A length-tension curve can also be constructed for a whole skeletal muscle. However, the FCT fibers of the skeletal muscle provide an additional component to the tension produced by the muscle fibers. As the muscle is extended beyond its resting length, the tension produced by the FCT fibers becomes greater and greater. Thus, the tension produced by a whole skeletal muscle increases greatly with increased length.
Oxygen is used by the mitochondria of the muscle to produce energy in the form of ATP.
As a muscle is used, its oxygen supply becomes depleted. Naturally, this depletion occurs more quickly in white striated muscle fibers than it does in red striated muscle fibers. With continued exercise, however, the oxygen becomes depleted in both types of fibers.
However, ATP can still be formed, but much less efficiently, in a sequence which is anaerobic (without oxygen). In this anaerobic sequence, the glucose is only partially decomposed. The ultimate product of the anaerobic sequence is lactic acid.
Lactic acid accumulates in the sarcoplasm of the muscle fibers. As this occurs, the muscle becomes stiffer and is no longer able to function well. This condition is called fatigue. An oxygen debt has been built up during the anaerobic production of ATP. This debt must be paid before the muscle will be able to function properly again.
Tonus is a state of semicontraction of the musculature of the body. The degree of tonus varies considerably with the state of health and exercise of the individual. Tonus serves to remove the slack from the skeletal muscles so they can act immediately when called upon.
Also, at the joints, tonus serves to keep the opposing surfaces of the bones close together. This helps to prevent injury to the articular cartilages during muscular contractions.