G. Homeostasis – Muscle contraction is an important homeostatic device 1. Oxygen debt – During exercise blood vessels dilate and blood flow increases a. If the activity is too great and the oxygen supply is limited then pyruvic acid cannot enter Krebs b. Pyruvic acid is then converted to lactic acid to access energy – anaerobically c. 80% of lactic acid is diffused to the liver and is converted back to glycogen d. 20% inefficiency results in debt 2. Fatigue – The inability of muscle to maintain the strength of a contraction Occurs as a lack of O 2, depletion of glycogen or as a result of lactic build up 3. Heat production – 85% of the total energy released during contraction is used to maintain body temperature

H. Kinds of contractions – dependent on the frequency of stimuli ·Muscle fiber contraction is “all or none” ·Within a skeletal muscle, not all fibers may be stimulated during the same interval ·Different combinations of muscle fiber contractions may give differing responses ·Graded responses – different degrees of skeletal muscle shortening 1. Twitch – a. Single, brief contraction b. Not a normal muscle function

2. Tetanus (summing of contractions) a. One contraction is immediately followed by another b. The muscle does not completely return to a resting state c. The effects are added – stimuli/sec ·Unfused (incomplete) tetanus ·Some relaxation occurs between contractions ·The results are summed

· Fused (complete) tetanus – stimuli/sec · No evidence of relaxation before the following contractions · The result is a sustained muscle contraction

3. Treppe - a. Skeletal muscle contracts with more force to the second stimulus b. Staircase phenomenon I. Isometric & Isotonic Contractions 1. Isometric – muscle does not shorten during the contraction 2. Isotonic – the muscle’s length changes during contraction. Define: Muscle tone Recruitment Fast fibers (white) Slow fibers (red) Muscle fatigue