Unit 4 Part 1 Notes


Unit 4 Part 1 (Chapter 6)


The Muscular System




  • 40 – 50% of body weight (120 lbs. of weight = 50-60 lbs. muscle)

    • Height Weight Chart
      • Not a good indicator for active and highly active individuals
    • BMI Calculator
      • Solid indicator for body fitness except for maybe extreme weightlifters
  • Movement occurs when muscles shorten (contract)



I.  Movements Produced by Contractions


  • Flexion – Closing the angle at a joint.

  • Extension – Opening the angle at a joint.

  • Rotation – Movement around a longitudinal axis.


  • Abduction – Moving a body part away from the midline.

  • Adduction – Moving a body part toward the midline.

  • Circumduction – Circular movement allowed at a ball and socket.

  • Supination – Palm up position.

  • Pronation – Palm down position.

  • Dorsiflexion – Toes brought upward.

  • Plantar flexion – Toes brought downward.


  • Inversion --  Turning the foot medially (sole turned inwards)

  • Eversion --  Turning the foot laterally (sole turned outwards)





II. Muscle Tissue


  • Made of muscle fibers (myofibrils) and filaments (actin and myosin)



1) Skeletal



        - Striated (Has dark bands)

        - Voluntary

        - Attached to bones

 - Multinucleate


2) Cardiac


        - Striated

        - Involuntary

        - Found only in heart

 - Intercalated Discs

- Uni-nucleate

- Does not get tired



3) Smooth (Visceral)


        - Non-striated

        - Involuntary

- Uni-nucleate

- Peristalsis

        - Found in hollow organs

            a.  Blood vessels

            b.  Intestines

            c.  Bronchioles


III.  Structure of Skeletal Muscle


  • Extend from one bone to another.

  • Origin - Attachment site to stationary bone.

  • Insertion – Attachment site to moveable bone.

  • Body – Bulk of muscle tissue.

  • Tendons secure muscles to bone.


IV. Microscopic Structure


  • Consist of fibers


1) Thin myofilaments – Formed by the protein actin. (Fast twitch) (green fibers in animation)

2) Thick myofilaments – Formed by the protein myosin. (Slow twitch)

  • Sarcomere – Basic unit of contraction in muscle fiber. (purple fibers in animation below.)


V.  Functions of Skeletal Muscle


1) Movement


  • Move by pulling on bones.

  • Movement occurs when groups of muscles work together by contracting with others relaxing.

  • Prime Mover – Primary muscle involved in movement.

  • Synergist – Assists in movement.

  • Antagonist – Muscles which relax.

    • Ex. Flexing upper arm

    - Prime mover – Biceps

    - Synergist – Brachialis

    -Antagonist - Triceps


2) Posture


        - Done by tonic contractions (Muscle tone)


3) Heat Production


       - Contraction of muscle fibers produces heat through breakdown of ATP.


  • Hypothermia – Decrease of body temperature.




VI.  Motor Unit



  • Contractions occur only after stimulation by nerve impulses.

  • Motor neurons – Nerve cells that stimulate muscle fibers.

  • Chemicals (neurotransmitters) are released by motor neuron at neuromuscular junction (area where muscle and nerve comes together).


VII.  Muscle Stimulus


  • Threshold Stimulus / Action Potential - Minimal level of stimulation needed to cause a muscle fiber to contract.

  • Respond fully or not at all. “All or None

  • However, number of muscle fibers contracting can be executed to varying degrees

    • This is why you can bend body parts at different degrees and angles


VIII.  Types of Skeletal Muscle Contractions


1) Twitch – Quick, jerky response to a stimulus.


2) Tetanic – Sustained and steady twitch (just a few areas of muscle undergo contractions).


3) Isotonic – Normal muscle contractions at a joint.


4) Isometric --- Contraction that does not always produce movement.


          - Increased tension which, when repeated, increases muscle tone and strength.


IX.  Role of Other Body Systems in Movement


  • Muscle functioning depends on proper functioning of other systems.

  • Disorders of nervous system can result in paralysis,

    • Tourette’s Syndrome, Parkinson’s Disease, and Huntington's Disease.

  • Disorders of skeletal system can result in disabling effects on muscular system.        

    • Ex. Arthritis, Rickets, and Osteomalcia


X.  Fatigue


  • Results from over-stimulation of muscle cells without adequate rest.


1) During exercise, stored ATP required for muscle contractions is depleted.

2) Formation of additional ATP results in rapid consumption of oxygen and nutrients.

3) Body then switches to lactic acid fermentation to obtain energy. (Burning of sugars in muscle tissue)

4) Lactic acid accumulation results in soreness/cramping.

5) Oxygen debt (labored breathing) occurs which removes excess lactic acid. 


XI.  Aerobic Respiration


  • C6H12O6 + 6O2 ------->  6H2O + 6CO2 + 36-38 ATP


  • Glucose + Oxygen ------>  Water + Carbon Dioxide + energy molecules


  • Respiration occurs when muscles have enough oxygen to break down glucose to make lots of ATP energy molecules.

  • Major source of “fuel” for the body

  • Occurs in the mitochondria

  • This is why muscle cells have high amounts of mitochondria inside of them


XII. Anaerobic Respiration


  • C6H12O6  ------->  Lactic Acid + 4 ATP

  • Glucose + no oxygen ------------>  Lactic Acid + few ATP


  • Respiration that occurs in the absence of oxygen.

  • Very little energy is made

  • Lactic Acid is made as a byproduct

    • Builds up in the muscles and causes cramping

    • You breathe harder when you exercise to try and get rid of the lactic acid in your muscles.  It’s your body’s way of telling you to slow down or stop


XIII.  Effects of Exercise


  • Properly balanced exercise and nutrition leads to improved muscle tone, better posture, more efficient heart and lung, higher threshold for fatigue, and better self esteem.

  • Disuse Atrophy – Shrinkage of muscle due to prolonged inactivity.

  • Hypertrophy – Increased muscle mass due to increased use.

    • Strength training (contracting muscles against heavy resistance) enhances hypertrophy.

  • Endurance (Aerobic) training – Does not result in hypertrophy but increases ability of muscles to contract over extended period of time               

    • (increased number of blood vessels -> increased blood flow -> more oxygen and nutrients).

Boston Marathon Winner:  Robert Cheruiyot