Fig. 3. Analysis of tibia placement in horizontal walking. (A) The end of the tibia (dark gray dots) and the center of mass (COM; white dot) were digitized using motion analysis software. These points were used to calculate the distance of the end of the tibia during the stance phase to the COM (r; white line) and the angle of the tibia relative to the COM (; angle of white line and black line). (B) Leg placement values were plotted in a polar graph (r, ) for three successive steps in an intact aged animal (60 weeks post-adult molt). The start of the arrow shows the anterior extreme position (AEP), and the arrowhead represents the posterior extreme position (PEP). (C) As a comparison, leg trajectories for steps of the same animal shown in B were plotted after `tarsus catch' was evident (week 63). This graph shows that tibia placement is altered slightly in trials with `tarsus catch'. (D) Leg placement values in a 1-week-old adult. (E) Summary polar plot. Values of r and during the AEP and PEP were averaged in the same-aged animals in trials before and after `tarsus catch' developed. AEP and PEP values of the left legs were converted to values between 0° and 180° for statistical analysis. Dotted lines between the AEP and PEP are only used to link the points within a single leg. Subtle differences between trials before and after `tarsus catch' has developed are present (see Table 1). However, these differences could not be the cause of `tarsus catch'.