1. EXCITATION CONTRACTION COUPLING

2. Summary of Mechanics
1 Muscles pull; they don’t push
2 Muscle lengthen by being yanked on by antagonists or gravity
3 Muscle force can be graded by recruitment of motor units
4 You activate small motor units first: the size principle
5 Muscle force can be graded by repetitive stimulation
6 Muscle force can be graded by changing length, but who cares
7 Muscle velocity is inversely related to muscle force: you can
be strong or fast but not both at the same time
8 Muscle power peaks at 1/3 maximum force
9 Pinnate muscle fibers develop more force at lower velocity
because of the angle
10 Muscles fatigue: they drop force on continued use
11 Muscles are heterogeneous based on contractile properties
Slow twitch (S)
Fast fatigue resistant (FR)
Fast Intermediate (FI)
Fast fatiguable (FF)

3. Summary of contractile mechanisms
1 Muscle cells are highly organized
2 Myofibrils consists of interdigitating hexagonal arrays of filaments
3 Thick filaments are mainly myosin: A bands
4 Thin filaments are actin + tropomyosin + TnI + TnC + TnT: I bands
5 Sliding filaments explains the length-tension curve
6 Cross-Bridge cycling couples ATP hydrolysis to force or shortening
7 Myosin isoforms have different turnover numbers
8 Muscles can be classified by their myosin isoforms
Type I (slow twitch)
Type IIa (fast twitch oxidative)
Type Iib (fast twitch glycolytic)
9 Costameres may transmit force from myofilaments to muscle
exterior through the cytoskeleton

4. The Ca transient trails the muscle action potential and precedes force development

5. SR release and uptake activities are separate

6. SR Ca release produces a Ca transient and A-M activation

7. SR Ca reuptake shuts down A-M interaction

8. The Ca transient is a switch for A-M activity

9. Tropomyosin spreads TnC control of A-M interaction along the actin filament

10. Occupancy of TnC by Ca trails the Ca transient but precedes force development

11. The series-elastic model explains twitch time and tetany

12. Summation of Ca transients allows force to catch up with TnC-Ca saturation