Hamstring Muscles: Functional Anatomy Guide

The average gym rat uses the term “hamstrings” to refer to the muscles on the back of thigh, whose primary function is knee flexion.

If you want to get really technical, “hamstrings” technically refer to the tendons on either end of the posterior thigh muscles, namely the semitendinosus, semimembranosus and biceps femoris (which includes both a long and a short head). That said, I think it’s perfectly acceptable to use “hamstrings” to refer to the muscle group itself. And I’ll do just that in this post.

Hamstring Muscles

The three hamstring muscles alone make up the classification of muscles known as the posterior compartment of the thigh.

The hamstrings are found medial to the gracilis and adductor magnus, and lateral to the vastus lateralis.

The semitendinosus lies superficial to the semimembranosus, though the semimembranosus is so wide that part of it is left uncovered on either side of semitendinosus; these muscles constitute the medial hamstring.

The biceps femoris long head is superficial to the outer edge of the semimembranosus. Both the long head and the short head of the biceps femoris constitute the lateral hamstring.

All heads of the hamstrings originate on the ischial tuberosity of the pelvis, except the biceps femoris short head which arises from the posterior femur.

The lateral hamstring runs inferolaterally and inserts on the proximal lateral tibia. The medial hamstring runs inferomedially and inserts on the proximal medial tibia and fibula.

The biceps femoris and semitendinosus have a parallel fiber orientation and fusiform shape, while the semimembranosus has oblique-oriented fibers and a unipennate muscle architecture.

Also Called

  • Hams
  • Hammies
  • Back of the thigh

Origin, Insertion, Action & Nerve Supply

Muscle Origin Insertion Action Nerve Supply
Semitendinosus Ischial tuberosity Medial surface of the superior tibia
  • Knee flexion
  • Hip extension
  • Knee internal rotation (when knee is flexed)
  • Hip internal rotation (when knee is flexed)
Tibial division of sciatic nerve part of tibia (L5-S2)
Semimembranosus Posterior medial condyle of the tibia
Biceps Femoris Long Head
  • Lateral part of the head of the fibula
  • Lateral collateral ligament
  • Lateral condyle of the tibia
  • Knee flexion
  • Hip extension
  • Knee external rotation (when knee is flexed)
  • Hip external rotation (when knee is flexed)
Biceps Femoris Short Head Linea aspera and lateral supracondylar line of the femur
  • Knee flexion
  • Knee external rotation (when knee is flexed)
Common fibular [peroneal] division of the sciatic nerve (L5-S2)

Exercises:

Note: The table below is limited to exercises involving knee flexion and/or hip extension, where the hamstrings are a prime mover.

That said, many other exercises train the hamstrings indirectly but nonetheless intensely. These include many gluteus maximus exercises (e.g. deadlift, glute bridge) and lower spinal erector exercises (e.g. bent-knee good morning, back extension).

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Stretches & Myofascial Release Techniques:

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Self Myofascial Release Techniques

When using these techniques, give special attention to the common trigger points shown in the image below.

Hamstring trigger points

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Common Issues:

  • Excessively Lengthened and Overactive Hamstrings: The hamstrings are excessively lengthened yet overactive in individuals with lower crossed syndrome (LCS). It is unusual for muscles to be long yet overactive, since excessive lengthening typically causes inhibition. The hamstring muscles are chronically lengthened due to anterior pelvic tilt moving the hamstrings’ origin (ischial tuberosity) upward, increasing its distance from the insertion. The hamstrings respond by becoming overactive because they must eccentrically contract to resist further anterior tilt in order to have retain some degree of pelvic stability and limit further lordosis to protect against lower back or other injuries. The gluteus maximus is supposed to be the main muscle in charge of resisting anterior pelvic tilt, but because it is reciprocally inhibited by the hip flexors in LCS, the hamstrings are forced to take on this role. The hamstrings also compensate for the glutes during movement, becoming synergistically dominant over them in hip extension. All of this tension on the hamstring muscles can lead to injury under the right – or should I say wrong – circumstances. Minor (Grade 1), moderate (Grade 2) and severe (Grade 3) muscle strains, as well as tendinosis, are among the most common hamstring injuries.

Training Notes:

  1. Consider the following information if you have excessively lengthened and overactive hamstring muscles.
    • Avoid doing static stretches for the hamstrings. Remember that although the hamstrings may feel short or inflexible, they are actually already longer than they should be. Static stretching will further lengthen the muscle, which only makes it weaker and less able to do its already-overbearing job of maintaining some degree of pelvic stability. Thus, static stretching can increase the likelihood of a hamstring strain. Or, a lower back injury may occur if too much stress is transferred onto the lower spine because the hamstrings fail to provide sufficient pelvic stability.
    • Use soft tissue release techniques on the hamstrings. Unlike static stretching, this won’t lengthen your hamstrings. But it can alleviate the feeling of tension and help you move better by freeing up restricted tissue. It’s a good idea to do this on a regular basis, but especially before any exercises with significant hamstring involvement.
    • Perform soft tissue release techniques and stretches for the hip flexors and quadriceps as part of a daily mobility routine, and prior to lower body training. Inhibiting and lengthening these tight muscles allows the hamstrings and glutes to fire properly, which in turn encourages proper hip position.
    • Chances are your quadriceps are significantly stronger and more developed than your hamstrings. The previous bullet helps, in part, to remedy this by doing stretching/soft tissue release on the quads. However, it’s also necessary to change your exercise selection so that you’re doing more hip-dominant exercises and less quad-dominant exercises.
    • Train the hamstrings, but train the glutes harder. Although the hamstrings are weak, the root cause of their weakness is likely the glutes. Therefore, it’s important that you choose exercises that involve hip extension – not just knee flexion (i.edon’t limit yourself to just leg curls). This way you can train both the hamstrings and the glutes. If you were to isolate the hamstrings with leg curls while ignoring the glutes, you could potentially make the situation worse: the glutes would become even more synergistically inhibited by the hamstrings during hip extension,which would make a hamstring strain more likely during high intensity hip-dominant activities such as heavy deadlifts or sprinting.
    • Strengthening your abs is an essential complement to strengthening your glutes. These are the two most important muscles for achieving and maintaining a neutral pelvis and spine, which in turn allow the hamstrings to calm down and return to their normal resting length.
    • Work on integrating a neutral pelvis and neutral spine into your training. As discussed above, strengthening the abs and glutes, and releasing/stretching the hip flexors will help with this. However, a major part of achieving proper alignment and stability of the pelvis and spine is by teaching your body how to do it. This entails first learning how to get to neutral while simply standing (activate glutes and lower abs to tuck bottom of pelvis under torso; tense abs to prevent excessive lordosis; keep torso upright and head high). Then, you must train yourself to do this throughout the day, when standing, walking and during exercise.
    • In your day-to-day life, try to avoid or modify the positions and activities, which contribute to the postural and movement habits that encourage dysfunction of the hamstrings (and other muscles). This, for one, means to spend less time sitting down. And also, to work on integrating neutral pelvis/spine when standing, sitting and walking around throughout the day.
    • If you religiously follow the advice in the above bullet points, you should be well on your way to resolving the imbalance with your hamstrings and making major improvements in your posture. However, you need to do more  if you have lower crossed syndrome, since it’s the source of the problem. See my post on how to correct lower crossed syndrome (article coming soon).
  2. Sometimes tightness in the hamstrings is the result of sciatic nerve entrapment occurring in the hamstrings or somewhere higher up in the body/spine. If the entrapment is the result of a soft tissue entrapment, then the solution may be as simple as doing some sciatic gentle nerve glide exercises. However, the problem could be more serious, such as nerve impingement caused by a bulging or herniated disc. As such, it’s very important to see a qualified medical professional before trying anything, if you think the problem is nerve-related.
  3. The following bullet points provide general advice and tips to improve your hamstring training and build bigger and stronger hamstrings. Most (though not necessarily all) of this is directed towards those who don’t have any noteworthy muscular imbalances/postural issues affecting the hamstrings.
    • Increase your training volume on hamstring exercises by doing more total sets and reps. Or you can increase your training frequency; so if you’re only doing hamstring exercises once per weak, start doing them twice per week.
    • The hamstrings are made up of predominantly fast twitch muscle fibers, meaning that they are best recruited by high load and/or explosive movements. So, most of your sets should be done for a relatively low number of reps (e.g. 8 or fewer) using heavy weight and/or explosive speed.
    • Do single leg hamstring exercises to avoid developing strength and muscular imbalances between your left and right leg. I like the single leg Romanian deadlift, since it also works the often weak gluteus medius and it trains core stability as well.
    • If you want to get the most out of your hamstring training, you should be training your glutes at least as hard. First off, strong glutes protect you against hamstring muscle strains. Second, strong glutes mean you’re able to use heavier weights during exercises involving hip extension, meaning you can better overload your hamstrings.
    • Even if you don’t have overactive hamstring muscles, it’s still a good idea to release them before doing any heavy lower body training. Since the hamstrings will have some lingering tension in them after training them, performing some post-workout static hamstring stretches can be helpful for quickly regaining any lost flexibility or mobility.
    • The vast majority of people should train the hamstrings more than the quadriceps. The quadriceps tend to be the stronger and more muscular of the two muscle groups, especially among weight trainees; hence, focusing more on the underdeveloped hamstrings will help to correct this imbalance. However, even those lucky few with no ham/quad imbalance should do slightly more hamstring training if they want to maintain their balanced thigh musculature. The is because, despite equal training for the hams and quads, it’s generally easier to develop quadriceps dominance for a number of reasons (e.g. quads tend to be easier to recruit; people get to get more excited about quad-dominant lifts over ham/glute-dominant lifts and therefore put more effort into them; it’s easier to compensate with other muscles on hamstring exercises e.g. using lower spinal erectors too much on RDLs or good mornings; quads may become short stiff if you do a lot standing or sitting during the day due to their involvement in knee extension and hip flexion).

About the Author Alex

Hey! My name is Alex, and I'm the owner and author of King of the Gym. I started this website back in late 2009 during college, and it has been my pet project ever since. My goal is to help you learn proper weight training and nutrition principles so that you can get strong and build the physique of your dreams!

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