Inactivity-Gated Fall-Like Detection Using Quaternion-Based Orientation Change on an ESP32–LSM9DS1 Wearable IMU Node

False alarms remain a practical limitation of wearable inertial fall-detection systems, particularly when rapid non-fall movements produce acceleration or angular-velocity peaks comparable to fall-like events. This paper presents an inactivity-gated event logic implemented on a low-cost ESP32–LSM9DS1 wearable IMU node. The logic combines three sequential gates: dynamic-motion detection, quaternion-based orientation-change confirmation and post-event inactivity verification. Motion descriptors include acceleration magnitude, angular-velocity magnitude, jerk, stillness ratio and the angular distance between pre-event and current orientation quaternions. Five functional scenarios were tested at the gate level: static rest, slow orientation change, impulse without posture change, dynamic non-fall movement and controlled fall-like movement. The observed decisions followed the intended sequence: non-fall cases did not generate an alarm, whereas the controlled fall-like event generated an alarm. The study demonstrates a compact, interpretable and embedded-oriented decision structure for early-stage wearable IMU safety-monitoring prototypes.