Gait analysis and evaluation of knee buckling risk following total knee arthroplasty

Abstract

[Purpose] Total knee arthroplasty (TKA) can alleviate pain and improve daily functioning in patients with knee osteoarthritis. However, postoperative decreases in knee extensor strength and limitations in joint range of motion (ROM) may increase the risk of knee buckling. This study aimed to investigate gait characteristics and mechanical conditions potentially associated with knee buckling in patients following TKA. [Participants and Methods] This study was conducted on patients following TKA. Based on the inclusion criteria, this study included 21 patients (25 knees; postoperative day 22.0 ± 10.5) with data for knee joint position sense, knee-extension strength, tibial acceleration, knee flexion angle, and electromyography (EMG) of vastus medialis, vastus lateralis (VL), rectus femoris, tibialis anterior, and lateral head of the gastrocnemius during gait. EMG signals were normalized using the maximal voluntary contraction method. Participants walked six steps at a self-selected speed and the second gait cycle on the operated side was analyzed. [Results] Knee joint position sense and knee flexion angle at initial contact (IC) showed a moderate positive correlation. Moderate negative correlations were observed between postoperative days and VL activity and between maximum vertical direction tibial acceleration and VL activity. [Conclusion] Decreased position sense may affect knee joint control at IC, potentially manifesting as an increased knee flexion angle and suggesting the involvement of mechanical conditions associated with knee buckling. Furthermore, VL muscle activity during gait decreased with increasing postoperative days, and greater VL muscle activity was associated with gait patterns characterized by reduced vertical tibial acceleration.

INTRODUCTION

Total knee arthroplasty (TKA) is a common surgery for knee osteoarthritis (OA). Knee OA results from degeneration of the articular cartilage between the femur and tibia, resulting in decreased knee function and weight-bearing pain that affects daily activities. Knee deformity can also influence the ankle joint, leading to flatfoot deformity, reduced proprioception, and impaired balance^{1, 2)}. Grades III and IV of the Kellgren–Lawrence (K–L) classification are considered advanced and end-stage knee osteoarthritis (OA), and surgical interventions, including total knee arthroplasty (TKA), are indicated based on the patient’s activities of daily living (ADL)³⁾. In particular, TKA has been shown to achieve favorable outcomes, providing pain relief and improved functional activity in patients.

Despite these benefits, the invasiveness of TKA can lead to postoperative inflammation, pain, range of motion (ROM) limitations, and muscle weakness⁴⁾. Quadriceps weakness is a particularly important factor affecting gait stability and thus a major focus of physical therapy⁵⁾. Such weakness likely occurs due to preoperative knee-extension ROM limitations, postoperative inflammation, and alignment changes⁶⁾. Weakness of the quadriceps is considered a contributing factor to knee buckling during walking and stair climbing. Knee buckling refers to the sudden collapse of the knee under weight-bearing, causing the joint to give way. This phenomenon is particularly likely to occur when the knee is unable to support body weight during walking or stair ascent and descent, increasing the risk of falls due to impaired postural stability. Although pain and knee instability are also associated with knee buckling⁷⁾, quadriceps weakness is recognized as one of the most important contributing factors. The quadriceps muscles play a key role in knee extension and help prevent buckling caused by excessive knee flexion.

Thacher et al.⁸⁾ compared postoperative knee buckling associated with different intraoperative nerve blocks and reported that adductor canal blocks are preferable to femoral nerve blocks. Similarly, Fujita et al.⁹⁾ conducted a comparison of nerve blocks and, consistent with Thacher et al., also supported the use of adductor canal blocks. However, while the effect of surgical manipulation on knee buckling has been examined, few studies have specifically investigated knee buckling and gait analysis in patients undergoing TKA. While evaluating the risks associated with surgical techniques is important in clinical practice, it is also essential to assess the risks related to postoperative movements and functional activities. Therefore, this study evaluated gait characteristics and knee buckling risk in postoperative TKA patients using EMG and tibial acceleration measurements.

PARTICIPANTS AND METHODS

This study identified 31 patients (7 males, 24 females, 39 knees) who underwent TKA and were hospitalized between June and August 2025. Exclusion criteria included 2 patients (4 knees) with neurological disorders, cerebrovascular disease sequelae, or severe motor paralysis, 2 patients (2 knees) who were more than 50 days postoperative, and 8 patients (8 knees) with missing data. Ultimately, 21 patients (6 males, 15 females, 25 knees) met the inclusion criteria and were included in the analysis.

This study was conducted in accordance with the Declaration of Helsinki and followed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guidelines. Ethical approval was obtained from the JCHO Gunma Central Hospital Ethics Committee (Approval No. 2025-001). All participants provided informed consent.

For each participant, the following variables were assessed: knee joint position sense, maximal knee-extension strength (hereafter referred to as knee-extension strength), and, during gait, electromyography (EMG), tibial acceleration, and knee flexion angle. In this study, “knee buckling” was defined as the sudden collapse of the knee under weight-bearing during walking. It refers to a condition in which excessive knee flexion occurs during the stance phase due to an inability to support the body. Although actual episodes of knee buckling were not directly induced, knee flexion angle and tibial acceleration during the stance phase of walking were measured to evaluate mechanical conditions potentially associated with knee buckling. EMG during walking was measured to investigate gait characteristics in patients following TKA. Measurements were obtained using a multichannel telemetry system, WEB-7000 (Nihon Kohden, Tokyo, Japan; 1,000 Hz). Participant height, weight, body mass index (BMI), age, and postoperative days were obtained from electronic medical records.

Knee joint position sense was measured using an electronic goniometer attached to the thigh and shank in accordance with the basic and moving axes for knee ROM defined by the Japanese Association of Rehabilitation Medicine¹⁰⁾.

The measurement procedure was based on the method reported by de Oliveira et al¹¹⁾. Participants began in the supine position with the lower leg hanging off the edge of the bed. The participants were asked to allow their lower legs to hang freely within a pain-free range, and a support was placed behind the foot to maintain the knee in approximately 80° flexion.

The examiner then passively moved the knee to a position between 50° and 60° of flexion and held it for 3 seconds to allow the participant to memorize the angle. The participant subsequently reproduced the angle actively and held it for 1 second. The absolute difference between the passively set angle and the actively reproduced angle was calculated and used as the representative value.

Knee extension strength was measured using a handheld dynamometer (μTas F1, ANIMA Corp., Tokyo, Japan). Participants sat with the hip and knee flexed at approximately 80°, a pad was placed against the anterior surface of the distal lower leg while manual resistance stabilized the device.

Surface EMG electrodes were placed on the vastus medialis (VM), vastus lateralis (VL), rectus femoris (RF), tibialis anterior (TA), and lateral head of the gastrocnemius (LG) to measure muscle activity during the loading response (LR) phase of the stance period in gait. Surface EMG electrodes were placed on the muscle bellies of VM, VL, and RF at the distal one-third of the thigh. Electrodes for the TA and LG were placed on the muscle bellies at the proximal one-third of the lower leg.

Based on the method reported by Olin and Gutierrez¹²⁾, a triaxial accelerometer was attached to the distal portion of the tibial tuberosity in a seated position. Consistent with previous research¹³⁾, the X-axis was defined as the horizontal direction relative to the long axis of the tibia (+: superior, −: inferior), the Y-axis as the lateral direction (+: left, −: right), and the Z-axis as the vertical direction (+: anterior, −: posterior) (Fig. 1). Only the horizontal and vertical directions were analyzed in the present study.

Fig. 1.

Configuration of each axis of the triaxial accelerometer.

Participants walked barefoot at a comfortable speed for six steps; data for the second step using the operated limb were analyzed. Knee flexion onset recorded using a EXILIM EX-100 high-speed camera (CASIO, Tokyo, Japan; 120 fps) was synchronized with that measured using the electronic goniometer (time zero). MVC for VM, VL, and RF was recorded concurrently with maximal knee-extension strength testing. TA MVC was measured during resisted dorsiflexion, while LG MVC was measured during resisted plantarflexion in a prone position. After full-wave rectification, the EMG signals were normalized to the maximum voluntary contraction (%MVC).

For the measurement of the loading response phase, the duration from heel strike of the target limb to toe-off of the contralateral limb was determined from still images captured by a high-speed camera synchronized with the EMG. This time window was then used for the analysis of EMG signals and tibial acceleration.

ImageJ software was then used to analyze the knee flexion angles at initial contact (IC) and at the end of loading response (LR), as well as their differences.

Statistical analysis was performed using SPSS version 31 (IBM Corp., Armonk, NY, USA). The normality of the data was assessed using the Shapiro–Wilk test. Pearson’s correlation was used to examine relationships between normally distributed continuous variables, while Spearman’s correlation was applied for non-normally distributed or ordinal variables. Correlation analyses were performed to investigate associations among knee joint position sense, knee extension strength, EMG activity, postoperative days, weight, BMI, tibial acceleration, knee flexion angles at IC and LR during gait, and the difference between the two angles (Δflexion angle). Statistical significance was set at p<0.05.

RESULTS

Participants’ characteristics (mean ± SD) are as follows: age 76.0 ± 5.0 years; height 155.4 ± 8.16 cm; weight 56.9 ± 10.8 kg; BMI 23.4 ± 3.24 kg/m². The average postoperative days were 22.0 ± 10.5. Knee-extension strength was 11.6 ± 3.36 kgf and the position sense error was 5.39° ± 2.83°.

Both horizontal direction and vertical direction tibial accelerations peaked during LR and mid-stance (MSt). Figure 2 shows an example of the horizontal and vertical direction tibial acceleration throughout the stance phase. Maximum horizontal and vertical direction tibial accelerations (mean ± SD across all participants) were 1.81 ± 0.37 G and 0.23 ± 0.26 G, respectively. Knee flexion was 21.44° ± 6.60° at IC, 23.8° ± 6.65° at the end of LR, and the difference was 2.34° ± 3.07°.

Fig. 2.

Tibial acceleration in the horizontal direction and vertical direction during the stance phase of gait. Orange: horizontal direction, blue: vertical direction.

Correlation results are shown in Tables 1, 2, 3, 4. A positive correlation of 0.428 was observed between joint position sense and knee flexion angle at IC. Positive correlations were also observed between VL and VM muscle activity (r=0.732) and between VL and RF muscle activity (r=0.488) during the loading response phase of LR. These correlations reflect coordinated quadriceps muscle activation during stance, which plays a role in knee stabilization under load in postoperative TKA patients. Negative correlations were observed between postoperative days and VL activity (r=−0.398) and between maximum vertical direction tibial acceleration and VL activity (r=−0.479). Although not significant, the correlation between LR knee flexion angle and maximum vertical direction tibial acceleration approached significance (r=−0.376, p=0.064).

Table 1. Correlation between maximum each direction tibial acceleration and each factor.

	Maximum horizontal direction acceleration	Maximum vertical direction acceleration
BMI	0.055	0.264
Knee extension strength	0.129	0.199
Postoperative days	0.088	0.039
Joint position sense	0.066	−0.173
IC knee flexion angle	−0.24	−0.342
LR end knee flexion angle	−0.125	−0.376
Dflexion angle	−0.218	−0.079

* Indicates significance at p<0.05 (two-tailed). Pearson correlation coefficient was applied. BMI: body mass index; IC: initial contact; LR: loading response.

Table 2. Correlations between knee flexion angles and angle differences during the stance phase of gait and knee extension strength and position sense.

	IC knee flexion	LR end knee flexion	Δflexion angle
Knee extension strength	−0.181	−0.084	0.222
Joint position sense	0.428*	0.333	−0.199

* Indicates significance at p<0.05 (two-tailed). Pearson correlation coefficient was applied. IC: initial contact; LR: loading response.

Table 3. Correlation of muscle activities during the loading response.

	VM	VL	RF	TA	LG
VM	-	0.732*	0.327	0.377	0.136
VL	0.732	-	0.488*	0.296	0.220
RF	0.327	0.488*	-	0.551*	0.105
TA	0.377	0.296	0.551*	-	0.238
LG	0.136	0.220	0.105	0.238	-

*Indicates significance at p<0.05 (two-tailed). Spearman rank correlation coefficient was applied. VM: vastus medialis; VL: vastus lateralis; RF: rectus femoris; TA: tibialis anterior; LG: lateral head of the gastrocnemius.

Table 4. Correlations between muscle activities and various factors during the loading response.

	VM	VL	RF	TA	LG
Postoperative days	−0.211	−0.398*	−0.142	0.173	0.123
Joint position sense	0.002	−0.111	−0.014	0.095	−0.312
Maximum horizontal direction acceleration	−0.184	−0.329	−0.049	0.111	0.142
Maximum vertical direction acceleration	0.312	−0.479*	0.027	0.220	0.125

* Indicates significance at p<0.05 (two-tailed). Spearman rank correlation coefficient was applied. VM: vastus medialis; VL: vastus lateralis; RF: rectus femoris; TA: tibialis anterior; LG: lateral head of the gastrocnemius.

DISCUSSION

This study aimed to clarify gait characteristics in TKA patients. Our finding of a moderate positive correlation between position sense and knee flexion angle at IC suggests that impaired position sense leads to increased knee flexion at foot contact. Knee buckling is a phenomenon that occurs when the knee joint flexes excessively. A clear threshold of knee flexion angle at which knee buckling occurs has not yet been established. However, abnormalities in knee flexion behavior have been reported to be associated with the evaluation of knee joint instability¹⁴⁾. Previous studies have suggested that patients with knee osteoarthritis tend to adopt a knee-stiffening gait strategy, which increases walking knee joint stiffness^{14, 15)}. Furthermore, Gustafson et al.¹⁴⁾ reported that a smaller knee flexion angle at initial contact (IC) is one of the factors contributing to lower walking knee joint stiffness. Based on these findings, the association observed in the present study between impaired position sense and increased knee flexion angle at IC may represent a compensatory response aimed at increasing knee joint stiffness. Moreover, such gait characteristics may suggest a potential relationship with knee buckling.

This study also aimed to investigate gait characteristics in patients following TKA. There was a moderate negative correlation between postoperative days and VL activity, as well as between maximum vertical tibial acceleration and VL activity. These findings suggest that, as postoperative recovery progresses, the predominance of VL muscle activity is suppressed. It has been reported that VL is primarily active in knee flexion positions, whereas VM plays a greater role in the knee extension range¹⁶⁾. Improvements in knee extension range of motion after surgery may enhance VM output, which in turn could reduce the mechanical load placed on the VL. Furthermore, Xue et al.¹⁷⁾ reported improvements in proprioception following TKA. Such improvements in proprioception may contribute to enhanced neuromuscular control and lower limb muscle function, potentially leading to a reduction in the predominance of VL muscle activity.

Our finding of a negative correlation between maximum vertical tibial acceleration and VL activity suggests that, in patients following TKA, as stiff-knee gait decreases and a clear double-knee action is achieved, predominant VL activation decreases. This pattern may reflect improved shock absorption and load distribution during the stance phase, potentially contributing to enhanced knee stability. From the perspective of knee-buckling risk, the increased involvement of other lower limb muscles and proprioceptive control may reduce the load on a single quadriceps component, suggesting a more efficient and stable gait strategy under weight-bearing conditions.

While the present findings have focused on predominant VL activity, VM activation should also be considered. One possible factor influencing VM function is the surgical approach used in TKA. In this study, all participants underwent the medial parapatellar approach, which requires incision of the medial patellofemoral retinaculum and partial detachment of the VM. This procedure may lead to postoperative VM dysfunction due to inflammation, pain, and reduced muscle gliding, particularly in the early postoperative period. Consequently, in the early postoperative period, VM dysfunction may have been facilitated by factors such as swelling, pain, and impaired muscle gliding, which may have resulted in predominant VL activation to preserve knee joint control during gait. Furthermore, as postoperative recovery progresses, improvements in patellar mobility, knee extension stability, and neuromuscular control may facilitate more balanced quadriceps activation, thereby reducing the predominance of VL activity. Such changes could contribute to improved control of knee flexion during the loading response phase, which is considered a critical period for the occurrence of knee buckling. Although no significant differences in knee flexion angle during LR were observed in the present study, these neuromuscular adaptations may still play an important role in mitigating the risk of knee buckling during gait.

Although not statistically significant, a moderate negative correlation was observed between knee flexion angle at the end of the LR and maximum vertical direction tibial acceleration (p=0.064). This suggests that greater knee flexion during LR is associated with smaller maximum vertical tibial acceleration, indicating a possible strategy in which increased knee flexion serves to limit anterior tibial tilt.

This study has several limitations. First, the participant selection criteria should be considered. The postoperative period was limited to within 50 days after TKA; however, postoperative recovery progresses substantially over time, and gait strategies may vary accordingly. Narrowing the postoperative time window or stratifying participants based on postoperative days may help to clarify gait characteristics at different stages of recovery.

Second, no participants in this study exhibited actual episodes of knee buckling. This may also be influenced by the postoperative period included in the study. Future studies incorporating comparisons across postoperative time points, as well as assessments of subjective sensations of knee buckling or instability, may allow for a more direct examination of factors associated with knee-buckling risk.

Furthermore, although this study focused on the relationships between proprioception and knee extensor strength, other motor-related factors—such as knee extension deficits, motor imagery, and neuromuscular control—were not examined. Including these factors in a more comprehensive assessment may further enhance the understanding of gait control and knee-buckling risk in patients following TKA.

Funding

This study received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Conflict of interest

The authors have no conflicts of interest to declare relevant to this article.