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What's in the muscle? Semitendinosus

Just like last week we’re staying in the area of the hindquarters. The muscle that is taking the spotlight this week is the Semitendinosus.

The Semitendinosus is another superficial muscle located in the horse’s hindquarters. It is one of the three muscles that make up the area that is commonly known as the hamstrings. It lays in between its stronger hamstring counterpart Semimembranosus and the Biceps Femoris muscle.

The Semitendinosus originates from two points, it originates ventrally from the first two caudal vertebrae and the caudal border of the Sacrosciatic ligament. The second head originates from the Ischiatic tuberosity which is its primary origin. It has insertion points at the tibia crest, the deep fascia of the lateral hind limb and the calcaneus, one of the bones of the hock.

Its function is to extend the hip and hock joints as well as flexing the stifle. It thus functions as an antagonist to the Tensor Fascia Latea discussed earlier. Hence, this is how dysfunction in one muscle causes the other to compensate.

In some horses, the Semitendinosus appears hypertrophied with a clear defined poverty line. What can be the causes of this?

1️⃣ Poor posture. Horses have a very efficient stay apparatus that allows them to stay and even sleep upright using minimal muscular effort. However, today many horses do not use their bodies efficiently, which causes other muscle groups such as the hamstrings to overexert in an effort to balance the body. Instead of minimal muscular effort, the body compensates by recruiting several additional muscle groups - such as the hamstrings - to provide an incline against gravity and manage its body parts.

Poor posture can have many causes such as dysfunctional anatomy, birth trauma, hoof/teeth balance and of course tack and training. Most training techniques focus on the big gymnastic muscles that produce movement. However, these muscles are usually poorly innervated and does do not have much capacity for ‘muscle memory’. Cybernetic muscles on the other hand are usually short muscles that are richly innervated and stabilize. Hence, to train correct posture and self-carriage that allows the horse with optimal efficiency and minimal effort, it is quintessential to not just address the gymnastic muscles, but also the cybernetic ones.

To that extent, it is most effective is to train these muscles passively in your management as well. Think about it: even if you would train your horse one hour each day, your horse will still be on its own 23 hours. A handy rider can put the horse mechanically into balance for that one hour, but the horse will collapse again when it’s left alone for the next 23 hours. Not very efficient is it? Instead, creativity allows us to address the horse’s brain and training of cybernetic muscles into our management via passive physio. Train smarter, not harder!

2️⃣ More Push than Carrying Capacity. In line with the first point, the Semitendinosus usually hypertrophies the more the horse increases speed. It is important to not confuse speeds with forward. On the contrary, a fast horse usually isn’t a forward.

The hypertrophy of the semitendinosus is common in disciplines such as racing as horses are bred and trained for the purpose of speed. However, hypertrophy of the hamstring for a dressage horse usually just shows bad posture and lack of self-carriage. The horse is not able to provide vertical impulse with the front limbs which leads to a lack of balance. In an attempt to not fall over, the horse simply has to increase speed as such.

Apart from hypertrophy, there are two common other common clinical causes of overall dysfunction in the Semitendinosus:

1️⃣ Scar tissue. The effect of scar tissue is hugely underestimated. Remember, the skin is a highly innervated organ and scar tissue can be a 1000x more sensitive than regular tissue. Manual therapies and complimentary treatments such as high power laser combined with a good rehabilitation plan can be of help.

2️⃣ Fibrotic Myopathy. The semitendinosus appears as a long strap and as such is susceptible to this condition that is caused by injury to the hamstring muscles at the tendinous insertion. It leads to a gait abnormality and is often caused by abrupt turns and sliding stops.

3️⃣ Trauma. Blunt trauma’s to the Ischiatic tuberosity, such as bumps during floating, can dislodge the Semitendinosus from its origin (May-Davis) .

See you next week!

Want to know more? Come joint us for a dissection or biomechanics assessment module listed in the events!

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