Fig. 4. Relationship between changes in the length of muscle fascicles and the whole muscle–tendon unit during muscle lengthening in one subject. (A–C) The relationship between changes in the lengths of muscle fascicles and the whole muscle–tendon unit when the knee was at 20°, 60° and 90°, respectively. The relationship at the three different knee angles are very similar. (D) The three relationships superimposed, and the fitted line (in red) using Eqn 3 (see Materials and methods). The vertical red arrow shows slack length of the whole muscle–tendon unit, and the horizontal red arrow indicates the slack length of the muscle fascicles.