A strain wave gear mechanism includes: a casing; an elliptical drive shaft; an outer ring arranged coaxially around the elliptical drive shaft and rotationally fixed to the casing, the outer ring having a toothed inner circumference; a flexible ring mounted around the elliptical drive shaft radially inwards of the outer ring, the flexible ring having a toothed outer circumference. Rotation of the elliptical drive shaft causes elliptical rotation of the flexible ring relative to the outer ring such that teeth of the flexible ring engage with teeth of the outer ring at points corresponding to the major axis of the ellipse of the drive shaft to cause movement of the flexible ring in a direction opposite to rotation of the drive shaft. The outer ring is rotationally fixed to the casing via a solenoid operated disconnect gear configured to selectively disconnect the outer ring from the casing.