Why do I keep getting a hamstring injury? A scientific analysis

Hamstring injuries are the most common injuries in professional football. The number of hamstring injuries has steadily increased by 4% since 2001, today they account for up to 24% of all injuries in men's football. 15 to 30% of all athletes with a hamstring injury suffer another hamstring injury (recurrence) when they return to sport too soon. As a competitive athlete, you may recognise this feeling. You have been rehabilitating for weeks, you feel strong but suddenly it goes wrong again. The frustration and uncertainty is great. Why does this keep happening?

In this article, we explain the possible reasons. And what you can do about it. We give you an insight into how it arises and an understanding of what factors you can influence.

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What makes hamstrings so injury-prone?

The hamstrings consist of the biceps femoris, semitendinosis and semimembranosis. The muscles have a biarticular function. That is, they can deliver force across two joints, in the case of the hamstrings, the hip and the knee. This dual function means that there is continuous switching between different tasks during movement. Under great speed or force, this is vulnerable.

During running or sprinting, the hamstrings have an important function of slowing down the leg just before the contact moment on the ground. The muscle extends while simultaneously delivering force. We call this an eccentric contraction. This puts a great strain on the hamstring group.

Without top speed trip a hamstring injury

A major misconception about hamstring injuries is that they always occur during full effort or maximum sprinting. Indeed, you would expect the long, explosive runs at top speeds to cause the most injuries. But this is far from always the case. Almost half of all hamstring injuries in top-level football today occur during moderate-intensity acceleration. So these are not hardcore sprints, but sudden accelerations from standstill or from a moderate running pace. This can be when you need to quickly switch when the ball is lost or react to a misplayed pass. In these kinds of situations, your body has to make a series of complex movements without the preparation time that you have in a planned sprint. It is precisely this unexpected change of direction that puts a great eccentric load on the hamstrings. It is exactly this combination of unpredictability, time pressure and eccentric load that makes the risk of injury so high. So although there need not be a maximal effort, we still describe this mechanism as a so-called sprint-type hamstring injury. Eccentric load during rapid acceleration.

Besides this sprint type, we can also classify hamstring injuries into stretch type hamstring injuries. A sometimes simple movement with the leg to control or block a ball can put the hamstring in an extended position while delivering (sub)maximum power. So a stretch type injury does not necessarily always result from extreme exertion either.

Sarcomeres, the basis of muscle contraction

Sarcomeres are the smallest functional units that actually provide movement. To understand why some athletes keep getting the same injury, we first need to understand how these microscopic structures work.

A sarcomere is about 2.5 micrometres long at rest. This is so small that you need about 40,000 of them in succession for 1 centimetre of muscle length. These structures lie back-to-back in long chains within each muscle fibre. When your muscle contracts, all these sarcomeres shorten at the same time, ultimately resulting in the movement we can see.

The structure of a sarcomere resembles a sliding system. It consists of two types of protein filaments:

Myosin (thick filaments) - proteins with small heads that can attach
Actin (thin filaments) - along which myosin moves
Z-lines - the boundaries of each sarcomere, to which actin is attached

When a muscle needs to contract, the myosin heads grab onto the actin filaments and pull them together. This process repeats itself at lightning speed and in perfect synchronisation throughout all sarcomeres.

Why do I keep getting a hamstring injury?

There are a few factors why a hamstring injury is more likely to occur or return. Some players rarely or never get injured while others roll from one injury to the next. Some of these risk factors can be influenced while others cannot. But if you know what you can work on, you can minimise the risk.

1. Eccentric force

Research shows that players with an eccentric force below 337 Newtons have a more than 4.4 times higher risk of hamstring injury. The main issue here is how much force you can deliver while your hamstring extends. When sprinting, accelerating and braking quickly, the hamstring has to deliver force while lengthening the muscle. When this is not possible enough, you are more likely to find yourself in situations where the hamstring can no longer distribute forces properly.

2. Why short muscle fascicles make you injury-prone

A fascicle is a bundle of muscle fibres. The longer these bundles are, the more stretch the muscle can accommodate before it enters the danger zone, so to speak. Research shows that players with muscle fascicles shorter than about ten centimetres in the biceps femoris are over four times more likely to suffer a hamstring injury. This is not without reason. It has nothing to do with bad luck or coincidence but with biomechanics. A short fascicle has less ability to stretch. So this muscle bundle is more likely to reach maximum stretch than a longer bundle. Short fascicles also contain fewer sarcomeres in series, which means each sarcomere has to absorb more stretch and can be damaged faster.

Muscle fibres differ in length between people, and even between different muscles in the same body. For example, a long muscle fibre might contain 2,000 sarcomeres in series, while a short muscle fibre might have only 1,000. This seems like a small detail, but it has major implications for injury susceptibility.

Imagine a muscle needs to stretch 10 centimetres during a sprint. For a long muscle fibre with 2000 sarcomeres, each individual sarcomere only needs to stretch 0.05 millimetres. But in a short muscle fibre with only 1,000 sarcomeres, each sarcomere has to stretch 0.1 millimetres. compare this to a force you have to divide with fewer people to lift something, everyone has to work harder.

When sarcomeres are stretched beyond their comfort zone, what we call the “yield point”, microscopic cracks form in the Z-lines. These tiny damages accumulate, especially when the muscle has to endure these extreme stretches repeatedly, such as when sprinting or jumping.

This is trainable, but then the intensity of the exercise must be heavy enough to lead to structural adaptation. The nordic hamstring curl is effective in this, as this exercise creates sufficient eccentric tension in the end position to create new sarcomeres in series.

It is a slow biological process that takes eight to 12 weeks of consistent training to actually have an effect, which also disappears when you stop. Intensity, duration and, above all, loyalty to the programme determine how effective it is. This is precisely where things often go wrong in practice. The nordic is often performed too carelessly, too quickly or too irregularly. Many players also drop out prematurely because the exercise is heavy, causes considerable muscle pain and the legs can feel tired and stiff for days afterwards. For these reasons, clubs do not use it enough, despite all the knowledge. It is uncomfortable and does not always fit into a busy match schedule.

Yet this remains one of the few truly modifiable factors proven to lower injury risk. And there are ways to make this a good fit. By smartly periodising, starting with low volumes, controlling tempos and using alternatives on days with higher training loads.

Quadrant of doom

Australian research shows that the combination of these two factors in particular extremely increases injury risk. Players with biceps femoris fascicles shorter than about ten centimetres and an eccentric hamstring strength of below 337 Newtons are exactly in this vulnerable zone.

These two factors greatly reinforce each other. A short fascicle has little room to stretch during rapid extension. But insufficient eccentric force means you can't slow down that extension either. The combination means you can't catch much. On the pitch, this translates to players making seemingly normal actions but simply not having enough buffer to slow down and safely absorb the forces.

3. Age and injury history

Older age and previous hamstring injuries are two of the main risk factors for hamstring injury recurrence. Large datasets, such as the UEFA Elite Club Injury Studies, have shown this link for years. As players age, recovery ability decreases and explosive power gradually goes down. As a result, hamstrings have less of a buffer to absorb peak loads. Injury history also plays a big role. In fact, in several studies, it is the strongest predictor of a new injury. Players with a previous hamstring injury are two to six times more likely to re-injure the hamstring.

At a damage to muscle or tendon transition, forms scar tissue. This is not normal muscle tissue but a jumble of collagen fibres criss-crossing each other - very different from the nicely organised parallel muscle fibres. This scar tissue:

Is less elastic than healthy muscle tissue
Disrupts force distribution through the muscle

But this is not a lost cause. As discussed earlier, both eccentric power and fascicle length are modifiable. This is precisely where the scope for addressing these risk factors lies. Research shows that when players develop sufficient eccentric power and build longer fascicles, the effect of age virtually disappears and the risk of injury history is greatly reduced. In practical terms, this means that in older age, you should actually invest in smart, well-timed and specific strength training. When you do this structurally and purposefully, a person can effectively train themselves out of the risk group, so to speak.

4. Asymmetry in strength

Differences in eccentric hamstring strength between left and right sides are also a risk for injury. Players where one hamstring can deliver significantly less force than the other side are more likely to be injured than those with a more even distribution. Measurements of Nordic force and isokinetic tests show that asymmetry above 10 to 15 per cent is associated with a higher risk of hamstring injury. The issue is not total leg strength, but rather the specific eccentric function of the hamstring itself. When that force is unevenly distributed, timing and load changes during sprinting, braking and directional changes. This explains why asymmetry is considered an independent risk factor and why players with a previous injury with often still asymmetry have a significantly higher risk of new injury.

The power of the nordic hamstring curl

The Nordic hamstring curl is by far the best researched exercise in injury prevention for the hamstrings. There is strong evidence that this exercise is effective in injury prevention. In the first phase of the movement, when you slow down the movement to the max, the hamstrings work quasi-isometrically. The muscle fibres do not lengthen much, but the mechanical tension on the muscle fibres is already very high. This part builds control and tension, but does not yet provide the main structural adjustments.

The real power of nordic only starts at the so-called breakpoint. This is the moment where you can no longer slow down the movement and the hamstrings have to work under high load to slow down your fall. Van Hooren's research shows that in precisely this braking phase, the fascicles of the biceps femoris lengthen quickly and forcefully, with high peak loads that you can hardly achieve in other exercises. This rapid extension under high tension is exactly the stimulus that makes fascicles lengthen, as new sarcomeres are added in series. This is one of the few proven ways to structurally reduce hamstring fragility. The nordic hamstring curls thus has a long quasi-isometric phase(70-85%), followed by an extremely heavy eccentric braking phase that is responsible for the positive training effect.

The nordic hamstring curl mainly activates the semitendinosus and biceps femoris short head in the first phase and actually shifts the load in the braking phase to the biceps femoris long head and semimembranosus, the muscles most often injured during sprinting. Thereby, the exercise seems exceptionally in tune with the mechanism of occurrence of a hamstring injury.

Why you need more than just the nordic hamstring curl

Although the Nordic hamstring curl provides a unique stimulus for fascicle length and eccentric strength, additional exercises are also necessary for a complete picture. Recent biomechanical studies clearly show that different exercises have totally different muscle activation. The single-leg deadlift, for example, makes the biceps femoris long head work at a greater fascicle length throughout the movement, but with hardly any length change. That means the muscle builds strength in a longer position, but does not receive the same stimulus as in the Nordic hamstring curl. The single-leg novel chair is right in between: moderate fascicle change, medium forces and an activation pattern very similar to the quasi-isometric loading of the hamstrings in the late phase of sprinting.

These differences are important because no single exercise trains all functions or patterns of the hamstrings at the same time. An effective programme combines all these elements: extension, braking, hip dominant strength, knee angle dominant control and angle specific tension. Only then will you build the complete load capacity needed to actually prevent recurrent hamstring injuries.

How to proceed now?

Hamstring injuries seem unpredictable, frustrating and long-lasting, but they don't have to be guesswork. There are certainly factors that you cannot change, such as your age or the fact that you have had a hamstring injury before. But the key components that determine whether you get injured again can certainly be influenced. Eccentric strength, fascicle length, asymmetry together make up your load capacity. These are not vague or elusive concepts, but clear parameters that you can work on with the right training.

It is never a guarantee but you can drastically reduce the chances of recurring hamstring injuries. And that's not chance, not fate and certainly not bad luck. It's smart training and understanding what your body needs to keep performing at the highest level.

Sources

Ruben Luijkx

Ruben Luijkx, Master of Science and co-owner of Fysio Fitaal, specialises in sports-related complaints, ultrasound diagnostics and manual therapy. Writing on physiofitaal.nl, he introduces you to the expertise and professionalism of Fysio Fitaal in Tilburg.

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