Muscle Architecture Quiz

Muscle Architecture

• Muscle architecture can vary, with different types of muscles having different designs (e.g. strap, unipennate, multipennate, fusiform, bipennate)
• Pennate muscles have varying architectures, with long vs. short fascicles and highly pennate vs. parallel fibers
• Muscle injury is more likely to occur at the muscle-tendon junctions

Muscle Architecture Summary

• Long fibers/fascicles allow for large range of motion (ROM), resulting in more work (F x d)
• Long fibers/fascicles have high shortening speeds
• Short fibers have lower metabolic cost (less ATP use)

Fiber Length and O2/ATP

• Longer fibers have more sarcomeres in series, resulting in higher ATP use to generate tension
• Shorter fibers are optimum for low metabolic cost, examples: gastrocnemius and soleus for running

Fiber Length and Speed

• Longer fibers have more sarcomeres in series, resulting in slower shortening velocity
• Examples: hamstrings (biceps femoris, semimembranosus, semitendinosus), vastus lateralis

Fiber Length and Work

• Longer fibers have a greater range of shortening (d), resulting in more work performed
• Examples: gluteus maximus, hamstrings, quadriceps, pectorals, biceps brachii

Muscles

• Muscles are the motors that drive movement around joints, through planes of motion, about axes
• Muscles are incredible, producing huge forces with minimum mass

Muscle Structure - Myosin

• Myosin has a heavy chain (head, neck, and tail) and light chains that influence function
• The head/neck (motor) domain bends to pull on actin
• Motor domain is very short (20 nm), resulting in low gear movement (need many strokes to pull actin)

Muscle Structure - Myosin

• Myosin molecules arrange themselves in a unique way, with tails together and heads at the ends
• Myosin 'walks' along actin to cause muscle contraction (cross-bridge model)

Muscle Structure - Sarcomere

• Actin and myosin are the major constituents of the fundamental unit of muscle: the sarcomere
• Sarcomere has a lattice structure

Muscle Properties

• Myosin heads have to rotate, then detach, recover, and reattach to actin (cross-bridge cycling)
• Faster cycle rate results in less force produced
• Muscle properties will be discussed further in Lecture 6, where current understanding of muscle contraction will be challenged

Muscle Properties - Eccentric

• Eccentric portion of curve: Force and power increase (negatively), but have different shape
• Greater force in eccentric contractions due to:
  • Extra force per cross-bridge as they're lengthened
  • More cross-bridges attached because of higher energy required for detachment
• Cross-bridge model can't predict eccentric force accurately; it over-predicts both force and energy cost