Controlling a flexible robot arm driven by McKibben artificial muscles with direct transmission is delicate. The usual PID controller rapidly reveals itself to be inadequate and robust control tools are unavoidable. Classical sliding control, although robust, generates chatter. Several solutions are available to attenuate this phenomenon, among them the twisting and super twisting algorithms, which belong to the 2-sliding control set. It will be shown when to use the equivalent control and the effect of a noised sensor signal on control performance. Also, the use of an additional discontinuous term that increases robustness, performance and stability is put forward. Experimental results are presented and discussed.