Your heart is essentially a pump, designed to circulate blood throughout your body. Your bloodstream carries nutrients and oxygen to cells throughout your body, and carries away carbon dioxide and other byproducts of cellular metabolism. Many things can interfere with your heart's ability to pump efficiently, including excess unhealthy body weight.
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Defining Normal Heart Rate
Your heart rate is simply the number of times your heart beats per minute. It can be determined by feeling your pulse on areas where the artery is close to the skin, such as your wrist or either side of your neck. An adult's normal resting heart rate (RHR) falls between 60 and 90 beats per minute. An athlete who is in great shape may have a RHR between 40 and 60. In comparison, a heart that beats over 90 times per minute is defined as an abnormally high rapid heart beat, also known as tachycardia.
Work Load and Heart Rate
Your heart rate increases and decreases depending on the amount of demand placed on your muscles. When the demand is high, your heart pumps harder to deliver extra oxygen to your muscles for metabolism. Carrying extra dead weight in the form of fat increases your muscles' workload, causing an elevated heart rate whenever you do physical activity. It is not so much your total body weight that counts, but the ratio of fat to lean mass, known as your body composition.
Lean Mass and Mechanical Work
When your body weight is made up of mostly muscle, the work load of doing physical activity is diminished because your capacity for doing mechanical work increases with more muscle mass. Muscle tissue is metabolically active, meaning that chemical work is being done in muscle tissue at all times. Fat tissue is similar to the fuel in the tank of your car. It does not contribute to mechanical work unless recruited for energy. Fat is storage fuel, and as such, it adds to the workload of your muscles, which must carry its weight. When muscle work increases, your heart rate increases to supply oxygenated blood to muscle tissue.
Weight and Resting Heart Rate
When at rest, excess fat may cause your heart to work overtime because your extra body weigh restricts the flow of blood through your arteries and veins. A strong healthy heart beats more slowly because it delivers a greater volume of blood with each beat. If you are sedentary and out of shape, your heart, which is also a muscle, may be weakened from inactivity. To compensate for its weakened state, it must beat more frequently to meet your body's oxygen demands.