Humanoid robots are highly redundant systems with respect to the tasks they are asked to perform. This redundancy manifests itself in the number of degrees of freedom of the robot exceeding the dimensionality of the task. Traditionally this redundancy has been utilised through optimal control in the null-space. Some cost function is defined that encodes secondary movement goals and movements are optimised with respect to this function, subject to fulfilment of task constraints. Until now design of cost functions has been carried out on an ad-hoc basis and has required time-consuming hand-tuning to ensure that the desired (or acceptable) behaviour is realised. Here we present a novel approach for designing cost functions for optimal control in the null-space by exploiting recent advances in statistical machine learning. The behaviour of a (kinematically or dynamically controlled) mechanical system performing some task is observed and separated into task- and null-space components. The null-space component is then modelled as a first order differential equation with the cost as the independent variable. Numerical solution of this equation provides training data for a statistical learning algorithm that is used to build an open-form model of the cost function. Results are presented in which the reconstructed function is used to replace that of the original control scheme and the resultant behaviour, for the same set of tasks, is compared.