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What's in the muscle? Biceps Femoris

My last post started with the introduction of the Hamstring group so today’s spotlight

is for the : Biceps Femoris Muscle

The Biceps Femoris is a superficial muscle and the most powerful of the three muscles that make up the Hamstring group – it’s two counterparts being Semitendinosus and Semimembranosus.

It is a complicated muscle that originates with two heads that quickly forms a single muscle mass that soon divides into three parts which are visible in their division as it descends the limb.

Its origins and insertions have been cause for confusion and controversy in anatomy texts. As such, I adhere to the findings upon dissections from which it stems that the muscle has two origins. The points of the first origin include the 3rd, 4th and 5th sacral dorsal spines, the 1st caudal vertebrae, the gluteal fascia, and both the Sacroiliac and Sacrotuberous ligaments. It’s 2nd and primary origin arises from the Ischiatic Tuberosity.

The muscle inserts at the patella, the lateral patellar ligament, the femur, the tibia crest, the fascia latae and the calcaneus. It thus, has a similar origins and insertion points as its hamstring counterpart the Semitendinosus.

The Biceps Femoris is a multi-functional muscle. It extends the hip, hock and stifle, but also flexes the latter one! As such, it is one of the few muscles that can both flex and extend the SAME joint.

In quite some (performance) horses, this muscle feels tight and is under a lot of strain. This can be of various reasons, the most common being:

1️⃣ Stifle dysfunction. As mentioned earlier, the muscle has the dual task of both flexing and extending the stifle. Due to its insertions on the patella, it is an important muscle for patella function as it assists to pull the patella to the side and releasing it from the trochlea. As such, stifle dysfunction has a huge impact on the Biceps Femoris.

In addition, when you say stifle, you automictically say hock and hip well. So stifle dysfunction can be both a primary, but also a secondary cause to issues in the hock and hip joint.

Finally, in this line I should also note that the stifle is also hugely linked to the horse’s jaw and hyoid apparatus and well as to feet balance so these could also be indirect causes.

2️⃣ Discipline use. Strenuous exercises are known for causing tears in the muscle tissue and fascia. Associated disciplines that put a lot of strain on this muscle – especially in the higher levels - include hunting, showjumping, dressage, eventing, western, polo and racing. For example, in dressage horses, the three portions of the muscle can ‘split’ as the result of fascia tears from the moment they are asked to perform canter pirouettes as these moves require high collection. Upon palpation this separation of the three portions of the muscles feel like holes within the structure.

As such, the question is whether this should be considered a pathology, or the inevitable consequence of use of the body within a certain discipline at (top) sport levels.

3️⃣ Fibrotic Myopathy. Although less common, fibrotic myopathy can be found in the Biceps Femoris. The muscle often compensates during soreness of the limb which can cause hypertonicity in the muscle heads, which is a muscle tightness. The hypertonicity makes the muscle more susceptible to tears, especially during gallop.

Melanoma in the Biceps Femoris muscle

4️⃣ Injury & Pathology. A flip or a sideways fall could result in direct tears of scar tissue in the muscle. Less serious injuries such as leaning on a halter or leaning against the back rail of the float could also result in damage as the horse can start to ‘hang’ in its muscles. In grey horses, melanomas could develop in the muscle and interfere with it’s function. Other pathologies include for example tying up, metabolic syndromes and various muscular and internal conditions.

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