Journal of the Anatomical Society of India

: 2021  |  Volume : 70  |  Issue : 2  |  Page : 116--118

A unilateral anomalous muscle with an uncommon nerve supply interconnecting the biceps femoris and the gastrocnemius

Oladiran Ibukunolu Olateju 
 School of Anatomical Sciences, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, Republic of South Africa

Correspondence Address:
Dr. Oladiran Ibukunolu Olateju
School of Anatomical Sciences, Faculty of Health Sciences, University of the Witwatersrand, 7 York Road, Parktown 2193, Johannesburg
Republic of South Africa


This case study describes the morphology and embryology and hypothesizes the muscle action based on the morphology of an anomalous muscle that extends from the long head of the biceps femoris to the medial head of the gastrocnemius muscles. It has an uncommon nerve supply via the medial sural cutaneous nerve which is mainly sensory in humans. This variation is different from other similar variations involving the gastrocnemius and biceps femoris muscles. The case report will be of benefit to diagnosis, clinical training, and surgical procedures as well as contribute to knowledge on the type of nerve fiber carried by the medial sural cutaneous nerve or its components.

How to cite this article:
Olateju OI. A unilateral anomalous muscle with an uncommon nerve supply interconnecting the biceps femoris and the gastrocnemius.J Anat Soc India 2021;70:116-118

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Olateju OI. A unilateral anomalous muscle with an uncommon nerve supply interconnecting the biceps femoris and the gastrocnemius. J Anat Soc India [serial online] 2021 [cited 2022 Sep 25 ];70:116-118
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The most common variation of the gastrocnemius is the presence of an additional head that inserts into the popliteal fossa. This is called the third or accessory head of the gastrocnemius muscle.[1],[2],[3],[4] This abnormally placed muscle, with variable proximities to the popliteal artery or vein, may compress the popliteal vessels therefore causing problems that may require surgical interventions.[2],[3],[4],[5],[6] Another variation that often involves one or two hamstring (biceps femoris or semitendinosus) muscles is tensor fascia suralis or ischioaponeuroticus. This muscle usually arises from either or both hamstring muscles and then inserts by blending with the fascia of the leg.[7],[8] The present case study describes the morphology and embryology and hypothesizes the function of the anomalous muscle based on its morphology and location. This muscle variation is rare and has an uncommon nerve supply via the medial sural cutaneous nerve.

 Case Report

A 71-year-old male cadaver (from bequests/donation process) of South African of European ancestry was used during a routine dissection for the undergraduate medical students, permitted under the Human Ethics number: W-CJ-140604-1. After removing the skin and the deep fascia (fascia lata) to reveal the popliteal contents, a nerve-like structure initially thought to be a tibial nerve or its variant was observed in the fossa on the left leg. By tracing the tendon proximally and distally, the whole length of the anomalous muscle [[Figure 1]a – indicated by arrows] measuring approximately 43 cm in length became obvious. The anomalous muscle extended from the media side of the long head of bicep femoris to the medial head of the gastrocnemius muscles. For morphological description, the anomalous muscle [Figure 1]a, [Figure 1]b, [Figure 1]c had three observable and distinguishable parts – proximal, intermediate, and distal.{Figure 1}

The proximal part, presumably the muscle origin about 17 cm from the ischial tuberosity [[Figure 1]b – indicated by no-colour filled arrows], was fleshy, flattened mediolaterally and had a muscle belly that was about 2 cm wide and 16 cm long. It attached to the bicep femoris via a tendinous aponeurosis and tapered distally to become tendinous (intermediate part) at about 33 cm from the ischial tuberosity. This part was supplied through intramuscular branches of the sciatic nerve innervating the bicep femoris.

The intermediate part [[Figure 1]a – indicated by white arrows] was tendinous and laid in line with the tibial nerve within the popliteal fossa. Its tendon was thick and strong, measured about 12 cm long. This tendon connected the two ends of the anomalous muscle (i.e. the two bellies). Distally, it deviated slightly to the medial side to attach to the muscle belly of the distal part (at about 45 cm from the ischial tuberosity).

The distal part had a muscle belly and flattened anteroposteriorly. It measured about 1.8 cm wide and 15 cm long [[Figure 1]c – indicated by black arrows]. It rested entirely on the medial head of the gastrocnemius with the medial sural cutaneous nerve and the small saphenous vein medial to it. Its fibers blended with the superficial fibers of the medial head of the gastrocnemius muscle (at about 60 cm from the ischial tuberosity) with no visible tendinous aponeurosis. This part was presumed to be the insertion of the anomalous muscle. On careful examination, this part received a small muscular branch from the medial sural cutaneous nerve [Figure 1]d which was positioned underneath the muscle belly.


Several variations involving human skeletal muscles are common. These variations are often due to abnormalities during the embryonic development of skeletal muscles. All progenitor cells of the limb skeletal muscles are derived from the somites[9] which were initially formed after a series of migration and de-epithelization aided by several transcription factors.[10] Thereafter, the muscle masses undergo sequence of orientation, mitosis, lengthening, and delamination to form the individual muscles of the limbs.[9] It is thus believed that a failure in the sequence of events (for example incomplete delamination or disorientation) would result into variations sometimes seen in humans.[9],[10]

Most variations do not always present with severe clinical problems; thus, they usually remain unnoticed. Some however cause serious problems to structures within the proximity of the muscle variation that may require surgical intervention.[2] A good example is the third head of the gastrocnemius muscle which may compress the popliteal neurovasculatures.[1],[2],[3],[4],[6] Morphologically, it is improbable that the intermediate tendon (or its other parts) of the present variation would have caused any clinical problems in the subject while he was alive despite its close proximity to the tibial nerve.

The tensor fascia suralis has no associated clinical problem. It could arise from either the semitendinosus or the biceps femoris or from both but ends as a tendon that blends with the fascia of the leg.[7],[8] A similar variation was reported by Kumar and Bhagwat[11] which consisted a proximolateral (attachment on the medial side of the long head of biceps femoris), a proximomedial (attachment on the medial side of the semitendinosus), a middle (fleshy and tapered distally to form a somewhat “W” shaped intersection), and a distal part (fleshy and inserted between the two heads of gastrocnemius muscle). Another anomalous muscle arising from the medial side of the long head of the biceps femoris but fusing only with the semitendinosus has also been described.[12] A variation similar to the latter was also reported by Somayaji et al.,[13] but the muscle inserted superficially into the tendocalcaneus.

The present case differs morphologically from other variations in that the anomalous muscle only attached to the long head of the biceps femoris before blending superficially with the medial head of the gastrocnemius where it received a muscular branch from the medial sural cutaneous nerve – a striking feature of this anomalous muscle. The medial sural cutaneous nerve arises from the tibial nerve and it is the main trunk that forms the sural nerve. In addition, the sural nerve formation is highly variable; thus, it is often called a sural nerve complex.[14] Sural nerve transmits sensation from the skin on the posterolateral area of the lower third of the leg. However, the sural nerve may also carry motor fibers which are considered rare in humans.[14],[15] This uncommon innervation of a muscle variation involving the gastrocnemius and biceps femoris has not been reported in the literature. This is of clinical relevance in that surgeons need to investigate the complexity of sural nerve formation and the type of fibers (sensory or motor) it conveys before harvesting the nerve as an autograft. The sural, lateral, or medial sural cutaneous nerve could be harvested depending on factors such as the length of graft required and the complex nature of sural nerve formation.[15] In this scenario, for example, it would be the medial sural cutaneous nerve that would be spared. Other clinical relevancies are in research on nerve conduction studies as well as in nerve biopsy for diagnosing neuropathies of unclear underlying cause after conventional diagnostic assessments.[16]

It is reasonable to think that muscle variants may have no specific function (i.e. redundant muscle slip) because muscle variations often occur by accidents during development. Morphologically, this anomalous muscle could be said to have an origin (proximal end) and an insertion (distal end). The muscle belly thickness and the strong intermediate tendon signaled that this muscle when it contracts and acts together with other hamstrings would play a synergistic role during knee flexion. It must be emphasized that documenting the exact functions of muscle variations in cadavers may remain difficult, but their morphological features may provide cues for hypothesizing their action. Furthermore, the rarity of a variation or when present could be unnoticed due to no clinical problem that may warrant medical investigation.


This anomalous muscle is rare, and its uncommon innervation further shows that the medial sural cutaneous nerve carries motor fibers which may innervate an anomalous muscle when present. Awareness of such variation or its innervation will be of benefit during nerve conduction studies, diagnosis, and surgeries. It is also hypothesized that the anomalous muscle may have acted as a synergist during knee flexion.


The author is grateful to the School of Anatomical Sciences, University of The Witwatersrand, for granting access to the human collections. This case report would not be possible without the aid of those who donated their bodies toward the advancement of medical science.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.


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