Gsellman’s poor posture contributed to a 5.19 earned run average over 119⅔ innings last season — a year after, as an unheralded prospect, he helped save the Mets’ injury-plagued 2016 season. After starting for much of his first two years in the majors, Gsellman was briefly demoted to the bullpen last season, when a dropping arm caused his trademark sinking fastball not to sink as usual.

“It moved side to side instead of down,” he said.

So this off-season, with the help of Cressey and Rye in Jupiter, Fla., and a physical therapist, Judy Seto, the director of sports performance at Select Physical Therapy in Los Angeles, Gsellman fixed his body. A Los Angeles native, Gsellman made three visits a week to work on his right shoulder with Seto, who had supervised his rehabilitation after the surgery on his left shoulder.

Rye, who talked to Gsellman last summer about a hamstring strain, drew up an off-season program for Gsellman in person. Rye said past problems with Gsellman’s lower body had affected the way he generated and absorbed the force of pitching. An improvement with his legs would alleviate some stress on his shoulder.

This winter, Gsellman said, he stopped doing powerlifting moves like traditional squats, and instead emphasized leg workouts that helped both strengthen and loosen his limbs. When he addressed his hamstring deficiency, he said, it helped prevent knee pain he had felt in the past.

Gsellman also underwent many massages and did stretching to loosen soft tissue in and around his shoulder.

“In addition to spending time improving his rotator-cuff strength, we focused on getting his upper back and rib cage to sit and move in better positions, which in turn helped his shoulder,” Rye said.

It was all eye-opening for Gsellman.

“I wonder sometimes: How did I make it through the minor leagues?” he said. “I didn’t know anything about my body then. I rededicated myself to learn about my body, and it’s helping with the day-to-day now.”