We analyzed the response to strain of an infinite polyalanine chain in the α-helical conformationusing density functional theory. Under compressive strain the α-helix is found to undergo structural transitionsto a π-helix when the length of the helix is reduced by more than 10%. Under tensile strain the structurechanges into a 310-helix when the length is stretched by more than 10%. Our analysis of these transitionsshows that they proceed essentially in two steps: At first there is mainly a length change, and only withsome delay the helix twist adjusts.