1. Function is to move and position skeletal muscles and joints to produce movement or force
   1. This is seen through lengthening and shortening!
2. Floating Topic
3. So, how do muscles contract?
   1. Sliding Filament Theory
      1. Cross-Bridges are formed between Thick and Thin Filaments (known as Myosin and Actin, respectively). These cross-bridges occur in the sarcomere, the functional unit of muscle fibers. As the cross-bridges are formed, the sarcomeres are shortened, which collectively shorten the muscle. This can be visually seen in a concentric contraction!
         1. Types of Contractions
            1. Isometric Contractions
            2. Wall Sit
            3. Eccentric Contractions
            4. Extension of Curls
            5. Concentric Contractions
            6. Contraction of Curls
4. So, how do muscles generate force?
   1. M = r x F
5. M = Moment
   1. Also known as torque
6. Fiber Length
   1. The more sarcomeres there are IN A SERIES the more the muscle can shorten
7. So, how do muscles adapt?
   1. Changes of Activity Level
      1. Prolonged Lengthening
         1. Can induce production of additonal sarcomeres
         2. Hypertrophy
         3. Peak contractile force increased
         4. Changes in mRNA to make transition from type II to type I
      2. Prolonged Shortening
         1. Produced from immobilization
            1. Accelerates atrophy
            2. Accelerates loss of sarcomeres
         2. Changes in mRNA to make transition from type I to type II
      3. Aging
         1. Sarcopenia
   2. Sustained Changes
      1. Resistance Training & Strength Training
         1. Results in hypertrophy
         2. Results in increase in force production
         3. Increase in cross-sectional area
            1. Larger increase in type II
      2. Disuse Atrophy
8. F = Contractile Force
   1. Muscle Size
      1. Cross-Sectional Area**
         1. Anatomical CSA
            1. Pennate
            2. Parallel
         2. Physiological CSA
            1. Pennate
            2. Parallel
   2. Muscle Stretch
      1. Muscle Length-Tension Relationship**
         1. 1. Sarcomere shortened, unable to generate contractile tension
            1. High muscle tone
         2. 2. Resting Length
            1. Area where sarcomere can optimally stretch and contract
         3. 3.
            1. Muscle tension decreasing, but passive tension increasing
         4. 4.
            1. Sarcomere fully lengthened
   3. Level of Muscle Fiber Recruitment
      1. Stimuli
         1. Twitch
            1. Singular, low intensity stimulus
         2. Tetanic
            1. Rapid, repetitive stimulus
   4. Contraction Velocity
      1. Change of Muscle Length / Time
         1. Isometric Contractions
            1. No change in length
            2. No velocity, but produce a maximum contractile force
         2. Concentric Contractions
            1. Shortening
            2. Slow shortening produce more force than rapid shortening contractions
         3. Eccentric Contractions
            1. Lengthening
            2. Produce most force
   5. Motor Unit Types
      1. Motor Unit Type
         1. Slow Fibers
            1. 1. Slow Contraction Rate 2. Low Tension Generation 3. Low Fatigability 4. Small alpha motor neuron diameter
         2. Fast Fatigue Resistant
            1. 1. Moderate contraction rate 2. Moderate tension generation 3. Moderate fatigability 4. Moderate sized alpha motor neuron diameter
         3. Fast Fatigable
            1. 1. Fast contraction rate 2. High tension generation 3. High fatigability 4. Large alpha motor neuron diameter
9. r = Muscle's moment arm from point of rotation
   1. Anatomical Muscle Moment Arm
      1. The true moment arm by the sine of the angle of application which can also be estimated and measured directly.
   2. Moment Arm
      1. Perpendicular distance from muscle force to the axis
      2. Moment-Muscle Length Relationship
10. So, what does muscle look like?
    1. Gross Anatomy
       1. Layers
          1. Muscle Type
             1. Pennate: shorter and denser fibers
             2. Unipennate
             3. Bipennate
             4. Multipennate
             5. Parallel: longer fibers
             6. Fusiform
             7. Strap