Muscle Force

• Isometric: muscle remains same length
• Isometric contractions:

  • constant number of cross bridges formed
  • partial or maximal contraction
  • muscle and antagonist contract with equal force
  • Belly: shortens
  • Tendon: lengthens
  • MTU: same
• Concentric: muscle shortens
• Concentric contractions:

  • cross bridges cycling as muscle shortens
  • contraction velocity is positive
  • Belly: shortens
  • Tendon: lengthens
  • MTU: same
• Eccentric: muscle lengthens
• Eccentric contractions:

  • passive structures help increase force
  • Contraction velocity is negative
  • Belly: lengthens
  • Tendon: lengthens
  • MTU: lengthens
• Fascia within muscles is made of collagen
• Passive tension is not affected by velocity
• Force developed in muscle = sum of force in all sarcomeres + force produced in passive structures of muscle
• Force Production:

  A) Muscle

  B) Sarcomere

  C) Tendon

  
• 4 determinates of muscle force production:

  1. Length of muscle
  2. Contraction velocity
  3. Level of activation
  4. Time since onset of activity
• Force-Length Relationship

  • More muscle is stretched beyond resting length, generates more force in passive elastic structures
  • maximum force in sarcomere occurs when maximum number of cross bridges are formed
  • Longer sarcomere means more cross bridges
  • Most force when myosin and actin overlap without actin also overlapping

  
• Force-Activation Relationship

  • activation modulated by number of active motor units
  • activation modulated by motor neuron firing rate
  • as activation increases, level of force increases
• Force-Time Relationship

  • force increases over time until peak is reached
  • smaller motor units are recruited first at lower activation level
  • Type I fibres usually recruited first
• Force-Velocity Relationship

  A) Isometric

  B) Eccentric

  C) Concentric

  
• Force-Velocity Relationship

  • Isometric
  • near maximal contraction
  • no resistance to contraction
  • Concentric
  • As resistance to shortening increases, speed of shortening decreases
  • Time needed for cross bridges to form
  • Eccentric
  • if load exceeds force generated, muscle will lengthen and more force is produced
  • elastic components help release more force
• Force-Length-Velocity relationship: all work in parallel for anything other than isometric contractions