When Marlins third baseman Martin Prado began his second stint on the disabled list in 2017 with a right hamstring strain, his manager was frank.
"Any time I say, 'Hamstring,' you know it's not going to be good," Don Mattingly said in early May.
It was textbook. After singling with two outs in the seventh inning of a May 7 game against the Mets at Citi Field, Prado tried for third on a Derek Dietrich double. But he felt a twinge in his hamstring as he rounded second, then limped into third. Prado felt a little something else to go along with the pain.
"Just frustration," he said postgame. "I don't think there is another word."

It's a feeling many MLB players understand. This season, there have already been 31 DL placements that list the word "hamstring" in the transaction, well on pace to eclipse the 57 from 2016. The victims include many All-Stars: Prado, Yoenis Cespedes, Jean Segura, Justin Turner, Ian Kinsler, Carlos Gomez, Hunter Pence, Yunel Escobar, Kendrys Morales, Troy Tulowitzki and most recently, Neil Walker and Carsten Sabathia. The vast majority of those injuries happened on the basepaths, while Sabathia felt a telltale twinge in his left leg while pushing off the mound during the fourth inning of his start against the Angels on Tuesday night.
"Baseball is pretty sedentary compared to other sports," says Dr. Kevin Wilk of Champion Sports Medicine in Birmingham, Ala., who is also the longtime director of rehabilitative research at the American Sports Medicine Institute. "You are pretty still, and then you have a sudden burst, whether it's chasing a fly ball, sprinting from home to first or first to third, or a pitcher moving to cover first base. The hip is extended because they're trying to stretch it out, and they come up lame because the hamstrings weren't ready to handle the force."

Hamstrings, plural. There are actually three hamstring muscles up the back of each thigh -- the semitendinosus, semimembranosus and biceps femoris, from the inside to the outside. At the top, or proximal end, the three muscles come together, form the hamstring tendon and attach at the base of the pelvis on the ischial tuberosity, or the sitting bone. They run down the back of the thigh, cross the knee joint and attach distally to the tibia and fibula bones in the lower leg. The hamstring muscle group is responsible for the flexing of the lower leg at the knee.
Injuries occur when there is a muscular imbalance, when the hamstrings are not adequately warmed up or are fatigued, or when -- like on the basepaths -- there is a sudden need for speed.
During sprinting, the hamstrings fire eccentrically -- they contract when they are in a lengthened position -- to slow the lower leg and prepare the foot for ground contact. And while today's bigger, stronger players love the gym and train hard all offseason, the forces upon the hamstring when an athlete is in an all-out sprint simply cannot be recreated in the gym.

"If we don't sprint, we can expect to see more hamstring injuries," says Shane Rye of Cressey Performance in Jupiter, Fla. "Guys need to use the weight room as a tool but also not neglect sprinting, movement and mobility."
Rye points out that baseball players often have tight hips and glutes from the constant rotation involved with hitting and throwing, and the hamstrings are forced to compensate and become overworked.
"A hamstring is a slave muscle," Rye says. "If your glute isn't working well, the hamstring picks up the slack. If the external rotators of the hip aren't working well or your pelvis is rotated forward, the hamstrings pick up the slack. When other muscles don't do their job, the hamstrings do it for them."
Most hamstring strains in baseball occur between the middle of the muscle and the proximal attachment at the pelvis. As with all muscle strains, they are rated by grade; Grade 1 is a mild strain or pull, Grade 2 a partial tear and Grade 3 a complete tear, in which the hamstring tendon lifts completely away from the bone. Lower-grade injuries can be treated with simple rest, with dry needling to relax angry muscle fibers and reset nerve fibers, or with biologic solutions such as platelet-rich plasma (PRP) or stem-cell injections. Depending on severity of the injury, players may return to play in as little as a few days or as long as a few months. Full tears may need to be repaired surgically and sutured back to the bone, with a return to play in three to four months.

Think about this: A ballplayer strides into the on-deck circle, bat slung over his shoulder. Sometimes, there are two bats. Other times, the bat is weighted, or has a weighted doughnut over the barrel. The heavier load of the weighted bat stimulates the nervous system and increases muscle activation when swinging a lighter bat, thereby increasing bat speed. After a few warm-up cuts, the ballplayer is ready to step into the batter's box and swing the bat. But he is woefully unprepared to run out of it.
"From a positive standpoint, guys should know they're going to hit the ball and they're going to have to run," Wilk says. "And so therefore, you'd better prepare your whole body -- and not just your arms -- to swing the bat."
But when was the last time you saw a player in the on-deck circle throw in some high-knees or butt-kicks or other dynamic stretches to prepare his hamstrings for sprinting? It happens rarely, if at all, despite the fact that the player has likely spent the last little while sitting on the bench with his hamstrings in a shortened, contracted position. That is a recipe for a hamstring injury.
And as Mattingly said, it's not going to be good.