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1) Latent Period - AP moves through Sarcolemma, causing Ca2+ release from SR

2) Contraction Phase - Ca2+ binds, cross-bridge formation, power stroke, tension builds to peak

3) Relaxation Phase - Ca2+ actively transported back into SR, Ca2+ levels fall, active sites are covered, tension falls to resting levels.

1) Summation - Increasing force of contraction of muscle fibers within muscles

2) Recruitment - Increasing number of muscle fibers contracting.

1) Treppe

2) Wave Summation

3) Incomplete Tetanus

4) Complete Tetanus

Strength of contraction increased by increasing number of Motor Units stimulated.

Small Motor Units activated first, Larger Motor Units recruited as needed.

Some stimulated and held in Tetanus while additional units recruited.

Some motor units are always active, not enough to cause movement. Helps stabilize bone & joints, maintain body temperature. Heightened tone accelerates recruitment during voluntary contraction.

1) Fast Fibers = White Fibers = Fast Twitch

2) Slow Fibers = Red Fibers = Slow Twitch

3) Intermediate Fiber = Fast Twitch Oxidative, Fast Resistance

- Contract (reach peak twitch tension) very quickly, strong contractions

- Majority in body

- Large in diameter

- Densely packed myofibrils

- Large glycogen stores

- Few mitochondria

- Fatigue rapidly, use a lot of ATP

- ½ diameter of fast

- 3X longer to reach peak tension

- More mitochondria, less glycogen

- Specialized to contract for extended periods, slow to fatigue

- Surrounded by extensive capillaries:  O2

- Contain red pigment:  myoglobin
   - Related to hemoglobin
   - Binds oxygen

Mid sized

Most closely resemble fast fibers

Few myoglobin

Extensive capillary network

More resistant to fatigue than fast

Muscle growth from heavy training
   - Increases diameter of muscle fibers
   - Increases number of myofibrils
   - Increases mitochondria, glycogen reserves
   - Occurs in muscles stimulated to near maximal tension

Lack of muscle activity
   - Reduces muscle size, tone and power
   - Fibers smaller and weaker:  proteins broken down and replaced with fibrous tissue (connective tissue)
   - Initially reversible
        > If muscle dies then losses are permanent
        > Thus physical therapy is important for those who are unable to move