Swaying slower reduces the destabilizing effects of a compliant surface on voluntary sway dynamics

Abstract

The ability to control weight shifting (voluntary sway) is a crucial factor for stability during standing. Postural tracking of an oscillating visual target when standing on a compliant surface (e.g. foam) is a challenging weight shifting task that may alter the stability of the system and the muscle activation patterns needed to compensate for the perturbed state. The purpose of this study was to examine the effects of surface stability and sway frequency on the muscle activation of the lower limb, during visually guided voluntary postural sway. Seventeen volunteers performed a 2-min voluntary sway task in the anterior-posterior direction following with their projected center of pressure (CoPAP) a periodically oscillating visual target on a screen. The target oscillated at a frequency of 0.25 Hz or 0.125 Hz, while the participants swayed on solid ground (stable surface) or on a foam pad (unstable surface), resulting in four experimental conditions. The electromyogram (EMG) of 13 lower limb muscles was measured and the target–CoPAP coupling was evaluated with coherence analysis, whereas the difference in the stability of the system between the conditions was estimated by the maximum Lyapunov exponent (MLE). The results showed that slower oscillations outperformed the faster in terms of coherence and revealed greater stability. On the other hand, unstable ground resulted in an undershooting of the CoPAP to the target and greater MLE. Regarding the EMG data, a decreased triceps surae muscle activation at the low sway frequency compared to the higher was observed, whereas swaying on foam induced higher activation on the tibialis anterior as well. It is concluded that swaying voluntarily on an unstable surface results in reduced CoPAP and joint kinematics stability, that is accomplished by increasing the activation of the distal leg muscles, in order to compensate for this perturbation. The reduction of the sway frequency limits the effect of the unstable surface, on the head and upper body, improves the temporal component of coherence between CoP and target, whereas EMG activity is decreased. These findings might have implications in rehabilitation programs.

Neuromechanics of Dynamic Balance Tasks in the Presence of Perturbations

“…We detected maximal joint moments between 100 and 500 Nm, which are substantially higher compared to the joint moments reported for postural swaying (Hess et al, 2006 ) and equivalent to running (Stefanyshyn and Nigg, 1998 ; Arampatzis et al, 1999 ), jumping (Nikolaidou et al, 2017 ), and landing (Walsh et al, 2011 ; Werkhausen et al, 2018 ). Recently, we found greater movement instability and higher EMG activity of the leg muscles in unstable ground conditions during postural tracking of an oscillating visual target (Patikas et al, 2019 ). The higher activation of the leg muscles was the consequence of the increased movement instability during postural swaying on unstable ground and was interpreted as a compensation mechanism to ensure balance in the presence of external perturbations (Patikas et al, 2019 ).…”

Perturbation‐based exercise for prevention of low‐back pain in adolescent athletes

“…It has been reported that external perturbations increase movement instability and challenge the neuromotor system during motion. [32][33][34] As a response, the neuromotor system modifies the motor control and increases the systems robustness (ie, ability to cope with perturbations 32,33 ). Recently, we found that, in the presence of perturbations, the central nervous system generates fuzzier, less unstable, and less complex basic activation patterns of muscle groups, which makes the motor execution less prone to the influence of disturbances.…”

“…Challenging conditions, as for example the exposure to external perturbations, enhance the demand for the sensorimotor system to perceive sensory signals and to generate appropriate motor commands. It has been reported that external perturbations increase movement instability and challenge the neuromotor system during motion 32‐34 . As a response, the neuromotor system modifies the motor control and increases the systems robustness (ie, ability to cope with perturbations 32,33 ).…”

Standing on unstable surface challenges postural control of tracking tasks and modulates neuromuscular adjustments specific to task complexity

“…In the presence of external mechanical perturbations, the local dynamic stability during walking 10 , 12 , 13 , running 10 and balance tasks 7 , 14 decreases. At the same time, electromyographic (EMG) activity and muscle coactivation increases 14 – 16 , indicating a modulation of the neuromotor control in the presence of perturbations. Furthermore, during perturbed conditions the basic activation patterns of muscle groups become wider increasing the temporal overlap of activation between chronologically adjacent muscle synergies 10 , 17 , 18 .…”

“…Oullier et al 26 using the moving room paradigm found a decrease in the room-head coupling with increasing room frequency depicting a demand-dependent deterioration of visual coupling performance. While the effect of unstable surfaces on postural control during quiet stance has been widely investigated 14 , 21 , 27 , 28 , there is still a gap in the literature regarding possible modulations of motor control in the presence of perturbations during more complex tasks, such as visually guided postural sway, especially when the target is moving with high complexity.…”