Biomechanics Exam #2 (Muscle Phy.) Flashcards

individual cell with multiple nuclei

Fundamental unit within each muscle fiber

shorten the muscle & generate force

provide supporting structure for contractile proteins

• Generate passive tension when stretched
• Provide internal & external support/alignment for the muscle fiber
• Assist in force-transfer

structural support & elasticity to the muscle

• Epimysium
• Perimysium
• Endomysium

tough outer covering that separates the muscle belly from other structures in the body

divides the muscle into fascicles & allows room for blood vessels & nerves to travel

• surrounds individual muscle fibers.
• Helps convey force of contraction
• Shows sight of metabolic exchange between fibers and capillaries

Fusiform & pennate are most common

• Amount of active proteins available to generate a contraction force
• cut perpendicular through the muscle belly, divide muscle volume/muscle length

• PCSA = V/L
• more force generating capacity + more/high pennation (assuming similar morphology)

• Angle of orientation between muscle fibers & tendon
• 0 degrees = 100% of force transfer; 30 degrees = 86%
• *most human muscle has pennation angles between 0 & 30 degrees*

• As you stretch a muscle passively, both the tendon (series) and extracellular CT (parallel) generate a spring-like resistance

• Muscles lose ability to contract due to lack of overlap of actin/myosin.
• passive resistance still provides force to stabilize joints

• elasticity also allows some level of energy storage
• Muscles also demonstrate viscoelastic properties
• The higher the velocity of stretch = higher the passive stiffness
• Elastin+KE+Muscle contraction = we can jump+sprint

• Requires stimulation from nervous system
• Sliding filament theory

• amt of active force depends (in part) on the length of the muscle at that instant • (tells us how much crossbridging is occurring)
• ideal resting length =length that allows the most crossbridging!

• As a sarcomere is lengthened/shortened (past resting length)
• => less potential crossbridges=> less potential force generation

• Elbow
• Hip

• Different for each muscle group
• Muscle length & muscle moment arm are constantly changing!

1. concentric
2. eccentric
3. isometric

• Internal (muscle) torque > external (load) torque to produce
  movement.
• Muscle length shortens

• muscles are driven by the nervous system, internal torque < external torque
• The muscles are forced into lengthened positions

• length of muscle remains the same.
• Internal torque = external
  torque.

• muscle force is inversely
  proportional to velocity of muscle shortening
• Limited by speed of crossbridging

directly proportional to the velocity of muscle lengthening

1. A greater average force per crossbridge, because they are pulled apart
2. A more rapid reattachment of crossbridge formation
3. Passive tension produced by viscoelastic nature of the stretched series & parallel elastic element
4. —-—
5. the metabolic costs & EMG activity are less
6. Because eccentric contractions (assume force is equal) require less
   muscle fibers

Work/time

• in ventral spinal cord
• can be stimulated by multiple inputs, but
  primarily the decending cortical neurons (messages from your brain)

alpha motor neuron and all of the muscle fibers it
innervates

• Motor units are recruited according to the amount of force required
  for the task
• Innervation ratio: Depending on the muscle, each alpha motor neuron may innervate between 5-2000 muscle fibers

• Smaller motor neurons are recruited before bigger
  motor neurons
  • allows for smooth & controlled increments in
    force development

• longer twitches (hence “slow fibers”), hand/eye
• lower force,
• more fatigue-resistant

• short twitches, thigh/hip
• higher force output
• easily-fatiguable

• force produced by a muscle is dependent
  on subsequent sequential action potentials
  • “how hard the CNS is driving a muscle”
• Need more force produced = CNS sends more signals
• No relaxation btn twitches = fused tentany
• dependent upon the length of the twitch! (slow MU = tentany at lower freq)

High force per crossbridge ==> less motor units required

Lower force per crossbridge ==> more motor units required

Higher rate coding

• Measuring the sum of the change in voltage from all action potentials sent to an activated muscle
• indwelling (fine wires into the muscle) or surface
  (electrodes on skin)

• nervous system solving movement problems
  • Force, joint angles, goals, motivation, fears, etc.
• nervous system activates the fewest muscles or muscle fibers possible for joint action control
  • Henneman’s Size Principle
  • Rate Coding
  • Energy Efficiency