def compute_II_fr(stretch,muscActivation,species):
	""" Compute the firing rates of the II afferent fibers.

	Uses the experimetnally derived model devloped by Prochazka (1999 (2)).
	Keyword arguments:
	stretch -- muscle stretch in mm.
	muscActivation -- normalized muscle activation [0 , 1].
	"""

	bias = 80
	xCoef = 13.5
	musCoeff = 20

	#compute firing rates
	fr_II = bias + xCoef*stretch + musCoeff*muscActivation
	if fr_II<0: fr_II=0

	if species == "rat" or species == "mouse": return fr_II
	elif species == "human": return fr_II*0.2
	else: raise(Exception("Invalid species"))