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The unfolding of proteins induced by a large mechanical force pulling the termini apart have been widely investigated during the past decade. In contrast, the equilibrium properties of a protein in the presence of a small force have not been clearly determined or related to the nonequilibrium response of the same protein under a large force. We show through atomistic numerical simulations that the equilibrium free-energy landscape of an immunoglobulin domain changes in a complex fashion that cannot be described in terms of one-dimensional projections, as generally assumed in the interpretation of mechanical (un)folding experiments, or predicted from the unperturbed free-energy landscape. We also show that the assumption that the relative free-energy between a pair of states varies linearly with the force leads to parameters that do not correspond to those computed from the equilibrium simulations. [ABSTRACT FROM AUTHOR]