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Get the masses moving! The American College of Sports Medicine (ACSM), and the American Heart Association (AHA) have published updated recommendations for physical activity and public health (Haskell et al 2007). In 1995, the Centers for Disease Control and ACSM issued the original recommendation that “every US adult should accumulate 30 minutes or more of moderate-intensity physical activity on most, preferably, all days of the week”(2). The intention was to increase the participation in leisure time physical activity in a largely sedentary population. Some experts referred to this as “exercise lite”(2). This is the minimum amount of activity necessary to help prevent chronic diseases. In time, other organizations published their version of physical activity guidelines and the general public became confused: Is it 30 minutes or 60 minutes of activity? Or is 20 minutes of exercise within a specific target heart rate range? ACSM and AHA’s updated guidelines clarify the seemingly contradictory guidelines. Most of you on our email list are endurance athletes, and certainly don’t need a threat to be made on health in order for you to move some. The guidelines presented here are below what some of you accomplish in a single day. However, you are the minority, less than half of the U.S. population attain the minimum amount of activity necessary for good health and Massachusetts doesn’t fair much better compared to the rest of the country - only 52% of Commonwealth residents meet the standard.
Only 26 percent of U.S. adults engage in vigorous leisure-time physical activity three or more times per week (defined as periods of vigorous physical activity lasting 10 minutes or more). About 59 percent of US adults do no vigorous physical activity at all (Lethbridge-Çejkuet al, 2005) . From a public health perspective, this is discouraging news since physical activity has such a positive effect on the prevention of chronic diseases. So why am I preaching to the choir? You all value and appreciate the benefits of physical activity. Most people may never do a triathlon, but you don’t have to in order to realize the health benefits of physical activity. Athletes are in a powerful position to promote all types of physical activity, not just competition. It may be the encouragement from an “Ironman“ that inspires a few to take a walk everyday. Hopefully you will share this information with some of your sedentary friends and get them moving! Updated Recommendation Statement Aerobic Activity - “To promote and maintain health, all healthy adults aged 18-65yr need moderate-intensity aerobic physical activity for a minimum of 30 minutes on five days each week or vigorous activity for a minimum of 20 minutes three days each week. Also, combinations of moderate and vigorous activity can be performed to meet this recommendation” (Haskell et al 2007). Muscle Strengthening Activity – “to promote and maintain good health and physical independence, adults will benefit from performing activities that maintain or increase muscular strength and endurance for a minimum of two days per week” (Haskell et al 2007). This may be where many endurance athletes fall short. Participating in activity above and beyond recommendations provide additional health benefits (Haskell et al 2007). So if we can encourage our sedentary counterparts to become more physically active through moderate activity that is great, but if we can get them to take up triathlon, even better! What is the difference between moderate and vigorous intensity activity? Exercise intensity can be defined in terms of metabolic equivalent (METS). One MET is the amount of energy expended at rest, moderate intensity activity is any activity that is 3-6 METS and anything greater than six METS is considered vigorous. For example walking 3.0 mph is moderate physical activity (3.3 METS) and Jogging at a 5.0 mph pace is considered vigorous activity (8.0 METS). If you want to see what MET levels you are working at with different activities, check out this web site that lists just about every activity you can think of. http://prevention.sph.sc.edu/tools/compendium.htm (Ainsworth, 2002) Using METS you can also calculate energy expenditure (calories) multiply the number of minutes participating in the activity by the METS, divide by 60 to convert to MET/hours, and then multiply by your body weight in kilograms (divide your weight in pounds by 2.2 to get kg). In conclusion, being physically active is important, and all Americans need to accumulate 30 minutes or more of moderate intensity physical activity at least 5 days per week or vigorous activity for 20 minutes three times per week. And everybody should engage in activities to improve their muscular strength and endurance a minimum of two times per week. These aren’t guidelines to train for an Ironman, or even improve fitness; rather they are intended to impact public health by getting the sedentary to become moderately active. Motivate someone today to become physically active! On the National Health Observances calendar, September or Steptember was America On the Move's month-long celebration highlighting the benefits of active living and healthy eating. 1. Ainsworth BE. (2002, January) The Compendium of Physical Activities Tracking Guide. Prevention Research Center, Norman J. Arnold School of Public Health, University of South Carolina. Retrieved [08.24.2007] from the World Wide Web. 2. Haskell, WL., Lee, IM., Pate, RR., Powell, KE., et al (2007) Physical activity and public health: Updated recommendation for adults from the American College of Sports Medicine and the American heart Association. Med Sci Sports Exerc, 39, 8 1423 – 1434 3. Lethbridge-Çejku M,and Vickerie J. (2005). Summary health statistics for U.S. adults: National Health Interview Survey, 2003. National Center for Health Statistics. Vital Health Stat 10(225). 4. Pate, RR., Pratt, M., Blair, S., Haskell, WL., Macera, CA., et all (1995). A reciommednation from the Centers for Disease Control and Prevention and the American College of Sports Medicine. JAMAFeb, 273; 402-407.
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Intro to Exercise Physiology: The Metabolic Cart Measuring energy expenditure is important for people interested in health, fitness, and performance. Knowing how much energy a person expends throughout the day certainly has implications for weight loss. An individual’s maximal ability to produce energy can be used to determine physical fitness. Moreover, measuring energy expenditure can also be used for evaluating and predicting performance. Sounds great, but how is it done? Energy expenditure can be measured through direct or indirect calorimetry. Direct calorimetry measures the body’s rate of energy metabolism (heat production). This involves the use of a highly sophisticated chamber. The walls of the chamber contain copper tubing through which water is passed. The subject sits in the chamber; the heat from the body radiates to the walls and raises the temperature of water. The amount heat required to raise the temperature of 1 liter of water, one degree Celsius is called a kilocalorie or simply calorie. The same calorie we see on the back of food packages. The problem is this device is expensive, cumbersome, and inapplicable for most exercise situations. More frequently, energy expenditure is measured through indirect calorimetry. Indirect calorimetry is based on the principle that all energy releasing processes in the body, ultimately, rely on oxygen. Measuring a person’s oxygen consumption during physical activity gives a highly accurate estimate of energy expenditure. Correlation studies with direct calorimetry show that for each liter of oxygen consumed there is a release of 4.82 kCals of energy. How does indirect calorimetry work? The most common method is called open-circuit spirometry. A subject inhales ambient (from the environment) air that has a constant composition of 20.93% oxygen (O2) , .03% carbon dioxide (CO2), and 79.04% nitrogen. The changes in the composition of the gases in the expired air indirectly reflects energy metabolism. The Douglas Bag method is considered to be the Gold Standard for analyzing respiratory gas exchange. The Douglas bag method involves collecting expired gases in an air tight bag and withdrawing a precise amount of gas form the bag for analyzing. The problem is that it is time consuming and requires repeated measurements. Today, we have modern analyzers that can analyze gas exchange breath by breath. The process has become completely automated and there is very little skill required in collecting the gases. A subject is fitted with a low resistance, two way non-rebreathing valve. This type of valve ensures that only room air is being inhaled by the subject and all expired air is going into the machine to be analyzed. Expiratory air passes through a pneumotachometer or for some types of machines, a turbine. These measure air volume. In order to accurately calculate oxygen consumption, air volume must be measured precisely. In some types of machines the gases end up in a mixing chamber from which the gas is sampled from and in some systems the gas is sampled right from the mouth. Each type offers its advantages. The difference in O2 concentration in the inhaled air (FIO2) versus O2 concentration in the exhaled air (FEO2) represents the amount of oxygen consumed by the body (VO2). Oxygen consumption can be measured at rest to determine resting metabolic rate and it can be measured during constant steady state exercise to determine exercising metabolic rate or exercising energy expenditure. During steady state exercise, if we calculate the ratio of the volume of CO2 produced versus the volume of O2 consumed we can determine if fat or carbohydrate is being used for fuel. The metabolic cart is often used with athletes to measure their maximal ability to utilize oxygen (VO2max or maximal oxygen consumption). Measuring an athlete’s VO2max is the gold standard for assessing cardiorespiratory fitness. We know that international caliber endurance athletes have VO2max values around 80 ml/kg/min. VO2max testing can be used as a barometer to gauge an athletes potential to be competitive at the local, regional, national, or international level. Recently I read a newsletter article that stated that by knowing an athlete’s VO2max we can’t use the information for exercise prescription. This can’t be further from the truth. The fact is that everything boils down to oxygen consumption in the muscle. Let me explain. For years we have used heart rate as an indicator of exercise intensity. We would predict or measure maximal heart rate and take percentages of maximal heart rate to determine a target heart rate range. If during exercise our heart rate wasn’t within the desired heart rate range, then we would not be stressing the body to the degree necessary to cause adaptations. The reason we use heart rate as an indicator of training intensity is because it is directly related to oxygen consumption. As oxygen consumption goes up, heart rate follows right along with it. Since we can’t carry a MET cart along with us on the bike (although there are some portable systems) we use heart rate as a surrogate measure of our oxygen consumption. The weakness here is that sometimes the relationship is lost. Such as during exercise in hot humid environments where there is a large increase in heart rate out of proportion to oxygen consumption. Because of that aberration, some cyclists prefer to use power for monitoring exercise intensity. But guess what, that’s right, power is directly proportional to oxygen consumption. You need more power? Well the muscle will need to consume more oxygen to produce the energy to produce the power. Of course not all energy production uses oxygen, there are anaerobic pathways. But they have limited capacities; even in a sprint triathlon 99% of the energy production is aerobic. Figure 1 shows the results of a graded maximal exercise test where VO2max was measured and how the data can be used for exercise prescription. On the figure, I have marked VO2max, 50% VO2max, and 80% VO2max and the power and heart rate associated with those percentages of VO2max. 50% VO2max represents the workload that would be appropriate for long slow distance training. 80% VO2max represents the lactate threshold. Training below this would be tempo training and above the threshold would be threshold or VO2max training. Very useful information indeed! In conclusion the MET cart can provide some valuable information about an individual’s resting, submaximal, and maximal metabolism. We can categorize an individual’s fitness, determine energy expenditure, and evaluate the fuel usage. This information would be helpful for weight loss, training, for predicting performance, or to determine race pace during endurance events. Figure 1: results of a graded exercise test |
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