Methods
This observational, cross-sectional study was designed to investigate early indicators of OA using kinematic analysis. The research protocol was approved by the Sangji University Institutional Review Board (1040782-230814-HR-09-117), and all participants provided written informed consent prior to the commencement of the study.

The study population consisted of 43 recreational table tennis players (86 legs in total), ranging in age from 40 to 70 years, who had participated in amateur table tennis for more than 5 years. The Early Osteoarthritis Questionnaire (EOAQ) was used to identify the presence or absence of early osteoarthritis symptoms among participants ( Migliore et al., 2023 ). The experimental group ( n = 42 legs; 25 males and 17 females) included individuals responding “frequently” or “rarely” to the initial two EOAQ items. The control group ( n = 44 legs; 21 males and 23 females) consisted of those answering “never” to these questions.

Exclusion criteria were established to maintain a focused study population and minimize confounding factors. Individuals were deemed ineligible if they had experienced a lower-extremity injury within the previous six months, had a history of hip surgery, rheumatoid arthritis, diagnosed osteoarthritis, or neurological conditions. Detailed participant characteristics and study flow are presented in Table 1 and Figure 1 , respectively.

Notes:

EOA early osteoarthritis HD horizontal displacement SU step up SD step down

early osteoarthritis

horizontal displacement

step up

step down

The EOAQ, a recently developed instrument for assessing early-stage knee OA, served as the primary screening tool. The 11-item questionnaire was structured into two domains: Clinical features (two items) and patient-reported outcomes (nine items). Each item offers three response options reflecting symptom frequency over the preceding six months: “Never”, “Rarely (one to three episodes)”, and “Frequently (more than three episodes)”. The EOAQ was designed to capture subtle symptomatic and functional alterations characteristic of incipient knee OA, thereby facilitating early detection and intervention ( Migliore et al., 2023 ).

Each participant completed three SU and SD trials per leg. To ensure proper execution, the participants underwent thorough familiarization with the testing protocol, including detailed instructions and practice attempts. All the tests were performed barefoot to eliminate footwear-related variability. The leg testing order was randomized to mitigate potential order effects.

For the SU test, participants initiated the movement with one leg positioned on a 20 cm height step box while maintaining the alignment of the hip, knee, and foot. The participants received both visual demonstrations and verbal instructions on SU performance, without specific guidance on knee and hip alignment. The test was concluded when the participant raised their non-positioned leg until heel contact was made with the step-box surface. The SD test began with the participants seated with their feet and knees parallel to the hip width. As in the SU test, the participants were provided demonstrations and verbal instructions without specific alignment cues. The test was completed when the participant lowered the non-stance leg until heel contact was achieved with the floor in front of the step box. This protocol was designed to capture the natural movement patterns during functional tasks, potentially revealing early indicators of knee osteoarthritis.

Kinematic data were collected using a high-resolution video. A smartphone (iPhone 15; Apple Inc., Cupertino, CA, USA) equipped with 4 K video capability (2,556 × 1,179 pixels, 240 fps) was used. The device was mounted on a tripod, 60 cm above the ground level and positioned 250 cm anterior to the participants. Post-collection, video recordings were processed using specialized motion analysis software (Kinovea ® version 0.8.15; Kinovea, Bordeaux, France). A 2D video analysis was selected for this study based on its clinical feasibility, accessibility, and demonstrated utility in previous research examining frontal plane kinematics ( Osis et al., 2016 ; Weon & Ha, 2024 ). While 3D motion capture systems offer greater precision, the 2D approach allows for more straightforward implementation in clinical settings without requiring specialized laboratory equipment.

The landmarking of anatomical points was performed manually by an experienced physical therapist with over 10 years of clinical experience. Yellow spherical markers were placed directly on the specified anatomical landmarks prior to video recording. A 2D video analysis was used to quantify the horizontal displacement of five key anatomical landmarks in the frontal plane during the SU and SD tests ( Figs. 2 and 3 ). The horizontal displacement of the five key anatomical landmarks included that of the pelvis (PHD), femur (FHD), knee (KHD), lower leg (LHD), and ankle (AHD). Yellow spherical markers were affixed to the anterior superior iliac spine (pelvis), femoral midpoint (femur), patellar center (knee), tibial tuberosity (lower leg), and superior aspect of the navicular bone (ankle).

Using Kinovea software, marker trajectories were tracked throughout the SU and SD tests. During data processing, these markers were tracked manually within the Kinovea software by the same researcher to ensure consistency in trajectory analysis. The maximum horizontal displacement was quantified for each marker ( Figure 3 ) ( Weon & Ha, 2024 ). Displacement was calculated as the distance between the marker’s initial position and its point of maximum excursion during the SU and SD tests. The lateral displacement was recorded as a positive value, whereas the medial displacement was assigned as a negative value.

This analytical approach provides insights into joint stability and movement patterns. The magnitude and direction of the displacement values offered valuable information: larger positive values indicated greater lateral movement, larger negative values signified greater medial movement, and values approximating zero suggested minimal joint movement and enhanced stability. Analysis of the PHD, FHD, KHD, LHD, and AHD allowed for a comprehensive assessment of postural control and joint stability during the SU and SD tasks, potentially revealing postural control strategies and compensatory mechanisms.