Fitness refers to your heart muscle. The stronger your heart, the more fit you are. The only stimulus that makes any muscle stronger is to exercise that muscle against increasing resistance. To make your skeletal muscles stronger, you have to lift heavier weights or press against greater resistance in any weight-bearing exercise. The only way that you can strengthen your heart muscle is to exercise against greater resistance also.
When you use your legs, your leg muscles squeeze blood from the veins near them toward your heart. Then, when your leg muscles relax, the veins near them fill with blood. This alternate contraction and relaxation of your leg muscles acts as a second heart pushing huge amounts of blood towards your heart. To pump the extra blood from your legs to your heart and then to your body, your heart muscle has to squeeze harder and faster. The harder you exercise, the more blood is pumped by your legs to your heart, and in turn, the harder your heart has to work to push it out towards your body, so your heart has to beat faster and with more force to do more work.
Fitness is determined more by how hard you exercise than by how long you exercise. Exercising at a casual pace does not do much to strengthen either your heart or your skeletal muscles. When you work harder, more blood returns to your heart, and this increased amount of blood fills the inside of your heart and stretches it, so your heart has to pump against greater resistance and the heart muscle becomes stronger.
Dear Dr. Mirkin: Does everyone need to drink eight glasses of water a day?
Drinking lots of water just means you will spend a lot of time running to the bathroom. All foods contain water, and all food is converted to energy, carbon dioxide and water. You can get most of the fluid the body needs from food, and you only need to drink enough water to prevent constipation.
When you eat, the pyloric valve at the end of your stomach closes to keep food in the stomach. Then the stomach takes fluid that you drink and food that you eat and mixes them into a soup. Then the soup passes to the intestines and remains a soup until it reaches your colon. Only then is the fluid absorbed to turn the soup into solid waste in the colon. If you do not have enough fluid in your body, your body extracts extra fluid from your stool, which makes it hard and can cause constipation.
A reasonable amount for a healthy human is one cup of water or any other fluid with each meal. If you have a problem with constipation you may not be drinking enough water (see my report on constipation), but if you are not constipated, you are getting plenty. You'll also want to replace fluids whenever you sweat a lot, particularly when you exercise or in hot weather. Drink water whenever you feel thirsty, but there's no benefit from forcing yourself to drink eight glasses of water a day.
Dear Dr. Mirkin: Why are some men so much more aggressive than others?
A partial answer may come from a study in Biological Psychology (March 2005), showing that the ratio of a man's second finger to his fourth finger appears to predict aggression. The amount of testosterone a baby is exposed to in his mother's uterus determines how long his fourth finger grows. So men who have been exposed to high levels of testosterone in utero have a larger ratio of the fourth ("ring") finger to the second ("index") finger and therefore are more prone to physical aggression later in life. The author of this study states: "This study shows that events in the womb have subtle effects on children's personality." Previous studies have shown that men with higher ratios of the fourth to second finger may be better in sports and are usually more dominant and masculine. On the other hand, these men are at higher risk for autism and immune deficiency. Men have larger finger ratios than women. The authors cautioned that these findings link a larger finger ratio to "a tendency toward physical aggression". They do not show that all men with larger ratios are more aggressive.
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