AG

Lec 19

Muscle types and performance

  1. Muscle contractions

    1. Concentric contractions

      1. Muscle is actively shortening

    2. Eccentric contractions

      1. Muscle is actively lengthening

        1. Ex. quadriceps active just after heel strikes ground while knee flexes

        2. Ex. arm flexors active to control fall of object

    3. Isometric contractions

      1. Muscle actively held at fixed length

    4. Passive stretch

      1. Muscle passively lengthening

        1. Ex. hamstrings stretched when bending over (passive tensions)

        2. Muscle activation NOT required

        3. Very little energy required

        4. Boback does wall sit

  2. Control of force production

    1. Recruitment of sufficient motor units

    2. Stimulation frequency (# of stimulations per second)

  3. Rate of stimulus determines force of contraction


  1. Axon terminal- muscle cell interface (aka motor end plates) can be repaired after injury/ pathology

  2. Low level tension in muscles

    1. Muscles can produce low level tension at rest to maintain posture

      1. Neck, trunk (muscles along outer part of spinal cord), limbs

    2. Proprioception

      1. Ability of CNS to sense relative position of the body

      2. Used in physical therapy to retrain balance after injury- may be necessary to repair neuromuscular connections

    3. Tai Chi 

      1.  slow movements enhance proprioceptive abilities in mind/body integration

  3. Training effects

    1. Atrophy- disuse of muscle

      1. Muscle fibers (cells) shrink in diameter

      2. Actin and myosin are actively removed

    2. Hypertrophy- overload training

      1. Actually damage proteins- respond by building more tissue 

      2. Has a finite limit

        1. exception= anabolic steroids. Trick your body into minimizing catabolic action and maximizing anabolic actions

    3. Greg valentino

      1. Minimize catabolic effects by inhibiting cortisol (hydrocortisone) produced naturally by the adrenal gland 

    4. Hyperplasia- increase in cell number

      1. Humans typically dont do this as adults

        1. Exception injection of IGF/Human growth hormone

      2. Birds have been shown to exhibit hyperplasia in pectoralis muscle

    5. Belgium blue cattle

      1. Have a mutation in their myostatin gene which normally limits muscle growth

      2. Physical appearance is due to hyperplasia

    6. Cross section of leg muscles (slow twitch vs fast twitch)

Muscle fiber types

  1. Fast twitch, glycolytic (white color)

    1. Lots of force in small amount of time

    2. Responds to weight training

    3. Greatest force production (of all 3)

    4. Anaerobic(without oxygen) fiber type- not many mitochondria giving it light color

    5. Large fatigue factor- but recovers easily

  2. Slow twitch, oxidative (red color bc lots of mitochondria, uses oxygen in electron transport chain to produce atp in a cell)

    1. Least amount of force

    2. Can sustain contraction over long time

    3. Highly anaerobic- tons of mitochondria and contain lots of myoglobin

    4. Fatigues slowly- days to weeks to recover

  3. Fast twitch, oxidative

    1. Most of us have this as most of our fibers 

    2. Moderate force production

    3. Relatively aerobic- many mitochondria 

    4. Moderate fatigue resistance, relatively fast recovery 

  4. Muscle fiber types determined by genetics

    1. You can alter the properties of muscle fibers through training

      1. Fast twitch oxidative fibers can become more oxidative with endurance training

    2. Heritability (h2) of muscle fiber type is >50% (fairly heritable)

Sprinters have fast twitch oxidation