Andre Seyfarth

Adaptive gait control under external perturbations requires coordinated modulation of both local muscle activation and global muscular organisation. To capture these multi-level adaptations, this study integrates co-contraction share analysis, which characterises distributed muscle contributions beyond conventional antagonist-pair co-activation indices, with principal component analysis (PCA) as an analytical framework for examining electromyographic (EMG) responses to frontal–plane angular perturbations during overground walking. EMG signals are recorded from 10 subjects on distal and proximal lower-limb muscles during the pre-perturbation stride (PPS), perturbation stride (PS), and subsequent stride (SS) under four perturbation conditions: midstance ipsilateral, midstance contralateral, touchdown ipsilateral, and touchdown contralateral. Co-contraction share is quantified using summed values, peak values, and peak locations across defined gait phases, while PCA is used to characterise synergistic activation patterns. Perturbation-related modulation is observed across all strides with effect magnitudes ranging from moderate to very large. PPS generally shows more stable activation patterns. PS exhibits the strongest perturbation-induced changes, whereas SS shows either partial recovery or persistence of altered activation depending on muscle group and perturbation condition. Distal muscles demonstrate the largest and most sustained effects, particularly in co-contraction magnitude and timing, indicating a dominant role in immediate stabilisation and post-perturbation recovery. Proximal muscles exhibit earlier, but generally smaller adjustments, consistent with whole-body coordination. PCA reveals fewer significant changes than co-contraction share analysis, possibly reflecting both preservation of global coordination structure and lower sensitivity to localised perturbation effects. These findings support a hierarchically organisation of perturbation responses and demonstrate the value of combining co-contraction share analysis with PCA for characterising muscular adaptation during perturbated gait. The identified control strategies may provide useful design principles for adaptive locomotion control to perturbation in bio-inspired legged robotic and wearable assistive technologies.

This study examines how users sustain effective interaction with assistive technologies when artifact modification is constrained and breakdowns are consequential. Prior research offers limited insight into how users develop novel interaction patterns before routinization. We introduce technique innovation as a novel, enacted interaction pattern that becomes reliable through repeated use without altering the artifact. Drawing on a qualitative multiple-case study based on 18 interviews with para-athletes, we identify 38 technique innovations and a recurring process through which they emerge and stabilize. Most identified techniques improved interaction reliability rather than enabling new actions. Our findings contribute by (1) conceptualizing technique innovation as a distinct post-adoptive behavior and (2) advancing a system-level account of interaction breakdowns, including inter-technology misalignment.

Design codes for pedestrian-induced vibration on footbridges treat each pedestrian asa prescribed vertical force on the deck. This convention ignores the feedback by which deck motion modifies the gait, and at resonance overpredicts the measured field amplitudes.This paper introduces a load model for vertical human-structure interaction that closes this feedback loop. The per-step force each pedestrian exerts on the bridge splits into two parts. The first is the classical rigid-ground walking force. The second is an interaction component proportional to the deck velocity, which acts on the deck like an amplitude-dependent damper. At resonance this damping is negative and the pedestrian injects energy into the deck. The response self-limits at a finite equilibrium amplitude, because the negative damping weakens as the deck amplitude rises. Away from resonance the coupling reduces to the positive damping the pedestrian's own body contributesA main advantage of the proposed model is that its stability criterion and equilibrium amplitude require no prescribed driving force, unlike classical moving-force models. Furthermore, its parameters transfer to any bridge through the first-mode frequency. Another advantage is that for groups the per-pedestrian damping contributions add linearly across the deck.Without recalibration, the model reproduces the reported field amplitudes of four independently documented footbridges within ratios 0.93 to 1.43, while classical moving-force models overestimate them by factors 3.3 to 33.

Design codes for pedestrian-induced vibration on footbridges treat each pedestrian as a prescribed vertical force on the deck. This convention ignores the feedback by which deck motion modifies the gait, and at resonance overpredicts the measured field amplitudes.This paper introduces a load model for vertical human-structure interaction that closes this feedback loop. Each pedestrian is modelled as a single-degree-of-freedom oscillator whose leg stiffness varies over the gait cycle. On a vibrating deck this periodic stiffness couples the pedestrian to the deck motion. The per-step force on the bridge is then the classical rigid-ground walking force plus an interaction component proportional to the deck velocity, which acts on the deck like an amplitude-dependent damper. At resonance this damping is negative and the gait pumps energy into the deck. The response self-limits at a finite equilibrium amplitude, because the negative damping weakens as the deck amplitude rises. Away from resonance the damping stays positive.A main advantage of the proposed model is that its stability criterion and equilibrium amplitude require no prescribed driving force, unlike classical moving-force models, and that its parameters transfer to any bridge through the first-mode frequency, with the amplitude scales of the coupling tied to the comfort thresholds of design codes. A second advantage is that for groups the per-pedestrian damping contributions add linearly across the deck.Without recalibration, the model reproduces the reported field amplitudes of four independently documented footbridges within ratios 0.93 to 1.43, while classical moving-force models overestimate them by factors 3.2 to 33.The framework yields a closed-form stability criterion and a self-limiting equilibrium amplitude, both obtained without coupled crowd simulation and suited to footbridge serviceability assessment.

Human walking is inherently three-dimensional, requiring coordinated control in multiple planes of motion. Among these, controls in the sagittal and frontal planes play central roles in propulsion and balance. While sagittal plane control has been extensively modelled, the control mechanisms underlying frontal plane stability remain less understood. In this study, we hypothesize that frontal plane control can be achieved through a simple yet biologically inspired strategy: modulating hip compliance in response to ground reaction forces (GRFs). To validate this, we extend our previously proposed bipedal walking control framework, termed Concerted Control, to three dimensions and implement this control law at both the joint and muscle levels in bipedal models. Across a wide range of walking speeds (0.7–1.8 m s-1), both implementations yield dynamically stable three-dimensional gaits without any movement constraints, and their hip abduction–adduction torque profiles closely match human data, achieving cross-correlation values above 0.86 in most conditions. These results demonstrate that GRF-based hip compliance modulation is a parsimonious and robust control principle for frontal plane stability. More broadly, our findings highlight the potential of GRF-driven compliance control as a unifying strategy for neuromechanical modelling and bio-inspired bipedal locomotion.

Evaluating bioinspired design principles in wearable assistive devices provides a unique opportunity to interrogate our understanding of the critical factors that enable agile, stable, and economical human movement. We introduce the BiArticular Thigh EXosuit (BATEX), a wearable device integrating two morphological features found in biological legged systems: biarticular muscles and elastic tissues. BATEX employs two biarticular springs spanning the hip and knee to emulate the human rectus femoris and hamstring muscles, creating beneficial synergy to enhance walking economy. This design enables two energy-shuffling mechanisms: temporal (spring-like storage/return at a joint) and spatial (strut-like transfer across joints). In walking experiments at 1.3 m/s with N = 9 participants, a single compliant biarticular spring yielded a 7% metabolic cost reduction compared to walking without BATEX. Individually optimized configurations further improved metabolic reduction to 9%. BATEX morphology allowed users not only to off-load biological joint power (Assist) but also to increase total power (Augment). Across all exosuit configurations, the mechanical impact of the exosuit was reflected by a significant correlation between changes in users’ biarticular muscles’ activity and changes in net metabolic rate. In sum, compliant-biarticular exosuit architectures can concurrently assist and augment human lower-limb joint function, providing significant metabolic savings during walking.

Passive exosuits offer potential for reducing the metabolic cost of walking, but evaluating their effectiveness is challenging due to difficulties in measuring internal biomechanics and muscle-level energetics. This study investigates the metabolic effects of the BATEX, a passive biarticular thigh exosuit assisting hip and knee flexion/extension. The exosuit features a spring parallel to the rectus femoris (RF) and a spring parallel to the hamstring (HAM). We combined experimental data from ten healthy adults walking on a treadmill under various exosuit conditions (different stiffness and configurations) with EMG-informed musculoskeletal simulations to estimate muscle-level behavior. The simulations aimed to replicate experimental metabolic measures, analyze cost fluctuations across the gait cycle, and elucidate muscle-tendon energetic responses, comparing individuals experiencing reductions (positive responders, PR) versus increases (negative responders, NR) in metabolic cost. Simulation results successfully replicated experimental whole-body metabolic cost trends for most participants (8/10, ${r} \gt {0}.{7}$ , ${p}\lt {0}.{05}$ ), showing comparable overall net reductions primarily driven by significant savings during the swing phase. Combining RF and HAM springs resulted in complex, non-additive interactions. Muscle-level analysis identified both targeted effects on hip/knee flexors/extensors and non-targeted effects on ankle and hip ab/adductor muscles, with responses differing markedly between PR and NR groups. Distinct energy-saving mechanisms were associated with each spring type (RF: reduced activation; HAM: reduced fiber velocity). The differing responses of the PR and NR groups to the same device setup motivate personalization, and our muscle-level insights offer a foundation for future device refinement.

Walking on sloped terrain requires substantial mechanical and control adaptations for effective energy management compared to level ground locomotion. The Virtual Pivot Point (VPP) hypothesis explains sagittal plane angular momentum regulation during level walking, but its validity in slope walking remains unexplored. This study combines human experiments with template-model simulations to investigate how the VPP strategy is modulated during slope walking. Participants walked on an instrumented ramp at various inclinations (0°, ± 7.5°, ± 10°), while a 2D spring-loaded inverted-pendulum model with a trunk segment simulated the task. Experimental results confirmed that the VPP is a robust feature of slope walking ( R ² > 0.975). The simulation reproduced the change in hip torque and trunk adaptations by modulating VPP position. Results of this study indicate that VPP position and trunk dynamics could afford stability and energy management on gentle slopes, but to robustly navigate steeper ramps, humans recruit a multi-joint strategy where the knee and ankle joints play a crucial role in managing the energetic demands of sloped terrain. Beyond advancing our understanding of locomotor control, these insights have practical implications for the design of exoskeletons that adapt to uneven terrain.

Background/Objectives: This study investigated the biomechanics of a hip exoskeleton during sit-to-stand transitions. Methods: Eleven participants performed the task under three conditions: without the exoskeleton (No Exo), wearing the exoskeleton without assistance (Exo Off), and wearing it with hip extension assistance (Exo On). Results: The analyses revealed that joint angles (hip, knee, and ankle) and vertical ground reaction forces were comparable across all conditions. However, Exo Off significantly increased transition time, whereas Exo On did not differ significantly from No Exo. Additionally, both exoskeleton conditions led to increased integrated EMG (iEMG) activity in the rectus femoris, vastus medialis, and gluteus maximus—likely due to the added device mass. Notably, iEMG analysis revealed a significant reduction in gluteus maximus activity in Exo On compared to Exo Off. Conclusions: Despite providing only moderate torque assistance (0.12 Nm/kg), the results suggest that well-timed exoskeleton support can partially reduce the physical demands of sit-to-stand transitions. However, the observed reduction in gluteus maximus activity was limited, likely reflecting the combined effects of the assistance strategy, including its magnitude and timing, user adaptation and training, postural demands due to device weight and external torques, and mechanical constraints such as potential joint misalignment. Further research is needed to optimize hip exoskeleton support for daily activities.

Mental images such as foci of attention can significantly enhance the quality of movements, providing a positive effect on human performance, e.g., in dancers or athletes. Thirteen participants (height = 161 ± 13 cm, mass = 46.4 ± 17.3 kg, and age = 21 ± 8.4 years) with varying levels of experience in classical ballet were divided into three groups (amateur, professional, and children). Each participant performed three sauté en suite jumps, followed by an instruction to imagine “taking the floor with them” during the jump. The study aimed to assess the effect of this external focus on jumping performance using biomechanical modeling. Results showed a statistically significant increase in jump height and an expanded range of motion in the hip and knee joints after the intervention, suggesting a positive influence on movement quality. However, results varied among groups, with no significant change in leg stiffness across participants, though tendencies appeared within each group. These findings indicate that an external focus of attention could be a useful tool in dance pedagogy, enhancing performance quality across experience levels and supporting individual progress. The study recommends further research to explore the full impact of psychologically effective instructions on various aspects of physical performance.