Module 1 Basics of Muscles and Joints — Comprehensive Notes

Biomechanics of Bone and Tissues

  • Biomechanics overview

    • Biomechanics examines the structure, function, and motion of the systems that make up a living organism.
    • Helps understand how body tissues and different structures move together.

  • Bone composition

    • Bones are mostly made of collagen and calcium.
    • Two main bone components: cortical bone and cancellous (spongy) bone.
    • Cortical bone
    • More dense with greater calcium mineral content than collagen.
    • Provides rigidity and strength against forces.
    • Located in areas requiring strong support (e.g., the shaft of long bones like the femur).
    • Cancellous bone (spongy bone)
    • Higher collagen content.
    • Found around the marrow cavity and at the ends of long bones.
    • Absorbs load and helps soften impacts at joints (e.g., near the hip and femoral head).
    • Function of the end of long bones with cancellous bone: absorb joint impact during activities like walking.

  • Bone marrow

    • Bone marrow is fatty tissue inside bones.
    • Produces blood components (blood cells and platelets) housed within bones.

  • Articular cartilage (hyaline cartilage)

    • Dense connective tissue located on the ends of long bones.
    • Purpose: protect articulating surfaces and absorb forces at joints.
    • Structure: cartilage layer is about 2−4 mm2{-}4\ \text{mm} thick and has zones/layers; closer to bone becomes denser, increasing resistance to compression.
    • Mature cartilage is avascular (lacks blood vessels) and lacks nociceptors (pain receptors).
    • Healing: difficult because blood supply is needed for healing; damage can be asymptomatic until damage is significant.
    • Example condition: osteoarthritis – wear and tear leading to degeneration of cartilage, causing knee pain in older adults and potentially knee joint replacement.

  • Joint components related to bones and tendons

    • Ligaments
    • Dense connective tissue surrounding or within joints.
    • Stabilize bone-to-bone connections.
    • ACL (anterior cruciate ligament): common knee injury that destabilizes the knee when torn.
    • Tendons
    • Connect muscle to bone.
    • Main purpose is to transfer the force of muscle contraction to bone, enabling joint movement.
    • Next modules will cover ligaments and tendons relevant to specific joints (e.g., shoulder).

  • Joint capsules and aponeuroses

    • Joint capsule
    • Dense fibrous sleeve surrounding a synovial joint.
    • Provides passive stability and creates a sealed compartment containing lubricating fluid (synovial fluid).
    • Synovial joints
    • Contain lubricating fluid to facilitate smooth joint movement.
    • Aponeurosis
    • Broad fibrous insertion connecting adjacent muscles.
    • Example: rectus sheath (abdominal region).
    • There are multiple aponeuroses in the body; details will be discussed for various locations.

  • Muscles: types and structure

    • Types of muscle tissue
    • Skeletal (striated) muscle: voluntary movement; contraction causes movement of the skeleton.
    • Cardiac muscle: forms the myocardium of the heart; striated but shorter fibers; contracts in a wave-like pattern to pump blood.
    • Smooth (visceral) muscle: involuntary; non-striated; found in organs like intestines and esophagus; contractions are slower and automatic.
    • Skeletal muscle specifics
    • Generally moves bones with a fleshy belly (contractile tissue).
    • Some skeletal muscles are not strictly bone-to-bone movers (e.g., facial muscles); mostly they move bone segments.
    • Called striated due to visible bands.
    • Muscle attachments
    • Origin: attachment that moves the least, usually proximal and stable.
    • Insertion: attachment that moves more, usually distal and mobile.
    • Some muscles can act in both directions, depending on movement.
    • Muscle anatomy terminology in practice
    • Epimysium: outer protective layer of the muscle.
    • Perimysium (often misnamed as paramecium in informal usage): surrounds muscle fascicles.
    • Endomysium: surrounds individual muscle fibers (not elaborated in-depth in this lecture).
    • The epimysium connects with tendons to provide fixation points.
    • Note: in this transcript, the term paramecium is mentioned, but perimysium is the standard term.
    • Palpation and surface anatomy
    • Emphasis on palpating origins, insertions, and muscle bellies to identify structures and assess function.
    • Future module will include palpation practice and surface anatomy (landmarks).
    • Bony landmarks
    • Landmarks on bones that protrude under the skin, useful for locating muscles and for fitting orthotics or splints.
    • Upper-extremity landmarks are particularly useful for orthotics and splinting.
    • Muscle fiber arrangements (pennation and variants)
    • Pennate muscles: fibers oriented obliquely (slanted).
      • Multipennate: more than two fiber groups (e.g., deltoid, infraspinatus).
      • Bipennate: two sets of oblique fibers (e.g., rectus femoris, lumbricals).
      • Unipennate: fibers on one side of a tendon (e.g., semimembranosus, tibialis posterior).
    • Fusiform muscles: fibers parallel to the line of force, enabling a straight pull.
    • Sphincter muscles: circular pattern that closes or regulates opening (e.g., orbicularis oculi around the eye; anal sphincter).
    • Motor memory and coordination
    • Motor memory: learned patterns of movement through practice.
    • Muscles work together as agonists, antagonists, fixators, and synergists.
    • Agonist (prime mover): provides the most force for a movement.
      • Example: elbow flexion – brachialis is the primary mover; biceps brachii acts as a synergist.
    • Antagonist: performs the opposite movement; must relax to allow the agonist to move (e.g., triceps during elbow flexion).
    • Post-stroke considerations: disruption in balance and increased muscle tone can impair these patterns.
    • Muscles crossing joints
    • Some muscles cross a single joint (e.g., brachialis).
    • Others cross multiple joints (e.g., flexor digitorum profundus crosses the forearm to the wrist and multiple hand joints) and affect several joints.
    • Muscle contractions (types)
    • Isometric: muscle contracts without length change; e.g., holding a phone steady.
    • Isotonic: muscle changes length with movement; includes:
      • Eccentric: lengthening under tension (e.g., lowering a mug from the mouth).
      • Concentric: shortening under tension (e.g., bringing a mug to the mouth).

  • Joints: types and mechanics

    • Joint types
    • Synovial joints: mobile and designed for purposeful movement.
    • Fibrous joints: limited or no mobility; primarily stability (e.g., skull sutures in early life).
    • Cartilaginous joints: limited mobility; stability-focused (e.g., pubic symphysis).
    • Synovial joint variations and movements
    • Ball-and-socket: spherical surface of one bone fits into a concave surface of another; three axes of rotation; example: glenohumeral (shoulder).
      • Movements include flexion/extension, abduction/adduction, internal/external rotation, and circumduction.
    • Ellipsoid (condylar): oval convex surface into a concave basin; two axes of rotation; example: radiocarpal (wrist).
      • Movements: flexion/extension, abduction/adduction (radial/ulnar deviation).
    • Hinge: allows flexion/extension around a single axis; examples include humeroulnar joint (elbow).
    • Saddle: modified condyloid with convex/concave surfaces; two axes; example: carpometacarpal (CMC) joint of the thumb.
    • Gliding (plane): two flat surfaces slide against each other with minimal movement; example: intercarpal joints in the wrist.
    • Pivot: single axis rotation where one bone rotates around another; example: atlantoaxial joint (c1–c2) allowing head side-to-side movement.
    • Osteokinematics vs. arthrokinematics
    • Osteokinematics: gross movement of bones relative to each other (visible externally, e.g., elbow flexion).
    • Arthrokinematics: smaller, internal joint surface movements between bones that form the joint; often not externally visible.
    • Translation: a subset of arthrokinematics where surfaces move in the same direction (e.g., slides).
    • Common arthrokinematic movements:
      • Compression: surfaces press together.
      • Distraction: surfaces pull apart.
      • Gliding: ends of bones move parallel to each other.
      • Spin: rotation movement of a bone around its axis.
    • Example: knee involves subtle rotational arthrokinematics during movement, alongside visible osteokinetic flexion/extension.

  • Practical implications and study tips

    • Palpation and surface anatomy
    • Practice locating muscles by palpation to understand function and movement performance.
    • Use surface anatomy cues and bony landmarks to guide assessment and treatment planning.
    • Relevance to orthotics and splinting
    • Knowledge of surface landmarks supports accurate orthotic fitting and splint design.
    • Educational resources and next steps
    • Supplemental videos ( Osmo sis platform ) available for additional explanations.
    • Upcoming topics include goniometry and manual muscle testing (MMT), which are essential hands-on measurement techniques.

  • Summary of key terms to remember

    • Bone components: cortical bone, cancellous (spongy) bone, bone marrow.
    • Cartilage: articular (hyaline) cartilage; avascular and nociceptor-free in maturity; thickness ~ 2−4 mm2{-}4\ \text{mm}.
    • Joint types: synovial (mobile), fibrous (stability), cartilaginous (stability).
    • Synovial joint examples: ball-and-socket, ellipsoid, hinge, saddle, gliding, pivot.
    • Muscle tissue types: skeletal (voluntary), cardiac (myocardium), smooth (involuntary).
    • Muscle anatomy: origin, insertion, belly; epimysium, perimysium (often misnamed in informal usage), endomysium.
    • Fiber architecture: fusiform, pennate (uni-, bi-, multi-), sphincter.
    • Muscle actions: agonist, antagonist, synergist, fixator.
    • Contractions: isometric, isotonic (concentric, eccentric).
    • Arthrokinematics vs osteokinematics; translation movements: compression, distraction, gliding, spin.

  • Final notes

    • This lecture covers foundational concepts for module one, focusing on bones, joints, and muscles, with emphasis on structure, function, and clinical relevance.
    • The instructor notes that future modules will expand on specific joints (e.g., shoulder), palpation techniques, and practical assessment methods (goniometry and manual muscle testing).
    • If something is unclear, additional resources (anatomy software, Osmo sis, and supplementary videos) are available to reinforce understanding.