pH-responsive hydrogels are capable of converting chemical energy to mechanical work. To optimize their use as actuators, their response when operating against an external load must be fully characterized. Here, the actuation strain of a model pH-sensitive hydrogel as a function of different constant loads is studied. The experimental actuation strain, produced by switching the pH from 2 to 12, decreases significantly and monotonically with increasing initial tensile load. Two models are developed to predict the actuation strain as a function of applied stress. Simple mechanical models based on the change in hydrogel modulus and cross sectional area due to the change in pH are unsatisfactory as they predict only a small change in actuation strain with increasing external stress. However, the model based on the elastic and mixing free energy functions derived from the Flory-Huggins theory is found to accurately account for the actuation strain as a function of stress.