Many Functional Neuromuscular Stimulation (FNS) systems employ the strategy of co-contracting agonist/antagonist muscle pairs in order to stabilize posture and to resist internal and external disturbances. While increasing stimulation can improve posture control, it comes at a potentially high cost of increased energy expenditure. The suitability of a given co-stimulation level will depend upon the needs of a specific task. In this work, we have developed and investigated a scheme for providing the FNS system user with the ability to adjust co-stimulation levels to meet the needs of a specific task. Musculoskeletal models were used in a series of computer simulation studies to investigate the effect of co-stimulation level on disturbance resistance capability. Based on a cost function, the optimal open-loop co-stimulation level was determined for different weights on effort and controller performance. The results indicate that providing the user with the ability to modify co-stimulation level may be an effective means of adjusting posture in a task-dependent manner.