Spontaneous breathing effort of a mechanically ventilated patient can seriously deteriorate the treatment outcome if it is not taken into account when choosing the appropriate settings. This paper presents an improved spontaneous breathing effort model that can be used to develop improved mechanical ventilation algorithms. Through an indicative experimental study it is shown that this model accurately describes the spontaneous breathing effort of a healthy human test subject. In comparison to the commonly used sinusoidal halfwave effort model it is shown that the proposed model improved fitting quality by a factor two.