New York, Feb 24 (IANS): Standing with heels planted allows humans more swinging force when fighting, but the heel-down posture also makes us bad at fleeing, says a study.

In contrast, many other species of mammals, including most primates, stand, walk and run with their heels elevated, and on the balls of their feet or toes, a posture important for quickness, said the study published in the journal Biology Open.

The findings suggest that aggression played a role in shaping stance of modern humans.

"This story is one more piece in a broader picture, a suite of distinguishing characteristics that are consistent with the idea that we're specialised at some level for aggressive behaviour," said lead author of the study David Carrier from University of Utah in the US.

The physiological traits that confer advantages in fighting are different from those required for other tasks, such as running.

"Certain species tend to be good at fighting or fleeing, but not both," Emily Carrington from US National Science Foundation (NSF) said.

"This study provides insight into the basis for this trade-off. Animals that use their heels to plant their feet firmly to the ground, like bears, badgers and great apes, are able to deliver stronger blows to their opponents," Carrington added.

The heel-up stance, called digitigrade and unguligrade, seen in animals from wolves to horses and deer, increases the economy of running by lengthening the leg and improving the storage and recovery of energy in the tendons and ligaments of the lower limb.

The heel-down, or plantigrade, posture shared by great apes and other species, such as bears, wolverines and some rodents, is less specialised for running, however.

The researchers hypothesised that the evolution of great apes' plantigrade stance had to do with how apes climb and forage in trees.