This lecture will address a number of the underlying principles in the field of exercise science. This principles provide the foundation for the adjustments your body must make in response to the physical stress incurred during a single bout of exercise. These principles are also involved in the many training adaptations associated with participation in a regular exercise program. The first principle relates to the concept of homeostasis. This principle provides the basis for why our body needs to make many adjustments in a number of physiological and biochemical systems during exercise. Homeostasis can be defined as the tendency of the body to maintain a stable internal environment for cells by narrowly regulating critical variables such as pH or acid base balance, oxygen tension, blood glucose concentration, body temperature, etc. Any disruption to optimal homeostatic conditions will elicit multiple regulatory responses by the body In an attempt to bring disrupted variables back to normal levels. For example, when you ascend the high altitude, due to the low oxygen pressure in the inspired air, oxygen levels in your blood drop below desired levels. As a result, you're nervous, endocrine, cardiovascular and respiratory systems, makes adjustments in an attempt to compensate for this disruption in homeostasis. Well, as you can imagine, engaging in physical exercise is a very powerful disruptor of normal resting homeostasis. The more intense the exercise bout, the greater the disruption in homeostasis. Your muscles and blood can become more acidic. Blood oxygen and glucose levels must be regulated to prevent them from falling below normal levels. Body temperature increases activating thermal regulators processes. This are just a few of the necessary adjustments the body must make in response to the stress imposed by a single bout of exercise. This figure depicts many of the systems and tissues that must respond and adjust to exercise. The brain, the lungs and the respiratory system, heart and blood vessels, the cardio vascular system, muscles, kidney, liver etc., are most respond. Let's look a one example how body adjust. Showing here are the major components of the cardio vascular system. In order to ensure that the proper amounts of oxygen and nutrients are being delivered to the working muscles. The heart must pump more forcefully and the blood vessels to the muscles must dilate to increase local blood flow. The nervous and endocrine systems are the major regulators involved, responsible for the increase in both heart rate and thereby increasing cardiac output and the pumping capacity of the heart. While circulating hormones and local factors cause the blood vessels in the muscles to dilate. The nervous system also redirects or shuns blood away from less critical tissues such as the stomach, to the working muscles. Together, these cardiovascular adjustments ensure that the muscles are receiving adequate blood flow to support their energetic needs. The overload principle, defined here, provides the underlying validation for all training adaptations associated with both endurance and strength training. As stated, if you habitually overload a system it will respond and adapt. Basically, when you engage in physical activity, the stress imposed by a single bout of exercise elicits an immediate or an acute response by the body, as already discussed with homeostatic disruption. However, if you exercise three to five times a week for several months, the body will make long-term or chronic adaptations to the repeated stress of regular exercise. An example of the overload principle is shown here. In response to weeks and months of endurance training, a classic chronic adaptation is an increase in mitochondrial number, and oxidative capacity in skeletal muscle. The primary signal shown here, are activated acutely during exercise. After weeks of being repeatedly activated, chronic adaptations are made in the pathway responsible for mitochondrial biogenesis, thereby increasing their numbers. This is just one example of the many long term training adaptations elicited from the overload principle. Specificity principle is very straight forward. It states that only the system or body part repeatedly stressed will adapt to chronic overload. An example of the specificity principle is given here. When you do bench presses for weeks and months, only the chest muscles recruited will show improvements in strength. Other muscle groups not involved will show no training adaptations. Also, strength training will have no effect on those mitochondrial adaptations mentioned above for endurance training. Further, as your cardiovascular system is only marginally recruited during strength training, it will show little to no long term adaptations. Thus the overload principle will only apply to the system or body part used while exercising. The principle of reversibility is also straightforward, whereas overloading will result in training adaptations, inactivity, or detraining, will result in a return to baseline, or pre-training levels. This relates to the use it or lose it expression, which is commonly stated. Once the chronic stimulus for regular training has been removed, any adaptations made during training will eventually return to baseline or pre-training levels. Such a response is typical when someone will stop training due to illness or injury. Shown here is a classic example of the reversibility principle. Previously, sedentary individuals were endurance trained for eight weeks. The standard markers for endurance training were measured. These include markers of mitochondrial oxidative capacity mentioned previously and maximal oxygen uptake or VOT max. As can be seen, as per the overload principle, eight weeks of endurance training result in an increases in all of these variables. However, when these same individuals stopped all training for a period of six weeks, notice mitochondrial oxidative capacity rapidly return to pre-training values, while maximal oxygen uptake had a more gradual decline. Thus once the stimulus of regular exercise training has been removed, you will eventually lose any previous training adaptations. The principle of individuality relates to the genetic or hereditary component of training adaptations. It states that while the physiological responses to a particular stimulus ar largely predictable, the precise responses and adaptations will vary among individuals. In other words, if we were to initiate an endurance training program on two individuals, who are the same age, sex, and fitness level, we can predict the directions of training adaptations, but the magnitude will likely differ. Based upon genetic characteristics, one individual maybe more responsive to the training stimulus than the other and demonstrate larger increase in such variables as mitochondrial oxidative capacity and maximal oxygen uptake. Identical twins, who have similar genetic material, their response to a training program would be more uniform than two unrelated individuals. In summary, the body's responses to a single bout of exercise are governed by the principle of homeostasis. Training adaptations, both for health and performance, are influenced by the overload, specificity, reversibility, and the individuality principles.
