Physical interaction via a rigid stick between walking partners significantly improved stability after slip perturbations and accelerated recovery, with causality analysis showing increased directional influence from the assisting to the perturbed partner.
Key Findings
Results
Physical interaction between walking partners significantly improved stability after slip perturbations, particularly with rigid coupling.
Twelve dyads walked on a dual treadmill while one or both partners received brief, unanticipated anteroposterior perturbations.
Coupling conditions included holding a rigid stick or an elastic band between partners walking side by side.
The distance between centre of mass and minimal moment axis was used as a measure of instability.
Rigid coupling provided greater assistance and accelerated recovery compared to elastic coupling.
Results
Solo perturbations affecting only one partner led to better stabilization than simultaneous perturbations affecting both partners.
Perturbations were delivered as slips and trips in both solo (one partner perturbed) and simultaneous (both partners perturbed) conditions.
The undisturbed partner played a key role in stabilizing the perturbed partner during solo perturbation conditions.
This finding underlines the role of the undisturbed partner as an active source of mechanical assistance.
Simultaneous perturbations presumably removed the stabilizing contribution of the unperturbed partner.
Results
Granger-Geweke causality analysis revealed increased directional influence from the assisting (unperturbed) to the perturbed partner during perturbation recovery.
Granger-Geweke causality was applied in the frequency domain to assess directional interactions between partners.
Causality analysis showed asymmetric information flow, with the assisting partner exerting greater influence on the perturbed partner.
This directional influence was more pronounced under rigid coupling conditions.
The causality findings suggest that mechanical coupling is accompanied by meaningful information exchanges between partners.
Methods
Whole-body motion capture and ground reaction forces were used alongside the centre of mass-minimal moment axis distance as the primary instability measure.
The study employed a dual treadmill setup to allow independent perturbation delivery to each partner.
Whole-body motion capture was combined with ground reaction force measurements.
The distance between centre of mass and minimal moment axis (CoM-MMA distance) served as the quantitative measure of postural instability.
Both slip and trip perturbation types were examined across coupling conditions.
Results
Simple mechanical coupling via a rigid stick between partners was more effective than elastic coupling for assisting balance recovery after perturbations.
Two types of physical coupling were compared: a rigid stick and an elastic band.
Rigid coupling enhanced assistance and accelerated recovery relative to elastic coupling.
The differential effect between coupling types highlights the role of mechanical stiffness in inter-personal stabilization.
Both coupling types were compared against conditions involving perturbations during dyadic walking.
What This Means
This research suggests that when two people walk side by side while physically connected — such as by holding a stick together — they can help each other recover from unexpected balance disturbances more effectively than when walking alone. The study tested twelve pairs of people walking on a treadmill while one or both received sudden, unexpected pushes or pulls (simulating slips or trips). The researchers found that being connected by a rigid stick provided the most benefit, helping the person who lost balance to recover faster and more stably than when connected by an elastic band or not connected at all.
An important finding was that when only one person in the pair was disturbed, recovery was better than when both were disturbed at the same time. This points to the critical role of the undisturbed partner: having a stable companion who can provide a physical anchor makes a real difference. Using a mathematical technique called Granger-Geweke causality, the researchers also showed that information and mechanical influence flowed directionally from the stable (assisting) partner to the perturbed one, suggesting that the physical connection is not just passive but actually carries meaningful signals between the two people.
This research matters because it provides scientific grounding for practices like guiding an elderly or balance-impaired person by hand or with a cane-like connection. It suggests that simple, low-tech physical links between a helper and someone at risk of falling could meaningfully improve stability during walking, and that the rigidity of such a connection influences how effective the assistance is. Future assistive devices or rehabilitation tools might be designed with these principles in mind.
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Gayrard S, Watier B, Dusfour G, Gowrishankar G, Lalles A, Lagarde J. (2026). Instability analysis of perturbed gait in dyads: holding a stick together provides effective assistance and is accompanied by information exchanges.. Journal of the Royal Society, Interface. https://doi.org/10.1098/rsif.2026.0206