Knee osteoarthritis (KOA) is a chronic degenerative joint disorder characterized by progressive damage to articular cartilage, subchondral bone, and the synovial membrane (1). It ranks among the top five causes of disability in older adults (2), with a global prevalence exceeding 645 million individuals (3).

Obstacle negotiation presents distinct challenges for older adults with KOA, as this task exacerbates pain and elevates fall risk. Stepping over obstacles elicits greater pain intensity compared to level walking (4), a hallmark symptom of KOA (5). Approximately 50% of falls in this population occur during obstacle crossing (6), frequently resulting in fractures or mortality (7). Foot clearance, defined as the vertical distance between the foot and the obstacle during the swing phase of gait, is critical for fall avoidance, as most trips occur due to inadvertent contact between the swinging limb and the obstacle. Reduced foot clearance and stepping height is strongly associated with tripping and falls (8). Additionally, patients with KOA exhibit longer single-leg support time and slower gait speeds (9), with a 0.1 m/s decrease in velocity linked to a 10% decline in physical performance capacity (10). These gait alterations may arise from decreased lower limb flexion angles in the leading leg, reduced vertical impulse from the trailing leg during swing initiation, or insufficient propulsive force to maintain gait speed (11, 12).

Standard KOA interventions include pharmacological therapy, surgical procedures, and physical agent modalities (13). Pharmacological analgesics provide transient pain relief but are associated with gastrointestinal and cardiovascular adverse effects (14), while surgical options may be contraindicated in older adults due to comorbidities (15). Physical therapies are favored for their rapid efficacy and safety profiles (16), with transcutaneous electrical nerve stimulation (TENS) being a widely recommended physiotherapy (17). TENS delivers electrical currents via cutaneous electrodes to modulate peripheral pain pathways by activating large-diameter Aβ afferent fibers, which enhance inhibitory interneuronal activity in the dorsal horn of the spinal cord (18). However, TENS exhibits limited analgesic duration and modest effects on functional outcomes (19).

The limited analgesic and functional efficacy of TENS in KOA may be attributed to its exclusively peripheral mechanism of action. According to the classic gate control theory, conventional peripheral physiotherapy (e.g., TENS) modulates pain by activating large-diameter Aβ afferent fibers, which enhance inhibitory interneuronal activity within the dorsal horn of the spinal cord, thereby “closing the gate” to nociceptive input (20). However, the gate control theory also posits a central inhibitory pathway. Aβ fiber signals are rapidly transmitted to the brainstem and cortex, where descending projections modulate spinal gate dynamics via the periaqueductal gray and rostral ventromedial medulla (20). While TENS targets peripheral nociceptive pathways, it fails to directly address central sensitization, a hallmark of chronic KOA pain and disability (21).

Central sensitization in KOA arises from sustained nociceptive input driven by synovial inflammation and sterile inflammation of local soft tissues, which activates peripheral nociceptors and triggers increased neurotransmitter release at primary afferent terminals in the spinal dorsal horn (22). This persistent afferent barrage enhances the responsiveness of nociceptive neurons, leading to heightened pain sensitivity and exaggerated responses to mild stimuli, perpetuating chronic pain (22). Additionally, central sensitization may contribute to functional impairments, including altered gait patterns that further elevate fall risk (23).

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that delivers low-intensity direct current via surface electrodes (anode and cathode) positioned over targeted cortical regions (24). By inducing subthreshold shifts in neuronal membrane polarization, tDCS modulates cortical excitability: anodal stimulation enhances excitability, while cathodal stimulation suppresses it. This neuroplastic modulation may attenuate central sensitization-related pain hypersensitivity by activating descending inhibitory pathways in the spinal dorsal horn (25).

Overall, TENS provides transient pain relief via peripheral mechanisms but has limited effects on functional outcomes (19). In contrast, tDCS modulates cortical excitability and central pain processing, potentially improving gait patterns through enhanced neural plasticity (25, 26). A combined tDCS + TENS intervention may synergistically address pain and gait deficits in older adults with KOA by integrating peripheral analgesia with central neuroplasticity enhancement. However, no prior studies have evaluated this approach. Therefore, this study aimed to investigate the effects of a 6-week tDCS combined TENS intervention on pain relief and obstacle-crossing gait improvement (including foot clearance and crossing velocity) among older adults with KOA, compared to TENS alone. We hypothesize that (1) both TENS + tDCS and TENS alone will reduce pain scores and improve obstacle-crossing gait patterns (i.e., increased foot clearance and crossing velocity) in older adults with KOA, and (2) TENS + tDCS intervention will demonstrate superior efficacy compared to TENS alone in reducing pain and enhancing gait adaptability.

All dependent variables exhibited normal distribution, as verified through Shapiro–Wilk tests (p > 0.05). Chi-square tests revealed no statistically significant differences in sex (p = 0.795) and the side of the more affected leg (p = 0.795) between the two groups. Independent t-tests indicated no statistically significant differences in age (p = 0.828), height (p = 0.196), body mass (p = 0.055), and body mass index (p = 0.078) between the groups (Table 1).

The purpose of this study was to verify the effect of tDCS combined with TENS on pain and gait patterns during stepping over obstacle among older adults with KOA. These results supported hypotheses # 1 and 2, by pointing out that both interventions relieve pain and improving gait patterns during stepping over obstacles among older adults with KOA, while tDCS + TENS training has better effects.

Our study showed that both interventions were effective in reducing pain, while tDCS + TENS had better effects in older adults with KOA. The finding is supported by a previous study, which indicated that combination of tDCS and TENS is more effective in relieving pain for individuals with KOA than using TENS or tDCS only (35). According to the classic gate control theory, pain perception is regulated by a gating mechanism in the spinal cord. In the dorsal horn, central transmitting cells, known as T-cells (transmission cells), act as relays for nociceptive signals. Glial cells and inhibitory interneurons function as a “gate” by modulating input to T-cells. This gate is regulated by signals from large-diameter and small-diameter fibers, the former closes the gate while the later opens the gate. TENS works by activating large-fiber fibers, thereby enhancing the inhibitory effect of interneurons and closing the gate to block peripheral nociceptive input (20, 36). However, in chronic KOA, persistent peripheral inflammation may lead to central sensitization. tDCS, in contrast, promotes neural plasticity by modulating thalamocortical circuits and synaptic plasticity, which reduces abnormal neuronal discharges associated with central sensitization and alleviates chronic pain-related hyperalgesia (37). TENS reduces peripheral nociceptive input, while tDCS suppresses central pain amplification. Together, these interventions synergistically attenuate pain by targeting both peripheral and central mechanisms.

Our study demonstrated that both interventions were effective in increasing foot clearance during obstacle-stepping in older adults with KOA, while tDCS + TENS yielded superior effects to TENS alone, likely due to two factors. Firstly, adequate toe clearance relies on sufficient hip and knee flexion and ankle dorsiflexion (38); our secondary outcomes revealed that combined tDCS and TENS significantly increased these joint angles during obstacle-stepping in this population, supported by prior research showing both TENS and tDCS can improve joint range of motion during functional tasks (39). Secondly, adequate toe clearance may also stem from higher vertical impulse in the trailing leg and increased spanning height, as our secondary outcomes indicated that combined tDCS + TENS outperformed TENS alone in this regard. During obstacle-stepping, limb control depends on joint movement; TENS alleviates knee pain via spinal gating mechanisms (Aβ-fiber activation) and endogenous β-endorphin release, which reduces pain-induced inhibition of quadriceps and gastrocnemius activation (40). This improves knee flexion and ankle dorsiflexion during the swing phase while simultaneously strengthening push-off forces and increasing spanning height through enhanced muscle activation. The generated greater vertical impulse directly results in improved foot clearance during obstacle-stepping. tDCS effects target central neural networks to amplify motor control and propulsion efficiency. Anodal tDCS applied over the primary motor cortex (M1) contralateral to the affected knee enhances corticospinal tract excitability (41), increasing the firing rate of pyramidal neurons and improving the synchronization of motor unit recruitment in lower limb muscles. This heightened cortical drive optimizes joint movement control, enabling more precise regulation of hip, knee, and ankle angles, strengthening push-off forces and increasing spanning height, ultimately enhancing foot clearance. Together, these interventions synergistically amplifies both joint mobility and propulsion efficiency, thereby achieving greater foot clearance during obstacle-stepping. tDCS enhances neuronal firing rates by boosting excitability in intracortical and subcortical networks and promoting synaptic plasticity (42), thereby improving joint movement control and strengthening push-off forces, ultimately enhancing foot clearance. Together, these interventions synergistically amplifies both joint mobility and propulsion efficiency, thereby achieving greater foot clearance during obstacle-stepping.

Our study demonstrated both interventions were effective in increasing obstacle crossing velocity in older adults with KOA, while the tDCS combined TENS is more effective than TENS alone. The support time and time main effect of propulsive impulse from the secondary outcomes supported both tDCS + TENS and TENS increases crossing velocity. Firstly, tDCS improved motor unit synchronization in the plantarflexors, increasing the rate of force development during push-off (41). This allowed the support leg to generate sufficient propulsion in less time, shortening support duration without compromising stability. TENS reduced pain-related muscle inhibition, allowing the quadriceps and gluteals to better stabilize the knee and hip during support, reducing the need for prolonged ground contact (40). Sencondly, tDCS induces cortical depolarization in the primary motor cortex (M1), increasing corticospinal tract excitability (41). This neuromodulatory effect refines the coupling between cortical motor commands and peripheral muscle activation, minimizing delays between neural input and mechanical output. This effect improves motor unit synchronization of plantarflexor muscles, enhancing push-off force generation during terminal stance phase. TENS activates Aβ-fiber mediated spinal gating and promotes β-endorphin release. These mechanisms reduce in pain-related muscle inhibition directly enhance joint kinematics during propulsion (40). By relieving quadriceps inhibition, TENS enables fuller knee extension during late stance, a movement that increases the leverage of the support leg. This extended knee position shifts the body's COM forward relative to the support foot, amplifying the mechanical advantage for generating forward momentum (11). Simultaneously, reduced inhibition of plantarflexors promotes greater ankle plantarflexion during push-off, which augments the force of propulsion (12). However, the lack of time by group interaction in propulsive impulse indicated that there are other factors attribute to the superior effects of tDCS combined TENS intervention, which may be attribute to the two factors. Firstly, tDCS augments proprioceptive signal integration through anodal stimulation of the primary motor cortex (M1), which increases neuronal excitability in this region, thereby improving the processing of proprioceptive inputs from muscles and joints (e.g., signals from muscle spindles and Golgi tendon organs) (43). This enhanced integration refines real-time perception of limb position and movement dynamics, enabling patients to rapidly adjust foot trajectory and joint angles during stepping. By minimizing positional errors that typically induce deceleration or hesitation, the intervention optimizes movement efficiency, ultimately elevating overall crossing speed.Secondly, tDCS applied over the primary motor cortex (M1) enhances multi-joint coordination through synchronized activation of synergistic neural networks across the ankle, knee, and hip joints by modulating corticospinal excitability and thalamocortical connectivity (44). This optimization reduces kinematic delays during obstacle negotiation, such as inadequate toe clearance or excessive swing-leg flexion, leading to smoother gait transitions and increased overall speed.

This study has limitations. First, there was no follow-up after the 6-week intervention; it cannot be determined how long the effect of the intervention on relieving pain and improving gait patterns during stepping over obstacles in older adults with KOA lasted. Second, the obstacle height was set at only 20% of leg length; incorporating additional heights could enhance the generalizability of the findings. Third, this study included older adults with both unilateral and bilateral KOA, but the gait patterns for stepping over obstacles may differ. Additionally, this study highlights the temporal aspects of ground reaction forces during gait but doesn't explore how individual joints contribute to gait pattern differences. Future research should integrate kinematic and kinetic data to better understand biomechanical changes in KOA patients. Machine learning, which has been effective in identifying key gait features and subtle movement differences (45, 46), could be particularly useful. Using these techniques may help clarify each joint's role in gait variability, improving diagnostic accuracy and guiding targeted interventions.

Both the combination of tDCS and TENS and TENS-only were effective in relieving pain and improving gait patterns during obstacle crossing in older adults with KOA, while the combination of tDCS and TENS had superior efficacy. These findings support the integration of tDCS as an adjunctive neuromodulatory strategy to amplify the therapeutic benefits of TENS in this population.