A very simple way to measure knee flexion at home is to measure the distance between your hips and heels when your knee is fully flexed.
Reference chart for knee flexion following total knee arthroplasty: a novel tool for monitoring postoperative recovery, BMC Musculoskeletal Disorders
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Medicina, Free Full-Text
Range of Motion Measurement: Knee Flexion
Example time-aligned timeseries of sensor and video knee angle flexion