Muscle function is based upon three main principles, shown below. These principles, either alone or in conjunction, can be used to explain the properties and functions of muscle, and they are true for all muscles.
3 Principles of Muscles
1. Total muscle force is proportional to the number of attached actin-myosin cross bridges.
2. There are several different energy substrates that provide the energy for force production.
CP = creatine phosphate (creates ATP indirectly via the enzyme creatine phosphokinase or CPK)
The Y-axis shows the relative power or force produced by each energy substrate. The X-axis is time during which the muscle is in use.
3. Muscle force is controlled by the central nervous system (CNS), which acts on motor units, not whole muscles or muscle fibers. A motor unit is defined as the muscle fibers innervated by a single α motor neuron; it is essentially groups of muscles that perform a certain motor function.
A researcher conducted an experiment involving stimulating a single muscle fiber with an electrode and measured the resultant force output. The electrical stimulation lasted until the force generated by the muscle significantly decreased, or for 60 minutes, whichever came first. After many trials, the researcher discovered three different types of muscles, labeled A, B, and C.
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