In this case, {HEAD ~ the first single leg grounded period} corresponds to {TAIL ~ the current single leg grounded period} 
corresponds to
In this case , { HEAD ~ the first single leg grounded period } corresponds to { TAIL ~ the current single leg grounded period }
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL