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.