Fig. 1

The workflow of the study consisted of vibration signals measurement for six consecutive trials of the knee injections (A). Three needle sizes were used (B). Experimental setup was done by mounting the accelerometer firmly on the medial collateral ligament just below the medial border of patella, about the mid-patellar level (C). An empty syringe was placed at supero-lateral knee, ready for the 2-ml air injection, and a linear ultrasound probe was positioned on the quadriceps tendon above the patella (D)