Locomotor resilience through load-dependent modulation of muscle co-contraction

Timing of the co-activation of antagonist muscles affects foreleg movements during mantis strikes

The foreleg movements during mantis strikes can be determined by three joint angles: the prothorax-coxa (P-C), coxa-trochanter (C-T), and femur-tibia (F-T). To understand how foreleg movements are controlled by muscle activities in the Chinese mantis, we combined motion analysis with electromyogram (EMG) recordings from three antagonist pairs, one at a time: the coxal promotor and remotor in the P-C joint, the trochanteral extensor and flexor in the C-T joint, and the tibial extensor and flexor in the F-T joint. During the P-C joint angle increase, a burst of spikes was observed in the EMG recordings from the coxal promotor. The promotor burst was accompanied by a burst of spikes in the coxal remotor. Co-activation of antagonist muscles, the extensor and flexor, was also observed at the C-T joint, but not at the F-T joint. The burst duration of the promotor or extensor mainly determined the peak angle of the P-C, C-T, and F-T joints. Co-activation of antagonists observed during P-C and C-T joint angle increases likely plays a role in adjusting the peak joint angle; the peak joint angle became smaller as remotor (or flexor) bursts occurred earlier. Variation in the timing of antagonist bursts may finely regulate the peak joint angle.

Mechanosensory encoding of forces in walking uphill and downhill: force feedback can stabilize leg movements in stick insects

Force feedback could be valuable in adapting walking to diverse terrains, but the effects of changes in substrate inclination on discharges of sensory receptors that encode forces have rarely been examined. In insects, force feedback is provided by campaniform sensilla, mechanoreceptors that monitor forces as cuticular strains. We neurographically recorded responses of stick insect tibial campaniform sensilla to "naturalistic" forces (joint torques) that occur at the hind leg femur-tibia (FT) joint in uphill, downhill, and level walking. The FT joint torques, obtained in a previous study that used inverse dynamics to analyze data from freely moving stick insects, are quite variable during level walking (including changes in sign) but are larger in magnitude and more consistent when traversing sloped surfaces. Similar to vertebrates, insects used predominantly extension torque in propulsion on uphill slopes and flexion torques to brake forward motion when going downhill. Sensory discharges to joint torques reflected the torque direction but, unexpectedly, often occurred as multiple bursts that encoded the rate of change of positive forces (dF/dt) even when force levels were high. All discharges also showed hysteresis (history dependence), as firing substantially decreased or ceased during transient force decrements. These findings have been tested in simulation in a mathematical model of the sensilla (Szczecinski NS, Dallmann CJ, Quinn RD, Zill SN. Bioinspir Biomim 16: 065001, 2021) that accurately reproduced the biological data. Our results suggest the hypothesis that sensory feedback from the femoro-tibial joint indicating force dynamics (dF/dt) can be used to counter the instability in traversing sloped surfaces in animals and, potentially, in walking machines.NEW & NOTEWORTHY Discharges of sensory receptors (campaniform sensilla) in the hind legs of stick insects can differentially signal forces that occur in walking uphill versus walking downhill. Unexpectedly, sensory firing most closely reflects the rate of change of force (dF/dt) even when the force levels are high. These signals have been replicated in a mathematical model of the receptors and could be used to stabilize leg movements both in the animal and in a walking robot.

A hierarchical model for external electrical control of an insect, accounting for inter-individual variation of muscle force properties

Cyborg control of insect movement is promising for developing miniature, high-mobility, and efficient biohybrid robots. However, considering the inter-individual variation of the insect neuromuscular apparatus and its neural control is challenging. We propose a hierarchical model including inter-individual variation of muscle properties of three leg muscles involved in propulsion (retractor coxae), joint stiffness (pro- and retractor coxae), and stance-swing transition (protractor coxae and levator trochanteris) in the stick insect Carausius morosus. To estimate mechanical effects induced by external muscle stimulation, the model is based on the systematic evaluation of joint torques as functions of electrical stimulation parameters. A nearly linear relationship between the stimulus burst duration and generated torque was observed. This stimulus-torque characteristic holds for burst durations of up to 500ms, corresponding to the stance and swing phase durations of medium to fast walking stick insects. Hierarchical Bayesian modeling revealed that linearity of the stimulus-torque characteristic was invariant, with individually varying slopes. Individual prediction of joint torques provides significant benefits for precise cyborg control.

Roles of muscle activities in foreleg movements during predatory strike of the mantis

Foreleg trajectory in the mantis strike varies depending on prey distance. To examine how muscle activities affect foreleg trajectory, we recorded strike behaviours of the Chinese mantis with a high-speed camera and electromyograms of the foreleg trochanteral extensor and flexor. At the approach phase of the mantis strike, the prothorax-coxa (P-C) joint elevated and the femur-tibia (F-T) joint extended. At the sweep phase, the coxa-trochanter (C-T) joint rapidly extended, then, the F-T joint rapidly flexed to capture the prey. At capture initiation, the C-T joint extended more with greater prey distance. After cutting the tendon of the trochanteral flexor, the C-T joint extended similarly to that of the intact foreleg but did not flex after it reached its peak angle. After cutting the tendon of the trochanteral extensor, the C-T joint did not extend as much as that of the intact foreleg. During rapid extension of the C-T joint, a burst of spikes from the coxal trochanteral extensor was observed in electromyograms. Among several parameters, burst duration was the best predictor of C-T joint angular change during strike. Unexpectedly, trochanteral flexor activity was also observed during rapid extension of the C-T joint. These results indicated that the coxal trochanteral extensor mainly contributed to the rapid C-T extension during strike, but other muscles also contributed at the beginning of extension. The trochanteral flexor appeared to contribute to C-T flexion by countering the rapid extension.

Adaptive load feedback robustly signals force dynamics in robotic model of Carausius morosus stepping

Animals utilize a number of neuronal systems to produce locomotion. One type of sensory organ that contributes in insects is the campaniform sensillum (CS) that measures the load on their legs. Groups of the receptors are found on high stress regions of the leg exoskeleton and they have significant effects in adapting walking behavior. Recording from these sensors in freely moving animals is limited by technical constraints. To better understand the load feedback signaled by CS to the nervous system, we have constructed a dynamically scaled robotic model of the Carausius morosus stick insect middle leg. The leg steps on a treadmill and supports weight during stance to simulate body weight. Strain gauges were mounted in the same positions and orientations as four key CS groups (Groups 3, 4, 6B, and 6A). Continuous data from the strain gauges were processed through a previously published dynamic computational model of CS discharge. Our experiments suggest that under different stepping conditions (e.g., changing "body" weight, phasic load stimuli, slipping foot), the CS sensory discharge robustly signals increases in force, such as at the beginning of stance, and decreases in force, such as at the end of stance or when the foot slips. Such signals would be crucial for an insect or robot to maintain intra- and inter-leg coordination while walking over extreme terrain.