Passive resting state and history of antagonist muscle activity shape

34 35 Limb movements can be driven by muscle contractions, external forces or intrinsic 36 passive forces. For lightweight limbs like those of insects or small vertebrates, passive forces 37 can be large enough to overcome the effects of gravity and may even generate limb 38 movements in the absence of active muscle contractions. Understanding the sources and 39 actions of such forces is therefore important in understanding motor control. We describe 40 passive properties of the femur-tibia joint of the locust hind leg. The resting angle is 41 determined primarily by passive properties of the relatively large extensor tibiae muscle and is 42 influenced by the history of activation of the fast extensor tibiae motor neuron. The resting 43 angle is therefore better described as a history dependent resting state. We selectively 44 stimulated different flexor tibiae motor neurons to generate a range of isometric contractions 45 of the flexor tibiae muscle, and then stimulated the fast extensor tibiae motor neuron to elicit 46 active tibial extensions. Residual forces in the flexor muscle have only a small effect on 47 subsequent active extensions, but the effect is larger for distal than for proximal flexor motor 48 neurons, and varies with the strength of flexor activation. We conclude that passive properties 49 of a lightweight limb make substantial and complex contributions to the resting state of the 50 limb which must be taken into account in the patterning of neuronal control signals driving its 51 active movements. Low variability in the effects of the passive forces may permit the nervous 52 system to accurately predict their contributions to behaviour. 53