Muscle Force

• Load


  Resistance to a muscle (i.e., what the muscle is pushing, pulling, lifting, etc.)
• Muscle tension


  Force generated by contraction of muscle

  
• Muscle tension may or may not result in change muscle length (e.g., isotonic vs. isometric contractions)

  
• Muscle tone


  Partial state of contraction of resting muscles

  
• Length-tension relationship


  • Tension generated by a muscle depends on how stretched out or contracted it was at the start (at rest)
  • Fiber extremely contracted à no room for filaments to slide
  • Fiber extremely stretched out à difficult for myosin heads to "get a grip" on actin
  • Few cross bridges able to form

  
• Muscle Twitch


  • One contraction-relaxation cycle to a SINGLE stimulus
  • Lasts < 0.1 s
  • Myogram – measures timing & strength of twitch

  
• Latent Period


  • Delay (~2 ms) between stimulus and visible contraction
  • Excitation (APs generated)
  • Excitation-contraction coupling: Ca2+ released & binds to troponin; tropomyosin moves, exposing active sites on thin filament
• Contraction Period


  1. Muscle acts on resisting load

  2. Tension develops & increases

  3. Contraction – sarcomeres contract

  
• Relaxation Period


  1. Return to baseline tension

  2. Relaxation – Ca2+ pumped back into SR

  3. Actin active sites re-covered by tropomyosin

  
• Slow-twitch fibers (Type I)

  • Slow to twitch
  • SR slow to release & reabsorb Ca2+
  • Myosin hydrolyzes ATP slowly
  • Fatigue-resistant

  
• Fast-twitch fibers (Type II)

  • Fast to twitch
  • Extensive SR releases & reabsorbs Ca2+ quickly
  • Myosin rapidly hydrolyzes ATP
  • Fatigue quickly

  
• Factors that affect twitch strength

  • Intensity of stimulus
  • Frequency of stimulus

  
• Increasing stimulus intensity

  1. Multiple motor unit summation/recruitment

  2. As stimulus intensity increases à more motor units recruited à twitch strength increases

  3. Asynchronous contraction occurs among different motor units

  4. Smallest units/fibers recruited first, followed by medium, then large

  
• Increasing stimulus frequency

  1. Temporal/wave summation

  2. If 2+ stimuli arrive close together à new stimulus arrives before previous twitch ended

  3. 2nd twitch "piggybacks" on previous twitch à increased tension

  4. Muscle fiber does not have time to completely relax

  5. Ca2+ still available (not enough time for it to re-enter SR) à active sites remain exposed

  
• Isotonic contraction


  • Generates tension, muscle length changes
  • Concentric – muscle shortens
  • Eccentric – muscle lengthens (but still contracting)

  
• Isometric contraction


  • Generates tension, but no change in muscle length
  • E.g., holding weight mid-air with no joint flexion/extension
  • Important in posture, stability

  
• Whole Muscle Strength

  • Muscle size – increases by adding more myofibrils (more tension possible)
  • Size of active motor units
  • Number of motor units recruited (multiple motor unit summation)
  • Stimulus frequency (temporal/wave summation)
  • Length-tension relationship