Background: Ankle plantarflexor muscle impairment contributes to asymmetrical postural control poststroke. Objective: This study examines the relationship of plantarflexor electromyography (EMG) with anterior–posterior center of pressure (APCOP) in people poststroke during progressive challenges to standing balance. Methods: Ten people poststroke and 10 controls participated in this study. Anteriorly directed loads of 1% body mass (BM) were applied to the pelvis every 25–40 s until 5%BM was reached. Cross-correlation values between plantarflexor EMG and APCOP (EMG:APCOP) position and velocity were compared. Results: EMG:APCOP velocity correlations were stronger than EMG:APCOP position across all muscles (p < .01), and correlations were predominately stronger in the nonparetic compared with the paretic leg (p < .05). Increasing challenge to standing balance reduced asymmetry of EMG:APCOP relationships. Conclusions: These data suggest that sensory information reflected in APCOP velocity interacts more strongly with plantarflexor activity in people poststroke and controls than APCOP position. Furthermore, increasing challenge to standing balance reduces postural control asymmetry between legs poststroke.