Yuri P. Ivanenko

Auditory information plays a critical role in human posture and locomotion. Beyond its obvious contribution to spatial awareness, sound provides continuous cues about body orientation, movement dynamics and environmental context. This review summarizes current knowledge on how auditory inputs modulate postural stability, locomotor kinematics and muscle coordination through multisensory integration with visual, somatosensory and vestibular systems. We highlight evidence from behavioural, neurophysiological and clinical studies showing that auditory features, for example coming from spatialised sounds, rhythmic stimulation and auditory feedback, can influence gait timing, trajectory control and balance recovery. A dedicated section explores the interaction between the auditory and vestibular systems, focusing on shared neural pathways in brainstem and cerebellum that contribute to motion perception and equilibrium. We also discuss alterations of auditory-motor coupling in neurological and sensory disorders, including vestibular deficits, Parkinson’s Disease and developmental coordination disorders, and we consider the implications for auditory-based rehabilitation. Understanding how sound informs and stabilizes human movement may open new perspectives for multisensory training, neuroprosthetic design and fall prevention strategies.

The first two years of life are critical for the normal development of locomotor circuits. The emergence of independent walking (IW) during this period may reveal how the functional state of neural circuits develops in children with cerebral palsy (CP). In this cross-sectional study, we analysed the activation of eighteen lower limb muscles, corresponding neuromuscular modules, and spinal maps of motoneuron activity in 34 children with CP, in comparison with typically developing (TD) children. Consistency of common modules was used as a criterion for assessing the development of modularity of muscle activity patterns. We found a lack of maturation in children with CP prior to IW, which began late or never. These children exhibited diffused loci of motoneuron activity across lumbosacral segments prior to IW. They also showed two main basic modules, much like newborns, which were augmented to four main modules after IW. The findings indicate that, for IW, the developing spinal circuitry necessitates a differentiation of proximal and distal extensor activity, and an increase in the number of neuromuscular modules under the influence of corticospinal inputs. Early impairments in CP involve conservation of low dimensional control of locomotor movements until IW, suggesting a complex interplay between the maturation of neuromuscular modules and locomotor experience during a critical period of development.

Young infants manifest prominent neuromuscular responses to changes in muscle length, along with a variety of spontaneous movements. The first months of life are an important period during which sensorimotor integration and muscle tone gradually mature. In adults, muscle responses may also be observed when coordinated limb movements are transiently blocked. Given that infants normally exhibit spontaneous limb activity, here we examined whether a transient upper or lower limb block evoked consistent muscle responses while the infants were allowed to perform spontaneous movements with the other limbs. We examined polymyographic recordings in 12 bilateral arm and leg muscles in full-term and preterm infants (1–7 months old). Overall, muscle activity, its spectral characteristics, and agonist–antagonist coactivation were comparable before and after the block in both full-term and preterm infants, suggesting that the transient limb immobilization effect is not evident or consistent, as opposed to previously reported prominent muscle responses to muscle lengthening or shortening. The lack of consistent muscular responses to limb block supports the idea that individual limb motion during spontaneous movements is relatively independent of the control of other limbs, and that sensory input during changes in muscle length is more effective in revealing sensorimotor connections than its non-appearance.

Humans commonly engage in upper limb haptic interactions during bipedal locomotion, and the expansive use of our arms makes us unique compared to the quadrupeds we evolved from. Examples of these haptic interactions include walking while carrying an object, using environmental surfaces such as a railing to provide stability assistance, and holding hands while walking with another individual. These interactions may increase the complexity of our locomotor behaviors, such as when feedforward control is employed to dampen arm motion and dissipate reaction forces at heel contact to stabilize an object we are carrying. However, these interactions also increase the available sensory information in the upper limb and can be utilized to aid in locomotor adaptation. For instance, the interaction forces experienced when holding hands or during collaborative object transport can lead to an unconscious synchronization of gait patterns between the two individuals. Recent work has further suggested that upper limb haptic interactions may have clinical relevance for improving locomotion in pathological populations. This review brings a novel, integrative perspective by examining upper limb haptic interactions in locomotion across everyday, collaborative, and clinical scenarios. In particular, the review highlights the importance of studying upper limb haptic interactions from different viewpoints to gain insight into the neuromechanical control of adaptive locomotion, as well as to investigate how these interactions can be exploited for clinical use.

Introduction Powered lower-limb exoskeletons increasingly show promise for restoring mobility, inducing neuromuscular plasticity, and enriching quality of life in neurological populations (e.g., stroke, spinal cord injury [SCI], cerebral palsy [CP]). However, key challenges persist, namely: understanding mediators of training effectiveness, refining control paradigms for personalization, and assessing real-world deployment. With contributions spanning clinical trials, biomechanics, control mechanisms, and feasibility studies (Figure 1), this collection illuminates progress and remaining hurdles. Figure 1. Overview of key themes emerging from the Research Topic "Exoskeleton Gait Training". Populations studied across the included papers comprised children with cerebral palsy (CP), adults with stroke and spinal cord injury (SCI), and healthy participants. Primary outcomes included improvements in spatiotemporal gait parameters, neuromuscular (EMG) adaptation, cardiovascular benefits, and increased user satisfaction. Enabling factors that contributed to these outcomes were compliant device design, adaptive control strategies, therapist-guided sessions, and training conducted in real-world environments. Mechanisms and Control: Insights for Adaptive Design A central theme of this Research Topic is understanding the mechanisms and control principles that can guide the adaptive design of exoskeletons, as highlighted by several contributions spanning clinical feasibility, neurophysiological mechanisms, and integrative reviews. De Seta and Romeni provide a narrative review on closed-loop systems integrating exoskeletons with neuromodulation (e.g., spinal stimulation) and mobile brain imaging for individuals with neuromotor disorders, underscoring multisystem feedback and personalization as future standards. Hofstetter et al. describe a mixed-methods feasibility study of the HAL® Lumbar Type exoskeleton in long-term care, designed to evaluate usability, acceptance, and its potential to reduce caregivers' lower back strain during physically demanding tasks. The study aims to provide evidence for integrating assistive exoskeletons into routine care settings to support both staff well-being and quality of patient care. Shamantseva et al. examine the effects of transcutaneous spinal cord stimulation on quiet standing in healthy participants - a mechanistic study emphasizing the importance of considering cognitive aspects in balance control and suggesting a potential synergy with exoskeleton training. Avaltroni et al. contribute a physiologically-grounded review of spinal motor maps, highlighting how neuromechanical principles can shape exoskeleton control paradigms and link motoneuron activity to gait mechanics, neuroprosthetics, and therapies for locomotor impairments, with applications in interpersonal coordination, gait assessment, and exoskeleton training. Pediatric Cerebral Palsy: Adaptive Biomechanics in Natural Terrain Several contributions address pediatric cerebral palsy, highlighting how adaptive exoskeletons and biomechanical analyses can inform rehabilitation strategies across different levels of impairment and developmental stages. Tagoe et al. show that two sessions of untethered ankle exoskeleton training on varied terrain significantly improved assisted walking speed (+11 %) and unassisted speed (+8 %) in individuals with CP. Notably, stride length was the primary driver, yet muscle co-contraction remained unchanged—suggesting mechanical rather than neural adaptation during short training spans. The findings highlight the spatiotemporal benefits of an adaptive ankle exoskeleton for individuals with CP in real-world settings. Villani et al. analyze lower-limb EMG activity during exoskeleton-assisted walking in children, revealing distinctive developmental activation patterns that can inform age-adjusted control strategies. Even with full assistance, children's locomotor controllers interpret step-related afferent input to generate essential leg muscle activity, though with notable differences— reduced proximal muscle control, coactivation of lumbar and sacral motor pools, and weak distal extensor propulsion at push-off. This approach shows promise for both assessing exoskeleton performance and for guiding interventions that help children develop natural gait patterns and optimize systems for clinical use. Takahashi et al. report a case study of three children with cerebral palsy undergoing robot-assisted gait training with the pediatric-sized Hybrid Assistive Limb (HAL-2S). The intervention improved hip and knee extension during stance, enhanced muscle activation, and increased patient-reported performance and satisfaction, highlighting the feasibility and potential therapeutic value of HAL-assisted training in pediatric rehabilitation. In the other article, Avaltroni et al. evaluate a reciprocating exoskeleton system in children with severe CP (GMFCS levels IV–V), reporting improved standing and walking capacity alongside specific kinematic adaptations. These findings underscore feasibility even in high-need pediatric cases, allowing walking and training at home in children with severe CP, enhancing development, social interaction, and endurance, while being well-received by families. Neurological Injury: Stroke and SCI Rehabilitation Advances in exoskeleton research for stroke and spinal cord injury demonstrate both clinical efficacy and broader functional benefits, as shown by recent trials and feasibility studies included in this Research Topic. Jin et al. report a randomized clinical trial combining unilateral lower-limb exoskeleton training with conventional therapy post-stroke. Participants achieved faster gait recovery and balance improvements versus controls, highlighting clinical benefit. Nadorf et al. evaluate the ABLE personal-use exoskeleton in individuals with SCI for at-home and community activities, finding high feasibility and usability (in...

Given the cervical spinal cord’s role in locomotor and rhythmic upper limb tasks, its neuromodulation has emerged as an important area of study for understanding human spinal rhythmogenesis. We previously demonstrated that, under unloading conditions, arm muscle vibrostimulation can elicit non-voluntary upper limb oscillations. In this study, we investigated the effects of transcranial magnetic stimulation (TMS) of the motor cortex during both voluntary and non-voluntary (vibration-induced) rhythmic arm movements. We analyzed motor-evoked potentials, mean arm muscle activity, and kinematic parameters of arm movements, including cycle duration and shoulder and elbow joint angular oscillations. Motor-evoked potentials in proximal arm muscles were significantly modulated during both movement types. Notably, low-frequency TMS markedly enhanced non-voluntary arm oscillations, whereas its effect on voluntary movements was statistically non-significant. This differential response is likely due to the absence of characteristic supraspinal influences in sensory-induced spinal activation during non-voluntary movements. These findings align with previous evidence showing that supraspinal pathways facilitate rhythmogenesis in the lower limbs, and they now extend this concept to the upper limbs. Overall, our results suggest that therapies aimed at modulating cervical central pattern generators may benefit from the active engagement of supraspinal motor circuits.

Given the cervical spinal cord’s role in locomotor and rhythmic upper limb tasks, its neuromodulation has emerged as an important area of study for understanding hu-man spinal rhythmogenesis. We previously demonstrated that, under unloading conditions, arm muscle vibrostimulation can elicit non-voluntary upper limb oscillations. In this study, we investigated the effects of transcranial magnetic stimulation (TMS) of the motor cortex during both voluntary and non-voluntary (vibration-induced) rhythmic arm movements. Motor-evoked potentials in proximal arm muscles were significantly modulated during both movement types. Notably, low-frequency TMS markedly enhanced non-voluntary arm oscillations, whereas its effect on voluntary movements was minimal. This differential response is likely due to the absence of characteristic supraspinal influences in sensory-induced spinal activation during non-voluntary movements. These findings align with previous evidence showing that supraspinal pathways facilitate rhythmogenesis in the lower limbs, and they now extend this concept to the upper limbs. Overall, our results suggest that therapies aimed at modulating central pattern generators in the cervical spinal cord may benefit from the active engagement of supraspinal motor circuits.