The pneumatic muscle actuator (PM) with axial threads is a different idea from the McKibben PM. The muscle consists of an elastic bladder, similar to a tube, that is attached at either end to fittings. Inside the bladder longitudinal high resistance cables are inserted. Outside some circular rings stop the radial deformation in some points. When the internal bladder is pressurised the actuator shortens. The high power to weight ratio and the compliance of these actuators are attractive for many applications such as prosthetic and orthesis appliances. The working modality of these actuators is repeated contractions and extensions of the muscle. This repeated action is a heavy load for these actuators and leads to failure. The life span of the PM is often shorter than others actuators, such as pneumatic cylinders and electric drives. In this paper a model to predict the life of the PM is presented. Experimental results are carried out to validate the model and show the strong influence of the material used for the bladder. The silicone rubber shows a life of hundreds cycles at full contraction, while it reaches ten thousands cycles at partial contraction.