Unit 5: Mechanics of shoulder, spine and hip - Subjective Questions

The shoulder complex is one of the most mobile joint systems in the human body, comprising four articulations working together:

• Glenohumeral (GH) joint: A ball-and-socket synovial joint between the head of the humerus and the glenoid fossa of the scapula. It provides the greatest range of motion but is inherently unstable due to the shallow glenoid.
• Acromioclavicular (AC) joint: A plane synovial joint between the acromion of the scapula and the lateral clavicle. Allows small gliding and rotational movements.
• Sternoclavicular (SC) joint: The only bony attachment of the upper limb to the axial skeleton, connecting the clavicle to the sternum.
• Scapulothoracic articulation: A functional (not true) joint where the scapula glides over the thoracic wall, essential for full arm elevation.

Supporting structures:

• Glenoid labrum deepens the socket for stability.
• Rotator cuff muscles (supraspinatus, infraspinatus, teres minor, subscapularis) provide dynamic stability.
• Ligaments and capsule provide passive restraint.

This arrangement balances mobility versus stability, with dynamic muscular control compensating for limited bony stability.

The shoulder complex permits movement in three planes:

• Flexion / Extension (sagittal plane)
• Abduction / Adduction (frontal plane)
• Internal / External rotation (transverse plane)
• Horizontal abduction / adduction and circumduction (combined motions)

Scapulohumeral Rhythm:
During arm elevation, motion occurs simultaneously at the glenohumeral joint and the scapulothoracic articulation in a coordinated pattern.

• The classic ratio is approximately 2:1 — for every of arm elevation, about occurs at the GH joint and at the scapulothoracic joint.
• Thus, of a total of elevation, roughly comes from the GH joint and from scapular rotation.

Significance:

• Maintains optimal length-tension relationship of the deltoid and rotator cuff.
• Keeps the glenoid positioned under the humeral head for stability.
• Prevents impingement of the supraspinatus tendon under the acromion.

Disruption of this rhythm (e.g., due to muscle weakness or injury) leads to impingement and reduced elevation.

During abduction, the arm acts as a lever with the GH joint as the fulcrum. The forces involved are the weight of the arm (), the deltoid muscle force (), and the joint reaction force ().

Assumptions (example):

• Weight of arm acts at the center of gravity, distance from joint.
• Deltoid force acts at angle at distance from joint.

Moment equilibrium about the joint:

Solving for muscle force:

Because the deltoid's moment arm is small and is small (shallow line of pull), becomes very large — often several times body weight.

Joint reaction force is then found by force equilibrium:

Key findings:

• The joint reaction force can reach 0.8–0.9 times body weight at abduction when holding weights.
• Peak loads occur around 90° abduction due to unfavorable moment arms.
• This explains why rotator cuff and GH joint injuries are common in overhead activities.

The vertebral column consists of 33 vertebrae arranged in five regions:

• Cervical (7) — smallest, most mobile
• Thoracic (12) — articulate with ribs
• Lumbar (5) — largest, bear most load
• Sacral (5, fused) — form the sacrum
• Coccygeal (4, fused) — form the coccyx

Spinal Curvatures:

• Cervical lordosis (convex anteriorly)
• Thoracic kyphosis (convex posteriorly)
• Lumbar lordosis (convex anteriorly)
• Sacral kyphosis

These curves increase resistance to axial compression. Resistance is proportional to , where is the number of curves.

Components of a typical vertebra:

• Vertebral body — bears compressive load
• Vertebral arch (pedicles + laminae)
• Spinous and transverse processes — muscle/ligament attachment
• Articular processes (facets) — form facet joints
• Vertebral foramen — houses the spinal cord

Intervertebral discs (nucleus pulposus + annulus fibrosus) between bodies act as shock absorbers and permit motion.

The intervertebral disc (IVD) is a fibrocartilaginous cushion between adjacent vertebral bodies. It has two main parts:

• Nucleus pulposus: A gel-like central core (~80% water) that behaves like an incompressible fluid, distributing pressure evenly in all directions.
• Annulus fibrosus: Concentric layers (lamellae) of collagen fibers oriented in alternating directions (~ to horizontal), providing tensile strength and containing the nucleus.
• Cartilaginous endplates: Connect the disc to vertebral bodies and allow nutrient diffusion.

Load distribution mechanism:

• Under compression, the nucleus pulposus develops hydrostatic pressure and transfers load radially to the annulus.
• The annular fibers resist this tension, converting axial load into tensile stress.
• This mechanism allows the disc to act as a hydraulic shock absorber.

Load characteristics:

• Intradiscal pressure varies with posture: lowest when lying supine, higher when sitting, and highest when sitting and leaning forward while lifting.
• With age, the nucleus loses water content, reducing its load-distributing ability and increasing risk of disc herniation.

The spine permits movement in three planes, though the amount varies by region:

• Flexion / Extension (sagittal plane)
• Lateral flexion / bending (frontal plane)
• Rotation (transverse plane)
• Circumduction (combined)

Regional motion characteristics:

• Cervical spine: Greatest overall mobility; large flexion/extension and rotation (especially at atlanto-axial joint for rotation).
• Thoracic spine: Limited flexion/extension due to rib cage; permits rotation.
• Lumbar spine: Good flexion/extension; rotation is very limited due to sagittally oriented facet joints.

Factors limiting spinal movement:

• Orientation of facet joints — determines available direction of motion.
• Intervertebral disc height and elasticity.
• Ligaments (anterior/posterior longitudinal, ligamentum flavum, interspinous, supraspinous).
• Muscle tension and bony contact (e.g., spinous process contact in extension).
• Rib cage in the thoracic region.

The combined small movements between adjacent vertebrae sum to produce large overall trunk motion.

Spinal muscles are grouped by location and function:

Posterior (extensor) muscles:

• Erector spinae (iliocostalis, longissimus, spinalis): Primary extensors; maintain upright posture and control forward bending.
• Deep muscles (multifidus, rotatores, interspinales): Provide segmental stability and fine control; important for rotation.

Anterior and lateral (flexor) muscles:

• Rectus abdominis: Trunk flexion.
• External and internal obliques: Trunk rotation and lateral flexion.
• Transversus abdominis: Increases intra-abdominal pressure for spinal stability.
• Psoas major: Hip flexor that also loads the lumbar spine.

Functional roles:

• Movement: Producing flexion, extension, lateral bending, and rotation.
• Stabilization: The core muscles (transversus abdominis, multifidus, diaphragm, pelvic floor) create a stable base.
• Intra-abdominal pressure (IAP) mechanism: Contraction of abdominal muscles raises IAP, which acts like an internal cushion that reduces compressive load on the discs during lifting.

Balanced action of flexors and extensors is essential for spinal health and injury prevention.

Consider lifting a load with the trunk bent forward. Model the lower back as a lever pivoting at L5-S1.

Forces involved:

• Weight of upper body () acting at horizontal distance from L5-S1.
• Load being lifted () acting at horizontal distance .
• Erector spinae muscle force () acting at a small moment arm (~5 cm).

Moment equilibrium about L5-S1:

Solving for muscle force:

Because is very small (~) while and are large, becomes enormous.

Compressive force on disc (force equilibrium along spine axis):

where is the trunk inclination.

Key insights:

• Compressive loads on L5-S1 can exceed several thousand newtons even when lifting modest weights.
• Intra-abdominal pressure reduces by providing an additional extension moment.
• This explains why lifting with a straight back and load held close (small ) drastically reduces spinal load.

Intra-abdominal pressure (IAP) is the pressure within the abdominal cavity, generated by the coordinated contraction of the diaphragm, abdominal wall muscles, and pelvic floor.

Mechanism of load reduction:

• During lifting, contraction of the transversus abdominis and oblique muscles compresses the abdominal contents, raising IAP.
• This creates a pressurized 'cylinder' in front of the spine that generates an extension moment, effectively unloading the posterior spinal muscles.
• The result is a reduction in the required erector spinae force () and therefore in the compressive force () on the intervertebral discs.

Quantitative effect:

• The extension moment provided by IAP acts over the abdominal cross-sectional area at a distance from the spine.
• Studies estimate IAP can reduce lumbar compressive load by up to 10–40% during heavy lifting.

Practical significance:

• Explains the protective effect of bracing the core and the use of weightlifting belts.
• Highlights the importance of strong abdominal musculature for spine protection.

However, excessively high IAP can strain the cardiovascular system (Valsalva effect), so it must be balanced.

The hip joint is a ball-and-socket synovial joint between the head of the femur and the acetabulum of the pelvis. It is designed for weight-bearing and stability while permitting a wide range of motion.

Bony structures:

• Femoral head: Nearly spherical, covered in articular cartilage.
• Acetabulum: A deep cup formed by ilium, ischium, and pubis — provides much greater bony coverage than the shoulder's glenoid.
• Acetabular labrum: Fibrocartilaginous rim that deepens the socket and creates a suction seal.

Angles of importance:

• Angle of inclination (neck-shaft angle): Normally ~. Increase = coxa valga, decrease = coxa vara.
• Angle of anteversion: Normally ~– forward rotation of femoral neck.

Soft tissue stabilizers:

• Strong ligaments: iliofemoral (Y-ligament), pubofemoral, ischiofemoral.
• Joint capsule: Thick and reinforced.
• Surrounding muscles: gluteals, adductors, iliopsoas.

Stability vs. mobility:
The deep socket and strong ligaments provide high stability, while the spherical head allows movement in all three planes, making the hip a compromise favoring stability over the shoulder's extreme mobility.

The hip joint permits movement in three planes:

Sagittal plane:

• Flexion: Iliopsoas, rectus femoris, sartorius (~ with knee flexed).
• Extension: Gluteus maximus, hamstrings (~–).

Frontal plane:

• Abduction: Gluteus medius, gluteus minimus, tensor fasciae latae (~).
• Adduction: Adductor longus, brevis, magnus, gracilis (~).

Transverse plane:

• Internal (medial) rotation: Gluteus medius/minimus (anterior fibers), TFL.
• External (lateral) rotation: Deep external rotators (piriformis, obturators, gemelli, quadratus femoris), gluteus maximus.

Combined movement:

• Circumduction: Sequential combination of all the above.

Functional note:
The gluteus medius is especially important during walking — it stabilizes the pelvis in the frontal plane during single-leg stance, preventing the opposite side from dropping (a positive Trendelenburg sign indicates its weakness).

During single-leg stance (as in the stance phase of gait), the pelvis is balanced over the femoral head like a lever, with the hip joint as the fulcrum.

Forces involved:

• Body weight minus the stance leg () acts downward at the body's center of gravity, distance medial to the hip.
• Hip abductor force () (mainly gluteus medius) acts at distance lateral to the hip to prevent the pelvis from tilting.

Moment equilibrium about the femoral head:

Since the abductor moment arm is roughly half of the body weight moment arm (i.e., ), the abductor force is about twice body weight.

Joint reaction force ():
By vertical force equilibrium, the joint must support both the body weight and the abductor force:

This gives a hip joint reaction force of approximately 2.5 to 3 times body weight during single-leg stance.

Clinical significance:

• Explains why hip loads increase dramatically during walking.
• Using a cane in the opposite hand or carrying loads on the same side reduces the required abductor force and lowers joint load.

Both are ball-and-socket joints, but they differ significantly in design based on their functional demands.

Summary:

• The shoulder sacrifices stability for mobility, relying on muscles for control.
• The hip sacrifices some mobility for stability, using its bony architecture and ligaments to bear the body's weight during locomotion.

The rotator cuff is a group of four muscles that surround the glenohumeral joint and provide dynamic stability:

• Supraspinatus — initiates abduction; compresses head into glenoid.
• Infraspinatus — external rotation.
• Teres minor — external rotation.
• Subscapularis — internal rotation.

Stabilizing function:

• Their tendons blend with the joint capsule and compress the humeral head into the glenoid, resisting the upward pull of the deltoid.
• This maintains the center of rotation and prevents superior migration of the humeral head (which would cause impingement).

Force Couple Concept:
A force couple is a pair of forces acting in opposite directions to produce rotation or maintain balance.

• In the frontal plane: The deltoid pulls the humerus upward, while the inferior rotator cuff (infraspinatus, teres minor, subscapularis) pulls downward. Together they rotate the head smoothly without excessive superior translation.
• Scapular force couple: The upper trapezius, lower trapezius, and serratus anterior work together to rotate the scapula upward during arm elevation.

Clinical importance:
Weakness or tears in the rotator cuff disrupt these force couples, leading to instability, impingement, and reduced function.

Intradiscal pressure (load on the L3 disc, based on Nachemson's classic studies) varies significantly with posture:

Explanation of trends:

• Lying down eliminates most gravitational compression, so pressure is lowest.
• Sitting produces higher loads than standing because the lumbar lordosis flattens, shifting load to the discs and increasing the trunk's forward moment.
• Bending forward increases the moment arm of the upper body weight, requiring greater muscle force and increasing compression.
• Adding an external load while bent forward dramatically increases the moment and compressive force.

Practical implications:

• Prolonged sitting and forward-bent lifting are the most stressful for the discs.
• Maintaining lumbar lordosis and keeping loads close to the body reduces disc pressure.

Angle of Inclination (Neck-Shaft Angle):

• The angle formed between the axis of the femoral neck and the axis of the femoral shaft in the frontal plane.
• Normal adult value: approximately .
• Coxa valga: angle > (neck more vertical) — increases load on femoral head, lengthens the limb, reduces abductor moment arm.
• Coxa vara: angle < (neck more horizontal) — increases bending stress on the femoral neck (risk of fracture), increases abductor moment arm.

Angle of Anteversion:

• The angle between the axis of the femoral neck and the transverse axis of the femoral condyles in the transverse plane.
• Normal adult value: approximately – (femoral neck rotated anteriorly).
• Excessive anteversion: causes 'toeing-in' gait and internal rotation; may increase joint instability.
• Retroversion (reduced/negative angle): causes 'toeing-out' gait.

Biomechanical significance:

• These angles determine the moment arm of hip muscles, the congruency of the joint, and the distribution of stress across the femoral head and neck.
• Abnormal angles alter gait mechanics and predispose to joint degeneration and fractures.

Shoulder impingement occurs when soft tissues (mainly the supraspinatus tendon and subacromial bursa) are compressed within the subacromial space — the gap between the humeral head and the acromion/coracoacromial arch.

Biomechanical causes:

1. Reduced subacromial space:

   • During abduction (especially –, the 'painful arc'), the greater tuberosity of the humerus approaches the acromion.
   • Any narrowing (bone spurs, acromion shape) compresses the tendon.

2. Loss of humeral head depression:

   • Normally, the rotator cuff compresses and depresses the humeral head to keep it centered.
   • If the cuff is weak or fatigued, the deltoid's upward pull dominates, causing superior migration of the head and reducing the subacromial space.

3. Disrupted force couples:

   • Imbalance between the deltoid and rotator cuff force couple leads to abnormal humeral head translation.

4. Scapular dyskinesis:

   • Poor upward rotation of the scapula (weak serratus anterior / lower trapezius) fails to clear the acromion during elevation.

Consequences:

• Repeated compression causes tendon inflammation, degeneration, and eventual tears.

Prevention/management:

• Strengthening the rotator cuff and scapular stabilizers restores normal mechanics and the subacromial space.

The facet (zygapophyseal) joints are formed by the superior and inferior articular processes of adjacent vertebrae. Their orientation governs which movements are permitted or restricted in each spinal region.

Cervical region:

• Facets oriented at about to the horizontal plane, in the frontal plane.
• Permits flexion, extension, lateral flexion, and rotation — the most mobile region.

Thoracic region:

• Facets oriented in the frontal plane (~ from horizontal).
• Favors rotation and lateral flexion, but rib cage limits overall motion.

Lumbar region:

• Facets oriented in the sagittal plane (~ from horizontal).
• This orientation permits good flexion and extension but strongly limits rotation (only ~– per segment).

Key principle:

• Facet joints permit motion in the plane in which they lie and restrict motion perpendicular to it.
• Sagittal facets (lumbar) block rotation; frontal facets (thoracic) allow rotation.

Functional significance:

• Explains why the lumbar spine is vulnerable to rotational injuries and why most trunk rotation occurs in the thoracic region.
• Facet orientation also influences load-sharing between the facets and the intervertebral disc.

Because hip joint loading during single-leg stance depends on the balance of moments about the femoral head, external supports and loads can significantly alter joint reaction force.

Recall the equilibrium equation:

where = abductor force, = abductor moment arm, = body weight, = its moment arm. Joint force .

Effect of a cane in the OPPOSITE hand:

• A cane held in the hand opposite to the affected hip provides an upward push at a long moment arm from the hip.
• This creates an additional moment that assists the abductors, reducing the required .
• Result: hip joint force can decrease by up to 50%.
• A cane in the same-side hand is much less effective (short moment arm).

Effect of load carriage:

• Carrying a load on the same side as the stance hip reduces hip load because the load's moment partially balances the body weight moment (shifts center of gravity toward the hip).
• Carrying a load on the opposite side increases hip load, as it adds to the body weight moment the abductors must counter.

Practical guidance:

• Patients with hip pain should use a cane in the opposite hand and, if carrying items, hold them on the affected side.

The muscles acting on the shoulder complex can be classified into groups based on their attachments and functions.

1. Scapulothoracic (axioscapular) muscles — position and stabilize the scapula:

• Trapezius (upper, middle, lower fibers) — elevation, retraction, upward rotation.
• Serratus anterior — protraction and upward rotation (crucial for elevation).
• Rhomboids (major & minor) — retraction and downward rotation.
• Levator scapulae — elevation.

2. Axiohumeral muscles — connect trunk to humerus (prime movers):

• Pectoralis major — flexion, adduction, internal rotation.
• Latissimus dorsi — extension, adduction, internal rotation.

3. Scapulohumeral muscles — connect scapula to humerus:

• Deltoid — primary abductor (also flexion/extension by fibers).
• Rotator cuff (supraspinatus, infraspinatus, teres minor, subscapularis) — dynamic stability and rotation.
• Teres major — extension, adduction, internal rotation.
• Coracobrachialis — flexion and adduction.

Functional classification:

• Prime movers: deltoid, pectoralis major, latissimus dorsi.
• Dynamic stabilizers: rotator cuff.
• Scapular stabilizers/rotators: trapezius, serratus anterior, rhomboids.

Coordinated action of these groups produces smooth, stable movement through the full range.