Fitness Whether you do it to lose weight, to reach a fitness goal or — dare we say it? — just for fun, exercise changes you.

There’s the red face and the sweating, the pounding heart and pumping lungs, the boost to your alertness and mood, the previously nonexistent urges to talk about nothing but splits and laps and PBs.

But while we all know that staying physically active is essential to a long, healthy, productive life, we don’t often understand exactly what’s happening behind the scenes.

We asked the experts to take us through — from head to toe — what happens in the body when we exercise. Neuroscientist Judy Cameron, Ph.D., professor of psychiatry at the University of Pittsburgh School of Medicine, Tommy Boone, Ph.D., a board certified exercise physiologist, and Edward Laskowski, M.D., co-director of the Mayo Clinic Sports Medicine Center spill the beans on what gets and keeps you moving.

Muscles
The body calls on glucose, sugar the body has stored away from the foods we eat in the form of glycogen, for the energy required to contract muscles and spur movement.

It also uses adenosine triphosphate, or ATP, but the body only has small stores of both glucose and ATP. After quickly using up these supplies, the body requires extra oxygen to create more ATP. More blood is pumped to the exercising muscles to deliver that additional O2. Without enough oxygen, lactic acid will form instead. Lactic acid is typically flushed from the body within 30 to 60 minutes after finishing up a workout.

Tiny tears form in the muscles that help them grow bigger and stronger as they heal. Soreness only means there are changes occurring in those muscles, says Boone, and typically lasts a couple of days.

Lungs
Your body may need up to 15 times more oxygen when you exercise, so you start to breathe faster and heavier. Your breathing rate will increase until the muscles surrounding the lungs just can’t move any faster. This maximum capacity of oxygen use is called VO2 max. The higher the VO2 max, the more fit a person is.

Diaphragm
Like any muscle, the diaphragm can grow tired with all that heavy breathing. Some argue that as the diaphragm fatigues, it can spasm, causing a dreaded side stitch. (Others argue a side stitch is due to spasms of the ligaments around the diaphragm instead, while others believe the spasms to originate in the nerves that run from the upper back to the abdomen and arecaused by poor posture!) Deep breathing and stretching can alleviate the discomfort in the middle of a workout, and preemptive strengthening in the gym can ward off future issues.

Heart
When you exercise, heart rate increases to circulate more oxygen (via the blood) at a quicker pace. The more you exercise, the more efficient the heart becomes at this process, so you can work out harder and longer. Eventually, this lowers resting heart rate in fit people.

Exercise also stimulates the growth of new blood vessels, causing blood pressure to decrease in fit people.

Stomach & Intestines
Because the body is pumping more blood to the muscles, it takes some away from the systems and functions that aren’t top priority at the moment, like digestion. That can result in tummy troubles. Movement, absorption and secretion in the stomach and intestines can all be affected.

Brain
Increased blood flow also benefits the brain. Immediately, the brain cells will start functioning at a higher level, says Cameron, making you feel more alert and awake during exercise and more focused afterward.

When you work out regularly, the brain gets used to this frequent surge of blood and adapts by turning certain genes on or off. Many of these changes boost brain cell function and protect from diseases such as Alzheimer’s, Parkinson’s or even stroke, and ward off age-related decline, she says.

Exercise also triggers a surge of chemical messengers in the brain called neurotransmitters, which include endorphins, often cited as the cause of the mythical “runner’s high.”

The brain releases dopamine and glutamate, too, to get those arms and legs moving, as well as gamma-aminobutyric acid, or GABA, a prohibitive neurotransmitter that actually slows things down, to keep you moving in a smooth and controlled manner.

You’ll also likely feel better thanks to a bump in serotonin, a neurotransmitter well known for its role in mood and depression.

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