The best replacement for stiffness is strength.

The best alternatives to stiffness are rated as following:
★★★★★  strength
★★★  rigidity
★★★  hardness
★★  thickness
★★  modulus
★★  toughness
★  spring constant
★  natural frequency
★  swing speed
★  elasticity
★  tensile strength
  soreness
  ductility
  joint pain
  young's modulus
  compliance
  elastic modulus
  structural stiffness
  dynamic stiffness
  spring rate
  durometer
  modulus of elasticity
  material stiffness
  spasticity

Your alternative for stiffness is not listed? Suggest!
Speaking about stiffness, one probably shouldn't forget about arthritis.

Other search terms related to stiffness and their ratings are:
★★★★★ arthritis
★★★ rheumatoid arthritis
You have in mind a companion for stiffness and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of stiffness in all years. Click on graph for more.
stiffness
strength
rigidity