A cardiac fibre undergoes complex rhythmic contractions and relaxation for every pumping cycle. These motions are represented by two sets of differential equations:

 ϵ 𝑑𝑥/𝑑𝑡 = −(𝑥 3 + 𝑇. 𝑥 + 𝑏)  𝑑𝑏/𝑑𝑡 = 𝑥 − 𝑥𝑑 + (𝑥𝑑 − 𝑥𝑑)𝑢 

x(t) represents the change in length of a heart muscle fibre. b(t) represents the change in the strength of the electrical impulse initiating the systole.