"Even though human legs allow a wide repertoire of movements, when
people travel by foot they mostly use one of two locomotor patterns,
namely, walking and running. The selection of these two gaits from
the plethora of options might be because walking and running require
less metabolic energy than other more unusual gaits. We addressed
this possibility previously using numerical optimization of a minimal
mathematical model of a biped. We had found that, for a given step-
length, the two classical descriptions of walking and running,
‘inverted pendulum walking' and 'impulsive running', do indeed
minimize the amount of positive work required at low and high speeds
respectively ..."