Welcome to Module 2. In this module, we will examine the critical adjustments made by key physiological systems in response to a single bout of exercise. This will include the muscular, respiratory, cardiovascular, endocrine, and immune system. Additionally, training adaptations in these systems will be addressed. I will begin by discussing the muscular or skeletal system. There are three types of muscles in the body: heart or cardiac muscle, skeletal muscle, and smooth muscle such as the muscles surrounding your blood vessels that allow them to constrict or dilate. This video will focus on skeletal muscle. First, let's review the three major types of contractions performed by skeletal muscle. I will provide an example of each one. Isometric contractions are defined as no change in length of the muscle while tension is being developed. In a concentric contraction the muscle shortens during tension development. Lastly, with eccentric contractions, the muscle is actually lengthening while tension is being developed. Here's an example of an isometric contraction. Pictured is a woman planking. As she holds this position, her muscles are not changing length yet tension must be employed in the muscles recruited to prevent her from falling on her face. The most common type of muscle contraction is concentric. Shown here is the basic arm curl where the bicep muscles are shortening as the weight is being lifted up toward the shoulder. Finally, eccentric contractions involve the lengthening of the muscle while it develops tension. A good example is that of downhill running. While you are running downhill your quadricep muscles are actually lengthening while developing tension to resist gravity and prevent you from doing a faceplant. This type of contraction as we'll see in Module 3 is the main cause of muscle soreness experienced 8 to 48 hours after exercise. It is very important to understand that skeletal muscle is composed of three very different types of fibers that have a range of biochemical and physiological characteristics. Type 2x muscle fibers are considered our power fibers. They are recruited when we need to generate a lot of force, such as in jumping, sprinting, and lifting heavy weights. These fibers can also generate this force very quickly. Since they need to supply a large amount of ATP in a short period of time, they rely heavily on the anaerobic pathways for energy production. As a reminder, this includes ATP and cretin phosphate already present in the muscle plus the anaerobic breakdown of glucose to lactate. Also notice that since the bulk of their energy comes from the anaerobic pathways their mitochondrial numbers are very low. Finally, as it is difficult to maintain this type of power output for an extended period of time, Type 2x fibers fatigue rapidly. On the other extreme, Type 1 fibers can only generate moderate force at a much slower rate. These fibers are primarily recruited when we are engaging in submaximal exercise for an extended period of time such as distance running, swimming, and cycling. These fibers are high in mitochondria and as such use the aerobic pathway for ATP production. Also, as long as there is ample fuel, Type 1 fibers are slow to fatigue. Type 2a fibers are a hybrid of Type 1 and Type 2x fibers. Thus, they demonstrate characteristics from both of these fibers. Shown here are the motor units for each fiber type. A motor unit consists of the motor neuron and all the muscle fibers that it enervates. Notice again that the Type 2x fibers can generate a great deal of force quickly. As such, they are ideal for explosive exercises such as sprinting. However, as shown, they are quick to fatigue. On the other hand, Type 1 fibers generate a much weaker force but are more resistant to fatigue. Let's look at how the various muscle fiber types are recruited during exercise of increasing intensity. At the onset of exercise, when the workload is light intensity, the Type 1 fibers will be the primary fiber type recruited. This should make sense since at the easy workloads only a small amount of force is required and the Type 1 fibers are slow to fatigue, thus, an individual would be able to exercise at this intensity for a long period of time. However, as the exercise intensity increases to moderate and heavy, Type 2a and Type 2x fibers are also recruited to help generate the force required for these more difficult workloads. Please notice that even at the higher workloads the Type 1 fibers are still being recruited. At the highest workload we are dependent upon Type 2x fibers to provide the necessary power required but we will not be able to maintain this intensity for very long as the 2x fibers will soon fatigue. As a reminder, we discussed the crossover concept in Module 1. As the exercise intensity increases during the course of a graded exercise test, the reliance on carbohydrates as a fuel also increases and at some point crosses over becoming the preferred fuel. A major reason for this shift in fuel preference is the increasing recruitment of Type 2x fibers. As discussed, these fibers are low in mitochondria and rely heavily on the anaerobic utilization of carbohydrates. When we examine the fiber type composition of elite athletes, it is not surprising to find that distance runners have a much greater percentage of Type 1 muscle fibers and a lower percentage of both Type 2x and Type 2a fibers. The Type 1 fibers are ideal for distance athletes who compete at submaximal exercise intensities for extended periods of time. On the other extreme, sprinters who repeatedly engage in high power explosive exercise have a much greater percentage of both Type 2 fibers. An average individual will have a range of approximately 50 percent for Type 1 and Type 2 fibers dependent upon genetic makeup and activity levels. With regular endurance training, these individuals demonstrate a shift in their muscle fiber type from Type 2 to Type 1. Similarly, intense sprint training will result in a shift in the opposite direction from Type 1 to Type 2. Muscle fiber type can be determined from taking muscle biopsies from a specific muscle group. The sample can then be stained to determine fiber types contained within the muscle. Shown here are results from two very different elite athletes, a sprinter and an endurance athlete. As can be seen, the sprinter has a large percentage of Type 2 fibers shown in white while the distance athlete has a much greater percentage of Type 1 fibers which stained black. Finally, regular involvement in intense strength or resistance training will produce very different adaptations in muscle. I will discuss these mechanisms in detail in Module 3 but for now realize this type of training will result in an increase in the cross-sectional area, size, and strength of existing muscle fibers. This is called muscle hypertrophy and is defined as an increase in cross-sectional area of muscle due to an increase in contractile proteins. This occurs to a greater extent in Type 2 fibers but also occurs in Type 1 fibers. If you wish to review the 12 steps involved in the complete contraction of a skeletal muscle beginning with the arrival of an action potential and ending with a complete contraction, I have provided a link here. In summary, the three major types of contraction are isometric, concentric, and eccentric. The order of fiber recruitment during progressively increasing exercise intensity is Type 1 initially followed by Type 2a and Type 2x. Type 1 fibers rely more on aerobic energy sources while Type 2x rely more on anaerobic energy sources. Distance athletes have a greater percentage of Type 1 fibers while sprinters have a greater percentage of Type 2 fibers.
