The metabolic cost of postural variability and vestibular contributions during standing balance
R.G. Rienks (TU Delft - Mechanical Engineering)
E. van der Kruk – Graduation committee member (TU Delft - Biomechatronics & Human-Machine Control)
A.K. Silverman – Graduation committee member (TU Delft - Biomechatronics & Human-Machine Control)
P.A. Forbes – Mentor (TU Delft - Biomechanical Engineering)
M. Leeuwis – Mentor (TU Delft - Biomechatronics & Human-Machine Control)
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Abstract
Maintaining standing balance requires continuous sensorimotor control and is associated with a measurable metabolic cost. While minimization of energy usage is thought to influence human movements, it remains unclear how this applies to standing balance and how postural variability contributes to metabolic cost, because previous studies could not separate the effects of biomechanics from those of variability. This study aimed to quantify the metabolic cost of standing balance and to determine how this cost relates to postural variability independently of biomechanical alterations.
Twenty healthy participants performed seven standing conditions in which postural variability was manipulated using electrical vestibular stimulation (EVS) with varying frequencies and amplitudes. With EVS, postural responses were evoked without mechanically destabilizing the body. Metabolic cost was measured using indirect calorimetry, while postural variability was quantified using center of pressure (CoP) velocity and upper-body kinematics.
Free-standing resulted in an approximately 8% higher metabolic cost compared to externally supported standing, indicating the energetic cost of active balance control. Increasing postural variability through low-frequency EVS significantly elevated metabolic cost and revealed a positive relationship between variability and metabolic cost. In addition, vestibular stimulation and its reflexive muscle activity did not increase metabolic cost when postural variability remained unchanged. Furthermore, movements with comparable variability resulted in similar metabolic costs, regardless of whether they were reflexively induced or self-initiated.
These findings demonstrate that the metabolic cost of standing is primarily determined by postural variability rather than by vestibular input or reflexive muscle activity alone. The results support the hypothesis that humans adopt a preferred level of postural variability that is close to the metabolic minimum, consistent with principles of optimal control.