A crank apparatus (100, 100A, 200, 200A, 200B), comprising: a crank arm (110, 110A, 210, 210A, 210B), having at least one cavity (111, 211, 211A) on one of the surfaces (115, 116) of the crank arm (110, 110A, 210, 210A, 210B); at least one thin material layer (120, 220, 220A, 220B) having an exposed outer surface (122, 222, 222A, 222B), an embedded surface disposed on another surface of the at least one thin material layer (120, 220 220A, 220B) relatively away from the exposed outer surface (122, 222, 222A, 222B), and a plurality of lateral surfaces respectively connected between the exposed outer surface (122, 222, 222A, 222B) and the embedded surface; and at least one sensing element (130, 230, 230A1, 230A2, 230A3, 230A4, 230B1, 230B2, 230B3, 230B4); wherein the crank arm (110, 110A, 210, 210A, 210B) is manufactured of a material having non-uniform strain characteristic, the at least one thin material layer (120, 220, 220A, 220B) is manufactured of a material having uniform strain characteristic, the crank arm (110, 110A, 210, 210A, 210B) is adapted to be subjected to force and deformed, the at least one thin material layer (120, 220, 220A, 220B) is adapted to be deformed corresponding to the deformation of the crank arm (110, 110A, 210, 210A, 210B), and the at least one sensing element (130, 230, 230A1, 230A2, 230A3, 230A4, 230B1, 230B2, 230B3, 230B4) is adapted to measure the corresponding strain of the at least one thin material layer (120, 220, 220A, 220B) caused by a force applied to the crank arm (110, 110A, 210, 210A, 210B); characterized in that the at least one cavity (111, 211, 211A) of the crank arm (110, 110A, 210, 210A, 210B) comprises a plurality of protruded walls respectively blocking a portion of the at least one cavity (111, 211, 211A) and covering at least one edge of the at least one thin material layer (120, 220, 220A, 220B) in such a way that the movement of the at least one thin material layer (120, 220, 220A, 220B) along any three orthogonal axial directions in the crank arm (110, 110A, 210, 210A, 210B) is limited by the plurality of protruded walls and the at least one thin material layer (120, 220, 220A, 220B) is clamped and firmly embedded into the at least one cavity (111, 211, 211A) by the plurality of protruded walls, wherein the at least one sensing element (130, 230, 230A1, 230A2, 230A3, 230A4, 230B1, 230B2, 230B3, 230B4) is attached to the exposed outer surface (122, 222, 222A, 222B) of the at least one thin material layer (120, 220, 220A, 220B). a crank arm (110, 110A, 210, 210A, 210B), having at least one cavity (111, 211, 211A) on one of the surfaces (115, 116) of the crank arm (110, 110A, 210, 210A, 210B); at least one thin material layer (120, 220, 220A, 220B) having an exposed outer surface (122, 222, 222A, 222B), an embedded surface disposed on another surface of the at least one thin material layer (120, 220 220A, 220B) relatively away from the exposed outer surface (122, 222, 222A, 222B), and a plurality of lateral surfaces respectively connected between the exposed outer surface (122, 222, 222A, 222B) and the embedded surface; and at least one sensing element (130, 230, 230A1, 230A2, 230A3, 230A4, 230B1, 230B2, 230B3, 230B4); wherein the crank arm (110, 110A, 210, 210A, 210B) is manufactured of a material having non-uniform strain characteristic, the at least one thin material layer (120, 220, 220A, 220B) is manufactured of a material having uniform strain characteristic, the crank arm (110, 110A, 210, 210A, 210B) is adapted to be subjected to force and deformed, the at least one thin material layer (120, 220, 220A, 220B) is adapted to be deformed corresponding to the deformation of the crank arm (110, 110A, 210, 210A, 210B), and the at least one sensing element (130, 230, 230A1, 230A2, 230A3, 230A4, 230B1, 230B2, 230B3, 230B4) is adapted to measure the corresponding strain of the at least one thin material layer (120, 220, 220A, 220B) caused by a force applied to the crank arm (110, 110A, 210, 210A, 210B);