On 2D plane positive u values are displayed by the red color,
Ta by the green and stretching by the blue color.
This script uses simple explicit algorithm and
512×512 float32 textures with the Neumann (CLAMP_TO_EDGE)
boundary conditions. It makes 2 it time steps dt per frame.
Use Alt- r, s keys to control the script (surprisingly they work in Firefox only).
For small deformations the equilibrium equation in the displacement formulation is 
(μ + λ)∇divu + μΔu
+ f = 0, f = div(I Ta) = ∇Ta,
where u = (X, Y) is displacement of a point from equilibrium position,
μ, λ are Lame parameters (we put μ = 1 ),
Ta represents the active stress in cardiac tissue 
dTa/dt = ε(u) (kT u - Ta).
We consider tissue patch with fixed border uborder = 0.
Using 2-nd order finite differences we obtain the approximate equation
(2 + λ)(Xi+1,j + Xi-1,j - 2Xi,j ) +
Xi,j+1 + Xi,j-1 - 2Xi,j + (Yi+1,j+1 + Yi-1,j-1
- Yi+1,j-1 - Yi-1,j+1 )(1 + λ)/4 + h2Fxi,j = 0,
Fxi,j = (Tai+1,j - Tai-1,j )/2h. (*)
For weighted Jacobi relaxation  of (Xi,j , Yi,j ) on
n×n grid we rewrite equations (*) as
2(3 + λ)Xi,j = (2 + λ)(Xi+1,j + Xi-1,j ) +
Xi,j+1 + Xi,j-1 + (Yi+1,j+1 + Yi-1,j-1
- Yi+1,j-1 - Yi-1,j+1 )(1 + λ)/4 + h2Fxi,j
Multigrid algorithm is used next.
In this simulation λ = 1, ω = 4/5. You can set all parameters in the script
 Linear elasticity (Wiki)
 M.P. Nasha, A.V. Panfilov
Electromechanical model of excitable tissue to study reentrant cardiac arrhythmias
Progress in Biophysics & Molecular Biology 85 (2004) 501.
 William L. Briggs, Van Emden Henson, Steve F. McCormick
A Multigrid Tutorial
(look at "Tutorial Slides")
updated 2 Aug 2012