Internal and External Rotation: Anatomy, Joints, and Muscles

What is the anatomy of internal and external rotation?

Internal and external rotation are fundamental movements in human anatomy, describing the rotation of a limb around its longitudinal axis towards (internal) or away from (external) the midline of the body, primarily occurring at ball-and-socket joints like the shoulder and hip.

Understanding Rotational Movements

Rotational movements are critical components of human locomotion, sport, and daily activities. They occur when a bone spins or rotates around its own longitudinal axis.

• Internal Rotation (Medial Rotation): This movement involves the anterior (front) surface of a limb rotating inward towards the midline of the body. For example, at the shoulder, internal rotation brings the hand across the body.
• External Rotation (Lateral Rotation): This movement involves the anterior (front) surface of a limb rotating outward away from the midline of the body. At the shoulder, external rotation moves the hand away from the body.

These movements are typically observed in the transverse plane (or horizontal plane) around a vertical (longitudinal) axis. While significant at major ball-and-socket joints, subtle rotation also occurs in the spine and at the radioulnar joints of the forearm (pronation and supination are specific types of forearm rotation).

Key Joints for Rotation

The primary joints capable of significant internal and external rotation are multi-axial, allowing for a wide range of motion.

• Shoulder (Glenohumeral) Joint:

  • Joint Type: Ball-and-socket joint.
  • Bones Involved: The head of the humerus (upper arm bone) articulates with the glenoid fossa of the scapula (shoulder blade).
  • Movement: The humerus rotates medially or laterally within the glenoid fossa. This joint has the greatest range of rotational motion in the body.

• Hip (Acetabulofemoral) Joint:

  • Joint Type: Ball-and-socket joint.
  • Bones Involved: The head of the femur (thigh bone) articulates with the acetabulum of the pelvis.
  • Movement: The femur rotates medially or laterally within the acetabulum. While less range than the shoulder, hip rotation is crucial for walking, running, and changing direction.

• Spine (Intervertebral Joints):

  • Joint Type: Complex series of cartilaginous and synovial (facet) joints.
  • Bones Involved: Vertebrae of the cervical, thoracic, and lumbar spine.
  • Movement: Axial rotation of the trunk occurs through the cumulative small rotations between adjacent vertebrae, particularly significant in the cervical (neck) and thoracic (upper back) regions.

• Forearm (Radioulnar Joints):

  • While not typically termed "internal" or "external" rotation of the limb, pronation (palm down, turning the radius over the ulna) and supination (palm up) are specific rotational movements of the forearm and hand. These occur at the proximal and distal radioulnar joints.

Muscles of Internal Rotation

Muscles responsible for internal rotation typically originate on the trunk or scapula and insert onto the anterior or medial aspect of the humerus or femur, allowing them to pull the limb inward.

• Shoulder Internal Rotators:

  • Subscapularis: A rotator cuff muscle, originating from the anterior surface of the scapula and inserting on the lesser tubercle of the humerus. It is the primary internal rotator.
  • Pectoralis Major: A large chest muscle with clavicular and sternal heads, inserting on the lateral lip of the bicipital groove of the humerus. A powerful internal rotator and adductor.
  • Latissimus Dorsi: A large back muscle, originating from the thoracolumbar fascia and iliac crest, inserting on the bicipital groove of the humerus. It also extends and adducts the arm.
  • Teres Major: Often called the "Lat's little helper," originating from the inferior angle of the scapula and inserting near the latissimus dorsi on the humerus.
  • Anterior Deltoid: The front part of the shoulder muscle, originating from the clavicle and inserting on the deltoid tuberosity of the humerus.

• Hip Internal Rotators:

  • Gluteus Minimus: Located deep to gluteus medius, originating from the ilium and inserting on the greater trochanter of the femur.
  • Gluteus Medius (Anterior Fibers): Originating from the ilium and inserting on the greater trochanter. Its anterior fibers contribute to internal rotation.
  • Tensor Fasciae Latae (TFL): Originating from the iliac crest and inserting into the iliotibial (IT) band. It assists in hip internal rotation, flexion, and abduction.
  • Adductor Longus, Brevis, Magnus (Anterior/Oblique Fibers): These muscles of the inner thigh, originating from the pubis/ischium and inserting along the femur, can contribute to internal rotation, especially from a flexed hip position.

Muscles of External Rotation

Muscles responsible for external rotation typically originate on the trunk, pelvis, or scapula and insert onto the posterior or lateral aspect of the humerus or femur, allowing them to pull the limb outward.

• Shoulder External Rotators:

  • Infraspinatus: A rotator cuff muscle, originating from the posterior surface of the scapula and inserting on the greater tubercle of the humerus. A primary external rotator.
  • Teres Minor: Another rotator cuff muscle, originating from the lateral border of the scapula and inserting on the greater tubercle of the humerus.
  • Posterior Deltoid: The rear part of the shoulder muscle, originating from the spine of the scapula and inserting on the deltoid tuberosity.

• Hip External Rotators:

  • Piriformis: Originating from the sacrum and inserting on the greater trochanter of the femur. It is a key external rotator and can be implicated in sciatic nerve issues.
  • Gemellus Superior and Inferior: Small muscles located above and below the obturator internus, originating from the ischium and inserting on the greater trochanter.
  • Obturator Internus and Externus: Muscles originating from the obturator foramen of the pelvis and inserting on the greater trochanter.
  • Quadratus Femoris: A strong, flat rectangular muscle, originating from the ischial tuberosity and inserting on the intertrochanteric crest of the femur.
  • Gluteus Maximus: The largest and most superficial gluteal muscle, originating from the ilium, sacrum, and coccyx, inserting on the IT band and gluteal tuberosity of the femur. A powerful hip extensor and external rotator.
  • Sartorius: The longest muscle in the body, originating from the ASIS and inserting on the medial tibia. It contributes to hip external rotation, flexion, and abduction.
  • Iliopsoas (Psoas Major and Iliacus): Primarily hip flexors, but they can contribute to external rotation, especially from an extended hip position.

Importance in Movement and Injury Prevention

Understanding the anatomy of internal and external rotation is crucial for both optimizing performance and preventing injury. Imbalances or weaknesses in these muscle groups can lead to:

• Shoulder Impingement: Weak external rotators or overactive internal rotators can pull the humerus forward, impinging tendons under the acromion.
• Knee Pain: Poor hip external rotation strength can lead to excessive knee valgus (knees caving inward) during movements like squats or running, increasing stress on the knee joint.
• Lower Back Pain: Restricted hip rotation can force the lumbar spine to compensate, leading to increased stress and potential pain.

A balanced development of both internal and external rotators is essential for joint stability, efficient movement patterns, and overall musculoskeletal health.

Conclusion

Internal and external rotation are complex, multi-muscular movements fundamental to the function of the shoulder, hip, and spine. By understanding the specific muscles and joints involved, individuals can better appreciate the intricate mechanics of human movement and design more effective training and rehabilitation programs to ensure optimal joint health and performance.