Fig. 1. (A) Picture of the flat ultrasound probe attached to the leg with 3-motion analysis (CODA) markers rigidly attached to the probe head. The ultrasound probe images the leg in the sagittal plane. (B) A stick figure representing the leg used in the measurements. The blue lines represent the leg and foot, the green lines represent the lines joining the three markers attached to the ultrasound probe to measure its position and orientation relative to the leg, and the red line indicates the measured Achilles tendon length (from the calcaneous insertion to insertion on the MG, as determined with the ultrasound images). For an animation, see Animation 1 in supplementary material.