In biomechanics recent mathematical models allow to predict the muscular force response to functional electrical stimulations (in short, FES). The main concern of the present paper is to deal with the computation of optimized electrical pulses trains (for example in view of maximizing the final force response). Using the fact that FES are modeled as Dirac pulses, our problem is rewritten as an optimal sampled-data control problem where the control parameters are the pulses amplitudes and the pulses times. We establish the corresponding Pontryagin first-order necessary optimality conditions and we show how they can be used in view of numerical simulations.