SANTO DOMINGO, Dominican Republic — The ground shook when Gregory Polanco thundered past, pulling a metal sled piled with weights. As he ran across the grass, with twigs and leaves crackling under his sneakers, Polanco huffed through a high-elevation training mask designed to restrict oxygen intake and, in the case of this particular mask, to look like Darth Vader’s helmet.
Once he reached the trees, Polanco slowed and stopped. He removed the mask and gulped fresh air. He tugged at his sweat-soaked T-shirt. Again, his trainer instructed, so Polanco turned around and took off once more. It was 8 a.