In this video, I will discuss the application and significance of calorimetry. In Exercise Science, we are interested in quantifying or measuring the amount of energy expended during a single bout of exercise. This is useful for various reasons, including-- determine the energy cost for a wide range of physical activities, caloric expenditure for weight control, and type of fuel or substrate being used by the muscles during exercise. The technique of calorimetry is used to measure energy expenditure during exercise. Calorimetry is defined as the measurement of heat production, usually measured in units of calories. As all metabolic processes eventually result in heat production, such as a skeletal muscle during a contraction, we can estimate an individual's metabolic rate by measuring the rate of heat production. The actual measurement of heat production by the body is termed "Direct Calorimetry", and can be used to estimate one's metabolic rate both at rest and during exercise. Here is an example of a human calorimetry chamber. The heat produced by the body during exercise can be monitored by various thermal detecting devices. These chambers are extremely expensive and thus, are not very practical for clinicians, health care providers, and researchers alike. Indirect calorimetry is by far the most common method for estimating one's metabolic rate, specifically, by the measurement of oxygen consumption. It is based on the observation that oxygen is consumed by the body for energy production when your muscles are working. A direct relationship exists between the amount of oxygen consumed and the amount of heat produced. So, by measuring the rate of oxygen consumption, the indirect calorimetry, we can get a very good estimate of an individual's metabolic rate. The rate of oxygen consumption is indicated by V̇O2, the V indicates the volume of oxygen and the dot over the V implies a rate, in this case-- milliliters of oxygen consumed for a minute. Notice the significant increase of oxygen consumption when we go from a resting state to steady states of maximal exercise at a given intensity. This represents the indirect calorimetry, an increase in your metabolic rate compared to rest. In order to measure an individual's maximal oxygen consumption, which is the gold standard for assessing one's cardiovascular fitness, the individual must complete a graded exercise test to exhaustion. This is generally done on a treadmill or bicycle ergometer. This test begins at a very easy workload. Afterwards, the intensity is gradually increased every two to three minutes until the individual fatigues and can go no further. Oxygen consumption is measured at the end of each stage and plotted against workload. As can be seen, oxygen consumption increases with increase in grade, indicating a greater metabolic rate required for the extra work. Notice that in going from grade 5 to grade 6, oxygen consumption does not significantly increase despite the increased workload, indicating the individual has reached his or her maximum rate of oxygen consumption or V̇O2 max. As shown here, V̇O2 increases linearly with increasing workload until V̇O2 max is achieved. A hallmark training adaptation associated with regular endurance training is an increase in one's V̇O2 max. The reason for this increase relates to improvements in oxygen delivery, thus cardiovascular adaptations, as well as improvements in muscle mitochondrial oxygen utilization. We will discuss these training adaptations more thoroughly in a later module. Endurance or aerobic activities that recruit a large muscle mass over an extended period of time generally produce athletes with the highest values for V̇O2 max. As such, cross-country skiers and distance runners typically have the highest reported values for V̇O2 max. In addition to measuring oxygen consumption, a second feature of indirect calorimetry is that it allows us to measure the amount or volume of carbon dioxide produced by the body. This knowledge, coupled with the value for oxygen consumption, allows us to calculate the respiratory exchange ratio shown here. This ratio is simply the volume of carbon dioxide produced divided by the volume of oxygen consumed. This ratio is very useful as it provides valuable information on the type of fuel or substrate being used by the muscles during exercise. For example, if you are burning pure fat your respiratory exchange ratio will be 0.70. If you are burning pure carbohydrate your respiratory exchange ratio will be 1.0. This is valuable information for clinicians, researchers, and athletes alike. Here's a commonly used fat, the 16 carbon free fatty acid palmitate, used by muscles during exercise for fuel. Notice that the respiratory exchange ratio for its complete oxidation to carbon dioxide and water is 0.70. With the complete oxidation of glucose, the primary carbohydrate used by muscles during exercise, the respiratory exchange ratio is 1.0. Armed with this information researchers have determined that carbohydrates are used to a greater extent than fats for sprinting and high power activities while the use of fats is preferred for distance athletes. This will be discussed in detail later in this module. Indirect calorimetry can be used to estimate an individual's metabolic rate, both at rest and during exercise. Endurance training will result in an increase in maximal oxygen consumption. Respiratory exchange ratio can provide an indication of the type of fuel used by the muscles during exercise.
