This 1980 landmark paper by Kastelic, Palley, and Baer proposes a structural-mechanical model for the toe region of the tendon stress-strain curve. The model — called SSL (Sequential Straightening and Loading) — attributes the toe region to the fascicle's distribution of collagen fibril crimp angles, not to a single representative fibril. It was validated against rat tail tendon data at 3, 6, and 24 months of age.
Before this model, the toe region was either attributed to a single representative fibril or modeled with arbitrarily chosen statistical distributions that had no morphological basis. Kastelic's SSL model was the first to ground toe-region mechanics directly in observed fascicle structure.
For clinical practice, the toe region defines the physiological loading range for tendon. Activity within the toe region recruits fibrils progressively without generating high stress in any individual fibril — this is the biomechanical rationale for low-load, high-repetition loading protocols in early tendon rehabilitation.
As aging reduces crimp angle, the toe region shortens. Older tendons transition to high-stress linear loading at lower elongations, which helps explain the increased tendon injury risk with age.
This paper established the structural framework that underpins modern understanding of tendon preconditioning, stress-shielding, and the biomechanical rationale for preserving tendon tissue rather than resecting it.
This 1980 landmark paper by Kastelic, Palley, and Baer proposes a structural-mechanical model for the toe region of the tendon stress-strain curve. The model — called SSL (Sequential Straightening and Loading) — attributes the toe region to the fascicle's distribution of collagen fibril crimp angles, not to a single representative fibril. It was validated against rat tail tendon data at 3, 6, and 24 months of age.
Before this model, the toe region was either attributed to a single representative fibril or modeled with arbitrarily chosen statistical distributions that had no morphological basis. Kastelic's SSL model was the first to ground toe-region mechanics directly in observed fascicle structure.
For clinical practice, the toe region defines the physiological loading range for tendon. Activity within the toe region recruits fibrils progressively without generating high stress in any individual fibril — this is the biomechanical rationale for low-load, high-repetition loading protocols in early tendon rehabilitation.
As aging reduces crimp angle, the toe region shortens. Older tendons transition to high-stress linear loading at lower elongations, which helps explain the increased tendon injury risk with age.
This paper established the structural framework that underpins modern understanding of tendon preconditioning, stress-shielding, and the biomechanical rationale for preserving tendon tissue rather than resecting it.