Structural proteins/skeletal muscles are important:

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

Collagen & elastin

structural support & elasticity to the muscle

Skeletal Muscle Types

• Epimysium
• Perimysium
• Endomysium

Epimysium

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

Perimysium

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

Endomysium

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

Muscle Morphology

Fusiform & pennate are most common

Physiologic Cross-sectional Area (PCSA)

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

• PCSA EQUATION
• More PCSA =

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

Pennation Angle

• 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*

Generating Force: Passive Length-Tension
Curve

• Generating Force: Passive Length-Tension Curve
• Stretching + spring-like resistance

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

Muscles during Passive Length-Tension Curve

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

Passive Length-Tension Curve: Elastin

• 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

Generating Force: Active Length-Tension

• Requires stimulation from nervous system
• Sliding filament theory

Generating Force: Active Length-Tension (Sliding Filament Theory)

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!

Sliding Filament Theory (sarcomere)

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

The TOTAL length-tension relationship

Internal Torque-Joint Angle Curve

• Elbow
• Hip

Internal Torque-Joint Angle Curve (muscle)

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

Force-Velocity Curve

Force Velocity Curve: 3 contractions

1. concentric
2. eccentric
3. isometric

Concentric (FVC):

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

Eccentric(FVC):

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

Isometric(FVC):

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

Max effort concentric contraction (FVC):

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

Max effort eccentric contraction(FVC)

directly proportional to the velocity of muscle lengthening

Eccentric force production >> concentric
force production

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

Power equation

Work/time

Alpha Motor Neurons

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

motor unit

alpha motor neuron and all of the muscle fibers it
innervates

Motor unit recruitment

• 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

Henneman’s Size Principle

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

Small motor units

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

Large motor units

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

Fiber typing

Rate coding // Neural Drive

• 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)

Choosing between recruitment & rate-coding: eccentric activation:

High force per crossbridge ==> less motor units required

Choosing between recruitment & rate-coding: Concentric activation

Lower force per crossbridge ==> more motor units required

Choosing between recruitment & rate-coding: Rapid task

Higher rate coding

Electromyography (for muscle recruitment)

• 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)

The Law of Parsimony

• 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