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.