Stability of vertical posture explored with unexpected mechanical perturbations: synergy indices and motor equivalence

Abstract

We explored the relations between indices of mechanical stability of vertical posture and synergy indices under unexpected perturbations. The main hypotheses predicted higher posture-stabilizing synergy indices and higher mechanical indices of center of pressure stability during perturbations perceived by subjects as less challenging. Healthy subjects stood on a force platform and held in fully extended arms a bar attached to two loads acting downward and upward. One of the loads was unexpectedly released by the experimenter causing a postural perturbations. In different series, subjects either knew or did not know which of the two loads would be released. Forward perturbations were perceived as more challenging and accompanied by co-activation patterns among the main agonist-antagonist pairs. Backward perturbation led to reciprocal muscle activation patterns and was accompanied by indices of mechanical stability and of posture-stabilizing synergy which indicated higher stability. Changes in synergy indices were observed as early as 50-100 ms following the perturbation reflecting involuntary mechanisms. In contrast, predictability of perturbation direction had weak or no effect on mechanical and synergy indices of stability. These observations are interpreted within a hierarchical scheme of synergic control of motor tasks and a hypothesis on the control of movements with shifts of referent coordinates. The findings show direct correspondence between stability indices based on mechanics and on the analysis of multi-muscle synergies. They suggest that involuntary posture-stabilizing mechanisms show synergic organization. They also show that predictability of perturbation direction has strong effects on anticipatory postural adjustment but not corrective adjustments. We offer an interpretation of co-activation patterns that questions their contribution to postural stability.

Behavioral synergic relations between eye and postural movements in young adults searching to locate objects in room inside houses

“…Overall, these significant relations may be useful for successful task performance. This general finding goes in line with the general conceptualisation of 'synergies for action stability' already mentioned previously (Latash 2008;Park et al 2012;Yamagata et al 2018) and in populations affected by some diseases such as Parkinson's Disease (Latash and Huang 2015). Indeed, synergies are useful for goal-directed reasons, they are useful to stabilize the body for successful task performance.…”

“…Previously, it has been suggested that postural control may not be controlled for its own sake, but for the achievement of other goals (Stoffregen et al 1999(Stoffregen et al , 2000. Latash and colleagues have invoked the idea of synergies, specifically muscle synergies serving for action stability or stability of performance variable (Park et al 2012;Latash and Huang 2015;Yamagata et al 2018). Our proposal of synergy between eye and postural variables is the same as forwarded by these investigators as our variables exhibit task-dependent negative covariation to stabilize overall performance (stable posture).…”

Static, Dynamic, and Cognitive Fit of Exosystems for the Human Operator

“…The quickest reflexes leave no time for central-executive intervention, but slower reflexes offer wider margin for goals to change reflex expression (Kurtzer, 2015). Measuring reflexes usually involves asking participants to maintain stable postures, introducing often unexpected stimulation, and measuring the size, intensity, and direction of the reflex response (Yamagata et al, 2018). Exosystem design might benefit from leaving short-latency reflexes unconstrained and using longer-latency reflexes as points of entry through which exosystems might blend into dynamic movements.…”

Muscle synergies for the control of single-limb stance with and without visual information in young individuals

“…Literature reports that as difficult the task becomes as higher is the involvement of more proximal joints for the maintenance of balance, in particular the hip [ 5 , 14 ]. In experimental settings, the difficulty of the task is usually increased by increasing the magnitude of a perturbation, by decreasing the magnitude of the supporting surface or by changing the features of the supporting surface [ 5 , 23 , 24 , 26 ]. For example, it has been reported that by moving from a stable to an unstable surface, the angular displacement of the ankle was stable across all the testing condition, with the knee and hip displacement arising when the difficulty of the task was higher [ 5 , 45 ].…”