Hip Complex - Structure and Function Notes

Structure and Function of the Hip Complex

Objectives

Describe the bony and connective tissue structures related to the hip joint.
Relate these structures to muscle attachments and joint function.
Discuss the normal angle of inclination of the femur and the conditions of coxa valgus and coxa varus.
Define femoral torsion in terms of anteversion and retroversion.
Define the function of the acetabular structures and explain how their alignment affects hip joint stability and mobility.
Discuss the open and closed chain osteokinematics and arthrokinematics of the hip joint in femur-on-pelvis and pelvis-on-femur movement.
Identify the muscles of the hip joint, and describe their function as primary and secondary movers.
Explain how changes in hip position can alter the function of specific hip muscles.

Bony Structures of the Hip Complex

Pelvis:
- 2 Innominates, each made up of:
  - Ilium
  - Ischium
  - Pubis
- Posterior articulation: Right and Left innominates connect Lower Extremities to the axial skeleton by the sacroiliac (SI) joint
- Anterior articulation: Right and Left innominates connect at the pubic symphysis
Acetabulum (Concave):
- Proximal part of hip joint
- Formed by all 3 pelvic bones
Femur (Convex):
- Distal part of hip joint
- Greater Trochanter

Femoral Anatomy

Angle of Inclination:
- Angle formed by the femoral neck and the femoral shaft in the frontal plane
- Normal: $125$ degrees
Coxa Varus:
- Angle $< 125$ degrees
Coxa Valgus:
- Angle $> 125$ degrees
Changes alignment and forces!

Coxa Valgus

Coxa Valgus = hip dysplasia
Increased risk of femoral subluxation & dislocation

Coxa Varus

Often a result of trauma, infection, or underlying bony disease (rickets, osteogenesis imperfecta) or developmental disease
Associated with abnormal wear to joint surfaces

Femoral Anatomy - Femoral Torsion

Rotational relationship between the neck of the femur and the femoral shaft.
Position of the neck relative to the medial-lateral axis of the femoral condyles
Normal: $15$ degrees
Femoral Anteversion:
- Angle $> 15$ degrees
Femoral Retroversion:
- Angle $< 15$ degrees

Femoral Anteversion & Retroversion

Normal Femoral Neck Anteversion: $15$ degrees
Increased Femoral Neck Anteversion: $45$ degrees
Femoral Neck Retroversion: $0$ degrees

Hip Joint Structures - Acetabular Labrum

Increases articular surface by $30\%$
Distributes/diminishes the load
Causes suction-like force
Outer $1/3$ = vascular
Inner $2/3$ = avascular
Floor of acetabulum does not normally contact femoral head

Osteokinematics at the Hip- Femur on Pelvis

Flexion $0-120$ degrees
Extension $0-30$ degrees
Abduction $0-45$ degrees
Adduction $0-30$ degrees
Internal (Medial) Rotation $0-45$ degrees
External (Lateral) Rotation $0-45$ degrees

Why Does This Matter?

Activity and Hip Flexion ROM Needed:
- Ascending Stairs: $47-66$ degrees
- Descending Stairs: $26-45$ degrees
- Walking on level surfaces: $30-44$ degrees
- Sitting with knees flexed: $90-112$ degrees
- Putting on socks: $120$ degrees
- Squatting: $115$ degrees

Osteokinematics at the Hip: Pelvis on Femur

Lumbopelvic Rhythm = movement of the lumbar spine in relation to the pelvis
Ipsidirectional lumbopelvic rhythm
- Combined movement of pelvis and hip
- Bending forward = Pelvis rotates anteriorly and lumbar spine flexes
- Allows for increased hip flexion ROM!
Contradictional lumbopelvic rhythm
- Pelvis and spine rotate in opposite directions

Pelvic Rotation in the Sagittal Plane

Anterior Pelvic Tilt
Posterior Pelvic Tilt

Pelvic Rotation in the Frontal Plane

Trendelenburg Sign
- Weak hip abductors opposite the lower hip
- Lumbar spine will bend towards the weak side to compensate

Pelvic Rotation in the Transverse Plane

As one innominate moves forwards, the other moves backwards
The spine rotates opposite to compensate
Results in arm swing!

Arthrokinematics of the Hip

Convex Surface: Femur
Concave Surface: Acetabulum
Femur on Acetabulum = Roll and Slide in opposite directions
- Hip Flexion = femoral head glides posterior
- Hip Extension = femoral head glides anterior
- Hip Abduction = femoral head glides inferior
- Hip Internal rotation = femoral head glides posterior
- Hip External rotation = femoral head glides anterior

Hip Complex Muscles

Support & move $2/3$ of the body during weight bearing and gait
Precise function of hip muscles changes depending on position of hip joint
Example: Hip Adductors
- Help hip flexors when in neutral or hip extension
- Help hip extensors when in hip flexion

Multi-Joint Muscles at the Hip

Cross multiple joints
Optimal contraction: must lengthen at one joint while contracting at the other joint
Muscle shortened over both joints = actively insufficient
To stretch, must lengthen at both ends
Example: Rectus Femoris
- Hip and Knee flexed = acts as hip flexor
- Hip and Knee extended = acts as knee extensor

Multi-Joint Muscles: Psoas & The Spine

Psoas Major: lumbar vertebral bodies to lesser trochanter
Spine stabilizer AND hip flexor
Tightness = increased lumbar lordosis and anterior pelvic tilt
Prolonged sitting and weak gluts; frequent cause of low back pain

Hip Adductors Group

Major stabilizer during single leg movement
Adductor group inserts on the pelvis at the pubic ramus
Fiber orientation allows for function in sagittal plane, especially in high power situations
End range flexion = Adductors act as extensors
End range extension = Adductors act as flexors

Hip Abductors: Glut Med, Glut Min, and TFL

Glut Med
- Anterior fibers = abduct during flexion
- Posterior fibers = abduct during extension
- Hip in neutral= posterior fibers ER
- Hip in flex= All fibers IR
Glut Min
- Abduction, Flexion
- Hip in flex = IR
All abductors vital to stabilization in single leg stance
- Gravity causes adduction & abductors resist it
- Essential during stance phase of gait

Multi-Joint Muscles: TFL & ITB

TFL
- Iliac crest to ITB
- Action: Flexion, abduction, IR
- Works with gluts to reduce stress on femur
ITB
- TFL to lateral tubercle of tibia (Gerdy’s)
- Discussed more in Knee

Hip Extensors

Primary Extensors: Glut max, hamstrings, adductor magnus
Gluteus Maximus:
- Stair climbing, Running, Jumping
- Greatest force at $70$ degrees hip flexion
- Works to posteriorly tilt pelvis

Multi-Joint Muscles: The Hamstrings

Biceps Femoris, Semitendinosus, Semimembranosus
Extend hip and flex knee
When hip is extended and knee flexed to $90$ degrees, the hamstrings do minimal extension
Knee extension = more hamstring participation in hip extension

Multi-Joint Muscle Example: The Squat

Up Phase: Concentric Hip Extension, Concentric Knee Extension
- Primary movers: Hamstrings and Gluts extend the hip
- Rectus Femoris (and quads) extend the knee
Down Phase: Eccentric control of Hip Flexion, Eccentric control of Knee Flexion
- Primary movers: Hamstrings and Gluts control hip flexion
- Rectus Femoris (and quads) control knee flexion

Hip External (Lateral) Rotators

PGOGOQ
- Piriformis
- Gemellus superior
- Obturator internus
- Gemellus inferior
- Obturator externus
- Quadratus femoris
Line of force = transverse plane
Stabilizes posterior hip joint

Piriformis

Hip flexion $> 90$ , Piriformis becomes a medial rotator

Hip Internal (Medial) Rotators

No single muscle responsible for IR alone
IR muscles include glut med, glut min, TFL, adductor longus, adductor brevis, pectineus
Hip internal rotators rotate the pelvis on the swing leg towards the weight bearing stance leg

To Stretch a Muscle- Place it in its Most Lengthened Position!

What is the muscle’s action?
The most lengthened position is the opposite of the action!

What Position Would Stretch These Muscles?

Psoas Major
Gluteus Maximus
Rectus Femoris
Biceps Femoris
Piriformis
Tensor Fascia Latae

References

Samuels-Colt, V. 2025. Foundation in Kinesiology and Biomechanics, 2nd edition.
Clarkson, H. 2020. Musculoskeletal Assessment: Joint Range of Motion, Muscle Testing, Function, 4th ed.

Exam 1

40-50 questions
Multiple Choice
Paper & Pencil
Covers all material from Class #1 – Class #4
60-90 minutes
No Lecture after exam