Activity patterns and timing of muscle activity in the forward walking and 4 backward walking stick insect

34 Understanding how animals control locomotion in different behaviors requires understanding 35 both the kinematics of leg movements and the neural activity underlying these movements. 36 Stick insect leg kinematics differ in forward and backward walking. Describing leg muscle 37 activity in these behaviors is a first step towards understanding the neuronal basis for these 38 differences. 39 40 We report here the phasing of EMG activities and latencies of first spikes relative to precise 41 electrical measurements of middle leg tarsus touch down and lift-off of three pairs 42 (protractor/retractor coxae, levator/depressor trochanteris, extensor/flexor tibiae) of stick 43 insect middle leg antagonistic muscles that play central roles in generating leg movements 44 during forward and backward straight walking. Forward walking stance phase muscle 45 (depressor, flexor and retractor) activities were tightly coupled to touch down, beginning on 46 average 93 ms prior to, and 9 ms and 35 ms after touch down, respectively. Forward walking 47 swing phase muscle (levator, extensor and protractor) activities were less tightly coupled to 48 lift-off, beginning on average 100, 67, and 37 ms before lift-off, respectively. In backward 49 walking the protractor/retractor muscles reversed their phasing compared to forward 50 walking, with the retractor being active during swing and the protractor during stance. 51 Comparison of intact animal and reduced twoand one-middle-leg preparations during 52 forward straight walking showed only small alterations in overall EMG activity but changes 53 in first spike latencies in most muscles. Changing body height, most likely due to changes in 54 leg joint loading, altered the intensity, but not the timing, of depressor muscle activity. 55 56 57