Why are the hamstrings so vulnerable to injury and what can be done to protect them? By Dr Leon Creaney.
The hamstrings are the site for some of the most common injuries in running. At the 2007 IAAF World Championships in Osaka, 30 athletes reported hamstring injuries, representing 16 per cent of all injuries at the athletes’ clinic that week.
Everyone recognises that “shot in the back of the thigh” moment – “it’s his hamstring, it’s gone!” As a doctor working at my first major sports event, I’ll never forget the men’s 100m final at the Manchester Commonwealth Games in 2002 – both Dwain Chambers and Mark Lewis-Francis pulled up within metres of each other with hamstring cramp.
The hamstrings are a group of three muscles – medially (inner thigh) the semimembranosus and semitendinosus, and laterally (outer thigh) the biceps femoris, which has a short and long head. They are known as “bi-articular” muscles because they cross two joints – the hip and knee – and this gives rise to a phenomenon known as “Lombard’s Paradox”.
From the high-knee position in sprinting, the hip extends under the pull of the hamstrings, while the knee extends from the pull of the quadriceps. This is paradoxical, since in order for this to occur the hamstrings must be shortening at the top end (proximal), while lengthening at the bottom end (distal) – this is Lombard’s Paradox. This produces tremendous straining forces within the hamstrings, that have to be resisted or the muscles will tear.
The hamstrings lengthen considerably during sprinting – up to 110 per cent of their resting length in the case of the most frequently injured of the three – the biceps femoris. They also have to cope with considerable forces, greatest again in the biceps femoris, toward the end of the swing phase. Most of this force is generated by the muscle, but up to a third comes from the elastic recoil of the tendon.
Fatigue is a major risk factor for hamstring injuries. It’s been shown in studies of football matches that hamstrings are injured more frequently at the end of each half. In track and field, the analogous situation is sprinting at the end of a long session. Fitness is important too and most injuries occur in the first few races of the season when an athlete still isn’t quite race-fit. The strongest risk factor however is previous injury. Once a hamstring has been injured, it is far more likely to be injured in the same spot again unless it is rehabilitated properly.
Biomechanically, flexibility is important. A sprinter should be able to touch their toes comfortably, but stretching before a session or race has never been shown to particularly modify risk of injury – warm-up is actually what is more important. Interestingly, one study has shown that quadriceps flexibility is important too.
Risk factors for hamstring injury
» Lack of race fitness
» Previous hamstring injury
» Poor flexibility of hamstrings or quadriceps
Poor eccentric strength
» Weak at end-range (lack of strength when the muscle is in a lengthened position)
» Strength imbalance between left and right
» Dominance of quadriceps strength over hamstrings
» Subtle back injuries – such as slipped disc, weak core
Muscles can work in three different ways – isometrically (holding), concentrically (shortening) or eccentrically (lengthening against load). Eccentric strength has received a lot of research attention in recent years. We now know that during movement muscles cycle between concentric and eccentric phases – the so called “stretch-shortening cycle”. If a muscle is eccentrically weak, it’s less able to resist a stretching force, meaning it can tear when forcefully stretched, as in sprinting.
We know that if there is an imbalance from left to right, one side being weaker than the other, then it increases the risk of tearing for the weaker leg. We also know that if the quadriceps are very strong compared to the hamstrings, there is a risk of injury too.
The first thing most athletes ask is: “how long until I’m racing again?” Studies have revealed some guides to answer this. Lateral (outer thigh) injuries are worse than medial (inner thigh). However, if an athlete can comfortably walk at a normal pace 24 hours after the injury, this is a good sign.
More in-depth studies have been performed using MRI and ultrasound scans. The further down the leg the injury occurs, the better. An injury felt close to the origin of the hamstrings (at the buttocks), is much worse than one felt at the other end nearer the knee.
A very useful study was done on 18 Swedish sprinters at the Karolinska Institute. They assessed new hamstring injuries with MRI scans. They found that there is a high correlation between the position of the injury (as explained above) and the time to return to fitness. However, there is very little correlation with some of the more traditionally used markers of severity, such as flexibility and strength immediately after the injury (straight leg raise and knee bending). In other words you get a better idea of how bad the injury is by feeling around to see where it hurts than you do by seeing how much you can stretch or how forcefully you can bend the knee.
Prediction of good and bad hamstring injuries
» Time to return to walking without pain is less than 24 hours – good
» Medial (inner) hamstrings – good
» Lateral (outer) hamstrings – bad
» Proximal (upper thigh) – bad
» Distal (lower thigh) – good
» Flexibility and strength immediately after injury – does not predict severity accurately
» Size of the injury on an MRI or ultrasound scan – very accurate
In the same study the researchers found that sprinters often underestimate how long it will take for an injury to heal. Most of the athletes thought it would be less than six weeks. Although 90 per cent of the improvement does occur in the first six weeks, it can take six months for a hamstring injury to fully heal to the 100 per cent that is required for top speed.
We now know that it is important to start gently stretching after an initial rest of 2448 hours. This should be done within the limits of pain. Restricting movement or using crutches is not helpful. An athlete should try to walk normally. As soon as possible an athlete should progress from slow walking, to fast walking and then jogging. Each stage should be progressed if the previous one is relatively pain free. From this point onwards, a slight increase in running speed should be possible with each session. A sensible progression is shown in the table below. These progressions may take a few months and athletes have to go at their own pace.
The most important exercise is the eccentric “Nordic” hamstring exercise (see diagram). This exercise requires a partner, but eccentric hamstring curls are almost as good. Once the eccentric strength on the affected side is almost as good as the normal side, the strength training can change to lighter, faster, concentric exercises.
Accelerated rehab progression for hamstrings
(progress to the next level when pain-free at the previous stage)
Stage 1: Walking
Stage 2: Brisk walking
Stage 3: Jogging (40 per cent)
Stage 4: Striding over 100m up to 50 per cent of maximum running speed (10 reps)
Stage 5: Striding over 80m up to 60 per cent (eight reps)
Stage 6: Striding over 70m up to 70 per cent (six reps)
Stage 7: Accelerate over 60m up to 80 per cent (six reps)
Stage 8: Accelerate over 50m up to 90 per cent (six reps)
Stage 9: Full training up to 100 per cent
Stage 10: Race fitness
Rehabilitation of a hamstring injury
» RICE – Rest, Ice, Compression, Elevation for 24-48 hours
» Early stretching and mobilisation
» Early return to brisk walking and jogging
» Accelerated rehab progressions
» Eccentric loading – “Nordic” exercise or hamstring curls
» Rapid concentric loading (late stage when running 80 per cent or faster)
The research into eccentric training has shown a number of benefits. The technique helps prevent and also treats injuries. The optimum length of the muscle is increased, meaning an athlete has more strength at end-range – when the muscle is at its most vulnerable. Studies have also shown that sprinting times are improved.
The hamstrings have to produce extreme forces in sprinting and are subject to significant lengthening. At several points in the running cycle, the hamstrings are literally being pulled apart, so the risk of straining is very high.
Following an injury, the position of the injury is the best predictor of how long it will take to heal. As a rule-of-thumb, the closer to the top of the leg, the worse the injury will be.
After injury it is important to begin stretching early and mobilise as soon as possible. Gradual progression of eccentric strength and running speed is the mainstay of rehabilitation. Athletes often underestimate how long it can take for hamstring injuries to heal – coming back too soon is common. There is also a significant risk of re-tearing a poorly rehabilitated injury.
» Dr Leon Creaney is a medical doctor, veteran athlete and a specialist consultant in sport and exercise medicine. He works for the NHS and Army, as well as the EIS and UKA. His private clinics are based in Manchester and London. For more, see sportandexercisemedicine.co.uk