Gregory Polanco makes it seem so effortless, cruising from first base to third on a single or gliding deep into the right-field corner to snare a fly ball.

With a 6-foot-4 frame, Polanco generates above-average speed with his long, powerful stride. Yet years before Baseball America ranked him one of the 10 best prospects in the game, Polanco was a gangly teenager in the Dominican Republic who covered ground like a turtle.

“He was so big, but so weak,” said Rene Gayo, the Pirates' Latin American scouting director. “When I first saw him, I thought he'd be an average runner at best.”

Gayo recalled the first time he watched Polanco run the 60-yard dash, a mess of flailing arms and elbows, frantically pumping knees and an awkward gait. He was timed at 7.4 seconds.

A year or so later, Polanco ran the 60 again. Gayo looked at his stopwatch, then did a double-take.

It read 6.9 seconds.

“When I heard that, I said, ‘Hey, I'm fast,' ” Polanco said, grinning. “I knew then I could run.”

In the Dominican, Polanco's coaches wanted to make him a left-handed pitcher. So he ran every day with the other pitchers and slowly built up strength, which Polanco now credits for his speed.

“He just matured and became a man,” Gayo said.

It was not an instant transformation. Polanco did not simply wake up one morning with blazing speed. In rookie ball in 2010, Polanco still struggled with his body and batted .202. He also had 19 stolen bases in 53 games.