tension will increase in a graded manner over time in a stepwise fashion known as treppe. In this process, the muscle contractions become more efficient. The phenomenon of treppe results from a higher concentration of calcium ions in the sarcoplasm that can only occur when there is enough ATP in the muscle cell. There is rarely a time when the muscles are completely relaxed. There will always be some amount of tension necessary to maintain its contractile proteins. This is called muscle tone. This requires a complex interaction between the nervous system and skeletal muscle so that few motor units get activated at a time, which allows some motor units to recover while others are active. A lack of low-level contractions is referred to as hypotonia, which may occur when the CNS is damaged in some way. It can also happen if there is a loss of innervation to skeletal muscle. Hypotonic muscles will appear flaccid and will have weak reflexes. Excessive muscle tone is referred to as hypertonia, which is often accompanied by hyperreflexia. Hypertonia can be displayed as spasticity or muscle rigidity as is seen in Parkinson’s disease.
ENERGY EXPENDITURE AND MUSCLE ADAPTATION TO EXERCISE Physical training alters the appearance of skeletal muscle, changing muscle performance. In the same way, a lack of muscle use can result in decreased performance and a change in muscle appearance. Hypertrophy involves an addition of structural proteins to a muscle fiber, while increasing the cell diameter. The reverse occurs when structural proteins are decreased as well as the muscle mass; this is called atrophy. Agerelated atrophy of the muscles is called sarcopenia. Slow fibers are predominant in endurance exercises that require little force but will require numerous repetitions. These fibers will involve aerobic metabolism that allows for the maintenance of contractions over long periods of time. What endurance training does is modify these slow fibers to make them even more efficient by allowing for increased numbers of mitochondria and myoglobin in the muscle cell. Aerobic training will also result in an increase in capillaries around the muscle fiber, which is referred to as angiogenesis. These capillaries supply oxygen and remove 18
