A 41-day-old baby was "twisted or pulled" with such force he suffered injuries similar to a fall down the stairs, a doctor told a court.
Jody Simpson, 24, and Tony Smith, 46, of Sydney Road, Whitstable, deny assaulting, ill treating or neglecting a child to cause unnecessary suffering or injury.
X-rays showed the boy had 11 fractures.
Consultant paediatric radiologist Dr Joanna Fairhurst told Maidstone Crown Court he would have been in great pain.
She said his injuries were akin to a big fall, or "major road accident".
The baby was born on 8 October 2014.
At around the age of six weeks, he became very ill and X-rays in November and December revealed the extent of his injuries.
Giving evidence Dr Fairhurst, of University Hospital Southampton NHS Foundation Trust, said he suffered an infection after the first eight fractures, and three others were most likely caused because the bone was infected.
She told jurors: "A twisting or pulling force was used to cause some of the fractures. The amount of force required was well in excess of any force any normal carer would use.
"Every one of these fractures would have been a very painful injury.
"[He] couldn't crawl, couldn't roll on to his belly and pull himself along or even lift his head," Dr Fairhurst told the court.
"I have to conclude that the most likely way is that they were inflicted on him by an adult or somebody mature."
The trial continues.