a) Characteristics of Synovial Joints

1. Articular cartilage - Variously shaped articular surfaces covered by hyaline cartilage, which reduces friction and distributes compressive forces.
2. Joint cavity - A potential space between the articulating bones, with articular recesses that vary in width.
3. Joint capsule - An all-around closed fibrous capsule, which is reinforced by alar folds, synovial folds, and synovial villi. It has two layers:
   • Outer fibrous layer (stratum fibrosum)
   • Inner synovial layer (stratum synoviale) - produces synovial fluid
4. Synovial fluid - Highly viscous fluid produced by the joint capsule. It lubricates the joint, nourishes articular cartilage, and provides shock absorption.
5. Intra-articular structures - Present in some joints to improve congruency and increase force-absorbing surface area:
   • Menisci (e.g., knee) - crescent-shaped, wedge in cross section
   • Articular discs (e.g., sternoclavicular, temporomandibular joints) - divide the joint into two chambers
   • Labra (e.g., hip, shoulder) - wedge-shaped fibrocartilaginous rims that deepen the socket
6. Ligaments - Intra- and extracapsular ligaments act as primary joint stabilizers.
7. Muscles - The joint is crossed by muscles moving it in opposite directions (agonist/antagonist pair).
8. Synovial bursae - Fluid-filled sacs often located near the joint, sometimes communicating with the joint cavity, reducing friction between structures.

Note: Not all of the above are present in every synovial joint.

• General Anatomy and Musculoskeletal System, THIEME Atlas of Anatomy

b) Pump Handle and Bucket Handle Movements of the Thorax

Pump Handle Movement (Anteroposterior diameter)

• The posterior ends of ribs articulate with the vertebral column; the anterior ends are lower than the posterior ends.
• When the ribs are elevated during inspiration, this geometry causes the anterior end (and attached sternum) to move upward and forward - like the handle of an old-fashioned water pump being raised.
• This increases the anteroposterior (AP) diameter of the thorax.
• Predominantly involves the upper ribs (ribs 1-6), especially ribs 2-6.
• On depression (expiration), the sternum moves downward and backward.

Bucket Handle Movement (Transverse/lateral diameter)

• In addition to the anterior ends being lower than the posterior ends, the middles of the rib shafts are lower than both ends - like a bucket handle lying flat.
• When the rib shafts are elevated during inspiration, the middles of the shafts swing upward and outward laterally - like a bucket handle being lifted.
• This increases the transverse (lateral) diameter of the thorax.
• Predominantly involves the lower ribs (ribs 7-10), which have more lateral flare.

Both movements occur simultaneously during normal breathing, with pump handle predominating in the upper thorax and bucket handle in the lower thorax.

• Gray's Anatomy for Students

c) Classification of Muscles Based on Function

• Postural muscles (tonic) - Continuously active to maintain body posture against gravity (e.g., soleus, erector spinae). Rich in slow-twitch (Type I) fibers.
• Phasic muscles - Activated for rapid, forceful movements. Rich in fast-twitch (Type II) fibers (e.g., gastrocnemius, rectus femoris).
• Multi-articular muscles - Cross more than one joint and act on multiple joints (e.g., rectus femoris, hamstrings).
• Shunt muscles - Oriented to produce compression/stabilization across a joint rather than angular motion (e.g., brachioradialis).
• Spurt muscles - Produce primarily angular movement at a joint (e.g., brachialis).

• General Anatomy and Musculoskeletal System, THIEME Atlas of Anatomy

d) Motor Strategies of Motor Control

Three Core Postural Motor Strategies

1. Ankle Strategy

• Used when: Small, slow perturbations on a firm, wide surface
• Mechanism: The body sways as a single inverted pendulum, rotating about the ankle joints. Muscles contract around the ankle (gastrocnemius/tibialis anterior) to restore the center of mass (CoM) over the base of support without moving the feet.
• Base of support: Remains fixed
• Key feature: The feet stay in place; correction occurs from the ground up (distal to proximal muscle activation sequence)

2. Hip Strategy

• Used when: Larger, faster perturbations or on narrow/compliant surfaces
• Mechanism: Rapid flexion or extension at the hip joints shifts the CoM. Trunk and hip muscles are activated first (proximal to distal sequence), which is the reverse of the ankle strategy.
• Base of support: Remains fixed, but hip motion shifts the CoM rapidly
• Key feature: Produces a countermovement of the trunk and legs in opposite directions

3. Stepping (Change-in-Support) Strategy

• Used when: The perturbation is so large that the CoM cannot be recovered with the ankle or hip strategy
• Mechanism: A rapid step or multiple steps are taken to move the base of support under the displaced CoM. Similarly, the hand may grab a support to enlarge the base.
• Base of support: Changed/enlarged
• Key feature: Longest latency, allowing cortical input (choice of which foot, direction, avoiding obstacles)

Key Features of Automatic Postural Responses

• They are not simple stretch reflexes but synergistic activation of muscle groups in a characteristic sequence.
• Latency in humans: 90-120 ms in ankle muscles (longer than stretch reflex, shorter than voluntary reaction time).
• The nervous system selects the appropriate strategy based on the size and speed of perturbation, the surface conditions, and prior experience/anticipation.
• Anticipatory postural adjustments (APAs) also occur - the CNS pre-activates postural muscles before a voluntary limb movement to counteract the expected postural disturbance.
• Eric Kandel, Principles of Neural Science, 6th Edition; Scott-Brown's Otorhinolaryngology

e) Pronation and Supination

Definition

• Supination - Rotation of the forearm so the palm faces anteriorly/upward (anatomical position). The radius and ulna are parallel.
• Pronation - Rotation of the forearm so the palm faces posteriorly/downward. The distal end of the radius crosses over the ulna.

Plane of Movement

Muscles Responsible

Supination

Pronation

Accessory

• Anconeus - Causes slight abduction of the distal ulna during pronation to maintain the palm over the central axis.

• Gray's Anatomy for Students; General Anatomy and Musculoskeletal System, THIEME Atlas of Anatomy