Minimally invasive surgery has encouraged the development of steerable robotic surgical instruments. These instruments require a certain number of actuators in order to operate their various degrees of freedom. High bandwidth distal actuation is a desirable property for such instruments. McKibben actuators have been shown to be able to deliver good dynamic behaviour. In an effort to miniaturize artificial muscles, compact muscles with external diameter of 1.5mm have been introduced in the past. This work proposes a further step in miniaturization by noticing that the inner part of the muscle does not contribute to the muscle functioning. A modified muscle, named concentric muscle, is proposed. This muscle is built up in a concentric fashion around a tube that can serve as a working channel to pass through various types of devices. This new concept does not only allow to optimize the space inside an active miniature instrument but also provides a protective layer to the integrated devices and their wiring that are passed through the working channel.