In other words, the position and velocity of the center of {HEAD ~ gravity} at the start point may be the position and velocity of the center of {TAIL ~ gravity} at the current time as measured by sensors 
In other words , the position and velocity of the center of { HEAD ~ gravity } at the start point may be the position and velocity of the center of { TAIL ~ gravity } at the current time as measured by sensors
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH