Control of bipedal posture is highly automatized but requires attentional investment, the amount of which varies between participants and with postural constraints, such as plantar-flexor muscle fatigue. Elevated attentional demands for standing with fatigued plantar flexors have been demonstrated using a stimulus–response reaction-time paradigm. Recently, a direct relation between the regularity of center-of-pressure (COP) fluctuations and the amount of attention invested in posture was proposed, according to which more regular COP fluctuations are expected with muscle fatigue than without. To study this prediction, we registered anterior–posterior COP fluctuations for bipedal stance with eyes closed prior to and after a plantar-flexor muscle fatiguing exercise protocol in 16 healthy young adults. We quantified the magnitude of COP fluctuations with conventional posturography and its regularity with sample entropy. The magnitude of COP fluctuations increased significantly with fatigued plantar flexors. In addition, more regular COP fluctuations were observed with fatigued plantar flexors, as evidenced by significantly lower sample entropy values. These findings corroborated our hypotheses. Moreover, COP regularity assisted in qualifying the change in sway magnitude with fatigue. Whereas increased sway is customary taken to reflect impaired postural control, we interpret it as a functional, but attention-demanding adaptation to the alteration of important posture-specific information.