1. The 2 Parts of the Muscle Belly
   1. Muscle Fibers
      1. 1 muscle fiber contains: Several nuclei Several myofibrils
         1. Myofibrils are made up of: Sarcomeres.
            1. Sarcomeres are made up of: Actin filaments and myosin filaments that slide together and apart to create basic muscle contraction.
   2. Connective tissue
      1. There are 3 layers of connective tissue:
         1. Epimysium--surrounds entire muscle
            1. Perimysium--surrounds muscle fascicles
            2. Endomysium--surrounds individual muscle fibers
2. But how do muscle fibers actually contract?
   1. Cross bridges are formed between the thin actin myofilaments and thick myosin myofilaments, causing the myofilaments to slide between one another. This happens when an electrical signal comes from the motor neuron associated with the muscle and causes depolarization in the muscle fiber, which releases calcium. The excess calcium binds with troponin and this signals the actin and myosin to bond.
      1. Relaxed: lengthened sarcomere length. Contracted: shortened sarcomere length
         1. number of cross-bridges formed depends on abundance of actin and myosin and frequency of stimulus
3. Different Types of Muscle Architecure: Parallel and Pennate
   1. Parallel Muscle Architecure
      1. Parallel muscles are always composed of relatively long fibers that attach to tendons at the ends of the muscles.
         1. Subtypes of Parallel:
            1. Strap: parallel arch. with less prominent tendons, so less prominent taper at ends. Ex.--sartorius
            2. Fusiform: parallel arch. with more prominent tendons at each end, so very prominent tapers.
   2. Pennate Muscle Architecture
      1. Pennate muscles are always composed of relatively short fibers that insert obliquely into a tendon running throughout the muscle.
         1. Subtypes of Pennate:
            1. Unipennate: pennate arch. in which one tendon penetrates the muscle.
            2. Bipennate: pennate arch. in which two tendons penetrate the muscle.
            3. Multipennate: pennate arch. in which three or more tendons penetrate the muscle.
4. Compare and Contrast: Traits of Parallel and Pennate Muscles
   1. Amount of contraction possible
      1. Parallel muscles can shorten much more than pennate muscles because longer muscle fibers create greater contraction compared to total muscle size.
   2. Angle at which muscle fibers run
      1. Parallel fibers run parallel to the longitudinal muscle axis, while pennate fibers run oblique to the longitudinal muscle axis because they insert into the tendon within the pennate muscle.
   3. Length of muscle fibers
      1. Parallel muscles have much longer fibers than pennate muscles do, because parallel fibers run the entire length of the muscle to attach to tendons at each end.
   4. Size of joint excursion
      1. Parallel muscles can usually create a larger joint excursion than a pennate muscle can because of their longer fibers. However, this also depends on the muscle's moment arm.
   5. PCSA
      1. PCSA=physiological cross-section area. Pennate muscles have more fibers and a larger PCSA than parallel muscles, and this makes them capable of generating larger forces than parallel muscles of same size.
5. Muscle strength is measured by the equation: M=r x F
   1. M=the moment generated
   2. r=the muscle's moment arm
   3. F=the muscle's contractile force
6. 5 factors that influence a muscle's contractile force (F)
   1. Muscle size.
      1. The more fibers, the larger maximum force of contraction.
      2. This is because of the muscle's PCSA (physiological cross-sectional area)
      3. PCSA responds to changes in activity level overtime
   2. Stretch of the muscle.
      1. DEFINITION: whether the muscle is shorter or longer than its optimal length.
      2. The muscle's length is influenced by its elastic connective components.
      3. The muscles length influences the interaction of sarcomeres.
      4. A muscle that is stretched too long or too short cannot contract effectively.
   3. Level of muscle fiber recruitment.
      1. DEFINITION: how many muscle motor units are active.
      2. Electromyogram (EMG) reflects the number of active motor units and frequency of their activity. As muscle's mechanical advantage increases, EMG decreases
      3. In isometric (neither shortening, nor lengthening) contractions, number of motor units active is directly proportional to the force generated.
      4. In con/eccentric contractions, the effect of number of motor units active depends on the mechanical advantage of that specific muscle.
   4. Contraction velocity
      1. DEFINITION: how fast the muscle contracts.
      2. In concentric (shortening) contractions--speed of contraction is inversely proportional to force.
      3. In eccentric (lengthening) contractions--speed of contraction is directly proportional to force. Produce more F than isometric and concentric. Increases with the speed of contraction
   5. Motor unit types composing the muscle.
      1. Type I motor units--slow twitch.
         1. Generate less force, but do not fatigue.
      2. Type IIa motor units--moderate twitch.
         1. Generate faster, tire more easily.
      3. Type IIb motor units--fast twitch.
         1. Generate fastest, tire most easily.
7. Relationship of moment arm to muscle moment generated
   1. The length of the moment arm is directly proportional to the moment generated.
      1. Greatest moment arm--when muscle's angle of application is 90 degrees.
      2. Least moment arm--when muscle is at its most lengthened position.
8. 5 things that alter muscle function over time
   1. Prolonged lengthening
      1. Effect: protein synthesis and production of additional sarcomeres, creating hypertrophy (muscle growth).
   2. Prolonged shortening
      1. Effect: loss of sarcomeres, creating atrophy (muscle loss). Transition toward type II muscle fibers
   3. Resistance exercise
      1. Effect: hypertrophy, increase in cross-sectional area of muscles, protein synthesis showing type II fibers transforming to type I.
   4. Decreased activity
      1. Effect: atrophy, reduction in CSA of muscles, type I fibers begin to transfor to type II.
   5. Aging
      1. Effect: loss in total strength, fewer type II fibers, increased atrophy.
9. Organization of Muscle From Oatis Chapter 4
10. Illustrations of Muscle Types
11. Relationship of moment arm to joint excursion
    1. The length of the muscle's moment arm and the angular joint excursion are inversely proportional.
       1. Example: contraction of a muscle with a shorter moment arm produces a larger angular excursion than the same amount of contraction in a muscle with a longer moment arm.
12. IMPORTANT DEFINITIONS TO NOTE FIRST
    1. Moment arm of muscle: the perpendicular distance between the muscle and the point of rotation.
    2. Angle of application: angle between the line of pull of a muscle and the limb to which the muscle attaches.
    3. Anatomical moment arm of muscle: the distance along the bone between the muscle's attachment and the center of rotation of the joint. Related to true moment arm by the sine of the angle of application.
13. from human anatomy diagrams
14. Factors that Influence Muscle's Ability to Produce Motion
    1. length of fibers composing the muscle
       1. function of the architecture
    2. length of muscle's moment arm
15. Comparison of eccentric, isometric and concentric muscle strengths with elbow positions from Oatis, Chapter 4