FROM MY PT SCHOOL NOTES · FUNCTIONAL ANATOMY & BIOMECHANICS · CHAPTER 5
Why Do Tissues Respond Differently to Load and Time?
The same force can have very different effects on a tendon, ligament, bone or nerve. Material and geometry matter, and so does how the load is applied.
Don’t mix up stress, strain and deformation
Stress describes applied force per cross-sectional area. Strain describes relative deformation, often ΔL/L, while absolute deformation describes an actual change in dimension. A thicker sample can behave differently under the same total load because its area differs.
The course’s stress–strain plots introduce toe, elastic, plastic and failure regions. These curves are simplified material descriptions, not clinically safe loading limits for a particular patient.
See why time matters
Viscoelastic tissues show both elastic and time-dependent behavior. Creep describes additional deformation under a sustained load; stress relaxation describes reduced stress under a maintained deformation.
The response also depends on rate of loading, temperature, hydration, tissue structure and history. A single graph does not fully predict adaptation or healing.
Match your explanation to the tissue
The separate tissue lecture compares collagen-rich connective tissues, articular cartilage, bone and peripheral nerve structures. Their composition and loading roles are not interchangeable.
A useful clinical reasoning question is whether the problem concerns tissue capacity, loading frequency, direction of force or healing stage. Do not convert classroom failure loads into patient-specific exercise prescriptions.
Quick check: Why isn’t a measured tissue deformation the same as strain?
Deformation is an absolute dimensional change; strain is normalized to the original dimension, often expressed as a proportion or percentage.
Try the mechanics on a real task
Example: Two samples of the same tissue type are loaded with the same total force, but one has twice the cross-sectional area. Both are held under load for a prolonged interval.
Work through the reasoning
Answer: Stress (force per area) is lower in the larger-area sample, while time-dependent creep can still occur. Actual deformation also depends on material properties and loading history, not solely stress magnitude.
Common trap: Using force, stress, deformation and strain as interchangeable words. Remember: Specify the quantity, units and time conditions.
Sources: my original FABIO lectures: Properties of Biological Tissues; Tendons, Ligaments, Cartilage, Bones, Nerves. Explanatory examples are newly written. This is a conceptual mechanics lesson, not a treatment instruction. Publisher or lecture figures are not reproduced.
