We analyze the role of external forces (both chemical and mechanical) in the kinetics of motor proteins. Based on a generalized detailed balance condition, simple exponential force dependent transition rates are widely used to interpret the available data. Yet, the use of Fokker-Planck equations in continuous models allows for a direct insertion of the force. We describe an analytical approach, based on a renormalization group scheme, to calculate the force dependence of transition rates in a generic model. Our analysis shows that the simple exponential is a good approximation to the correct force dependence only at low values of forces and provided that the step sizes are very small. The law for the force dependent transition rates is tested on a set of data on kinesin, obtaining a good agreement with existing results and predictions for future experiments.