Each muscle fiber may contract completely, partially, or not at all, depending on the strength of the stimulus

The stimuli that would allow the muscle fibers to contract could vary in terms of intensity. For example, writing is a simple job and that it may require the muscles to contract only partially. However, other jobs such as lifting of heavy objects would require the muscles to be completely contracted. 

The two primary Stem Cells are Pluripotent (Embyotic) and Somatic (Adult).

Explanation:

Pluripotent stem cells can turn into any type of cell in an adult body.

Somatic stem cells can only differentiate into specific cells in an organ or tissue.