Files in this Data Supplement:

• Video 1 -

  Video 1. Ultrasound of muscle-tendon junction. Ultrasound images of the muscle tendon junction recorded in real time during the hopping movement of one participant. The muscle-tendon junction is defined in Fig. 2. The white lines across the bottom right hand part of the image is the line created by the sonomicrometry crystal attached to the surface of the probe. This crystal is stimulated at 5 MHz during the motion analysis collection period, creating the white lines and indicating the collection period. This allows for synchronisation of the ultrasound measurements with the motion analysis data.

• Video 2 -

  Video 2. Stick figure of the hopping movement and Achilles tendon length changes. This video demonstrates an example of how the Achilles tendon length change was measured. The blue lines represent the distances between markers attached the leg to define the position of the segments in three dimensional space. The triangle shape represents the foot segment, where the apex to the left of the image is the position of the calcaneous. The green lines represent the distance between the markers placed on the ultrasound probe. The red line represents the distance between the calcaneous marker and the projected position of the muscle-tendon junction measured from the ultrasound images and projected into three dimensional space depending on the position and orientation of the probe (as measured with the three markers on the probe). The aqua line represents the magnitude, orientation and position of the ground reaction force relative to the leg.