Spring-based passive ankle exoskeletons have been designed to emulate the energy conservation and power amplification roles of biological muscle–tendon units during locomotion. Yet, it remains unknown if similar assistive devices can serve the other elastomechanical role of biological muscle–tendon units — power attenuation. Here we explored the effect of bilateral passive ankle exoskeletons on neuromuscular control and muscle fascicle dynamics in the ankle plantarflexors during rapid, unexpected vertical perturbations. We recorded muscle activation and soleus fascicle length changes during hopping with and without exoskeleton assistance (0 and 76 Nm rad⁻¹) on elevated platforms (20 cm), which were removed at an unknown time. Our results demonstrate that exoskeleton assistance leads to a reduction in soleus muscle activation, increases in fascicle length change and decreases in muscle forces during perturbed hopping. These changes have competing effects on the mechanics and energetics of lower limb muscles, likely limiting the capacity for series elastic tissues to absorb energy. As we strive towards the design of wearable assistive devices for everyday locomotion, information regarding real-time muscle–tendon behavior may enable tunable assistance that adapts to both the user and the environment.