Let C be the family of compact convex subsets S of the hemisphere in R n with the property that S contains its dual S * ; let u ∈ S * , and let (S, u) = 2 ω n S θ, u dσ (θ). The problem to study inf (S, u), S ∈ C, u ∈ S * is considered. It is proved that there exists a minimizing couple (S, u) such that S is self-dual and u is on its boundary. More can be said for n = 3: the minimum set is a Reuleaux triangle on the sphere. The previous problem is related to the one to find the maximal length of steepest descent curves for quasi-convex functions, satisfying suitable constraints. For n = 2, let us refer to [Manselli P, Pucci C. Maximum length of steepest descent curves for quasi-convex functions. Geom. Dedicata. 1991]. Here, quite different results are obtained for n ≥ 3.