Buttocks

The buttocks are a pair of rounded, muscular structures located on the posterior aspect of the pelvis. They are composed of layers of skin, fat, and three primary muscles: the gluteus maximus, gluteus medius, and gluteus minimus. These structures provide cushioning and support for the pelvis during sitting and movement.

Location

The buttocks are located at the back of the pelvic region, extending from the lower back to the upper thighs. They lie on either side of the sacrum and coccyx, over the ischial tuberosities, and are bordered laterally by the hips.

Anatomy

The buttocks are anatomically complex structures consisting of muscles, bones, nerves, blood vessels, and connective tissues.^[6] They play a crucial role in providing support, stability, and movement for the pelvis and lower limbs. Below is a detailed breakdown of their anatomy:

Bones of the Buttocks

The skeletal framework of the buttocks is formed by the posterior parts of the pelvic bones and the upper femur:

Pelvic Bones

• Ilium: The largest part of the pelvis, forming the superior portion of the hip bone.
• Ischium: The lower, posterior part of the pelvis, where the ischial tuberosities (“sit bones”) are located, serving as weight-bearing points during sitting.
• Sacrum and Coccyx: The midline bones at the base of the spine that provide attachment points for ligaments and muscles.

Femur

The proximal part of the femur (thigh bone), including the greater and lesser trochanters, serves as attachment sites for the gluteal muscles.

Muscles of the Buttocks

The primary muscles of the buttocks are the gluteal muscles, which are divided into three layers:

Gluteus Maximus

• The largest and most superficial muscle of the buttocks.
• Originates from the posterior ilium, sacrum, and coccyx.
• Inserts into the iliotibial tract (IT band) and the gluteal tuberosity of the femur.
• Provides the rounded shape of the buttocks.

Gluteus Medius

• Located beneath the gluteus maximus, forming the upper lateral part of the buttocks.
• Originates from the outer surface of the ilium.
• Inserts into the lateral surface of the greater trochanter of the femur.

Gluteus Minimus

• The smallest and deepest of the gluteal muscles.
• Originates from the outer surface of the ilium, below the gluteus medius.
• Inserts into the anterior surface of the greater trochanter of the femur.

Other Associated Muscles

• Piriformis: A small muscle located deep to the gluteus maximus, running from the sacrum to the greater trochanter.
• Tensor Fasciae Latae (TFL): A small muscle on the lateral side of the hip that connects to the IT band.
• Hamstrings (Biceps Femoris, Semitendinosus, and Semimembranosus): Extend from the pelvis to the posterior thigh, partially overlapping with the buttocks.
• Deep External Rotators: Includes muscles like the gemelli, obturators, and quadratus femoris.

Fascia and Connective Tissue

Gluteal Fascia

• A thick connective tissue layer covering the gluteal muscles.
• Provides support and protection to the underlying structures.

Iliotibial (IT) Band

A strong band of connective tissue running along the lateral thigh, originating from the gluteal fascia and the tensor fasciae latae muscle.

Ligaments

Sacrotuberous Ligament: Connects the sacrum to the ischial tuberosity, forming part of the boundary of the greater and lesser sciatic foramina.

Sacrospinous Ligament: Connects the sacrum to the ischial spine.

Nerves of the Buttocks

The buttocks are richly innervated by several major nerves:

Sciatic Nerve:

• The largest nerve in the body, passing through the gluteal region.
• Emerges below the piriformis muscle and runs down the posterior thigh.

Superior Gluteal Nerve:

Innervates the gluteus medius, gluteus minimus, and tensor fasciae latae.

Inferior Gluteal Nerve:

Innervates the gluteus maximus.

Pudendal Nerve:

• Provides sensation to the perineum and external genitalia.
• Passes through the greater and lesser sciatic foramina.

Posterior Femoral Cutaneous Nerve:

Provides sensory innervation to the skin of the buttocks and posterior thigh.

Blood Vessels of the Buttocks

The buttocks are supplied by branches of the internal iliac artery:

Superior Gluteal Artery:

• Supplies the gluteus maximus, medius, and minimus muscles.
• Passes above the piriformis muscle.

Inferior Gluteal Artery:

• Supplies the gluteus maximus and deep gluteal muscles.
• Passes below the piriformis muscle.

Venous Drainage:

The superior and inferior gluteal veins accompany the arteries, draining blood into the internal iliac vein.

Lymphatic Drainage:

Lymph from the buttocks drains into the superficial and deep inguinal lymph nodes.^[4]

Skin and Subcutaneous Tissue

Skin:

• The skin of the buttocks is thicker and more elastic, providing cushioning.
• Contains sweat and sebaceous glands.

Subcutaneous Fat:

• A thick layer of fat beneath the skin contributes to the rounded appearance of the buttocks.
• Provides insulation and shock absorption.

Bony Landmarks

The buttocks contain several palpable landmarks important for anatomical orientation:

• Iliac Crest: The upper border of the ilium, forming the superior boundary of the buttocks.
• Ischial Tuberosity: The “sit bones” located at the lower pelvis, bearing weight during sitting.
• Greater Trochanter: The bony prominence on the lateral femur, serving as an attachment point for the gluteal muscles.

Function

The buttocks serve multiple essential functions related to movement, posture, support, and protection.^[2] Comprised of muscles, connective tissues, and fat, the buttocks play a critical role in various activities, from walking and running to maintaining stability and cushioning the pelvis. Below is a detailed breakdown of their functions:

Movement and Locomotion

The gluteal muscles of the buttocks are the primary drivers of lower limb movement. They contribute to several key actions:

Hip Extension

The gluteus maximus is the strongest hip extensor, enabling movements such as standing up from a sitting position, climbing stairs, running, and jumping.

Hip Abduction

The gluteus medius and gluteus minimus abduct the hip, moving the leg away from the midline of the body. This is crucial for lateral movements and maintaining balance.

Hip Rotation

• The gluteus medius and minimus medially (internally) rotate the hip.
• The gluteus maximus and deep external rotators (e.g., piriformis, obturators) laterally (externally) rotate the hip.

Stabilization During Walking

The gluteus medius and minimus stabilize the pelvis during the single-leg stance phase of walking or running, preventing it from tilting sideways.^[8]

Postural Support

The buttocks play a significant role in maintaining upright posture:

Pelvic Alignment

• The gluteal muscles and surrounding ligaments stabilize the pelvis, preventing anterior or posterior tilting.
• Weak gluteal muscles can result in postural abnormalities, such as excessive lumbar lordosis.

Spinal Stability

The gluteus maximus supports the lower back and sacroiliac joint, reducing strain during activities like lifting or carrying.

Cushioning and Protection

• The thick subcutaneous fat layer of the buttocks acts as a natural cushion, protecting the underlying bony structures (e.g., ischial tuberosities) during prolonged sitting.
• The soft tissue of the buttocks also absorbs shock and reduces pressure on the pelvis and lower spine during physical activities like running or jumping.

Weight Distribution

The buttocks distribute body weight evenly across the pelvis and thighs during sitting or standing. This reduces localized pressure on the ischial tuberosities and prevents discomfort or injury.

Stability and Balance

Dynamic Stability

• During activities like walking, running, or climbing, the gluteal muscles stabilize the hip joint and pelvis, ensuring smooth, coordinated movements.^[7]
• The gluteus medius and minimus are especially important for balance during single-leg stance and side-to-side movements.

Static Stability

The gluteal muscles maintain stability when the body is stationary, preventing swaying or falling.

Propulsion

The gluteus maximus generates powerful force for forward propulsion during high-intensity activities, such as:

• Sprinting or running.
• Jumping or leaping.
• Climbing steep inclines or stairs.

Rotation of the Pelvis

• The gluteal muscles facilitate the rotation of the pelvis during activities such as walking or twisting motions.
• This pelvic rotation increases stride length and efficiency in locomotion.^[5]

Thermoregulation

The fat and skin of the buttocks assist in body temperature regulation:

• Insulation: The subcutaneous fat layer helps retain heat in cold conditions.
• Sweating: Sweat glands in the skin of the buttocks facilitate cooling during physical exertion.

Role in Sexual Dimorphism

The size and shape of the buttocks are influenced by the distribution of subcutaneous fat, which varies between sexes due to hormonal differences. This contributes to secondary sexual characteristics and body aesthetics.

Functional Adaptation

The buttocks adapt to the demands of various activities, enabling versatility:

• Sports: The gluteal muscles generate explosive power for athletic activities like sprinting, jumping, and lifting.
• Daily Activities: Activities such as bending, sitting, standing, or carrying rely on the strength and stability provided by the buttocks.^[3]

Clinical Significance

The buttocks play a vital role in mobility, stability, and protection, making them susceptible to various clinical conditions and injuries. Key aspects of their clinical significance include:

Muscle Injuries

Strains or tears in the gluteal muscles, especially the gluteus maximus or medius, can result from overuse or trauma, leading to pain and impaired movement.

Sciatica

Compression or irritation of the sciatic nerve in the gluteal region, often caused by the piriformis muscle (piriformis syndrome), results in radiating leg pain, numbness, or weakness.

Bursitis

Inflammation of the ischial or trochanteric bursae, which cushion the gluteal region, can cause localized pain and discomfort, especially during sitting or walking.

Pressure Sores

Prolonged pressure on the buttocks, especially in immobile individuals, can lead to ulcers over the ischial tuberosities.^[1]

Gluteal Fat Atrophy

Conditions like lipodystrophy or prolonged corticosteroid use can lead to a loss of fat in the buttocks, affecting cushioning and aesthetics.

Injections and Medical Access

The buttocks are a common site for intramuscular injections. Care must be taken to avoid the sciatic nerve.