Capillary Growth An interesting experiment demonstrates the importance of circulatory changes in the improvement in muscular performance due to training. Prior to training the performance of muscles was measured trader ischemic, conditions (blood supply cut off) and with free circulation. These measurements were repeated after training and it was observed that the performance of the muscles under ischemic conditions was increased less than 30 per cent, while under conditions of free circulation the increase amounted to 100 per cent. The slow rate of the improvement in work performance indicated an actual growth of new capillaries rather than a simple dilatation of existing vessels. That the improved circulation in training is due in part to an increase in the number of capillaries in the active muscles is shown by the results of several careful experiments. Training guinea pigs to run increased the proportion of capillaries in heart muscle and in the gastrocnemius muscle by 40 to 45 per cent. There was no change in the number of capillaries in skeletal muscles not used in the training exercise. In the normal, growing heart the number of capillaries increases along with the increase in size of the muscle fibers. In young subjects the increase in vascularity due to training is added to the increase due to growth. Growth and training seem to be specific factors in the increase in capillarity of heart muscle. The increase in size of the heart muscle fibers (hypertrophy) which occurs in certain types of heart disease is not accompanied by a proportional increase in number of capillaries. The smaller increase in heart rate in exercise and the increased work output in trained individuals are both to be attributed to improved vascularization of the skeletal muscles. That the decreased heart rate in exercise is due to peripheral changes and not to changes in the heart itself is indicated by an experiment in which training the arm muscles did not lower the heart rate in exercise involving untrained leg muscles. This experiment furnishes objective evidence that training is specific, i.e., training in one type of activity does not necessarily improve performance in a different type of activity. Paralleling tire increased vascularization of tire heart and skeletal muscles in training is an increase in the capillary supply or the motor area of the cerebral cortex. In growing guinea pigs the vascularity of the motor cortex increases progressively with the development of motor activity. A period of training is accompanied by a marked increase in vascularity of the motor cortex and the ventral horn of the spinal cord at the level of the fifth cervical segment. The sensory areas of the cerebral cortex share in the increased vascularization.		Home Components of Physical Fitness Clinical Examination Differences Between Fit and Unfit Men Physiological Elements of Fitness The Step Test The Effects of Training on the Physiological Systems Work Output Physical Condition of Muscle Application of Muscular Force Maintaining Optimum Strength Tests of Muscular Strength Strength and appearance Strength as a measure of physical fitness Strength as affected by organic problems Strength as prophylaxis Role of inactivity in production of disease Lack of Sociability in Work Lactic Acid System Muscular Form Capillary Growth Heart Size Heart Rate Training produces physicochemical changes Muscular Strength Nervous Stimulation and Muscular Strength Quality of Champions Kinesthesis Balance Reaction Time Muscular Tension Visual Aim