Sacrotuberous Ligament Anatomy and Pelvic Stability

The sacrotuberous ligament is a strong, fan-shaped band of connective tissue that runs from the sacrum and tailbone down to the ischial tuberosity, the bony knob you sit on. It forms part of the back wall of the pelvis and plays a central role in stabilizing the sacroiliac joint, transferring load between the spine and the legs, and even acting as a sensory organ that helps your nervous system track the position of your pelvis. Despite being one of the largest ligaments in the body, it rarely gets attention outside of anatomy textbooks until it starts causing pain or shows up on an MRI after an injury.

Where It Sits and What It Looks Like

The sacrotuberous ligament (often abbreviated STL) connects the back of the sacrum and the lower part of the coccyx to the ischial tuberosity on each side of the pelvis. It is a flat, roughly triangular structure that fans out as it descends. A systematic review of morphological studies found that the ligament’s reported length ranges from about 6 to 9 cm, its thickness at the midpoint is roughly 3 to 4 mm, and its width at midpoint ranges from about 2 to 3.5 cm, though these numbers vary across studies and individuals.¹ The ligament lies deep to the gluteus maximus, so you cannot feel it through the skin under normal circumstances.

What makes the STL anatomically interesting is how extensively it blends into neighboring structures. Its upper end merges with the posterior sacroiliac ligaments. Along its inner edge, it often has a thin, membrane-like extension called the falciform process that reaches toward the ischial ramus. An anatomical study of cadaveric specimens found this falciform extension present in about 87% of cases, with variation in how far the membrane spread and whether it fused with the obturator fascia or continued toward the ischioanal fossa.² In roughly 13% of specimens, the falciform process was absent entirely. These variations matter surgically, because the falciform extension forms part of the tunnel through which the pudendal nerve passes.

Connections to Muscles and Fascia

The sacrotuberous ligament does not exist in isolation. It is woven into a broader web of muscles and connective tissue that spans from the lower back to the thigh. The gluteus maximus attaches partly to it. The long head of the biceps femoris, one of the three hamstring muscles, sends fibers into the ligament in many people. And the piriformis, a deep hip rotator, has connections to it as well. The same systematic review noted that while the STL’s bony attachment sites are fairly consistent across individuals, the extent of its connections with these surrounding muscles remains somewhat unclear and varies from person to person.³

One particularly important connection runs upward. At the base of the lumbar spine, all layers of the thoracolumbar fascia, the thick sheet of connective tissue that wraps around the muscles of the lower back, fuse together into a dense composite that anchors firmly to both the posterior superior iliac spine and the sacrotuberous ligament.⁴ This means the STL is not just a local pelvic structure. It serves as a junction point where forces from the trunk, the lower back muscles, and the legs all converge. When you bend, lift, or walk, the STL is part of the chain that distributes those forces across the pelvis.

How It Stabilizes the Sacroiliac Joint

The sacroiliac joint, where the sacrum meets the hip bones, bears enormous forces during everyday activities. Unlike the hip or knee, it is not designed for large ranges of motion. Instead, it depends on tight bony wedging and a network of ligaments to resist shear. The sacrotuberous ligament is one of the key restraints. A biomechanical study showed that loading the STL in the direction of hamstring or gluteus maximus pull significantly reduced forward tilting, or nutation, of the sacrum within the sacroiliac joint.⁵ In other words, the STL acts as a check-rein that keeps the sacrum from rocking forward too far under load.

Because the gluteus maximus and (in many people) the biceps femoris attach to the ligament, these muscles can dynamically tighten the STL during movement, effectively stiffening the sacroiliac joint when extra stability is needed. Research on the biceps femoris–STL interaction confirmed that forces applied to the hamstring tendon directly increased tension in the sacrotuberous ligament and, by extension, influenced sacroiliac joint movement.⁶ The practical takeaway is that muscles in your thigh can alter the mechanics of a joint in your pelvis through this ligament. That link has implications for hamstring training, pelvic instability, and even walking patterns, which the researchers noted as clinically relevant.

The STL as a Sensory Organ

Ligaments are usually thought of as passive straps, but the sacrotuberous ligament is packed with nerve endings that feed information to the brain. Histological studies of the sacroiliac region found nerves distributed to the sacrotuberous and sacrospinous ligaments, containing a mix of thick myelinated fibers (associated with proprioception and pressure sensing) alongside thin myelinated and unmyelinated fibers (associated with pain).⁷ This mix of nerve types means the STL can detect both position and pain.

Separate biomechanical and histological work confirmed that the sacrotuberous and sacrospinous ligaments contain ramifying nerve terminals that likely serve a proprioceptive function, giving the central nervous system real-time information about pelvic position.⁸ Additional histological analysis of the sacroiliac ligaments identified mechanoreceptors consistent with proprioceptive signaling, leading the authors to speculate that this information helps the nervous system optimize upper body balance at the sacroiliac joint.⁹

This sensory role is clinically meaningful. If the STL’s nerve endings are irritated, inflamed, or compressed, they can generate pain that is difficult to localize. Patients often describe it as deep buttock pain that does not fit neatly into any single diagnostic category. The ligament’s dual role as both a structural stabilizer and a sensor means that dysfunction can produce both mechanical instability and pain at the same time.

Sacrotuberous Ligament Pain and How It Gets Diagnosed

One of the trickiest aspects of STL-related pain is distinguishing it from sacroiliac joint pain, piriformis syndrome, hamstring tendinopathy, or sciatica. The symptoms overlap considerably because these structures sit close together and share nerve supply. A study of patients who had already been treated for sacroiliac joint dysfunction found that a majority still had lower buttock pain afterward. Diagnostic injections directly into the sacrotuberous ligament identified the STL as the pain source in about 58% of those patients.¹⁰ That is a striking percentage and suggests that the STL is an underappreciated contributor to persistent pelvic and buttock pain, particularly when sacroiliac joint treatment alone does not fully resolve symptoms.

Among those patients whose pain was confirmed as originating from the STL, a subset found relief from injections alone, while others needed targeted physical therapy. The treatment approaches included direct relaxation techniques applied to the sacrotuberous ligament, relaxation of the biceps femoris, and strengthening exercises for the gluteus maximus.¹¹ The fact that hamstring and glute interventions helped resolve STL pain reinforces how tightly the ligament is linked to those muscles mechanically. If the hamstrings are chronically tight or the gluteus maximus is weak, the STL may bear load it was not designed to carry on its own, and that persistent overload can become a pain generator.

The Pudendal Nerve Connection

The pudendal nerve, which supplies sensation and motor control to the pelvic floor, genitals, and perineum, passes through a narrow corridor partly formed by the sacrotuberous and sacrospinous ligaments. In pudendal nerve entrapment syndrome, the nerve gets compressed as it runs between or beneath these ligaments, causing pain in the perineum, difficulty sitting, and sometimes sexual or urinary dysfunction. The anatomical study that classified falciform process variants noted that the extent and morphology of the STL’s membrane-like extension directly influences the shape of the tunnel through which the pudendal nerve travels.¹²

In surgical decompression of the pudendal nerve, the sacrotuberous ligament sometimes needs to be partially or fully divided to free the nerve. A natural concern is whether cutting such a major pelvic ligament causes lasting structural problems. MR neurography imaging after surgery showed that divided sacrotuberous ligaments healed spontaneously and were continuous again on follow-up imaging, regardless of whether the surgeon had repaired them with grafts or left them to heal on their own.¹³ Interestingly, the healed ligaments were thicker than normal ones. The group that healed without grafts had a mean thickness of about 5 mm, while those repaired with grafts measured about 8 mm, both larger than an intact STL. The thickening is thought to reflect scar tissue reinforcing the repair site, which, while different in composition from the original ligament, appears to restore structural continuity.

The STL’s Protective Role in Hamstring Injuries

Proximal hamstring ruptures, where the hamstring tendons tear away from the ischial tuberosity, are serious injuries most common in athletes and in falls. When these tendons tear completely, they tend to retract down the thigh, and the degree of retraction influences surgical decision-making and recovery. The sacrotuberous ligament plays an unexpected role here. Because the STL connects to the biceps femoris and semitendinosus tendons at the ischial tuberosity, it can act as a tether when those tendons rupture.

An MRI-based study found that when the sacrotuberous ligament remained attached to the torn biceps femoris–semitendinosus tendon, the amount of tendon retraction was significantly less than when the STL connection was disrupted.¹⁴ Retraction was also greater when the semimembranosus tendon was ruptured alongside the others. From a surgical standpoint, less retraction means the repair is technically easier and the tendon can be reattached to bone with less tension, which generally improves healing. So the STL’s muscular connections, sometimes treated as an anatomical curiosity, turn out to have real consequences when things go wrong.

Ossification of the Sacrotuberous Ligament

In some people, the sacrotuberous ligament gradually turns to bone, a process called ossification. This is uncommon and most often discovered by accident on imaging done for other reasons. A case-report review described the condition as predominantly found in elderly males, and most affected individuals had no symptoms at all.¹⁵ The proposed causes include chronic mechanical stress on the ligament, metabolic factors, and tissue remodeling after injury or surgery.

When ossification does cause problems, it is usually because the bony ligament compresses the pudendal nerve. A separate case report described partial STL ossification occurring alongside ankylosis (fusion) of the sacroiliac joint on the same side and degenerative changes on the opposite side, suggesting that the ossification may reflect altered load distribution across the pelvis rather than a purely local process.¹⁶ In the vast majority of cases, however, no treatment is needed. The ossification is incidental and the ligament continues to serve its mechanical function, albeit as bone rather than as flexible connective tissue.

Why It Matters for Pregnancy and Pelvic Girdle Pain

During pregnancy, the hormone relaxin loosens pelvic ligaments to allow the birth canal to widen. The sacrotuberous ligament is part of this process. As the STL becomes more lax, the sacroiliac joint loses some of its passive stability, and the muscles that normally stiffen the ligament, particularly the gluteus maximus and the hamstrings, have to work harder to compensate. When they cannot keep up, the result is pelvic girdle pain, one of the most common musculoskeletal complaints during and after pregnancy. The sacroiliac joint is a frequent focus of this pain, and sources of sacroiliac pain include hypermobility, shearing forces, soft tissue injury, and inflammation.¹⁷

The STL’s dual role as a stabilizer and a sensory organ makes the pregnancy scenario particularly frustrating. The ligament is both less able to do its mechanical job and more likely to be irritated by the abnormal movement it can no longer control. Postpartum, the ligament typically regains its tension, but in some women the laxity persists, contributing to ongoing sacroiliac dysfunction. Strengthening the gluteus maximus, which loads the STL and stiffens the sacroiliac joint, is one of the most commonly recommended interventions for this pattern.

Practical Implications for Exercise and Rehabilitation

Understanding the sacrotuberous ligament changes how you think about certain common exercises and complaints. Hamstring tightness, for instance, is usually treated as a flexibility problem. But because the hamstrings can dynamically tension the STL, which in turn stiffens the sacroiliac joint, chronically tight hamstrings might actually be the body’s strategy for stabilizing a pelvis that lacks adequate ligamentous or muscular support. Aggressively stretching those hamstrings without addressing the underlying pelvic stability issue could make things worse, not better. The research on biceps femoris–STL interaction explicitly raised this point, noting that hamstring training could influence sacroiliac joint mechanics and that the link between “short” hamstrings and pelvic instability deserves clinical attention.¹⁸

For rehabilitation after sacroiliac joint dysfunction or STL-related pain, the evidence points toward a combination of approaches that respect this interconnected anatomy. Direct manual therapy to the ligament can help, but addressing the muscular players is often equally important. Gluteus maximus strengthening loads the STL in a way that improves sacroiliac stability, while targeted hamstring work can restore normal tension dynamics without over-tightening. The clinical data showing that some patients needed biceps femoris relaxation while others needed gluteus maximus strengthening to resolve STL pain underscores that there is no one-size-fits-all protocol. The right intervention depends on which part of the kinetic chain is failing.

The broader lesson from the sacrotuberous ligament is that the pelvis does not have isolated parts. A ligament that connects to the hamstrings, the glutes, the lower back fascia, and the sacroiliac joint while simultaneously feeding proprioceptive data to the brain is not a passive strap holding two bones together. It is a functional hub, and treating any pelvic complaint without considering it risks missing a significant piece of the puzzle.