Abstract: A prosthetic foot can allow a user to engage in different activity levels, for example, walking and running using the same prosthetic foot. The prosthetic foot can have an elongate sole member, a first upper member, and a second pre-compressed upper member such that a distal end of the second upper member is biased toward the first upper member. A spacer between the distal ends of the first and second upper members can facilitate sliding of the distal end of the second upper member during ambulation to vary the stiffness of the foot. The spacer can also improve the lever arm length of the second upper member by maintaining a gap between the first and second upper members throughout ambulation.