A contractile unit comprising a contractile hose extending between two spaced head pieces and adapted to perform a longitudinal contraction on being subjected to internal pressure, and a position sensor means responsive to the distance between the head pieces. The position sensor means possesses a microwave generator arranged on one of the head pieces and adapted to emit microwaves into the interior space of the contractile unit, an evaluating means being provided responsive to the distance between the head pieces using transit time measurement, phase comparison of the emitted and reflected wave or using determination of the resonant frequency.