It is disclosed a composite yarn structure (10) comprising: —a first element (15) comprising a coaxial flexible bi-component monofilament including a conductive component (20) and a thermoplastic component (30) exhibiting piezoelectric properties, —at least a second element (40) twisted around the first element (15), wherein the second element (40) has a lower elasticity with respect to the elasticity of the first element (15) such that, upon elongation of the yarn structure (10) in a first direction, the yarn structure (10) expands in a second direction, whereby the dimensions of the yarn are increased both in first and second directions to generate an additional force on said piezoelectric component of the first element.