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

Purpose Single-limb stance is a demanding postural task featuring a high number of daily living and sporting activities. Thus, it is widely used for training and rehabilitation, as well as for balance assessment. Muscle activations around single joints have been previously described, however, it is not known which are the muscle synergies used to control posture and how they change between conditions of normal and lack of visual information. Methods Twenty-two healthy young participants were asked to perform a 30 s single-limb stance task in open-eyes and closed-eyes condition while standing on a force platform with the dominant limb. Muscle synergies were extracted from the electromyographical recordings of 13 muscles of the lower limb, hip, and back. The optimal number of synergies, together with the average recruitment level and balance control strategies were analyzed and compared between the open- and the closed-eyes condition. Results Four major muscle synergies, two ankle-dominant synergies, one knee-dominant synergy, and one hip/back-dominant synergy were found. No differences between open- and closed-eyes conditions were found for the recruitment level, except for the hip/back synergy, which significantly decreased (p = 0.02) in the closed-eyes compared to the open-eyes condition. A significant increase (p = 0.03) of the ankle balance strategy was found in the closed-eyes compared to the open-eyes condition. Conclusion In healthy young individuals, single-limb stance is featured by four major synergies, both in open- and closed-eyes condition. Future studies should investigate muscle synergies in participants with other age groups, as well as pathological conditions.

Modulation of intracortical inhibition during physically performed and mentally simulated balance tasks

Purpose Action observation (AO) during motor imagery (MI), so-called AO + MI, has been proposed as a new form of non-physical training, but the neural mechanisms involved remains largely unknown. Therefore, this study aimed to explore whether there were similarities in the modulation of short-interval intracortical inhibition (SICI) during execution and mental simulation of postural tasks, and if there was a difference in modulation of SICI between AO + MI and AO alone. Method 21 young adults (mean ± SD = 24 ± 6.3 years) were asked to either passively observe (AO) or imagine while observing (AO + MI) or physically perform a stable and an unstable standing task, while motor evoked potentials and SICI were assessed in the soleus muscle. Result SICI results showed a modulation by condition (F2,40 = 6.42, p = 0.009) with less SICI in the execution condition compared to the AO + MI (p = 0.009) and AO (p = 0.002) condition. Moreover, switching from the stable to the unstable stance condition reduced significantly SICI (F1,20 = 8.34, p = 0.009) during both, physically performed (− 38.5%; p = 0.03) and mentally simulated balance (− 10%, p < 0.001, AO + MI and AO taken together). Conclusion The data demonstrate that SICI is reduced when switching from a stable to a more unstable standing task during both real task execution and mental simulation. Therefore, our results strengthen and further support the existence of similarities between executed and mentally simulated actions by showing that not only corticospinal excitability is similarly modulated but also SICI. This proposes that the activity of the inhibitory cortical network during mental simulation of balance tasks resembles the one during physical postural task execution.

A Simple Yet Effective Whole-Body Locomotion Framework for Quadruped Robots

In the context of legged robotics, many criteria based on the control of the Center of Mass (CoM) have been developed to ensure a stable and safe robot locomotion. Defining a whole-body framework with the control of the CoM requires a planning strategy, often based on a specific type of gait and a reliable state-estimation. In a whole-body control approach, if the CoM task is not specified, the consequent redundancy can still be resolved by specifying a postural task that set references for all the joints. Therefore, the postural task can be exploited to keep a well-behaved, stable kinematic configuration. In this work, we propose a generic locomotion framework which is able to generate different kind of gaits, ranging from very dynamic gaits, such as the trot, to more static gaits like the crawl, without the need to plan the CoM trajectory. Consequently, the whole-body controller becomes planner-free and it does not require the estimation of the floating base state, which is often prone to drift. The framework is composed of a priority-based whole-body controller that works in synergy with a walking pattern generator. We show the effectiveness of the framework by presenting simulations on different types of simulated terrains, including rough terrain, using different quadruped platforms.

Rate of concurrent augmented auditory feedback in postural control learning in adolescents

Introduction: The main objective of this study was to determine the rate of auditory concurrent feedback that best enhanced the learning of a continuous postural task in adolescents. Material and methods: A sample of thirty adolescents (13 to 14-years old) was used, who were assigned to three groups: i) control group (CG); ii) 100% auditory feedback group and iii) 67% auditory feedback group. The subjects performed a pre-test, practice, post-test and a retention (24 hours after the practice). In the postural control task subjects were instructed to remain on a seesaw (unstable in anteroposterior position) and keeping it as level as possible. Results: The results demonstrated that concurrent auditory feedback did not enhance the performance of the continuous postural task, although concurrent auditory feedback (both 100% and 67% rates) caused changes to the postural control strategies. Conclusions: From this it was concluded that 100% and 67% concurrent auditory feedback are more recommendable than no-feedback in adolescents’ postural control learning.

The Interaction Between Long-Term Memory and Postural Control: Different Effects of Episodic and Semantic Tasks

The aim of this experiment was to investigate the postural response to specific types of long-term memory (episodic vs. semantic) in young adults performing an unperturbed upright stance. Although a similar level of steadiness (mean distance) was observed, dual tasking induced a higher velocity, more energy in the higher frequency range (power spectral density), and less regularity (sample entropy) compared with a simple postural task. Moreover, mean velocity was always greater in the semantic than in the episodic task. The differences in postural control during dual tasking may result from the types of processes involved in the memory task. Findings suggest a spatial process sharing between posture and episodic memory.

Efectos de la auto-selección del foco de atención durante el aprendizaje de una tarea de control postural (Effect of self-selection of the attentional focus during a postural control task learning)

El objetivo principal de este estudio es analizar el efecto de la auto-selección del foco de atención sobre el aprendizaje de una tarea motriz. Cuarenta participantes se distribuyeron en cuatro grupos en función del foco de atención (en la tarea motriz o cognitiva) y la selección del foco (auto-selección o imposición). Todos los sujetos realizaron un pre-test, práctica, post-test y retención (24 horas después). Durante los test solo realizaron una tarea de equilibrio sobre una plataforma que generaba inestabilidad en el eje medio-lateral. Sin embargo, durante la práctica todos realizaron la tarea de equilibrio junto con una tarea cognitiva de memoria de trabajo. El grupo centrado en la tarea cognitiva (impuesto) mostró una mejora de todas las variables de la tarea motriz en la retención. La conclusión de este estudio es que la imposición del foco de atención sobre la tarea cognitiva favorece el aprendizaje motriz de una tarea de equilibrio.Abstract. The main aim of this study is to analyse the impact of the self-selection of the focus of attention on the learning of a motor task. Forty participants were divided into four groups according to focus of attention (focused on motor or cognitive tasks) and focus selection (self-selection or imposition). All subjects performed a pre-test, practice, post-test, and retention (24 hours later). During tests, participants performed the postural task on a balance platform with instability in the medio-lateral direction. However, during practices they all carried out the same postural task, together with a backward span digit test. The group which focused on the cognitive task (imposed) showed improvements in all variables at retention. As a conclusion, imposing the focus of attention in cognitive tasks favours motor learning.

Behavioral and Neural Correlates of Cognitive-Motor Interference during Multitasking in Young and Old Adults

The concurrent performance of cognitive and postural tasks is particularly impaired in old adults and associated with an increased risk of falls. Biological aging of the cognitive and postural control system appears to be responsible for increased cognitive-motor interference effects. We examined neural and behavioral markers of motor-cognitive dual-task performance in young and old adults performing spatial one-back working memory single and dual tasks during semitandem stance. On the neural level, we used EEG to test for age-related modulations in the frequency domain related to cognitive-postural task load. Twenty-eight healthy young and 30 old adults participated in this study. The tasks included a postural single task, a cognitive-postural dual task, and a cognitive-postural triple task (cognitive dual-task with postural demands). Postural sway (i.e., total center of pressure displacements) was recorded in semistance position on an unstable surface that was placed on top of a force plate while performing cognitive tasks. Neural activation was recorded using a 64-channel mobile EEG system. EEG frequencies were attenuated by the baseline postural single-task condition and demarcated in nine Regions-of-Interest (ROIs), i.e., anterior, central, posterior, over the cortical midline, and both hemispheres. Our findings revealed impaired cognitive dual-task performance in old compared to young participants in the form of significantly lower cognitive performance in the triple-task condition. Furthermore, old adults compared with young adults showed significantly larger postural sway, especially in cognitive-postural task conditions. With respect to EEG frequencies, young compared to old participants showed significantly lower alpha-band activity in cognitive-cognitive-postural triple-task conditions compared with cognitive-postural dual tasks. In addition, with increasing task difficulty, we observed synchronized theta and delta frequencies, irrespective of age. Task-dependent alterations of the alpha frequency band were most pronounced over frontal and central ROIs, while alterations of the theta and delta frequency bands were found in frontal, central, and posterior ROIs. Theta and delta synchronization exhibited a decrease from anterior to posterior regions. For old adults, task difficulty was reflected by theta synchronization in the posterior ROI. For young adults, it was reflected by alpha desynchronization in bilateral anterior ROIs. In addition, we could not identify any effects of task difficulty and age on the beta frequency band. Our results shed light on age-related cognitive and postural declines and how they interact. Modulated alpha frequencies during high cognitive-postural task demands in young but not old adults might be reflective of a constrained neural adaptive potential in old adults. Future studies are needed to elucidate associations between the identified age-related performance decrements with task difficulty and changes in brain activity.