Japanese robotics company SoftBank is buying robot companies from Google’s parent firm Alphabet.

SoftBank has announced plans to buy Boston Dynamics and Tokyo-based Schaft, two companies that design and manufacture robots with human movements, though details on the terms of the transactions remain unclear.

Still, SoftBank's shares rose to a 17-year high after the announcement.

“Smart robotics are going to be a key driver of the next stage of the information revolution, and ... Boston Dynamics [is] the clear technology leader in advanced dynamic robots,” SoftBank group chairman Masayoshi Son said in a statement. 

SoftBank is a big backer of the US$93 billion Vision Fund, the world's largest private equity fund, which invests in technologies expected to grow significantly in the near future like robotics and artificial intelligence.

Mr Son, Japan's richest man, says SoftBank is preparing for a data “gold rush” as the global economy becomes increasingly digitised.

Schaft, a University of Tokyo spinoff, makes bipedal robots that can negotiate uneven terrain.

“Robotics as a field has great potential, and we're happy to see Boston Dynamics and Schaft join the SoftBank team to continue contributing to the next generation of robotics,” an Alphabet spokesperson said.

Boston Dynamics builds robots that mimic human and animal movement.

“They’re advancing the state of the art in independent robotics. They are probably the leader in the US,” Arnis Mangolds, a robotics expert who has worked with Boston Dynamics, has told Reuters.

“But the problem is it's not ready for prime time, and very few people have a tolerance for that.”