The "lifelike" doll was dropped onto hard surfaces

Using a specially-created lifelike "six-week old" doll, they simulated a series of falls onto carpet, foam and concrete.

The results suggest the way the head twists on impact is likely to increase the damage caused.

But a British expert says other evidence suggests severe damage is "unlikely" from short falls.

Brain injuries are the most common cause of death in childhood, killing or hospitalising thousands in the UK each year.

The researchers were looking for evidence of "rotational deceleration" - rapid changes in velocity as the head contacts a hard surface before the body and then violently rebounds.

Previously, it had been thought that these rotational forces were not a significant factor during falls.

Signs of abuse

The greatest rotational forces were generated when the doll was shaken and the head struck a solid surface.

While accidental falls are a frequent cause of paediatric trauma, they are also a common explanation given by care-takers in suspected abuse cases Dr Susan Margulies, University of Pennsylvania

The researchers concluded that these would probably cause brain haemorrhages and other widespread injury.

But vigorous shaking of the doll without the impact produced much lesser effects, and the researchers suggested they were unlikely to cause severe or fatal injury.

Experts still maintain that shaking a baby is never safe - and other studies have pointed to the risks of brain damage involved.

Sensors helped work out the likelihood of injury

In the Pennsylvania study a typical "accidental" five-foot falls headfirst onto concrete produced enough rotational force to cause severe injury.

Dr Susan Margulies, who led the study, said: "Previously falls were considered relatively benign.

"Traumatic brain injury is the most common cause of death in childhood, and child abuse is believed to be responsible for at least half of infant brain injuries.

"While accidental falls are a frequent cause of paediatric trauma, they are also a common explanation given by care-takers in suspected abuse cases."

She said the findings could help doctors differentiate between genuine accidents and abuse.

Safer play

It could also help scientists protect children's brains more effectively in future.

Dr Margulies said: "Children are not just miniature adults.

"Learning more about paediatric brain injuries will help us develop protective devices - helmets, playground surfaces, car seats - that better meet their specific needs."

But a leading UK expert, Dr Chris Hobbs, from St James' University Hospital in Leeds, said that other studies had suggested that shorter falls were unlikely to cause severe injury.

He told BBC News Online: "It's a very difficult area to examine, but there are a lot of observational studies - of children who fall off beds and out of cots in hospital, for example, which show that severe injury from these falls is extremely unlikely."