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Anatomy & Biomechanics: Comprehensive Answer

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a) Primary and Secondary Muscles of Breathing

Overview

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Primary Muscles of Inspiration

1. Diaphragm (the chief muscle of breathing)

• A dome-shaped musculotendinous partition separating the thoracic and abdominal cavities
• Originates from the xiphoid process, costal margin (ribs 7-12), and lumbar vertebrae (L1-L3 via crura)
• Innervated by the phrenic nerve (C3, C4, C5)
• Action: During contraction, the dome descends, increasing the vertical diameter of the thoracic cavity; this reduces intrathoracic pressure, drawing air into the lungs
• Responsible for approximately 70-80% of tidal volume during quiet breathing

2. External Intercostal Muscles (11 pairs)

• Run obliquely downward and forward between adjacent ribs
• Innervated by intercostal nerves (T1-T11)
• Action: Elevate the ribs, increasing the anteroposterior and transverse diameters of the thorax (pump-handle and bucket-handle movements)
• Active during both quiet and forced inspiration

3. Intercartilaginous part of Internal Intercostals

• The parasternal portion of the internal intercostals (between costal cartilages) assists in rib elevation during inspiration, acting synergistically with the external intercostals

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Primary Muscles of Expiration

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Secondary (Accessory) Muscles of Inspiration

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Secondary (Accessory) Muscles of Forced Expiration

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Summary: Mechanics of Breathing

• Quiet inspiration: diaphragm + external intercostals
• Quiet expiration: passive (elastic recoil only)
• Forced inspiration: all above + SCM, scalenes, pectoralis, serratus anterior
• Forced expiration: abdominals (especially transversus) + internal intercostals (interosseous)

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b) Stability of the Elbow Joint

1. Humeroulnar joint (trochlea-trochlear notch) - hinge joint for flexion/extension
2. Humeroradial joint (capitulum-radial head) - ball-and-socket allowing flexion/extension
3. Proximal radioulnar joint (radial head-radial notch of ulna) - pivot joint for pronation/supination

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1. Bony Stability

• The congruent fit of the trochlear notch of the ulna around the trochlea of the humerus provides inherent bony constraint against valgus/varus forces, especially in full extension
• The olecranon locks into the olecranon fossa in extension, creating a firm mechanical block
• The coronoid process resists posterior displacement of the ulna in flexion
• In 90° flexion, the congruence decreases and ligamentous support becomes more important

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2. Capsular Stability

• The fibrous joint capsule encloses all three articulations
• Anteriorly it is thin and loose (allows full flexion); posteriorly it is also relatively thin
• The capsule is attached proximally to the humeral epicondyles and margins of the coronoid, radial, and olecranon fossae; distally to the coronoid process and olecranon of ulna
• Fat pads overlying the three fossae are pulled away by attached muscles during movement, preventing impingement
• The brachialis and triceps attach to the capsule and retract the fat pads during flexion and extension respectively (Gray's Anatomy for Students)

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3. Ligamentous Stability

Medial (Ulnar) Collateral Ligament (UCL)

• Triangular ligament with three components:
  • Anterior band - strongest; primary restraint to valgus stress throughout ROM (especially 30-120° flexion)
  • Posterior band - taut in flexion; resists valgus and rotational stress
  • Transverse (Cooper's) ligament - connects coronoid to olecranon; adds rotational stability
• The UCL is the primary valgus stabilizer of the elbow
• Commonly injured in overhead throwing athletes (medial epicondylitis or UCL tear)

Lateral (Radial) Collateral Ligament Complex (LCL)

• Components:
  • Radial collateral ligament (RCL) - from lateral epicondyle to annular ligament; resists varus stress
  • Lateral ulnar collateral ligament (LUCL) - most important component; primary restraint against posterolateral rotatory instability (PLRI); runs from lateral epicondyle to ulna (posterior crista supinatoris)
  • Annular ligament - strong ring encircling the radial head, binding it to the radial notch of the ulna; allows radial head to pivot during pronation/supination; blends with both the RCL and joint capsule (Gray's Anatomy for Students)
  • Accessory lateral collateral ligament

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4. Muscular (Dynamic) Stability

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5. Vascular and Neural Innervation

• Blood supply: anastomotic network from collateral and recurrent branches of brachial, profunda brachii, radial, and ulnar arteries (Gray's Anatomy for Students)
• Innervation: predominantly radial and musculocutaneous nerves; some contribution from ulnar and median nerves

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c) Active and Passive Stability of the Cervical Spine

1. Passive subsystem - osteoligamentous column (bones, discs, ligaments)
2. Active subsystem - muscles and tendons
3. Neural/control subsystem - mechanoreceptors and CNS feedback

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Passive Stability

Bony Anatomy

• 7 cervical vertebrae (C1-C7) with distinctive features:
  • C1 (Atlas): ring-like, no body; articulates with occiput (atlantooccipital joint) - allows 50% of flexion/extension
  • C2 (Axis): has the dens/odontoid process; atlantoaxial joint allows ~50% of total cervical rotation
  • C3-C7: typical vertebrae with uncinate processes (uncovertebral joints) that add lateral stability
• Articular facets of the upper cervical spine (C0-C2) have very little inherent bony stability and rely heavily on ligaments

Key Ligaments

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Active Stability (Muscular)

Deep Stabilizing Muscles (intrinsic/local stabilizers)

Superficial/Global Muscles (powerful movers)

Principles of Active Stability

• Deep muscles act first (feedforward activation) to pre-stiffen the spine before large movements
• Global muscles modulate gross movement and react to perturbations
• EMG studies confirm that scalene and SCM activity increases with posture changes and higher respiratory drive, reinforcing the dual role of some cervical muscles
• Injury, pain, or degeneration causes inhibition of deep muscles (especially longus colli) with compensatory overactivation of superficial muscles - resulting in reduced segmental control and increased injury risk

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d) Postural Alignment and Its Role in Efficient Weight Distribution

Definition of Posture

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Components of Postural Alignment

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

• Passes through the external auditory meatus
• Through the odontoid process of C2
• Through the bodies of lumbar vertebrae
• Through the anterior sacrum
• Slightly anterior to the knee joint (hip slightly extended)
• Slightly anterior to the ankle joint (lateral malleolus)

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Weight Distribution in Static Activities

• Body weight is distributed evenly between the two lower extremities
• The center of mass (COM) lies approximately at the level of S2, slightly anterior to the sacrum
• The base of support (BOS) is the area beneath the feet
• COM must remain within the BOS to maintain balance (static equilibrium)
• Minimal antagonistic isometric muscle activity (postural tone) is required - primarily from the antigravity muscles (spinal extensors, hip extensors, ankle plantar flexors)

• Forward head posture shifts the COM anteriorly, increasing the moment arm of the head on the cervical spine - for every 1 inch of forward head displacement, the perceived weight of the head on the neck increases by approximately 10 pounds
• Lumbar hyperlordosis shifts compressive forces to the posterior elements (facets)
• Lateral pelvic tilt creates asymmetric loading on the lumbar spine and hip joints
• Slouching shifts COM forward, straining the posterior thoracolumbar fascia and hip flexors

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Weight Distribution in Dynamic Activities

• Stance phase (CKC): The foot is fixed; the proximal segments (tibia, femur, pelvis) must maintain the COM over the BOS. Hip abductors (gluteus medius) prevent contralateral pelvic drop (Trendelenburg mechanism).
• Swing phase (OKC): The limb is free; COM is transferred over the contralateral stance leg.
• Normal gait requires about 1-2% body weight in hip abductor force to prevent pelvic drop.

• Spinal flexion shifts COM anteriorly; the erector spinae must generate large extensor moments
• Lumbar compressive forces can reach 8-10x body weight during heavy lifting
• Maintaining neutral lumbar lordosis during lifting (brace, don't flex) minimizes compressive loading

1. Ankle strategy - minor perturbations controlled by ankle dorsiflexors/plantarflexors (sway in place)
2. Hip strategy - larger perturbations; hip flexors/extensors shift upper body mass
3. Step strategy - very large perturbations; a step is taken to widen the BOS

• Functional: absence of pain, normal muscle tone, balanced kinetic chains, harmonious skeletal segment relationships
• Non-functional: pain, muscle dystonia, abnormal tension, kinetic chain imbalances - all leading to increased energy expenditure and injury risk

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e) Closed-Chain and Open-Chain Biomechanics of the Hip Joint

Definitions

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Hip Joint Anatomy Relevant to Chain Mechanics

• Transverse axis: flexion (140°) / extension (20°)
• Sagittal axis: abduction (80°) / adduction (20°)
• Longitudinal axis: internal rotation (40°) / external rotation (50°)

• Lifting leg: ~1.5× body weight
• Standing on one leg: ~3-4× body weight (with abductor muscle action)
• Running/jumping: ~8-10× body weight (Bailey and Love's Short Practice of Surgery)

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Open-Chain Biomechanics of the Hip

• Hip flexors (iliopsoas, rectus femoris) produce anterior rotation of the femur
• Hip extensors (gluteus maximus, hamstrings) produce posterior rotation
• No compressive ground reaction force is transmitted through the chain
• Muscle forces are primarily rotatory (torque-producing)
• Shear forces dominate at the joint

1. Swing phase of gait - the foot is off the ground; the hip flexes and extends freely without bearing weight; the pelvis is stabilized by the contralateral stance-phase hip abductors
2. Supine straight-leg raise - the pelvis is fixed on the table; the femur moves freely in space
3. Seated hip flexion (as in a hip flexion machine) - the foot hangs free
4. Prone hip extension - femur extends freely while prone
5. Supine hip internal/external rotation - femur rotates on a fixed pelvis
6. Swimming flutter kick - both hips in alternating OKC as feet are free in water

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Closed-Chain Biomechanics of the Hip

• Ground reaction force (GRF) is transmitted proximally through the chain
• Compressive joint forces dominate (vs. shear in OKC)
• Multiple muscle groups co-contract simultaneously
• The pelvis moves ON the femur (rather than femur on pelvis)
• Biarticular muscles undergo the concurrent shift (shorten at one end, lengthen at other)

1. Squat: Hip, knee, and ankle all flex simultaneously. Flexion at the hip CANNOT occur without knee flexion and ankle dorsiflexion. The rectus femoris shortens across the knee while lengthening across the hip; the hamstrings shorten across the hip while lengthening across the knee (concurrent shift). Gluteus maximus is the primary hip extensor during the ascent phase.
2. Single-leg stance / Stance phase of gait: The stance-leg foot is fixed; the ipsilateral hip abductors (gluteus medius and minimus) contract to prevent contralateral pelvic drop (Trendelenburg mechanism). The abductors generate a force ~3x body weight across the hip joint. This is the basis of the Trendelenburg test - weakness of hip abductors results in the contralateral pelvis dropping when standing on the affected leg. (Bailey and Love's)
3. Stair climbing: Hip extends as the pelvis rises over the fixed foot. Gluteus maximus and quadriceps generate extensor torques at the hip and knee respectively.
4. Sit-to-stand: Hip extends rapidly in CKC; simultaneous knee extension and ankle plantarflexion occur. The hip extensors (gluteus maximus) and knee extensors (quadriceps) work in synergy.
5. Running/jogging (stance phase): The planted foot experiences GRF up to 2-3x body weight; the hip must extend (gluteus maximus) while stabilizing the pelvis laterally (gluteus medius).

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Comparison: OKC vs. CKC at the Hip

• Load joints through compressive forces (more protective than shear)
• Reproduce functional movement patterns
• Recruit co-contraction for joint stability
• Engage proprioceptive mechanoreceptors more effectively
• Allow biarticular muscles to undergo the concurrent shift as they do in real activities

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• Gray's Anatomy for Students (ISBN 9780323934237) - Elbow joint anatomy
• General Anatomy and Musculoskeletal System, THIEME Atlas (ISBN 9781626237186) - Hip joint biomechanics
• Bailey and Love's Short Practice of Surgery, 28th Ed. (ISBN 9780367548117) - Hip joint forces
• Kenhub - Anatomy of breathing: muscles of respiration
• Physiopedia - Centre of Gravity, Kinetic Chain
• Panjabi MM (1992) - Spinal stability three-subsystem model
• Musculoskeletalkey.com - Cervical spine stability (80% muscular)
• NSCA Kinematic and Kinetic Chains reference
• PMC - Relationships between lower limb joint movements in open and closed kinematic chains (PMC5260548)