The tendon of insertion was 5.5 cm long and coursed through the tarsal tunnel before blending extensively with the fibers of the quadratus plantae muscle. At the level of the flexor retinaculum the inserting tendon of AFDL was located immediately lateral to the posterior tibial neurovascular bundle.
Conversely, AFDL is speculated to represent an evolutionary remnant of the portion of the flexor hallucis longus muscle that migrated from the leg to the plantar foot to become the medial head of the quadratus plantae muscle [16, 35, 36].
Caption: FIGURE 2: Dissection of left leg and foot to show accessory flexor digitorum longus muscle (AFDL: accessory flexor digitorum longus muscle; CT: calcaneal tendon; FDL: flexor digitorum longus tendon; FHL: flexor hallucis longus tendon; TP: tibialis posterior tendon; QP: quadratus plantae muscle).
In the sole, both fascicles of the FDALM follow the medial margin of the quadratus plantae muscle, and the internal fascicle merges with this muscle.
Once in the foot, this tendon is located medial to the quadratus plantae muscle, and merges with it.
On the other side, Knott (in LeDouble) has observed it ending at the os calcis or flexor hallucis muscle; and Reinhardt has described it originating at the os calcis and ending at the quadratus plantae muscle. According to Gantzer and Ringhofer (in Hwang & Hill et al.), this supernumerary muscle is originated at the fibula; according to Macalister (in Turner), Kolliker (in LeDouble), Wood, Testut, and Morestin (in LeDouble), at the peroneus brevis muscle; Theile (in Hwang & Hill et al.) has described its origin at the tibia; Turner has described its origin both at the tibia's medial margin and at the fibula and its end at the anterior surface of the quadratus plantae muscle; Davies-Colley and Dalton have observed other sites (in LeDouble).
At the point of junction of the medial and lateral head, the quadratus plantae muscle attaches.
Lesbre mentioned a small accessory muscle isolated from the base of the calcaneus, not identifying it as the quadratus plantae muscle. The same author described only one lumbricalis muscle of the pelvic limb.
The average number of the nerve fascicules for the abductor digiti minimi and quadratus plantae muscles