The 3 hamstring muscles (characteristics and functions)

The human body is a true feat of biological evolution in which the more than 650 muscles that make up our muscular system they allow us to have incredibly complex mechanical functions. Thus, given its importance, it should not surprise us that up to 40% of body weight corresponds to muscle mass.

Muscles are organs of the locomotor system that, being made up of muscle tissue and being connected to the nervous system, are provided with the ability to contract. And it is precisely the contraction and relaxation that allows them to fulfill their functions of transmitting movement to the bones (joined to them through the tendons), giving stability, protecting internal organs, generating heat, allowing the movement of the organs. internal, maintain a stable posture ...

And, obviously, absolutely all the muscles of our body are essential, but, without a doubt, they take on special importance in the extremities. And, in this context, the leg muscles are those that allow such an important mechanical function as standing. And of all the muscles of the lower extremities, there is a muscle group that stands out especially: the hamstrings..

The hamstrings make up a group of three muscles (biceps femoris, semitendinosus, and semimembranosus) that run along the back of the thigh and are essential to allow knee flexion and leg extension. And in today's article, hand in hand with the most prestigious scientific publications, we are going to analyze the morphological properties and mechanical functions of these three hamstring muscles. Let's go there.

What are skeletal muscles?

Before we go into depth and analyze the three hamstring muscles, it is essential that we put ourselves in context. And for this, we must know that the muscles of the human body are divided into three large groups: smooth muscles (those of involuntary control, such as those of the esophagus, stomach, intestines, uterus, blood vessels or urinary bladder) , the cardiac ones (those that constitute the heart) and, which are what interest us today, the skeletal ones.

Skeletal or striated muscles are those organs of the locomotor system that are made up of muscular tissue whose control of the contraction and relaxation of myofibrils is voluntary.. That is, we are the ones who consciously modulate the activity of microscopic filaments (made up of actin and myosin) with contractile properties found in the cytoplasm of muscle cells, also known as myocytes.

These myofibrils, which connect with the nervous system, guide the movement of muscle tissue while being inside the muscle cells or myocytes, which give rise to elongated and plurinucleated muscle fibers that make it possible for the muscle, as a whole, to become contract or relax.

These skeletal muscles make up 90% of the body's muscles and, being of voluntary control, they are those that allow the development of motor functions, especially locomotion. This is possible because thanks to the tendons, very resistant fibrous connective tissue structures that join the muscles to the bones, these muscles are inserted into the skeletal system (hence the name) in order to transmit force to the bone structures and allow body movement.

Thus, any muscle in the body whose contraction and relaxation activity we can voluntarily control is made up of striated muscle tissue, which, colloquially, is what represents the "flesh" of the body. These skeletal muscles are innervated by nerves of the somatic rather than the autonomic nervous system, which is what occurs with smooth and cardiac muscles.

And being involved in locomotion, it is clear that the leg muscles, which are essential for bipedalism, will be made up of these muscles. Thus, the quadriceps, abductors, adductors, tibialis, extensors, gastrocnemius (popularly known as twins), soleus and, of course, the hamstrings will be muscles that comply with this nature.

The hamstring muscles: what are they and what functions do they have?

Having a general understanding of what skeletal muscles are, we are more than ready to focus on what has brought us here today: the anatomy of the hamstring muscles. As we said, the hamstrings are a muscle group made up of three muscles that run along the back of the thigh (the face from behind) from the hip to the region below the knee.

These are three skeletal muscles (biceps femoris, semitendinosus and semimembranosus) that are innervated by the sciatic or sciatic nerve, the longest and most voluminous nerve in the human being that begins in the pelvis and continues vertically along the vertical face of the thigh until it bifurcates to the height of the popliteal fossa. Its function is essentially motor, controlling the activity of, among other muscles, the hamstrings.

So these hamstring muscles are essential for walking, jumping, running, dancing, and even extending your run, since in their locomotor function they act as knee flexors, hip extensors (one of their main functions is to restrain the natural tendency to flexion that the hip has when we support the body during walking) and leg flexors on the thigh when standing.

Even so, being a muscle group made up of three muscles that, despite coordinating and serving the same purpose, are different, it is important to analyze their morphological characteristics and physiological functions individually. Let's see then, the properties of the three hamstring muscles: biceps femoris, semitendinosus and semimembranosus.

1. Biceps femoris

The biceps femoris is the most lateral hamstring muscle of the three and is located in the posterior thigh. It is a muscle formed by two heads that, despite having a different origin and innervation, share the same insertion. This insertion is made in the head of the fibula, the bone that, together with the tibia, forms the bony component of the lower trunk region below the knee.

The long head of the biceps femoris originates from the medial aspect of the ischial tuberosity together with the semitendinosus muscle in what is known as the joint tendon. This long head is innervated by the tibial division of the sciatic nerve. For its part, the short head of the biceps femoris originates from the lateral lip of the lower third of the linea aspera and the supracondylar crest of the femur, being quite distal to the long head. This short head is innervated by the peroneal portion (it is the only portion of the hamstrings not innervated by the tibial division) of the sciatic nerve.

Be that as it may, the biceps femoris as a whole has the function of, acting at the level of the knee and hip joint (the long head acts on both joints, the short head only on the knee), allow hip extension, leg flexion, pelvic stability, gait cycle, thigh extension, and external rotation of the thigh and leg.

2. Semitendinosus muscle

The semitendinosus muscle is a fusiform hamstring muscle of the posterior compartment of the thigh that originates from the lower part of the long head of the biceps femoris and the medial part of the ischial tuberosity. It descends anteriorly to the knee to insert itself on the medial surface of the tibia.

It has the peculiarity that practically half of this semitendinosus muscle is formed by tendon (hence its name) and, together with the semimembranosus, it forms the superior and medial margin of the popliteal fossa. Its insertion in the ischium is located below the gluteus maximus and in the same plane as the biceps femoris.

Again, its functions are based on the hip and knee jointTherefore, the action of the semitendinosus muscle is the extension and internal rotation of the thigh, the stabilization of the pelvis, and the flexion and internal rotation of the leg. It should be noted that, in arthroscopy operations to reconstruct the anterior cruciate ligament after its rupture, it can be used as a plasty.

3. Semimembranosus muscle

The semimembranosus muscle is the last of the hamstring muscles that remains to be seen. I know deals with the innermost muscle or medial of those that are located in the back part of the thigh and originates in the ischial tuberosity, ending in the posterior surface of the tibia dividing into three tendons that allow this insertion.

It has the name it has because its tendon of origin has a membranous appearance. It is a muscle that supports the semitendinosus along its path and, in addition, fulfills important functions within the hamstring muscles: it allows the posterior extension of the hip (in the direction of the back), the flexion of the knee and the internal or medial rotation of this knee.