INTRODUCTION TO BIOMECHANICS (BASIC DEFINITIONS) MUST KNOW

Definitions

  • Bio mechanics – application of the principles of mechanics / physics to biological organisms
  • muscle action vs. function
    • action – motions produced by a muscle’s shortening; described in reference to axes and planes of body
      • – determined by architecture of joint and position of muscle around it
      • – pure mechanics which can be inferred
    • function – how the organism chooses to use a muscle
      • – can involve positive, negative or non- work (see below)
  • agonists (Gr., contestant)– muscles with an identical action; usually restricted to single axis or plane of reference
  • antagonists (Gr., against + contestant; lit. enemy) – muscles with opposite action; usually restricted to single axis or plane of reference
  • synergist (Gr., together + work) – muscles which act together to perform a function; can involve both agonists and antagonists
  • work (W) – the mechanical definition
    • – occurs when a force moves its point of application
    • – e.g., muscles work by moving myosin heads along actin filament
    • – thus W = (F) (X), where
      • F= force (newtons; N)
      • X = distance moved (meters; m); can be positive or negative relative to line of action of force
    • – work is measured in joules [J = (N)(m)]
    • – muscles generate tension to perform positive, negative or non- work
      • – positive work – muscle shortens while generating tension (i.e., X > 0)
      • – negative work – muscle lengthens while generating tension (i.e., X < O)
      • – non-work – muscle generates tension without changing length (X=0)

Some Physiology (gag!)

  • isometric and isotonic contractions
    • – limited to experimental conditions in which mechanical properties (either tension or change in length) of muscle are measured; i.e., not possible in vivo
    • isometric contraction – muscle length is fixed and tension is measured
      • – used to generate length/tension curves (see below)
    • isotonic contraction – muscle tension (load) is fixed and change in length (shortening) measured
      • – used to generate force (tension)/velocity curves (see below)
  • sliding filament theory and length/tension curve (see Figure 1)
    • – theory proposed by biophysicist Jean Hanson (1919-73) and physiologist Hugh Esmor Huxley (1924- ) in 1954
    • – states that during contraction thin filaments slide past thick filaments with no change in the length of either type of filament
    • – force for producing sliding of thin filaments is generated by the cross-bridges (formed by myosin heads)
    • – theory predicts that force output will be proportional to the degree of overlap between thick and thin filaments or, more specifically, the number of cross-bridges formed
  • biomechanical implications of the sliding filament theory:
    • 1.  for maximum force output (total tension) muscle should be positioned below its optimal length so that work (either positive or negative) will occur over peak of length/tension curve
    • 2.   muscles which produce the same action across a joint are typically arranged such that their optimal lengths occur at different joint positions thus permitting a nearly constant level of force output at all joint positions
  • velocity-force curves (see Figure 2)
    • – generated from series of isotonic contractions
    • – force and velocity are inversely related such that at zero (0) velocity maximum force is generated, and at maximum velocity zero (0) force is generated
    • – power output = force x velocity (rate of doing work)
      • – measured in watts (1N x 1m/s)
      • – is maximized at about 30% of maximum force

Preliminary concepts

  • 1.   Force output is proportional to cross-sectional area (see Figure 3)
    • – specifically F = total CSA x Specific Tension of muscle (N/cm2)
    • – thus muscles that differ in length but have equal CSA generate equal amounts of force
  • 2.   Excursion (distance a muscle can shorten) is proportional to fiber length (see Figure 4)
    • – maximum sarcomere excursion = 50% of resting length
    • – thus longer fibers will contract a greater distance
  • 3.  Velocity (distance of shortening/unit of time) is proportional to fiber length (assuming equal load)
    • – muscles of different fiber length will contract to 50% in same amount of time
    • – since excursions distances differ but time is constant, velocity is greater in muscles with longer fibers

Muscle Architecture

  • Muscle architecture refers to arrangement and length of muscle fibers w/i a muscle
    • – variation in muscle architecture can affect:
      • (1) excursion (distance a muscle can contract)
      • (2) velocity
      • (3) force, and
      • (4) line of action
    • – variety of classification schemes exist; none perfect (except mine); many primarily descriptive
    • – functionally 3 general types: parallel, triangular and pinnate based on fiber arrangement (see Figure 5)
      • 1) triangular – muscle fibers radially arranged
        • -specialized for altering line of action assuming non-uniform distribution of motor units
      • 2) parallel – muscle fibers are arranged parallel to line of action (muscle pull)
        • – specialized for excursion and/or velocity
      • 3) pinnate – muscle fibers lie at an angle to line of action (muscle pull)
        • – specialized for force production
      • N.B. Relationship between angle of pinnation (parallel fibers have an angle of pinnation = 0 degrees), fiber length and excursion is not simple; in fact in some situations pinnation actually can increase excursion
  • Advantage of pinnation / Disadvantage of parallel (see Figure 6)
    • – maximum force produced by a muscle is proportional to the sum of the cross-section of all its fibers
    • – for muscles of equal volume, more muscle fibers can be packed into a pinnate arrangement than a parallel arrangement
    • – since axis of contraction of muscle fibers not parallel to pull of muscle (line of action) some muscle force dissipated perpendicular to line of action
    • – thus force output = # of fibers x cosine of angle of insertion
    • – thus advantage of pinnation is to increase force output of a muscle by packing more fibers in a given volume of space
  • Cost of pinnation / Advantage of parallel (see Figure 7)
    • – excursion = length a muscle fiber can contract; function of fiber length
    • – for muscles of equal length, pinnate muscles have decreased excursion relative to parallel
    • – max. sarcomere shortening = 50% of resting length; thus max. excursion of muscle = 50% of fiber length
    • – parallel fibers can shorten to their maximum
    • – pinnate fibers cannot shorten to their maximum w/o dislodging themselves from their tendons
    • – thus pinnate muscle has shorter excursion

Lever mechanics

  • Muscles generate forces and skeletal elements apply these forces and thus serve a machines
    • – machine – device for transmitting forces from one point to another
    • – majority (but not all) of skeletal elements function as type of machine known as lever
    • – lever is a rigid bar (regardless of shape) which rotates about a fixed point (fulcrum)
    • – in levers forces work by creating rotational forces about the joints (fulcrum) known as moments; i.e.,
      • m = F x L, where
        • m = moment or torque
        • F = force; in this case muscle tension
        • L = Lever (or moment) arm; distance between force and fulcrum; lies perpendicular to line of action of force
  • Lever systems are most easily analyzed under the conditions of equilibrium (see Figure 8)
    • Force equilibrium: Fi x Li (in-torque) = Fo x Lo (out-torque)
      • – solving for Fo:
        • Fo = Fi x (Li/Lo)
      • – thus to maximize force-output of a lever system for a given muscle force (Fi):
        • 1) increase Li
        • 2) decrease Lo
      • – Li/Lo = lever advantage
    • Velocity equilibrium: Vo x Li = Vi x Lo
      • – solving for Vo
        • Vo = Vi x (Lo/Li)
      • – thus to maximize velocity-output of a lever system for a given muscle velocity (Vi):
        • 1) decrease Li
        • 2) increase Lo
      • – Lo/Li = gear ratio
  • Note that for a given muscle input (Fi) a muscle lever system can either:
      • a) maximize lever advantage (Li/Lo) and produce a stronger but slower force (Fo)
      • b) maximize gear ratio (Lo/Li) and produce a faster but weaker force (Vo)
    • – it cannot maximize both (inverse relationship)
    • – thus, there is a trade off between velocity and force in any lever system

Muscle fiber types

  • Quality of force production can be varied by using different types of muscle fibers
    • – vertebrate muscles can be broadly divided into slow and fast based upon speed of contraction
    • – slow fibers – specialized for prolonged tension generation
      • – typically generate small forces (due to small fiber CSA and low innervation ratio) at low metabolic cost (aerobic respiration)
      • – fatigue resistant due to high density of mitochondria and myoglobin
      • – 2 subtypes
        • 1) tonic – multi-terminal fibers; membrane cannot propagate an AP thus contraction is graded; limited to extra-ocular muscles in mammals
        • 2) slow twitch – single terminal fibers; widely distributed
    • – fast fibers – specialized for generating tension rapidly
      • – typically generate larger forces (due to larger fiber CSA and high innervation ratio) at high metabolic cost (use both aerobic and anaerobic respiration)
      • – different sub-types (2A, 2B, 2X) differ in myosin isoforms and fatigue resistance
  • Majority of muscles are of mixed fiber type composition being a combination of fast and slow fibers occurring in two arrangements
      • 1) mosaic – fast and slow fibers uniformly distributed
      • 2) compartmentalized – fiber types non-uniformly distributed into intramuscular compartments
    • – however, some muscles which are used for repetitive or constant tasks (e.g., posture) can be comprised nearly entirely of slow fibers
      • – e.g., soleu

      THANKS TO FLORIDA INTERNATIONAL UNIVERSITY

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