Tendon stiffness and Young's modulus both refer to the ratio of stress to strain with no difference in physical meaning.

 Tendon stiffness can only be measured during passive stretching and not during dynamic activities like running.

Tendon stiffness is defined as the ratio of extension to force applied and is independent of the tendon’s cross-sectional area.

Tendon stiffness is the ratio of applied force (N) to the extension (m) produced, while the modulus describes the relationship of stress to strain.