Head kinematic variability is minimal near preferred cadence and independent of the vestibular control of locomotion

The unstable nature of our bipedal posture requires continuous feedback to maintain internal estimates of self-motion and generate appropriate balance-correcting responses. This feedback control process involves the integration of multisensory information, including vestibular cues of head motion. Minimizing head motion variability may optimize the information transmitted by vestibular signals that are important for balance control and consequently drive vestibular contributions to locomotion which decrease as we move faster. In this study, participants walked outdoors at 40% to 140% of their preferred step cadence while we characterized head kinematic variability and vestibular-evoked balance responses to electrical vestibular stimulation, a common method to generate virtual signals of head movement. Head kinematic variability was lowest near participants' preferred step cadences (90-126 steps/min) and gait speeds (1.1-1.7 m/s) while vestibular-evoked responses decreased exponentially as step cadence and gait speed increased. Hence, the minima of head kinematic variability were close to preferred step cadences, near previously established minima for the metabolic cost of locomotion. The relationship between head kinematic variability and vestibular-evoked balance responses, however, was inconsistent across all step cadences, suggesting that head kinematic variability did not drive vestibular-evoked balance response magnitude. The observed reduction in the variability of head motion signals at the preferred locomotor cadence/speed may serve to improve our self-motion estimates and reduce information processing requirements to ensure effective navigation, thereby potentially contributing to the well-established minimum in metabolic cost near preferred cadence.