## 2 Materials and Methods
### Measurement Procedures  
A mixed factorial design was employed with gender (male, female) as the between-subject factor and chair type (stool, computer chair, ergonomic chair) and sitting strategy (comfortable, habitual, correct) as within-subject factors. The dependent variables were the mean and range values of three sagittal joint angles: HI, TA, and KA. To mitigate order and carryover effects, the sequence of chair types and sitting strategies was randomized for each participant. A total of 1350 trials were recorded (30 participants \(\times\) 3 chair types \(\times\) 3 strategies \(\times\) 5 repeated measurements). Participants were instructed to wear light, form-fitting clothing to facilitate the accurate placement of reflective markers over anatomical landmarks. Upon arrival, they were briefed on the experimental procedures and provided written informed consent. Anthropometric measurements were then taken, and markers were placed at standardized anatomical locations. Participants were familiarized with each chair and its adjustment features prior to testing. For each trial, the chair was preset by the experimenter to a randomized configuration (excluding the non-adjustable stool). Participants were subsequently instructed to adopt one of the three designated sitting strategies based on standardized guidelines. The operational definitions of the three strategies--comfortable, habitual, and correct--were developed based on previous ergonomic and postural studies that investigated self-selected and instructed seated behaviors in laboratory and occupational settings [14; 15; 34]. Although these strategies are not formally standardized, they have been widely used to reflect posture variability under different task demands and environmental contexts. To minimize subjective interpretation, a pilot test was conducted with five individuals not included in the final sample. Feedback from the pilot was used to refine and clarify the verbal instructions. All instructions were delivered using a standardized script by the same experimenter to ensure consistency across participants. * Comfortable sitting strategy: _In this test, we would like to capture your most comfortable sitting posture. Please adjust the seat and your body until you feel maximally relaxed, with minimal stress on any part of your body. Once you feel that the posture is most comfortable, please say 'YES'_. * Habitual sitting strategy: _In this test, we aim to record your habitual sitting posture. Adjust the seat and your body until you feel you've adopted your most natural and familiar sitting position. When this posture feels like your usual way of sitting, please say 'YES'_. * Correct sitting strategy: _In this test, we aim to capture what you perceive to be the correct sitting posture. Adjust the seat and your body to what you believe is the most appropriate and supportive posture, minimizing any bodily resistance or discomfort. Once you believe the posture is correct, please say 'YES'_. Once the participant confirmed the target postural strategy, they were instructed to maintain it for 5 s while the motion analysis system recorded the data. This process was repeated five times per condition. For adjustable chairs, the settings were randomized between trials; the stool remained unchanged. To minimize fatigue and maintain data quality, a rest period of no less than 2 min and no more than 5 min was provided between trials. This interval was intentionally kept within the 2-5-min range to capture postural variation over a short period while limiting the influence of external or confounding factors that may arise with longer delays. During breaks, participants were encouraged to stand, walk, or sit on a non-experimental chair. The total duration of the experiment, including setup, testing, and rest intervals, was approximately 2 h per participant. All instructions and interactions were delivered by the same experimenter using standardized scripts to ensure consistency.