SANTA CLARA -- During training camp in 2013, Justin Smith was going against guard Mike Iupati when something happened to his left shoulder. Smith wasn't sure what it was.
"I just knew it hurt like hell," the veteran defensive linemen said on Monday.
Seven months later, surgery revealed that a piece of bone had detached from the shoulder, and in Smith's words, became "lodged in the back of it."
The event showed both why Smith decided to step away from the game after 14 seasons and why the 49ers would have loved to have him back for one more. Despite playing in pain that season, Smith still was one of the better defensive linemen in the league, still made the Pro Bowl and started all 18 of the 49ers' games.
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But he said Monday that wear and tear on his upper body -- especially the left side of his upper body -- had become too much and that it was time to walk away.
"Where I play on the right side, all my contact comes on my left shoulder and left side," he said. "And it doesn't respond like I want it to respond anymore. You know, if you don't have the tools, you can't do the job. So it's time to go."
Smith also tore his left triceps tendon -- it became partially detached at the elbow -- late in the 2012 season. He played in the postseason and Super Bowl that season but was not nearly the same player he was a year earlier in the playoffs.
"You've got to go in with the right mindset and this and that," Smith said. "It's a young man's game. And you've got to be full of p--- and vinegar when you step on the field. And as you get older, it's harder to get that same intensity going week in and week out."
Despite those injuries, however, Smith mainatined his intensity and remained perhaps the most respected player in the 49ers locker room. He also continued to lead offseason workouts among the defensive players.
For those reasons, a number of team officials -- from former coaches like Jim Harbaugh and Vic Fangio to current head coach Jim Tomsula -- wanted him to return.
"I wanted to keep playing as well," Smith said. "But when you get on the bald tires, you're on the bald tires."