Solutions of the Hamilton-Jacobi equation H(x,−Du(x))=1, with H(⋅,p)
H\"older continuous and H(x,⋅) convex and positively homogeneous of
degree 1, are shown to be locally semiconcave with a power-like modulus. An
essential step of the proof is the C1,α-regularity of the
extremal trajectories associated with the multifunction generated by DpH