An actuator comprising: at least one strand (10a, 10b, 10c, 10d, 10e, 10f) made of shape memory material and having a length that is variable between a first length corresponding to a contracted state of the SMM strand and a second length corresponding to an extended state of the SMM strand, one of the contracted and extended states being an undeformed state and the other one thereof involving a predetermined lengthwise deformation of the SMM strand from the undeformed state, wherein the SMM strand is arranged to return to the undeformed state from the state involving a predetermined lengthwise deformation when it is heated to a predetermined temperature, the SMM strand comprising a fixed end attached to a fixed mechanical interface of a support structure, and a moving end attached to a mobile mechanical interface movable between first and second positions in such a way that the mobile interface may move from one of the first and second positions to the other thereof when the variable length of the SMM strand changes from one of the first and second lengths to the other thereof, electrical means (16a, 16b, 16c, 16d, 24a, 24b, 26) for heating said strand (10a, 10b, 10c, 10d, 10e, 10f) comprising at least one heating wire (16a, 16b, 16c, 16d) coiled around the strand; characterized in that the coiled heating wire (16a, 16b, 16c, 16d) forms a circular helix, the pitch of which is arranged to increase along with the length of the strand when the length of the strand changes from the first length to the second length, and arranged to decrease along with the length of the strand when the length of the strand changes from the second length to the first length, and in that the circular helix has a uniform slope ( b a) comprised in the range between 0.62 and 0.76, preferably in the range between 0.625 and 0.73, when the length of the SMM strand (10a, 10b, 10c, 10d, 10e, 10f) is the first length, so that any change of the arc length of the circular helix caused by the SMM strand (10a, 10b, 10c, 10d, 10e, 10f) extending from the first length to the second length, or contracting from the second length to the first length, amounts to no more than 0.2%. at least one strand (10a, 10b, 10c, 10d, 10e, 10f) made of shape memory material and having a length that is variable between a first length corresponding to a contracted state of the SMM strand and a second length corresponding to an extended state of the SMM strand, one of the contracted and extended states being an undeformed state and the other one thereof involving a predetermined lengthwise deformation of the SMM strand from the undeformed state, wherein the SMM strand is arranged to return to the undeformed state from the state involving a predetermined lengthwise deformation when it is heated to a predetermined temperature, the SMM strand comprising a fixed end attached to a fixed mechanical interface of a support structure, and a moving end attached to a mobile mechanical interface movable between first and second positions in such a way that the mobile interface may move from one of the first and second positions to the other thereof when the variable length of the SMM strand changes from one of the first and second lengths to the other thereof, electrical means (16a, 16b, 16c, 16d, 24a, 24b, 26) for heating said strand (10a, 10b, 10c, 10d, 10e, 10f) comprising at least one heating wire (16a, 16b, 16c, 16d) coiled around the strand; characterized in that the coiled heating wire (16a, 16b, 16c, 16d) forms a circular helix, the pitch of which is arranged to increase along with the length of the strand when the length of the strand changes from the first length to the second length, and arranged to decrease along with the length of the strand when the length of the strand changes from the second length to the first length, and in that the circular helix has a uniform slope ( b a) comprised in the range between 0.62 and 0.76, preferably in the range between 0.625 and 0.73, when the length of the SMM strand (10a, 10b, 10c, 10d, 10e, 10f) is the first length, so that any change of the arc length of the circular helix caused by the SMM strand (10a, 10b, 10c, 10d, 10e, 10f) extending from the first length to the second length, or contracting from the second length to the first length, amounts to no more than 0.2%.