It sounds like Nick is suffering from a infection of some sort. The name of this infection he may be suffering from is onychomycosis.
Autoimmune diseases, heart diseases, stroke, cancers, diabetes, chronic kidney disease, osteoporosis,Alzheimer's disease, and cataracts.
When you flex your muscles you are contracting them. When your muscle fibers gain tension through Actin and Myosin crossbridge cycling they may shorten due to the tension. This is called muscle contraction