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