Prosthesis (1; 101) or orthosis for a joint, such as an ankle, comprising: - a first body (2; 102), - a second body (3; 103), - an articulated joint (4; 104) between the first and second bodies (2, 3; 102, 103), the articulated joint (4; 104) allowing the rotation of the first and second bodies (2, 3; 102, 103) with respect to one another around a joint rotation axis (10; 110), - a locking mechanism (5; 105) configured to selectively lock the rotation between the first and second bodies (2, 3; 102, 103) in one direction, when it is in a locked configuration, - a transmission mechanism (8, 12; 108, 112) between the joint rotation axis (10; 110) and a lockable part (7, 15; 107, 115) of the locking mechanism (5; 105), such that a rotation of the joint rotation axis (10; 110) generates a movement of the lockable part (7, 15; 107, 115), the axis (15; 115) of the movement of the lockable part (7, 15; 107, 115) being shifted relative to the joint rotation axis (10; 110), wherein the transmission mechanism (8, 12; 108, 112) is configured to transfer load between the joint rotation axis (10; 110) and the lockable part (7, 15; 107, 115) of the locking mechanism (5; 105), and comprises a reducer configured to reduce effort to lock the rotation of the first body (2; 102) with respect to the second body (3; 103), and characterized in that the reducer includes a hoist (12; 112) comprising at least one pulley (13; 113, 113') and one rope (14; 114), the pulley or pulleys (13; 113, 113') being linked to the joint rotation axis (10; 110), the rope (14; 114) being linked to the pulley or pulleys (13; 113, 113') and to an anchoring part (11; 111) fixed to the second body (3; 103) while being attached to the lockable part (7, 15; 107, 115) of the locking mechanism (5; 105) - a first body (2; 102), - a second body (3; 103), - an articulated joint (4; 104) between the first and second bodies (2, 3; 102, 103), the articulated joint (4; 104) allowing the rotation of the first and second bodies (2, 3; 102, 103) with respect to one another around a joint rotation axis (10; 110), - a locking mechanism (5; 105) configured to selectively lock the rotation between the first and second bodies (2, 3; 102, 103) in one direction, when it is in a locked configuration, - a transmission mechanism (8, 12; 108, 112) between the joint rotation axis (10; 110) and a lockable part (7, 15; 107, 115) of the locking mechanism (5; 105), such that a rotation of the joint rotation axis (10; 110) generates a movement of the lockable part (7, 15; 107, 115), the axis (15; 115) of the movement of the lockable part (7, 15; 107, 115) being shifted relative to the joint rotation axis (10; 110), wherein the transmission mechanism (8, 12; 108, 112) is configured to transfer load between the joint rotation axis (10; 110) and the lockable part (7, 15; 107, 115) of the locking mechanism (5; 105), and comprises a reducer configured to reduce effort to lock the rotation of the first body (2; 102) with respect to the second body (3; 103), and characterized in that the reducer includes a hoist (12; 112) comprising at least one pulley (13; 113, 113') and one rope (14; 114), the pulley or pulleys (13; 113, 113') being linked to the joint rotation axis (10; 110), the rope (14; 114) being linked to the pulley or pulleys (13; 113, 113') and to an anchoring part (11; 111) fixed to the second body (3; 103) while being attached to the lockable part (7, 15; 107, 115) of the locking mechanism (5; 105)