Post-operative knee swelling (POKS) is a common impairment following anterior cruciate ligament reconstruction (ACLR) that can substantially affect recovery trajectories and long-term rehabilitation outcomes ( Ong et al., 2023). In Indian epidemiological studies, ACL injuries have been reported as the most frequent sports-related knee trauma among young male athletes ( John et al., 2016). ACL tears are among the most common ligament injuries of the knee and are often managed operatively, although nonoperative strategies remain viable in selected populations ( Bogunovic & Matava, 2013). ACL reconstruction remains the standard surgical approach for restoring knee stability and function in active individuals ( Musahl & Karlsson, 2019). Surgical stimuli can intensify the inflammatory cascade within the knee joint, leading to postoperative swelling. Additionally, the irrigation procedure conducted during ACLR, entailing the extraction of synovial fluid, could undermine joint lubrication, thus fostering an increase in knee swelling. Despite advancements in surgical techniques and rehabilitation protocols, PKOS remains elusive. Its association with pain, reduced range of motion (ROM), reduced quadriceps muscle strength, muscle inhibition, atrophy, and impaired functional outcomes remains poorly understood ( Kochhal et al., 2019).

Swelling after ACLR is frequently linked with neuromuscular functional changes such as arthrogenic muscle inhibition (AMI), which is hypothesized to result from the inflammatory response triggered by tissue trauma associated with surgery ( Palmieri-Smith et al., 2013; Sonnery-Cottet et al., 2019). Recent studies have also shown persistent impairments in quadriceps neuromuscular control and physical function extending well beyond the early post-operative phase ( Hunnicutt et al., 2020). Non-contact ACL injuries, which are the most common type, are often attributed to faulty landing mechanics, muscle imbalances, and neuromuscular deficits ( Shimokochi & Shultz, 2008). AMI is typically characterized by reduced activation and motor unit output in the vastus medialis and vastus lateralis muscles, leading to poor treatment outcomes, including persistent range of motion and strength deficits, postoperative stiffness ( Palmieri-Smith et al., 2013; Sonnery-Cottet et al., 2019). Consequently, assessing maximum voluntary contraction (MVC) of these muscles using Electromyography (EMG) is essential to provide objective data on strength and activation of the quadriceps muscles. Documenting the changes in MVC values can indicate strength improvement or deficits, guiding the rehabilitation and monitoring patient progress. However, no previous study explored the association between PKOS and MVC over time in ACLR population.

Previous studies demonstrated that knee swelling leads to significant quadriceps inhibition and strength deficits ( Palmieri-Smith et al., 2013). These findings are supported by a systematic review and meta-analysis confirming persistent deficits in quadriceps strength and volitional activation post-ACLR ( Lisee et al., 2019). However, these studies largely focus on experimentally induced knee effusion and do not fully capture the impacts of acute and chronic effects of PKOS following ACLR, including deficits in pain, ROM, MVC and strength due to AMI, persistent impact on functional outcomes. Another study reported that quadriceps strength deficit observed following ACLR is primarily due to muscle atrophy rather than neuromuscular inhibition ( Hobson, 2018). However, the study lacks objective measurement of maximum quadriceps strength and activation in the immediate postoperative period (<6 weeks).

Return to sport following ACLR has been shown to be influenced by knee impairments, pain, and functional limitations in the early post-operative period ( Lentz et al., 2012). The need for this study arises from the recognized gap in the literature concerning the acute effects of post-operative swelling on key functional outcomes. While there is substantial evidence ( Palmieri-Smith et al., 2013; Palmieri et al., 2004) indicating that knee swelling negatively affects muscle strength and functional outcomes in the later stages of recovery, there is limited data on its impact during the immediate post-operative period ( Norte et al., 2018).

Therefore, the purpose of this study is to examine the time course of PKOS following ACLR and its association with pain, ROM, MVC, and functional performance in people undergoing ACLR for primary ACL injury. Examining the association of PKOS of ACLR with pain, ROM, MVC, and functional performance early in the rehabilitation process is critical for developing targeted interventions that can enhance recovery trajectories. Targeted neuromuscular strategies aimed at restoring knee extensor strength are essential components of evidence-based ACLR rehabilitation ( Buckthorpe et al., 2019).

The demographic characteristics of the study participants are summarized in Table 1 and Table 2. Thirty-two subjects, comprising 27 males and 7 females, were recruited for the study, with a mean age of 28.47 ± 8.613 years ( Table 2). The age-wise distribution of participants is illustrated in Figure 10. In Table 1, the distribution of participants by age reveals that the majority (62.5%) fell within the 21–30 age range, followed by 31–40 years (15.6%), and participants aged 20 and below accounted for 9.4%. Those above 40 years constituted 12.5% of the sample. Table 2 displays the gender distribution, with males comprising the majority (84.4%) and females representing 15.6% of the total sample. The gender-wise distribution is depicted in Figure 11. These findings suggest that the study sample predominantly consists of young to middle-aged adults, with a greater representation of males.

The objective of the study was to find an association between swelling at the knee joint with pain, ROM, muscle strength and functional outcome after ACLR.

In our study, the maximum number of participants who underwent ACLR were from the age group of 20-30 years; this could be attributed to their active lifestyle in comparison to those of the other age ranges. There were more male than female participants in our study despite the fact that the incidence rates of ACL injury are reported to be 2.4–9.5 times higher in females ( Sanders et al., 2016; Lew et al., 2021). This could be as a result of higher participation of males than females in contact or high-intensity sports, which is considered as a risk factor for ACL injury ( Sanders et al., 2016).

The findings of our study suggested that girth of the knee and VAS on activity and at rest decreased from day 0 to week 6, however no significant association was seen between the two parameters, it was also noted that muscle inhibition persisted with presence of swelling, similarly studies have also suggested that in the absence of pain, quadriceps inhibition was noted in knee with artificially induced swelling ( Hopkins & Ingersoll, 2000; Wood et al., 1988). Though the mechanism behind muscle inhibition has been poorly understood, it is believed that swelling on its own can cause considerable quadriceps AMI ( Rice & McNair, 2010; Rice et al., 2014). Studies have revealed that intra articular swelling significantly reduces EMG activation and H reflex amplitude despite the absence of structural injury, pain or joint effusion ( Wood et al., 1988; Rice et al., 2009) Additionally, joint infusion has been shown to significantly increase group II joint afferent discharge in animal studies by raising intra articular pressure and activating stretch and pressure-sensitive mechanoreceptors ( Grigg & Hoffman, 1982; Wood & Ferrell, 1984). In the spinal cord, group I non reciprocal (Ib) inhibitory interneurons are known to be excited by group II joint afferents, inhibiting quadriceps alpha motor neurons and preventing complete activation of the muscle ( Iles et al., 1990; Lundberg et al., 1978). Recent theoretical models further support these findings, emphasizing both spinal and supraspinal pathways contributing to persistent AMI and its impact on quadriceps activation ( Norte et al., 2021). This is further corroborated by findings from experimental joint effusion models, which show that both pre- and post-synaptic spinal inhibition play key roles in arthrogenic quadriceps suppression ( Palmieri et al., 2004).

AMI takes a central role in the injury cycle. Athletes who sustain joint injuries have mobility and ROM deficiencies. Reduced ROM can be caused by pain, swelling, muscle spasms, or the inability of the muscles around the joint to contract effectively. This causes muscle atrophy, weakness, and a slower rate of functional recovery ( Hopkins & Ingersoll, 2000; Young, 1993). Recent studies have also shown that joint effusion can reduce corticomotor excitability of the quadriceps, contributing to persistent voluntary activation failure in the early phase of rehabilitation ( Lepley et al., 2015).

In the present study, it was found that there was a statistically significant correlation between girth and passive ROM for knee flexion for week 6 when measured at 10 cm and 5 cm superior to the joint line. Though there was no statistically significant correlation noted between girth and ROM at other time points, there was an improvement in the knee flexion ROM from day 0 to week 6 which could be explained by the decreased levels of interstitial and intracapsular swelling following rehabilitation ( Wilk et al., 2022). LEFS values were measured after 2 weeks and 6 weeks and it was seen that the mean values for LEFS increased from week 2 to week 6 for all the subjects. Similar longitudinal improvements in LEFS scores following ACLR have been reported previously, reflecting functional recovery over early rehabilitation periods ( Alcock et al., 2012). However, the results showed that there was no association between girth measured at different points and LEFS after 2 weeks and 6 weeks.

The reduction in ROM following ACLR is due to multifactorial reasons, furthermore, pain catastrophizing and kinesiophobia could play a role in active avoidance of movement due to fear of recurring pain or injury and this could impair daily functioning and can have a detrimental effect on rehabilitation outcomes in the long-term ( Österberg et al., 2013; Quartana et al., 2009). Patients may avoid actions that could potentially result in pain or re-injury which can in turn lead to unfavorable views towards one’s body and avoid participation in daily activities. After musculoskeletal injury and surgery, delayed recovery and discharge are frequently caused by pain catastrophizing and fear of pain ( Leeuw et al., 2007; Tichonova et al., 2016). A higher level of pain catastrophizing was highly correlated with a greater level of pain in the knee while performing activities of daily living before and after rehabilitation. A higher degree of kinesiophobia was strongly correlated with more difficulties experienced while carrying out daily activities ( Tichonova et al., 2016).

In the present study, it was seen that MVC values for rectus femoris, vastus medialis and vastus lateralis showed a gradual increase from day 0 to week 6 except from day 2 to day 3. A significant correlation was observed between girth measured at 15 cm, 10 cm and 5 cm superior to the joint line and MVC for the muscles at 6 weeks whereas other measurements for girth from day 0 to week 6 showed no association with MVC for rectus femoris, vastus medialis and vastus lateralis. This could be possibly due to swelling and alterations in skin impedance which can give rise to artifacts influencing EMG readings ( Jun Kimura, 2013). Similarly, in a study done by Drechsler WI et al. (2006), changes in isometric muscle strength, voluntary activation and EMG parameters were monitored in relation to pain, swelling and stability in the knee joint in 31 patients after ACLR and it was found that there was no association between quadriceps activation levels and swelling, which was measured at 1 and 3 months after ACLR ( Drechsler et al., 2006). The data from the current study supports this premise and expands on their work in that multiple factors could be responsible in influencing the MVC values after ACLR. Our study suggests that while swelling decreased over a period of time, parameters like pain, ROM, MVC and LEFS improved, but there was no statistically significant association found between these variables. These findings align with previous evidence showing that quadriceps function following ACLR may not always correlate directly with swelling or pain but can still influence patient-reported outcomes ( Lepley et al., 2018; Loyd et al., 2019). Furthermore, a recent longitudinal study further supported these trends, showing weak correlations between swelling and strength or function in the early weeks following ACL reconstruction ( Prabhakar, 2025). These observations highlight the clinical importance of restoring knee extensor strength through progressive neuromuscular rehabilitation after ACL reconstruction ( Buckthorpe et al., 2019).

The variables that were examined, including swelling, pain, ROM, MVC, and functional outcome, all improved overall, however swelling by itself had no significant correlation with any of the variables. Though swelling has its own effect that might influence the rehabilitation outcomes, its management should not be given any lesser importance. Therapeutic modalities such as neuromuscular electrostimulation have been shown to reduce post-operative knee effusion, pain, and swelling following ACL reconstruction, thus supporting the need for proactive swelling management ( Ediz et al., 2012).

The Department of Physiotherapy, Kasturba Medical College, Mangalore, conducted this longitudinal study from February 2022 to January 2023. Ethical approval was obtained from the Institutional Ethics Committee, KMC Mangalore (IEC KMC MLR01/2022/12), and the study was registered with India’s Clinical Trial Registry (CTRI/2022/03/041101).