1. Section 6.3

2. 1. Athletes who excel in a particular sport stay fit by exercising their muscles. 2. Muscles that are not used or are used only for weak contractions decrease in size or atrophy. “Use it or Lose it” 3. Atrophy occurs when body parts are placed in casts or nerve damage occurs. 4. Over time, atrophy can cause fibers to shorten, leaving body parts in contracted or contorted positions.

4. 5. Forceful muscular activity over a prolonged time causes muscles to increase in size due to an increase in the number of myofibrils. 6. However, this only occurs if the muscle contracts to at least 75% of its maximum tension. 7. This increase in muscle size is called hypertrophy.

5. 1. Muscle fibers metabolize ATP both aerobically (with O 2 ) and anaerobically (without O 2 ). 2. Some muscle fibers however utilize one method more than the other to provide myofibrils with ATP. 3. Slow-twitch fibers= aerobic 4. Fast-twitch fibers= anaerobic

6. 1. Have a steadier tug and more endurance 2. Motor units have a smaller number of fibers 3. Helpful in sports like running, biking, jogging, and swimming. 4. Since they utilize oxygen as their fuel, they tire only when the oxygen supply becomes limited. 5. Have many mitochondria and are dark in color because they contain myoglobin, the respiratory pigment found in muscle tissue

7. 6. They are surrounded by dense capillary beds and draw more blood and oxygen than fast- twitch fibers 7. Highly resistant to fatigue 8. Have a substantial reserve of glycogen (stored glucose) and fat that can be broken down to create ATP.

8. 1. Designed for strength not endurance 2. Motor units contain many fibers 3. Provide explosions of energy and are helpful in sporting activities like sprinting, weight lifting, swinging a golf club, or throwing a shot put 4. Have fewer mitochondria and are lighter in color 5. Accumulate lactic acid because of a lack of oxygen, which causes them to fatigue quickly

9. Comparison Chart