As Richard S says, suppleness (bendability) and agility (grace and swiftness), are better words to use to refer to physical characteristics.
While flexibility is a physical property, used to describe materials, if you describe a person as flexible we would automatically interpret the term in a different way. If you say,
'Emma is very flexible'
most people would understand this to mean - particularly in a work context - that she is willing and able to alter her schedule, work different hours and days, and so on.