In the above equation BW(t) is the body weight, t represents the discrete-
time increments for weight estimation and measurement;
A(t) represents the input of the model, namely Body Area; nti is
the number of time delays between each input i and their first
effects on the output; a(z1) is the nominator polynomial and
equals 1 þ a1z1 þ a2z2 þ þana zna ; bi(z1) is the denominator
polynomials linked with the inputs i and is equal to
b0i þ b1iz1 þ b2iz2 þ þbnbi znbi ; aj, bi are the model parameters
to be estimated; z1 is the backward shift operator, defined as
z1y(k) = y(k 1); na, nbi are the orders of the respective
polynomials.