Muscle Bio Week 2.docx

Muscle Bio Week 2 ***Week 2, Module 2: Exercise Intensity, Duration and Fuel Use***   Energy Systems of the Body +-------------+-------------+-------------+-------------+-------------+ |   | **Exercise | **Exercise | **Fuel | **Other | | | Duration** | Example**...

Muscle Bio Week 2 ***Week 2, Module 2: Exercise Intensity, Duration and Fuel Use***   Energy Systems of the Body +-------------+-------------+-------------+-------------+-------------+ |   | **Exercise | **Exercise | **Fuel | **Other | | | Duration** | Example** | Source** | notes:** | +=============+=============+=============+=============+=============+ | *Phosphagen | 10 seconds | 100m sprint | Creatine | - Max | | System* | or less | | phosphate | power | | | | | | output | | | | | | | | | | | | - Short | | | | | | supply | +-------------+-------------+-------------+-------------+-------------+ | *Anaerobic | 1-2 minutes | - Set of | Glucose/gly | - More | | Glycolysis* | | squats | cogen | supply | | | | | | | | | | - 400m | | - Lasts a | | | | sprint | | bit | | | | | | longer | | | | | | | | | | | | - Limited | | | | | | by | | | | | | lactate | | | | | | product | | | | | | ion | +-------------+-------------+-------------+-------------+-------------+ | *Oxidative | 90 minutes | 10km race | Glucose/gly | - Low | | Phosphoryla | | | cogen | \'power | | tion | | | | \' | | (carbs)* | | | | output | +-------------+-------------+-------------+-------------+-------------+ | *Oxidative | Unlimited | Long | FFA and | - Not | | Phosphoryla | | duration, | IMTG | useable | | tion | | sustained | | for max | | (lipids)* | | exercise | | power | | | | | | output. | +-------------+-------------+-------------+-------------+-------------+   - RBC have no mitochondria, so they rely solely on anaerobic metabolism - As exercise duration increases, it becomes are to produce max power output. - We cannot maintain max power output for more than a few seconds and it is impossible to produce max power on fat fuels. - Athletes will \"hit a wall\" when the body runs out of glycogen for fuel. - We only have enough glycogen stored for roughly 90 minutes of exercise, when done at a high intensity. - When the body swaps to fat for fuel, this is a slow process, as it requires oxygen and isn\'t effective for someone doing high intensity exercise   [Effects of Exercise Intensity and Duration on Fuel Use ] - Rate of energy produced = rate of energy required - When exercise occurs, fuel source is selected on what is going to best match the rate of energy required. - When the body has to switch to an alternate fuel source, it will then affect the individuals ability to meet the required intensity. - Intensity and duration of exercise determines the rate and amount of energy needed to fuel said exercise - Fats are more energy efficient and produced more energy per gram but are slower to be broken down than carbs - Fat see used during low intensity exercise as the rate of energy demand is lower. Carbohydrates the preferred fuel source for when energy is rapidly needed, however they produce less energy per gram. - Absolute intensity of exercise: power output or speed, relative to someone fitness level. Determines the total quantity of fuel used - Relative intensity: power output or speed, relative to someone\'s weight. Determines the proportions of fuel used. - At rest and during low intensity exercise, fats give the major proportion of energy requirements when in this state. - Fats are energy dense, they can produce 129 ATP molecules from one molecule of fatty acids compared to the 36 you get from glucose. - In moderate intensity exercise, 50% of energy is still from fat. However, at about 65% of VO2max, lipolysis reaches its limits. Past this, fat use declines and glycogen takes over. - One we reach a high exercise intensity, we swap to carb sources. This is due to the need for fast energy and a decline in oxygen.   [Exercise Intensity and Fuel Selection ] - The ***absolute intensity*** determines the total quantity of fuel required: e.g., cycling at 500 W requires more fuel than cycling at 250 W - The ***relative intensity*** determines the fuel mix (i.e., the proportion of fat and CHO used by the working muscles): e.g., cycling at 85% of VO2max requires a greater proportion of CHO than cycling at 25% of VO2max   *Rest and Low Intensity* - At rest, ***free fatty acids** (**FFAs**), *which are produced from the mobilisation and breakdown of triglycerides in adipose tissue (via a process known as ***lipolysis***), satisfy the majority of the energy requirements of skeletal muscle.\ At low exercise intensities, a small percentage of the muscle's fuel comes from CHOs, and this consists almost entirely of plasma glucose.\ During low-intensity exercise (25 -- 40% of VO2max), as intensity increases, the rate of FFA mobilisation in plasma matches the rate of FFA oxidation (in muscle), which means there is an increase in the utilisation of FFAs.\ This means that fats continue as the main energy source (\~55% of total energy expenditure) at low exercise intensities.   *Moderate Intensity* - During moderate-intensity exercise, more than 50% of total energy is derived from FFA oxidation.\ With an increase from 40% to 65% of VO2max, total fat oxidation reaches its peak, despite a slight decline in plasma FFA oxidation. This is because of an increase in the oxidation of fats stored within the muscle, known as **intramuscular triglycerides (IMTGs)**. - Once intensity reaches 65% of VO2max, there is very little, if any, further increase in lipolysis as the increasing blood glucose inhibits lipase activity (the enzymes involved in breaking down triglycerides), which inhibits lipolysis.\ Once intensity rises past 65% of VO2max, muscle glycogen usage increases significantly, while both IMTG and plasma FFA use decreases. Simultaneously, lipolysis is suppressed, and the contribution of FFA oxidation to total energy requirement of exercise further declines. *High Intensity Exercise* - When exercise intensity reaches 85% of VO2max, there is a further decline in total FFA oxidation compared to moderate-intensity exercise. This is due to the insufficient blood flow from adipose tissue to the blood stream, which reduces the concentration of FFAs in blood plasma and reduces overall fat oxidation. - Additionally, CHOs can supply ATP to working muscles faster than fat, which is why CHO is the preferential fuel during high-intensity exercise.** **During intense exercise, CHO fuels predominate, with muscle glycogen and glucose utilisation increasing exponentially with increases in relative exercise intensity. - Muscle glycogen therefore becomes the most important fuel source for high-intensity exercise (65 -- 110% of VO2max   *[Exercise duration and fuel]* *Low intensity exercise* - During prolonged exercise performed at low exercise intensities, fuel selection does not change considerably as exercise progresses, even if the exercise persists for 1-2 hours. - This is because energy needs of muscle can be met exclusively from the oxidation of the fats (FFA mobilised from triglyceride stores in adipose tissue), which is an almost unlimited source of energy in the human body   *Moderate high intensity exercise* - Conversely, during prolonged exercise performed at moderate-high intensities, as exercise duration progresses, the rate of CHO oxidation declines, while lipolysis and fat oxidation increases. - The body can only maintain high-intensity exercise for a certain amount of time (\~90 min at \~85% VO2max). Eventually, plasma glucose levels decline due to the inability of glucose output from the liver to meet the needs of skeletal muscle. - As glycogen stores become depleted with prolonged exercise, fat once again becomes the main energy source for skeletal muscle and exercise intensity must decrease to a level constrained by the body's ability to mobilise and oxidise fat. [Fuel Depletion and Fatigue ] CHO (glycogen) stores in the body are limited, and are often substantially less than the requirements of training sessions and competitions/matches undertaken by many athletes. The ***depletion of muscle and liver glycogen*** also often ***coincides with fatigue*** and reduced performance during endurance events and many team sports. Prolonged endurance exercise leads to muscle glycogen depletion, which is in turn linked to fatigue and reduced performance, making it difficult to meet the energetic requirements of training and competition. Because our CHO stores are limited, and depletion of these stores reduces exercise performance, any nutritional strategy that conserves endogenous CHO stores (and therefore promotes increase fat oxidation) can potentially improve exercise capacity. In fact, one of the adaptations to endurance (aerobic) training is an improved ability to use fat as a fuel, which \'spares\' the limited CHO stores for use later in exercise, thereby improving endurance   [Measuring Fuel Utilisation ] We use the oxygen in the air that we breathe to "oxidise" the macronutrients carbohydrate and lipids. Our cells then use the energy released from these oxidation reactions to recycle ATP to maintain their energy charge, one of the by-products of course is carbon dioxide which we exhale. We can use gas analysis equipment to measure how much carbon dioxide someone is producing and how much oxygen they are using. The respiratory exchange ratio is the ratio of carbon dioxide produced to oxygen consumed.                                          RER = CO2 produced : O2 consumed When measured in the rested state on a general "western" diet RER is usually around 0.8-0.85. A person's RER gives us an indication as to the proportion of fat and carbohydrate that their body is burning to power their activities. How is it that RER can give us this insight into our metabolism? We know that to aerobically burn carbohydrate we will produce 1 molecule of carbon dioxide for every oxygen used to burn that carbohydrate. Hence the respiratory exchange ratio for subsisting entirely on carbohydrate will be 1. This is highlighted in the equation below: However, to burn fat we need more oxygen to release all the carbon from the fatty acids (remember that fatty acids must be beta-oxidised before being oxidised in the mitochondria, this is because they contain less oxygen than carbohydrate). As a result the RER for fat is less than 1 in fact the RER for subsisting entirely on lipid sources is 0.7 (remember that ratios don't have units). This is highlighted in the equation below for the oxidation of palmitic acid: CH3(CH2)14COOH + 23O2 = 16CO2 + 16H2O (RER for palmitic acid = 16:23 or 0.7)   So if we had someone in a rested, **fasted** state and we measured their RER by assessing the composition of their exhaled air we would expect their RER to be close to 0.7 which would indicate that their body is predominantly utilising lipid as a fuel. However, if we had that person undergo an incremental exercise test we would see their RER shift closer to 1 as their metabolism shifted to become more reliant on carbohydrate to fuel their activity. By the time they reached their VO2max their RER would be at least 1. We can thus utilise the RER measured in an individual's exhaled air to estimate the proportion of fuel that they are burning to power the activity.