Updated: May 5, 2022
A pulled hamstring is the same injury as a hamstrings strain. It is also one of the most common injuries suffered in athletics including baseball. 8% of all Major League Baseball injuries were hamstrings strains from 2002-2008 according to Posner, et al. Before we dive too deeply into this topic it is important to understand the hamstrings muscles and what constitutes a strain.
This image, courtesy of learnmuscles.com, shows the three muscles of the hamstrings: the semimembranosus, semitendinosus, and biceps femoris. All three muscles originate from ischial tuberosity which is the bump you feel in the lower part of your glutes, sometimes referred to as your “sit bone”. There is a short head of the biceps femoris that comes from the femur itself. All 3 muscles attach at the knee, with the semimembranosus and semitendinosus attaching to the tibia and the biceps femoris inserting into the fibula. The tibia is the inner bone of your lower leg and the fibula is the outer bone. All 3 muscles bend your knee as well as extend your hip backwards. Interestingly, 79-84% of hamstrings strains involve the biceps femoris, and it is thought to play a more significant role in the action of the hamstrings.
There are many theories for why the hamstrings are so often strained. One of the reasons is that it is constantly stressed eccentrically, meaning when it lengthens. Muscles that are engaged while lengthening create greater tissue damage, which could lead to more strains. Also, there are a high amount of Type II muscle fibers in the hamstrings relative to other leg muscles. Type II muscle fibers are your “fast twitch” fibers. They receive less oxygen but have greater power potential. You recruit these muscles with springing or heavy lifting so they require greater power but have higher risk of strain and damage from this usage.
A strain refers to any damage to the muscle. There are 3 grades of strain. Grade 1 is minimal damage to the muscle fibers which can be regenerated relatively quickly. Grade 2 is a more significant strain that involves partial tearing of the muscle fibers and may include damage to surrounding tissue including blood vessels which causes bruising. A grade 3 strain is a complete tear of the muscle or tendon which results in significant weakness and may require surgery. It is important to remember that our bodies heal from muscle strains all of the time. It is a more advanced version of what happens when you lift weights and break down muscle to get stronger.
There are three main phases of healing for muscle strains: inflammation, proliferation, and remodeling. Inflammation lasts for 3-5 days typically and involves the tissue swelling and consolidating the damage in the area. Proliferation can last for several weeks in severe strains and involves healing factors flooding the injured area. Remodeling is the process by which scar tissue and other structures are formed for long term stability. Remodeling can take up to 2 years in significant strains.
Hamstring Injury Prediction
Due to the prevalence of hamstrings injuries, there is significant research into the prediction and cause of injuries to hopefully prevent them. The most common reason for a pulled hamstring is if you previously suffered the same injury. Research on Australian rules football and track and field athletes showed reinjury rates between 13.9% and 63.3% of hamstrings strains. Additionally, a previous hamstrings injury makes you 3.6X as likely to have a future injury. This can be based on a variety of factors, some of which we understand. It is possible that the athlete was rushed back too soon and remodeling of the area was not sufficient to withstand the forces of their sport. It is also possible to have a genetic disposition. Research shows that the angle of your muscles in the hamstrings can play a role in injury frequency. This is not something that you can modify, so you may be predisposed to injury.
Postural factors can also cause hamstrings strains. Running with an anterior pelvic tilt can lead to increased stress on the hamstrings as shown in the image below
It is important to remember that the muscles originate from the pelvis, meaning an anterior pelvic tilt increases the stretch on them. Try tilting your pelvis anteriorly or sticking your stomach out and stretching your hamstrings. You probably have a lot less mobility this way. Imagine running in this position and you can see why there is increased stress on the muscles.
Older age also increases injury risk. As we age, our muscles lose elasticity meaning that they do not respond to quick movements well. Also, our Type II muscle fibers decrease in number causing a loss of power. If you imagine an older person, they can be very strong, but they lose their quick twitch muscles which limits their power. Interestingly, African-Americans seem to have a greater likelihood of hamstrings strains, although the exact cause is unknown.
The most common misconception that I see involves people thinking that flexibility will prevent a pulled hamstring. This is not based on research. While this seems to contradict the previous information involving an anterior pelvic tilt, it is true. Flexibility simply gives you more range of motion to utilize, it does not help you maintain strength in that range. Think of sprinters and their build. They are typically very muscular and actually have less flexibility than you would imagine. They are acting as tense springs with little wasted motion. Running faster involves maximizing your time in contact with the ground because this is how you propel yourself forward. Increased flexibility leads to longer strides which will actually slow you down. It also stretches your hamstrings further, which may put them under increased stress. This summarizes the anterior pelvic tilt issue. If your hamstrings are already in a lengthened position in standing because of your pelvic orientation, then you are stressing them more in all ranges.
The logical assumption would be that if flexibility is not important then strength must be the key to injury prevention. This is actually not the case either. Research does not reveal a link between hamstrings strength and risk of strain.
So, what movement qualities actually prevent a pulled hamstring? Some research points to how you jump. In typical jump testing there are two types of jumps: a countermovement jump and a static jump. A countermovement jump involves quickly squatting and jumping as high as you can. A static jump is when you squat down and hold for a few seconds before jumping. A study by Henderson, et al. showed that a higher static jump actually increases the risk of pulling your hamstrings. This does not seem to make much sense, but it is important to pair it with another study performed by Venturelli, et al. The second study showed that a higher static jump relative to a countermovement jump increased hamstrings strain risk. Typically, most people jump higher with a countermovement jump because you are utilizing momentum and the plyometric potential of your muscles. Imagine bouncing on a trampoline: you will bounce higher if you drop from a higher height and immediately jump. If you could hold yourself at the bottom, you would lose the kinetic energy from the jump.
Remember earlier I mentioned that the hamstrings display the greatest density of Type II fibers in the leg? Type II fibers will be crucial for a countermovement jump as they will store and release energy much quicker than Type I fibers. One could hypothesize that a lower countermovement jump relative to a static jump shows that the individual does not possess or is not utilizing their Type II explosive muscle fibers. Either one of these situations would theoretically put the hamstrings at greater risk. These tests are in their infancy, but they show more practical relevance than simply testing how much weight you can curl with your legs on a machine.
Pulled Hamstring Symptoms
A pulled hamstring is relatively easy to diagnose. There are not many structures in the back of your thigh that could confuse you. Sometimes, the strain can be in the upper tendon of the hamstrings, so pain can be closer to the buttock region. Symptoms typically involve a sudden onset of pain in the back of the leg. The pain is worse with stretching the hamstrings or contracting them. Imagine passively extending your knee. This puts a stretching stress on the hamstrings which will aggravate the strain. If you tried to curl your leg against resistance you would engage the muscle, putting a different but still painful strain on it.
There will also usually be tenderness along the hamstrings. You will also have difficulty walking if the strain is bad enough that the muscle is damaged. Some bruising may even