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Fig. 4. Immediate reaction of the perturbed leg to obstacle contact. (Top) Sketch
of the experimental sequence. (A-C) Top view of leg movements before and after
perturbation in the three behavioural contexts: inner leg (A), straight
walking (B) and outer leg (C). Vectors indicate the displacement of the
femur-tibia joint (FTi; grey in i, light green in ii) and tibia-tarsus joint
(TiTa; black in i, dark green in ii) within 20 ms immediately before (i) and
after (ii) obstacle contact. (iii) The mean vectors. Note that vectors do not
indicate the action of these joints but rather their displacement caused by
the movement of the joints proximal to them. The direction of the FTi vectors
reflects the contribution of the ThC and CTr joints and, therefore, two motion
components only (protraction/retraction, levation/depression). The direction
of the TiTa vectors additionally includes a contribution of the FTi joint
(extension/flexion). Vector length indicates swing velocity. Before
perturbation, leg movements exhibit prominent differences between behavioural
contexts, indicating that different muscle groups were active at the moment of
obstacle contact. In contrast, the patterns of movement after obstacle contact
were all very similar, seemingly converging to a common position.
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