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By Chris Crowley and Henry S. Lodge, M.D.
In "Younger Next Year for Women," Chris Crowley and Henry S. Lodge, M.D., detail how exercise slows the aging process for all humans
Aerobic fitnessis all about making more energy in the muscles. That means building more mitochondria in your cells and bringing them more fuel and oxygen. Mitochondria are tiny engines in your cells that can burn either fat or glucose. It's like having a car that can run on either diesel (fat) or gasoline (glucose), depending on your needs: diesel for long-haul road trips, high-octane gasoline for speed. Your muscles prefer to burn fat most of the time, because it's a more efficient fuel, but for hard exercise you burn glucose.
At rest, and with light exercise, you burn 95 percent fat and 5 percent glucose. Most fat is stored around your belly and hips. Your body has to bring it to your muscles through your circulation. Fat has to be carried in special proteins called triglycerides.
Capillaries can handle only a few triglyceride molecules at a time. So each capillary can deliver only a trickle of fat to your mitochondria. With consistent aerobic training, your body builds vast new networks of capillaries to bring more fat to your muscles. Eventually, however, you are delivering as much fat as you possibly can, and if you want to go faster, or harder, you need to start bringing glucose to the mitochondria to use as a second fuel.
With harder exercise you keep burning fat in the background, but all the extra energy comes from burning glucose. Most of the glucose is stored in your muscles ahead of time, but your circulation gets a double workout, first bringing in more glucose and the oxygen necessary to burn it, then carrying away the exhaust, especially the carbon dioxide.
Steady aerobic exercise, over months and years, produces dramatic improvements in your circulatory system, which is one of the ways exercise saves your life. Exercise stresses your muscles, and they release enough cytokine-6, known as C-6, a chemical for inflammation or decay, to trigger cytokine-10, known as C-10, the master chemical for repair and growth. The C-10 released by the adaptive micro-trauma of exercise drives the creation of new mitochondria, the storage of more glucose in the muscle cells and the growth of new capillaries to feed them. Your muscles get hard as you get in shape because they're stuffed full of all the new mitochondria, capillaries and extra glucose.
You have two natural aerobic paces, easy and hard, and they depend on two very different muscle metabolisms, which are determined by the fuel you use. Low-intensity, light aerobic exercise burns fat, while high intensity, hard aerobic exercise burns glucose. It's a critical difference, because these two paces trigger the two distinct metabolisms of foraging and hunting, which are our essential physical rhythms. Those two activities consumed most of our waking hours in nature, and each one called for distinctly different body and brain functions. Never mind that you're walking through the park rather than foraging, or at spin class rather than hunting: light and hard aerobics are still the master control signals for C-6 and C-10.
As soon as you push your body a little harder, you start burning glucose in addition to fat, and you need more oxygen to do this. That means bringing more blood to the muscles, so your heart rate goes up. Any heart rate north of 65 percent means that you're burning glucose and that you've moved into a different metabolism.
Your body starts drawing on the glucose stored in your muscles, feeding it into your mitochondria to produce the extra energy you need to run and hunt. At some point, however, the glucose metabolism also has an upper limit. You can bring loads of oxygen into the blood and carry away a lot of carbon dioxide, but above a certain level of exertion, the chemicals just can't move between blood and muscle, or within the muscle, fast enough to keep up with the demand.
Your muscles become starved of oxygen and the glucose can?t burn all the way down to carbon dioxide. Instead, you build up a sludge called lactate, which is incompletely burned sugar and which shuts down your muscle function after a few seconds of exercise at peak levels (like sprinting the length of a football field). As with the switch from fat to glucose, the switch into "anaerobic" metabolism, where there isn't enough oxygen, has ripple effects throughout your body.
The concept that exercise intensity is a master signal that regulates chemistry throughout your body and brain is so important that it?s worth a closer look, starting with light exercise. Light aerobic exercise is long and slow exercise at an easy pace -- up to 65 percent of your peak heart rate. At this level, your muscles burn mostly fat, so it's your most fuel-efficient pace, the one you can keep up all day. It's the pace you once used for foraging and now use for walking miles -- for those times when speed doesn?t count, but mileage does. You might think it's a waste of time to exercise in this zone, but it's a wonderful pace. This is the metabolic zone where your body and brain heal and grow. It's the zone where steady, low-grade C-10 drives the slow, consistent growth of infrastructure: blood vessels and mitochondria in your muscles; repair and health throughout your body.
Think of growth and repair in terms of a public works project: building an interstate takes time, and new capillaries don't sprout up all at once after the first day at the gym. Your body thinks about it for a while, plans the route and organizes the materials before it starts construction. Your body also doesn't trust you, or, more accurately, it doesn't trust nature. If you fall off track for a little while, even for a distressingly short interval of sloth, construction stops. The real benefits of exercise come with months and years of sustained, steady growth.
This all happens automatically through a carefully choreographed chemical dance within your bloodstream and body. The C-10 pattern of long, slow exercise regulates literally dozens of chemical signals in your body and brain. Names you might recognize, like growth hormone, adrenaline and serotonin, and names you don't, like endothelial growth factor, tumor necrosis factor and platelet derived growth factor. The point is that long, slow exercise builds your muscles, heart and circulation, mobilizes your fat stores and then goes beyond that to let your body heal. Long, slow exercise is the opposite of the chronic inflammation of modern living. It's the tide of youth.
Lodge, left, and Crowley
Exercising hard enough to push your heart rate above 65 percent calls for a new fuel. You need more power than you can get from fat alone, so your muscles start to burn glucose. This shift into high gear changes your metabolism because harder exercise is the automatic signal that you've started to hunt.
Animals in nature never move out of their low aerobic zone unless they're hunting, being hunted or playing (rehearsal for the first two). Glucose is powerful but expensive fuel. Your body knows that if you're burning glucose, you must be hunting, which triggers a major metabolic shift that affects your muscles, brain, gut, immune system, kidneys, liver, heart and lungs.
All those days of long, slow exercise you did earlier built you a bigger engine. Now, with glucose added, those extra mitochondria and blood vessels are running on rocket fuel. This is the benefit of low aerobic training. It's a trick nature invented so you could catch the antelope, and it's why every athlete in the world does long and slow to build the base fitness for harder aerobics.
Nature's perspective on hard aerobic exercise is that you are designed to be an endurance predator, able to work with your pack to run down antelopes on the savannah.
Hard aerobics, working up a good sweat, is our favorite exercise rhythm because hunting brings out our youngest and best biology: strong, fast, energetic and optimistic all day long. That's why you should do low aerobic exercise a couple of days a week to build your base, and then go out and play hard on the high-aerobic fields the other days. Tell your body it's springtime, and remember that Title IX has always existed in nature. Female wolves run and hunt just as hard as males, lionesses do most of the hunting and female dolphins hunt in bursts of speed up to 45 mph.
Aerobic exercise is primarily about your muscles' ability to endure. Strength training is primarily about your muscles' ability to deliver power, which, surprisingly, has asmuch to do with a special form of neural coordination as actual strength. Strength training causes muscle growth, and that's important, but it's the hidden increase in coordination that changes your physical life. This is not eye-hand coordination; it's the coordination of fine muscle detail through the elaborate networks of nerves that link your brain and body.
Generally, we aren't aware of nerve decay as we get older, but it's the main reason our joints wear out, our muscles get sloppy and our ability to be physically alert and powerful begins to fade. And it is reversible with strength training. Each step we take, each coordinated movement, involves thousands of nerve fibers, which together form a neural network. The trouble comes when your muscles, brain connections and the controlling spinal reflex arcs get sloppy and weak from years of a relatively sedentary existence. The casual motion of daily life is not enough to turn on the C-10 of growth. You need to do strength training to cross that threshold for power and coordination, to get C-10 into your neural networks, into the meat of your muscles, into your joints and into your tendons. Lifting weights until you can't lift them anymore -- that really turns on C-10.
Strength training creates an intimate connection between your body and your brain. Your physical brain -- the remarkably complex, physical brain ? integrates the millions of messages coming up from your body and coordinates them with all the impulses it's sending down to move your muscles against resistance. Athletes have come to realize the benefits of strength training over the last 30 years. But the interesting thing is that the greatest advances have come not in the strength sports, like the shot put and weight lifting, but in the coordination sports -- the ones that require grace, skill and coordination, like figure skating and skiing. Those improvements are due largely to increased coordination and muscular integration, as well as the increased muscle power available for jumping and landing, developed through strength training.
The nerves that control your muscles contain thousands of individual cells, each of which splits further into hundreds and hundreds of tiny branches. And each branch goes to one -- and only one -- muscle cell. There are over a million muscle cells in the large muscles of your thighs, and perhaps 10,000 nerve cells, bundled into a couple of main nerves, controlling them all.
You have two types of muscle cells, built for strength or for endurance, and they are different. Muscle endurance cells are known as slow-twitch. They have more mitochondria, more endurance and less power. Muscle strength cells are known as fast-twitch: fewer mitochondria, less endurance and much more power. Each individual nerve cell sends all of its tentacles to either strength or endurance cells, but never both. This means that each individual nerve cell ends up signaling for either strength or endurance.
Remember that, in your thigh, for instance, each of these nerve cells has thousands of tiny branches, going to thousands of muscle cells, all either strength or endurance. With this as background, let's look at strength training vs. endurance exercise. When you walk, your body predominantly recruits endurance units and rotates through them so each one gets a rest period between steps, which means that each unit gets only a fraction of the exercise you think you?re giving it. Certainly not enough stress to generate the powerful regeneration of C-10.
As you start to run, your body uses more endurance units with each step. Each unit may be used every third step now, and that's enough stress to trigger high levels of C-6 and then C-10. If you're running up a hill, hard enough to go beyond the capacity of your endurance units, your body adds in strength units. The longer you run, the less rest time the endurance units get. The more strength you demand, the less rest the strength units get. They will fatigue, and the fatigue will damage them. Taking them to fatigue is what turns on the surge of C-6 -- the good stress of exercise that turns on C-10.
When you hire that personal trainer, he or she will eventually make sure you lift enough weight to cycle all the way through the reserve capacity of your strength cells. To use them 10 or 12 times in a row, and then do it again. Done right, you will drain them of all their energy and then force them to contract a few more times. That's the critical part; that's how you intentionally damage your muscle cells. Not your muscles, just the muscle cells. And you damage them quite a lot. On purpose. That's fine and what your body needs. Lots of C-6, lots of inflammation, and then lots of repair and growth. Your muscles will quiver and burn, which is not fun, but inside you will be forcing your brain to activate all your strength units. Do this for three sets, and you will have forced your body to damage all those strength units, which then forces it to repair all those strength units. Growth, strength -- youth.
This is why you shouldn't do strength training six days a week. If you've done it right, you've done some real damage. Unlike endurance units, which recover from aerobic exercise overnight, your strength units need to enter a 48-hour repair cycle. Two days a week of strength training is enough.
Excerpted from Younger Next Year for Women: Live Like You're 50 -- Strong, Fit, Sexy -- Until You're 80 and Beyond. Copyright 2005 by Chris Crowley & Henry Lodge, M.D. Used by permission of Workman Publishing Co., Inc. New York. All rights reserved.
© 2006-2007 GeezerJock Media, LLC All Rights Reserved. Posted with permission
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