Mathematical principles of reinforcement (MPR; Killeen, 1994) is a useful model for predicting and describing behaviour. MPR is based upon three general principles represented by three independent parameters in the model: (1) incentives elicit responses and the quality or amount of a reinforcer will effect organisms arousal and motivation to work for that particular reinforcer (specific activation; a in the model), (2) there are time and energy constraints on responding resulting in ceilings on response rates (constraint; δ), and (3) reinforcement only occurs to the extent that responses are associated with reinforcers (Coupling; c.). This experiment tested the ability of MPR to predict response rates when the minimum force requirement to make a response and the reinforcer duration was increased. Five hens responded on a geometric fixed ratio (FR) schedule in three conditions; low force requirement, small reinforcer (0.24 N, 2-s access), high force requirement, small reinforcer (1.1 N, 2-s access) and high force requirement, large reinforcer (1.1 N, 4-s access). Response rates were well described by a bitonic function as predicted. Response rates were higher and the hens ceased responding at higher FR values when the force requirement was low. Constraint (δ) was not significantly affected by the increase in force requirement. Interestingly, estimates of a were most effected by the increase in force requirement. Estimates of a were not affected by the increase in the magnitude of the reinforcer. It was concluded that doubling the incentive value may require more than doubling the amount of reinforcement. Additionally, there may be a need to distinguish between time to make a response and effort required with the latter impacting more on the motivation of the organism.