lower in individuals with knee OA compared with healthy controls (p < 0.001). Following surgery, an additional decline in strength was observed at six weeks post-TKA, with both muscle groups exhibiting further weakness. The early postoperative decline was more pronounced in the quadriceps than in the hamstrings (Table 2).

From the third postoperative month onward, a steady and statistically significant recovery in muscle strength was evident (p < 0.001). Both quadriceps and hamstring strengths improved progressively across subsequent follow-up periods. The quadriceps exhibited a progressive improvement from baseline, with mean increases of approximately 155.6% at 3 months, 223.1% at 6 months, and 269.2% at 12 months postoperatively, showing a slightly delayed but ultimately greater recovery trajectory compared to the hamstrings. The hamstrings followed a similar pattern of recovery, though with slightly lower percentage gains compared to the quadriceps (Tables 3 and 4).

Despite these substantial improvements, quadriceps and hamstring strength at 12 months post-TKA remained significantly lower than those of the healthy control group (p < 0.001) (Table 5). The temporal pattern of strength declines and recovery is illustrated in Fig. 2.

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Fig. 2 Temporal trend of quadriceps and hamstring strength (lb) before and after TKA compared with controls.

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3.3 Quadriceps and hamstring muscle flexibility after TKA

The hamstrings demonstrated reduced flexibility preoperatively in comparison to controls (p < 0.05). At 6 weeks post-TKA, hamstring tightness increased further (p < 0.05), a trend possibly attributable to acute postoperative stiffness and protective guarded movements. Significant improvements in flexibility were subsequently recorded at the 3, 6, and 12-month intervals (p < 0.05). Despite these substantial gains over the first postoperative year, hamstring flexibility at 12 months remained significantly lower than the normative data established by the control group (p < 0.05) (Fig. 3).

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Fig. 3 Temporal trend of hamstring flexibility before and after TKA.

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Preoperatively, the quadriceps exhibited significantly reduced flexibility compared with healthy controls (p < 0.05). Following surgery, a transient increase in quadriceps stiffness was observed at 6 weeks post-TKA. This was followed by a gradual and steady improvement across all subsequent follow-up periods, with the muscle group attaining near-normal flexibility by the 12-month mark. However, despite this recovery trajectory, quadriceps flexibility at 12 months post-TKA remained significantly lower than that of the healthy control group (p < 0.05).

4.1 Principal findings

These findings suggest that individuals with knee OA exhibited significant preoperative deficits in both quadriceps and hamstring strength and flexibility compared with healthy controls. This reduction reflects the chronic disuse and neuromuscular inhibition commonly associated with advanced OA ^30–32. Following TKA, these deficits initially worsened at six weeks postoperatively, indicating early postoperative weakness and muscle stiffness.^11,25 The quadriceps demonstrated a more pronounced decrease than the hamstrings, consistent with early postoperative muscle inhibition and reduced voluntary activation. However, both parameters showed a consistent and statistically significant recovery over the subsequent months. By one year after surgery, marked improvements were evident; yet quadriceps strength and flexibility remained suboptimal relative to controls. These findings suggest that despite surgical correction and standard rehabilitation, complete neuromuscular recovery of the quadriceps may require longer durations or more targeted interventions.

4.2 Recovery of muscle strength and its clinical implications following TKA

The pronounced reduction in quadriceps strength observed at six weeks post-TKA aligns with earlier studies by Mizner and Snyder-Mackler (2005) and Stevens-Lapsley et al. (2011), which attributed postoperative weakness to arthrogenic muscle inhibition, postoperative pain, and swelling ^12,33–35. Although strength recovery progresses over time, persistent quadriceps deficits indicate that standard rehabilitation programs may be insufficient.

The subsequent progressive recovery from three months onwards likely reflects reactivation of motor units, improved voluntary control, and hypertrophic adaptations through rehabilitation. Despite substantial recovery, persistent quadriceps deficits at 12 months highlight the potential inadequacy of conventional rehabilitation programs in fully restoring muscle function. This persistent deficit suggests that current rehabilitation strategies may be insufficient to fully restore maximal quadriceps force generation within one year, highlighting the need for targeted long-term strengthening interventions.^12,19,35,36

As the quadriceps play a vital role in dynamic knee stability and functional tasks such as stair negotiation, standing, and walking, residual weakness may hinder mobility and increase fall risk. To enhance long-term outcomes, late-phase rehabilitation should integrate eccentric resistance exercises, higher-intensity strength training, and neuromuscular electrical stimulation to optimize activation and muscle hypertrophy. Incorporating task-specific functional training may further promote symmetry between the operated and non-operated limbs ^35–39.

4.3 Flexibility recovery and biomechanical relevance after TKA

Preoperatively, restricted flexibility in both quadriceps and hamstrings was likely the result of chronic pain, adaptive shortening, and reduced use associated with OA. The transient increase in muscle stiffness noted at six weeks postoperatively can be attributed to surgical trauma, pain-related guarding, and limited early joint mobility.^40,41 Over subsequent months, flexibility improved progressively, possibly attributable to pain relief, restored alignment, and active stretching during rehabilitation. Adequate flexibility is essential for optimal knee mobility; reduced quadriceps extensibility may restrict knee flexion, impair gait, and delay functional milestones such as stair climbing and squatting.^42,43

Similarly, persistent hamstring tightness can impede knee extension and alter movement patterns. Previous studies have predominantly focused on postoperative strength outcomes, with limited attention to flexibility changes. The concurrent recovery of strength and flexibility observed emphasizes their interdependence, suggesting that enhanced extensibility supports better muscle activation and joint function.^43,44 Early incorporation of stretching and soft tissue mobilization is therefore essential to improve knee mobility and prevent compensatory movement strategies during recovery. The observed temporal relationship between strength and flexibility restoration suggests that improved muscle extensibility may facilitate enhanced contractile performance and functional recovery. These findings highlight the importance of including flexibility assessment as a complementary outcome in future studies evaluating postoperative rehabilitation efficacy ^45–47.

4.4 Clinical recommendations

The findings underscore the need for a phase-specific, integrated rehabilitation approach that simultaneously targets muscle strength and flexibility. During the early phase (0–6 weeks), rehabilitation should emphasize pain management, gentle quadriceps activation through isometric exercises, and soft tissue mobilization to prevent stiffness. In the intermediate phase (6 weeks–3 months), progressive resistance exercises, closed-chain strengthening, and targeted stretching of both quadriceps and hamstrings should be introduced to restore muscle balance and joint mobility. The late phase (3–12 months) should focus on achieving strength symmetry through higher-intensity and eccentric training, functional retraining, and maintenance of flexibility using dynamic stretching and activity-specific exercises. Adopting this structured and progressive approach may help overcome persistent muscle deficits and improve overall functional outcomes following TKA.

4.5 Limitations and future directions

Although this study provides valuable longitudinal insights into postoperative recovery following total knee arthroplasty, several limitations should be acknowledged. Attrition at the 12-month follow-up may limit the generalizability of long-term outcomes. Additionally, the absence of blinded outcome assessment may have introduced performance bias. Potential confounding factors, such as postoperative pain levels, variations in adherence to rehabilitation protocols, and individual differences in physical activity, were not explicitly controlled and may have influenced muscle strength and flexibility outcomes.

Future research should incorporate electromyographic assessments to elucidate underlying neuromuscular activation patterns contributing to postoperative strength recovery. Although repeated-measures ANOVA was used, future studies employing mixed-effects modeling may provide additional robustness in handling missing longitudinal data. Studies with larger sample sizes, improved retention at long-term follow-up, and extended observation periods would further enhance understanding of recovery trajectories and support the optimization of rehabilitation strategies following TKA.