A trigger assembly for an automatic injection device including a biased element and a housing, the biased element releaseable by operation of the trigger assembly for movement in a first axial direction relative to the housing, the trigger assembly comprising: a button pressable by a user, said button shiftable in the first axial direction relative to the housing from a first axial position to a second axial position, said button including a blocking element including a blocking surface and a recessed surface, said recessed surface being spaced in a first angular direction from said blocking surface; a first engagement surface disposed on the biased element and movable therewith; a lock member supported within said housing to be rotatable therein, said lock member including a button-engaging element; a second engagement surface disposed on said lock member for engagement with said first engagement surface, said engagement of said first and second surfaces restraining motion of the biased element in the first axial direction; at least one of said first and second engagement surfaces being ramped for camming effect between said first and second engagement surfaces; said blocking element, when said button is disposed at said first axial position, having said blocking surface in abutting relationship with said button-engaging element to prevent rotation of said lock member in said first angular direction, which prevention of rotation maintains said second engagement surface in engagement with said first engagement surface; and said blocking element, when said button is disposed at said second axial position, having said blocking surface clear of said button-engaging element to allow said lock member, under influence of the camming effect of said first and second engagement surfaces as the biased element is biased in the first axial direction, to rotate such that said button-engaging element moves in the angular direction toward said recessed surface, said first and second engagement surfaces disengaging as said lock member rotates to release the biased element for movement in the first axial direction. a button pressable by a user, said button shiftable in the first axial direction relative to the housing from a first axial position to a second axial position, said button including a blocking element including a blocking surface and a recessed surface, said recessed surface being spaced in a first angular direction from said blocking surface; a first engagement surface disposed on the biased element and movable therewith; a lock member supported within said housing to be rotatable therein, said lock member including a button-engaging element; a second engagement surface disposed on said lock member for engagement with said first engagement surface, said engagement of said first and second surfaces restraining motion of the biased element in the first axial direction; at least one of said first and second engagement surfaces being ramped for camming effect between said first and second engagement surfaces; said blocking element, when said button is disposed at said first axial position, having said blocking surface in abutting relationship with said button-engaging element to prevent rotation of said lock member in said first angular direction, which prevention of rotation maintains said second engagement surface in engagement with said first engagement surface; and said blocking element, when said button is disposed at said second axial position, having said blocking surface clear of said button-engaging element to allow said lock member, under influence of the camming effect of said first and second engagement surfaces as the biased element is biased in the first axial direction, to rotate such that said button-engaging element moves in the angular direction toward said recessed surface, said first and second engagement surfaces disengaging as said lock member rotates to release the biased element for movement in the first axial direction.