A mechanical device (1) for harvesting energy from a joint movement. The device comprises a double pivoted linkage (2) fixed to a strap (4a) adapted to be fitted on one limb of the joint. A first driven gear (8) is disposed in a housing (11) adapted to be mounted on the other limb of the joint and rotatably mounted on a first shaft (9). The first shaft is rigidly supported in the said housing. A second driven gear (14) is in mesh with the first driven gear. The second driven gear is mounted on a second shaft (15) over a one way clutch (16). The second shaft is rotatably held in the said housing. A first driver gear (18) is mounted on the said second shaft spaced apart from the said second driven gear. A charger gear (19) is in mesh with the first driver gear and mounted on a third shaft (20) which is rotatably held in the said housing over one way clutches (21). A spiral spring (22) is disposed over the said third shaft spaced apart from the charger gear with the inner end of the spiral spring fixed to the third shaft. A third driven gear (24) is disposed over the spiral spring with the outer end of the spiral spring fixed to the inner surface of the third driven gear. A second driver gear (25) is in mesh with third driven gear and connected to a load (26).