BIO 304 · Human Anatomy & Physiology
Motor Units & Muscle Mechanics
Muscular System · Module 6
A reference for the Motor Units video. Force grows two ways: by recruiting more motor units, and by firing each motor unit faster. Both are happening every time you lift a coffee cup.
How to use this sheet Toggle the toolbar above. Notes prints the full reference for review. Study prints as a fill-in-the-blank worksheet , print it, then write each definition while you watch the video or read your book. Quiz me is on-screen typing practice; type the term, click Reveal to check yourself.
- Define a motor unit and contrast small versus large motor units by function.
- Distinguish twitch, wave summation, unfused tetanus, and fused tetanus.
- Explain isotonic versus isometric contraction and concentric versus eccentric movement.
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Motor Unit Basics
Definition & size
- Motor unitone motor neuron + all muscle fibers it innervates
- Small motor unitfew fibers per neuron · fine control (extraocular ~3-5 fibers)
- Large motor unitthousands of fibers per neuron · gross force (gastrocnemius ~1000+)
Recruitment
- Size principlesmall motor units fire first; larger added as needed
- Recruitmentactivating more motor units to increase force
- All-or-none for one fiberfiber contracts maximally or not at all when stimulated
Fiber types
- Type I (slow oxidative)red, lots of mitochondria, fatigue-resistant · endurance
- Type IIa (fast oxidative)intermediate; aerobic + glycolytic
- Type IIx/b (fast glycolytic)white, anaerobic, fast and powerful, fatigues quickly
Mechanics
Twitch & tetanus
- Twitchresponse to one action potential · latent, contraction, relaxation phases
- Wave summationa second twitch adds onto the first if it arrives early
- Unfused tetanusrapid stimulation; partial relaxation between twitches
- Fused (complete) tetanusso rapid no relaxation; smooth sustained contraction
Length-tension
- Optimal lengthmaximum cross-bridge overlap → maximum force
- Too shortthick filaments collide with Z discs; force drops
- Too longtoo little overlap to form bridges; force drops
Types of contraction
- Isotonicmuscle changes length, load constant (lifting a weight)
- Isometricmuscle generates force without changing length (holding a plank)
- Concentricisotonic shortening (curl up)
- Eccentricisotonic lengthening under load (lower the curl — this is what makes you sore)
Fuel for contraction
- Direct ATPlasts ~2 seconds
- Creatine phosphate~10-15 seconds of fast ATP regen
- Anaerobic glycolysis~30-60 seconds; produces lactate
- Aerobic respirationminutes to hours; needs oxygen and substrate
Step 3 . Retrieval check
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