Muscle Tissue

There are three types of muscle tissue:
Skeletal muscle
Skeletal muscle is also called striated muscle because of a striping pattern that is observed microscopically. Skeletal muscle is the muscle that makes up the muscular system and moves bony levers to cause body movement. Skeletal muscle is described to be voluntary, as it is under voluntary, deliberate mind control.

Cardiac muscle
Cardiac muscle is a specific muscle of the beating or contracting heart. Cardiac muscle is considered to be involuntary and it even generates its own repetitive contractions. Cardiac muscle can be conrtolled indirectly by voluntary methods, such as thinking angry or frightning thoughts or exercising.

Smooth muscle
Smooth muscle lines the walls of hollow organs, airways and arteries. Smooth muscle contractions are involuntary (e.g., peristalsis during digestion).

ATP (adenosine tri-phosphate) is necessary for muscle contractions to occur. ATP is available from the following sources:

Free ATP floating in the intracellular fluid of the muscle cells allows a few seconds of muscular contraction.

ATP from creatine phosphate, which transfer its phosphate group to ADP to form ATP. The creatine phosphate stored in muscle cells allows for about 15 seconds of muscle contraction.

ATP from muscle glycogenolysis when glycogen stored in muscle cells is broken down to glucose and the glucose is metabolized by cellular respiration. Most rapid in the early stages of exercise.

ATP from plasma glucose and fatty acids during prolonged energy requirement comes from glycogen stored in the liver and fat stored in the liver and adipose deposits througout the body.

Cellular respiration is defined by three major metabolic pathways: Anaerobic Glycolysis, Anaerobic Respiration and Aerobic Respiration.

Anaerobic Glycolysis
Glycolysis without the presence of oxygen that metabolizes glycogen (yields 3 ATP) or metabolized glucose (yields 2 ATP). Pyruvate is an end product and when there is no oxygen available to accept Hydrogen ions, the pyruvate must accept the Hydrogen ions and convert to lactate. Note that glycolysis can also be considered the first step in Krebs Cycle of  Tricarboxylic Acid Cycle

Aerobic Oxidation (Krebs Cycle and Electron Transport Chain)
Aerobic Oxidation or oxidative phosphorylation can use carbohydrate, protein or fat as a substrate to produce ATP. The Krebs Cycle and the Electron Transport Chain are two combined processes that occur in mitochondria. Oxygen is the final acceptor of hydrogen ions to form water H2O. This process yields 36 ATPs from glucose and 38 ATPs from glycogen. Fat metabolism or beta-oxidation can yield over 100 ATPs, as each fat molecule is cleaved 2 carbons at time to form acetyl CoA, which enters the Krebs Cycle. Beta-oxidation also occurs in the mitochondria.

Type I (slow twitch) fibers are fatigue resistant and are more active during lower intensity, longer duration activities.

Type II (fast twitch) fibers contract and develop tension more quickly than Type I fibers. Type IIA fibers are good force generators and have some of the endurance feature of Type I muscle fibers. Type IIB fibers are powerful contractors, but have lower capillary density, myoglobin content, mitochondrial density, triglyceride storage, and oxidative enzyme activity,

Fiber type varies according to individual, leaving some individuals with a genetic tendency toward endurance activities and some toward high-speed intense strength activities. Even among individuals of either type, certain muscle groups present a certain type characteristic muscle fiber type. For example, the transversospinalis group of the back; which include rotatores, interspinales, intertransversarii, semispinalis, and multifidis; are primarily Type I muscle fibers designed for endurance core stabilization action for the spinal column. In the calf muscle, the soleus tends to be slow-twitch (Type I) and the lateral and medial gastrocnemius tend to be mixed fast-twitch and slow-twitch.