A few Red Sox and Patriots have dealt with hamstring injuries. Why do they have a tendency to linger?

There are a number of factors that go in to deciding whether an athlete is ready to return to play.

Rafael Devers
Red Sox third baseman Rafael Devers is assisted off he field after injuring himself while sliding into third base. –Barry Chin/Globe Staff

Boston sports fans could be forgiven if they’ve had their fill of hearing about hamstring injuries in recent weeks. In the midst of sweeping the Yankees earlier this month, the Red Sox placed catcher Blake Swihart and second baseman Ian Kinsler on the 10-day disabled list with pulled hamstrings. Less than two weeks later, on the same day that the Red Sox activated Kinsler, they placed third baseman Rafael Devers on the DL with a pulled hamstring. It is the second time this season a hamstring injury has landed Devers on the DL.

In Foxborough, wide receivers Kenny Britt and Jordan Matthews were released by the Patriots after being nagged by strained hamstrings.

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So, what’s with all the hamstring injuries?

First of all, it’s not actually a single string, but rather a group of three muscles that run along the back of the thigh. Most injuries heal without the need for surgery.

“Hamstring injuries have very varied causes. They are multifactoral, so it’s hard. You can’t point to one thing that could cause them,’’ said Mike McKenny, director of sports medicine at Northeastern University. “They are caused by high stress on the tissue, in explosive play, cut, and pivot situations.’’

There are three grades of hamstring injury:

Grade 1: a mild muscle pull or tear

Grade 2: a partial muscle tear

Grade 3: a complete muscle tear

The RICE protocol — rest, ice, compression, elevation —­ is effective in most cases where the hamstring is strained. Here is breakdown of the steps:

Rest: Take a break from the activity that caused the strain

Ice: Use cold packs for 20 minutes at a time, several times a day

Compression: Wear an elastic compression bandage to prevent additional swelling.

Elevation: Reduce swelling by reclining and lifting the leg higher than the heart.

That can be helpful for weekend warriors trying to make it back into the office on Monday, but what about professional athletes? Initially, they might start with the same course of action, although too much rest can be a bad thing.

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“If the injury is bad enough and there’s a lot of swelling, bleeding, discoloration and instability, and if they’re not able to walk, sometimes those [RICE] treatments can provide some benefit. But the research has started to tell us that the earlier you can get somebody moving, long-term it is actually better for them.

“In high performance athletes, — Division I athletes, professional athletes, Olympic athletes — if you rest too long, that can open them up to other risks associated with their sport, so we look at it more instead of just rest, we modify their activity away from the things that are aggravating their hamstring.’’

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So, while sprinting and jumping might be out of the question, the athlete may continue to work with weights, or walk, but not run, depending on a variety of issues.

“The earlier that you can safely get them moving and start building a more resilient athlete through rehab, the better,’’ said McKenney. “You want to make sure that you are rebuilding them to where they can sustain the rigors of the sport they are playing.’’

There are a number of factors that go in to deciding whether an athlete is ready to return to play.

“With our athletes typically it is not one specific test, it is a collection of a multitude of tests,’’ said McKenny. “So if we have an athlete that misses time, we have a number of conditioning tests and things that they have been participating in as a part of their strength and conditioning program, that we basically have as pre-injury measures to compare it to. So these are areas where they at least provide us with a starting point. When they return to those numbers and those ranges we know we are making progress.’’

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Reaching those numbers is a good indication that an athlete may be ready to return to action, but there are other factors as well.

“A big part of injury rehab is, is the athlete confident that they are able to do what they need to do? That’s on an individual basis,’’ said McKenney. “So we might have situations where the athlete meets all their numbers, so on paper they look like they are doing well, but when you talk to them they might not be confident in their athletic ability.’’

Even when the athlete’s performance and comfort level sync, there is still no definitive test that indicates they are ready to return to the field.

“You can’t accurately test baseball and you can’t accurately test football because they are team sports,’’ said McKenney. “They are reading and reacting to things that are going on in the field in real time that are unpredictable, and it is really difficult to replicate — short of them actually going into those environments and participating at that level.

“When Tom Brady takes the ball and he steps back, how many options does he have with that ball? It’s never the same. He might hand the ball off, he might pitch, pass, cut left or run it himself. Those are all possibilities and that is that unpredictability.

“Baseball is the same thing. Is there a throwing error? Did the outfielder trip trying to field the ball? Those things can really play into it. You see people running the bases on what seems routine plays, and then something happens in fielding, and so now the runner has to make a decision where they have to all of a sudden decelerate very quickly, that they weren’t planning to do, or accelerate because there are extra bases on the line, and that’s when you actually see injuries like this happen. That’s how quick it is, it’s that split second. But it’s hard to train for that specifically because you can’t reproduce that necessarily.’’

Bottom line, it’s best to err on the side of caution when dealing with hamstrings.

“Because of the nature of hamstring injuries, how they have a tendency to linger, you’ll see athletes might be removed from activities sooner, while that injury is still in that minor, manageable state,’’ said McKenny. “Where if they just let it go and try to play through it, you can take what is the shorter view, and now you’re talking about making the injury worse. I know that’s why hamstring injuries are typically on the more conservative side of treatment because of that, because they can follow athletes, because they do linger, if they aren’t managed early on.’’

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