The Plantaris muscle is in your calf that is believed to be used by our ancestors to grip with their feet. It is so weak that it is considered functionally obsolete and arguments have been made that humans are evolving it out. It is absent in 7-10% of the population worldwide.
The plantaris is one of the superficial muscles of the superficial posterior compartment of the leg, one of the fascial compartments of the leg.
It is composed of a thin muscle belly and a long thin tendon. While not as thick as the achilles tendon, the plantaris tendon (which tends to be between 30 to 45 cm in length) is the longest tendon in the human body. Not including the tendon, the plantaris muscle is approximately 5-10 cm long and is absent in 7-10% of the population. It is one of the plantarflexors in the posterior compartment of the leg, along with thegastrocnemius and soleus muscles. The plantaris is considered an unimportant muscle and mainly acts with the gastrocnemius.