Anticipatory muscle activations to coordinate balance and movement during motor transitions: A narrative review

Maintaining balance while moving is vital for day-to-day activities. A key challenge in the comprehension of human movement is to determine how muscles contribute to balance-movement coordination. Motor transitions, defined as movements executed between two steady balance states, are particularly interesting phases to study balance-movement coordination because a large, discrete change in whole-body momentum may disturb balance. During voluntarily-initiated motor transitions, anticipatory muscle patterns provide the biomechanical conditions that are favourable to both maintaining balance and executing the movement.

What are the mechanical consequences of anticipatory muscle activations for balance-movement coordination during voluntarily-initiated motor transitions?

We review the biomechanical contributions of the anticipatory muscle activations identified in the literature during four types of voluntarily-initiated motor transitions, through the prism of three balance mechanisms ('moving the centre of pressure (CoP)', 'counter-rotating segments', and 'applying new external force(s)'). In particular, we investigate how anticipatory muscle activations modulate whole-body centre of mass acceleration.

We show that the mechanical consequences of anticipatory muscle activations have been extensively described, but mainly using the 'moving the CoP' mechanism. Unlike their role during steady balance states, both 'moving the CoP' and 'applying new external force(s)' mechanisms create a required mechanical instability during the anticipatory phase of motor transitions. The 'counter-rotating' mechanism may act as a stabiliser during motor transitions, but additional research is needed to clarify this assumption.

This review establishes that muscle activation processes have different mechanical consequences for balance-movement coordination during the anticipatory phases of motor transitions, compared to steady balance states. Because the mechanical instability that is created can lead to falls, a better understanding of the mechanisms underlying motor transitions is needed to enable the design of more effective fall prevention programs and/or devices for population with balance deficits.