<p>Leg fidgeting, characterised by rhythmic lower limb movement while seated, is a spontaneous, low-intensity behaviour that may serve as a practical strategy to interrupt prolonged sedentary time. This study aimed to evaluate the feasibility of detecting leg fidgeting using a wearable thigh-mounted accelerometer, and to quantify its energetic cost in comparison to sitting, standing, and slow walking under controlled settings. Fifteen healthy adults (mean age = 35.6 ± 12 years; 33.3% male) completed five-minute bouts of sitting, fidgeting, standing, and slow walking while wearing a thigh-mounted accelerometer (SENS motion system). Behaviour classification was validated against direct observation, and energy expenditure was measured using breath-by-breath indirect calorimetry. The SENS classification of fidgeting was evaluated using sensitivity, specificity, and balanced accuracy metrics. Energy expenditure was compared across activities using linear mixed-effects models, controlling for age, gender, and BMI. A total of 305 min of activity data were recorded. Balanced accuracy for activity classification ranged from 90.6% (slow walking) to 99.1% (standing), with fidgeting classified at 95.0%. The energy expenditure of fidgeting (mean = 1.69 kcal/min) was significantly different from sitting (1.49 kcal/min), standing (1.47 kcal/min), and slow walking (mean = 4.10 kcal/min). This study demonstrates that leg fidgeting can be detected using wearable sensors under controlled conditions. Furthermore, leg fidgeting expends slightly greater energy expenditure compared to sitting and standing. Future research should examine its metabolic relevance in free-living settings and explore its role in daily movement patterns and in strategies to reduce prolonged sedentary time.</p>