A dry baseball weighs just over five ounces, roughly equivalent to your average smartphone.

But unlike a handheld device, the baseball causes damage disproportionate to its small size. Along the 60-foot, six-inch path it traces on every pitch lie generations of crippled arms that couldn’t handle the stress of throwing the little leather ball.

Those arms, whose injuries range from slight muscle strains to ruptured tendons, belong to pitchers of all ages. Hefty percentages of little leaguers, college players and pros suffer some form of arm harm during their careers.

And this all transpires in a sport largely devoid of physical contact. In a game in which collisions only occur through miscommunication, the prevalence of pitching injuries far exceeds the casual observer’s expectations.

But to practitioners of the pitch, and those who study them, the pitching windup is among the most destructive movements in sports. Both physically and psychologically, it occupies a unique niche in the pantheon of sports motions, says Julia Millon, a student athletic trainer who has interned with Cal’s football, track, women’s soccer and baseball teams.

“It’s the most fragile athletic performance I’ve ever worked with,” she says.

The windup’s frailty derives not from external factors but from the violence inherent to the motion.

“The pitching motion specifically is actually one of the most abnormal motions the human body can do,” says Deanna Rolando, a graduate assistant athletic trainer intern with the Cal baseball team. “You could have the best mechanics, you could have full strength of your rotator cuff, but if on any given pitch you change your arm slot slightly or your fingers go over the top of the ball, then an injury could occur.”

According to experts in the body’s biomechanics, the main moments of stress lie in two points of the pitching motion. The first comes when the pitcher’s arm is cocked, a point where the arm’s joints experience extreme torque as the pitcher coils before exploding towards home plate.

The second occurs just after the pitcher’s release, when the arm resembles a slingshot with no ammo. The extreme deceleration required to bring the arm to rest is a major cause of injury, Rolando says.

“The elbow and the shoulder of the throwing arm are pushed to their limits on every pitch,” says Dr. Glenn Fleisig, research director at the American Sports Medicine Institute. “The ligaments and tendons are weak links.”

For some, the damage can develop slowly. Minnesota Twins starter Francisco Liriano spent four months on and off the disabled list before undergoing ligament replacement surgery in 2006. For others, the injury can occur in the blink of an eye.

“My arm just went away,” says Cal lefty Justin Jones, who suffered a stretched nerve midway through a game last June. “With max effort, I could probably throw about 20 miles per hour. There was nothing there.”

The box score from that contest — June 11, against Dallas Baptist in the NCAA Super Regionals — shows Jones spinning the best game of his collegiate career: The 2010 freshman All-American threw just 75 pitches through six scoreless innings, surrendering only one hit and one walk while striking out three. The postgame notes give a reason for his abrupt removal — “Jones left the game … after experiencing a cramp in his left bicep” — but hardly capture the full story.

He spent the next day, when the pain was at its most intense, with his arm in a sling. Although the throbbing subsided the next day, he couldn’t flex his left bicep and didn’t throw a baseball for six months.

“The initial few days right after it happened were kinda shitty, no two words for it,” he says. “It was just like, what’s wrong, am I going to get to throw in the (College) World Series? Better yet, am I going to get to throw a baseball ever again?”

The road back, like many pitchers’ recoveries, was long and difficult. Jones, who his father said had never suffered an injury before the stretched nerve, required rest and a gradual strengthening of his atrophied left arm. It also required a mental makeover as he rebuilt the delivery he had honed since his first childhood toss.

“Pitching was foreign to me,” he says. “I had to pretty much start over and not worry about chasing the tail of who I used to be.”

As pitchers grapple with both sides of their recovery, trainers morph between physical therapists and sports psychologists, cajoling their patients toward full health.

“You have to recognize, are they having a bad day? Do they need to take the day off?” Rolando says. “If they’re not mentally prepared to move on to the next phase of their throwing program, they’re just not going to get any better.”

But as he strives to recover, a pitcher must also overcome the sense of invincibility he carries onto the mound each day.

“You have to respect your body. There’s no such thing as 110 percent,” Jones says. “You can’t think you’re better than (your injury), because it’s going to bite you in the ass.”

The consequences of returning too early can sometimes be as severe as the injury itself. Liriano tried to pitch through the pain, and he has never reached the dizzying heights of his pre-injury brilliance.

For Jones, who concedes, along with coaches and family, that he came back too early, the recovery can be easily traced in this season’s starts. Through his first six starts — the first on Feb. 17, eight months after the injury and barely two months after he began throwing again — he threw 34 2/3 innings with a horrid 5.45 ERA while striking out 19. But three weeks ago, Cal head coach David Esquer shifted him to the back of the rotation, providing him an extra day of rest in successive weeks. In his three starts since the change, the junior has thrown 22 2/3 innings, compiled a 2.38 ERA and struck out 13.

“I still think I’m healing from the injury,” he says. “I feel 100 percent, but I think the injury’s going to last a little while.”

An injury remains with pitchers long after the pain recedes and the sweat of the recovery dries. In conversation, Jones recalls the emotions and dates of his injury and recovery with perfect clarity; the moments when he almost lost his livelihood are permanently etched into his psyche.

“When you love something almost as much as you love yourself, it’s really hard to let it go,” he says. “To almost have (baseball) be taken away is really frightening.”

But to live in fear is to admit defeat. Even though he has experienced a catastrophic injury, Jones doesn’t let himself think about it while on the mound.

“You’re putting your arm on the line every time you go out. All it takes is one pitch and your career can be over,” he says. “But if you think about that, that’s just holding you back.”