Muscle tone conditioning is important to prevent muscle disabilities in sports field. Static stretching is one of the most popular way to change muscle tone. To evaluate how muscle tone changes during static stretching, we have proposed a new method using a wearable muscle hardness meter and a viscoelastic model called spring^pot, and validated the method with the rectus femoris. Keeping one posture can stretch some muscles at the same time, but wearing many sensing devices should be avoided because static stretching must be performed in a relaxed state. If some adjacent muscles show a similar change of muscle tone, we can evaluate the stretching using only one muscle hardness meter. As a simple case study, this paper presents the similarities of muscle viscoelasticity at the medial head and the lateral head of gastrocnemius. We measred the muscle reaction force from the medial head and the lateral head of gastrocnemius during static stretching, and calculated the viscoelastic ratio as a hardness index. The viscoelastic ratio of the medial head and the lateral head of gastrocnemius changed similarly in two-thirds of the all trials. This result suggests the possibility of evaluation with one muscle hardness meter for muscle tone of the medial head and the lateral gastrocnemius.
