The lumbricals
The lumbricals are four small muscles in the second layer of the plantar foot. From medial to lateral, they are numbered 1 to 4. As with quadratus plantae, the lumbricals have a close association with the tendinous slips of flexor digitorum longus. They originate from the four tendinous slips just anteriorly to where they divide from the common tendon of flexor digitorum longus. The lumbricals insert into the medial aspects of the extensor expansions of toes 2-5 and they function to flex the metatarsophalangeal joints and extend the interphalangeal joints. The lumbricals are difficult to isolate for functional testing.