The flexor digitorum accessorius (FDA), also known as quadratus plantae, is an intrinsic muscle of the second layer of the sole of the foot. It assists flexor digitorum longus (FDL) by neutralizing its oblique line of pull and enhancing the efficiency of digital flexion during gait. ^, Typically, the muscle arises by two heads—a fleshy medial head from the medial calcaneus and a lateral head that is often tendinous or aponeurotic from the lateral calcaneus—and inserts into the posterolateral aspect of the FDL tendon.

Anatomical variations of the FDA include absence, accessory slips, fusion with adjacent muscles, and variability in insertion patterns. ^, However, reversal of muscular and tendinous composition between the medial and lateral heads is rarely emphasized, despite its potential functional and clinical implications. The present report describes such a variation and discusses its relevance to plantar biomechanics, surgical anatomy, and imaging.

During routine dissection of the sole of the foot in a formalin-fixed, 70 year old male cadaver used for undergraduate teaching, the plantar aponeurosis and first-layer muscles were reflected to expose the second layer. The flexor digitorum brevis was removed carefully to visualize the underlying structures.

The flexor digitorum accessorius was identified and traced from its calcaneal origin to its insertion. An unusual morphological pattern was observed. The lateral head was well developed and muscular, while the medial head was predominantly tendinous, forming a fibrous band with minimal fleshy component. Both heads converged toward the tendon of flexor digitorum longus, contributing to its insertion in the usual manner.

The muscle was supplied by branches of the lateral plantar nerve, and no fusion with flexor hallucis longus or lumbricals was noted. The surrounding plantar neurovascular structures appeared normal. No other gross anatomical variations were identified in the foot.

The quadratus plantae functions as a biomechanical stabilizer by realigning the oblique pull of the flexor digitorum longus into a straight posterior direction, thereby improving digital flexion efficiency during the stance and push-off phases of gait. ^, Variations in the muscular architecture of FDA may therefore alter the distribution and magnitude of forces acting on the toes.

In the present case, the muscular lateral head is likely to generate a stronger corrective force compared to a tendinous structure, while the tendinous medial head contributes less dynamic contraction. This asymmetric force distribution may subtly influence toe flexion patterns, plantar pressure distribution, and intrinsic muscle balance, particularly during prolonged weight bearing or pathological gait conditions. ^,

From a clinical perspective, this variation has several implications. During plantar surgical procedures, including plantar fasciotomy, tendon transfer surgeries, and reconstructive approaches to the sole, a tendinous medial head may be mistaken for an abnormal fibrous band and inadvertently divided. Conversely, injury to a well-developed muscular lateral head may compromise toe flexion and postoperative foot function. ^,

Radiologically, asymmetric muscle bulk between the medial and lateral aspects of the quadratus plantae on MRI or ultrasonography may be misinterpreted as muscle atrophy, fibrosis, or partial tear if the possibility of anatomical variation is not considered. For anatomists and educators, this specimen provides a valuable demonstration of structural diversity within intrinsic foot muscles and reinforces the importance of correlating anatomy with function.