Table 1.
Definitions of variables and terms.
| symbol | definition | units | relation to other variables |
|---|---|---|---|
| M | body mass | kg | |
| F | compressive load on footpad | N | |
| A | ground contact area of a footpad | mm2 | |
| P | foot pressure | N m−2 or Pa | F/A |
| y | thickness of footpad | mm | |
| Δy | compressive deformation of footpad | mm | |
| σy | compressive stress on the footpad tissue: load on the pad standardized by contact area | N m−2 or Pa | F/A |
| εy | compressive strain in the footpad tissue: deformation of the pad standardized to its original thickness | (dimensionless) | Δy/y |
| Ey | compressive modulus of footpad tissue (a material property) | N m−2 or Pa | σy/εy |
| ky | compressive stiffness of the whole pad (a structural property) | N m−1 | F/Δy |
| U | strain energy storage, the area under load–deformation curvea | J |  |
| u | strain energy density, i.e. strain energy stored per unit volume, which can be calculated as the area under stress–strain curvea | J m−3 |  |
| bY | regression coefficient (or ‘scaling factor’) for the log-transformed value of a given variable, log Y, on log M (see equation (2.3)) | (dimensionless) | |
| scaling | of a variable, the pattern or manner of change that is associated with changes or differences in size. (Here, animal body size is measured as body mass, M.) | ||
| geometry | shape: the spatial arrangement of components. (Distinct from both size and material.) | ||
| material | of or relating to the substance of which an object or component is composed. | ||
| structural | of or relating to an entire object. (Structural properties of an object are a consequence of its size, geometry and material.) | ||
aIf allowance is made for curvature in the load–deformation (or stress–strain) curve the maximum total energy storage will be (slightly) lower than the quantity shown here, which assumes linearly elastic material.