Hamstring Tears: The Elastic Band Model
Hamstring injuries are very common in athletics, comprising 6-29% of all sport-related injuries. There is a 12-25% reinjury rate in european football players, and 12-30% re-injury rate in other sports. Nearly 1/3 of those with hamstring tears will be re-injured, usually within 2 weeks of returning to play. Current research has shown large difference in recovery rates depending on severity, location and mechanism of injury. However, treatment protocols have been relatively similar despite the variability in recovery rates.
Think of the hamstrings as an elastic band. As an elastic band is stretched, it stores potential energy. Similarly, the hamstrings store potential energy as they are lengthened, such as during walking and running. When an elastic band is let go, it releases that potential energy into kinetic energy, powering it through the air. At the end of its stretch cycle, the hamstrings release the stored potential energy into kinetic energy to propel your body forward. However, like with an elastic band, the hamstrings can get damaged by over stretching or by being pulled apart with high force.
1. End of swing phase injury ( 70% of hamstring injuries):
This injury occurs during high speed and force causing peak musculotendinous stretch at late swing phase. Biceps femoris, the outermost hamstring, is particularly vulnerable due to its shorter knee extension moment arm during this phase. It has been shown that the moment arm at the hip during swing phase is double that of the knee, which also makes the biceps femoris most susceptible. Therefore, biceps femoris is more commonly injured than the medial hamstrings, especially in these types of injuries. High speed runners and sprinters fall into this category. Injury usually involves the proximal myotendinous junction of the long head of biceps femoris. During a sprint, the hamstring are lengthening eccentrically, creating a prime condition for muscle strain.
2. Over stretching injury (30% of hamstring injuries):
These injuries commonly occur at the ischial tuberosity involving the proximal free tendon of the semimembranosis muscle. Overstretch injuries can occur during slow or fast stretch movements (such as in dancing, kicking, etc). The muscle injury occurs when the activated hamstring is lengthened beyond its optimal length. It is the magnitude rather than the force that causes this increased strain.
Diagnosis and Prognosis
Clinical tests mainly aim to find the location and severity of the tear when the mechanism of injury is already known. Injuries are graded by the amount of range of motion, pain, and weakness.
Clinical Tests vs MRI:
The use of an MRI is like having access to a great investment firm before diving into the markets. You will be better informed and it may direct your decisions, but it won’t necessarily change the outcome.
Professional sports teams have access to MRI to make a better judgment of the severity of injury, prognosis and RTS (usually done within the first 2 weeks of injury). There is much debate between practitioners and researches regarding the use of MRI to reliably predict prognosis and return to play. The bottom line is that studies have shown that MRI does not add significant value when determining an athlete’s return to sport. This is good, because the majority of us will not have access to an MRI for a muscle injury. Generally, the area of maximal pain on palpation gives a good indication of the location of the injury and the closer to the ischial tuberosity, the longer the rehabilitation period.
In general, expect these timeframes for rehab:
- Gr. 1 from 3 weeks – 3 months
- Gr. 2 from 6 weeks – 6 months
- Gr. 3 from 6 months – 1 year
1. Previous hamstring injury
- This is the greatest risk factor (shown to increase re-injury rates by 2-6 times)
- Are re-injury rates due to poor rehabilitation or intrinsic muscle characteristics?
- MRI studies have shown scar tissue presence for up to 1 year after return to sport
- Less tissue compliance, deformation and muscle force transmission leads to higher peak force at the proximal myotendinous junction
- Certain protocols (L-protocol) for hamstring rehabilitation have shown significantly reduced re-injury rates
2. Hamstring flexibility:
- There is a lack of consensus whether flexibility contributes to hamstring strains. Most studies are done in static, rather than dynamic, conditions and reduce the generalizability of the findings
- Overall, systematic reviews show a lack of consistency between hamstring flexibility and hamstring injury
3. Hip flexor flexibility:
- Identified as a risk factor in older players, specifically over 25 years of age.
- Possible mechanism:
- A tight hip flexor can pull the pelvis into anterior tilt, increasing the length of the activated hamstring muscle and subsequently increasing the risk of acute injury. The body compensates during running by increasing lumbar lordosis and anterior pelvic tilt to adjust for this lack of hip extension.
4. Hamstring Strength:
- Most studies measured strength through either knee flexor:knee extensor ratio’s or peak torque values versus the contralateral leg
- However, these focus on concentric strength whereas the mechanism of hamstring injury mostly occurs during muscle lengthening
- Studies using an eccentric strengthening protocol have shown significant reduction in re-injury rates
- Hamstrings act as springs throughout the gait/running cycle, called the stretch shortening cycle. From 45-90% of the gait cycle (swing phase) the hamstrings absorb energy
Possible Risk Factors
5. Core stability:
- Core stability is the interaction of passive and active structures and neuromuscular control. Research shows that neuromuscular control and endurance may be more important than strength in the core. Poor motor patterns and endurance may increase risk regardless of core strength. One possible explanation is that greater control of the lumbopelvic region may allow the hamstrings to function at the appropriate lengths during sport activites, reducing potential for injury
- There is evidence that fatigue can lead to greater injury rates in the hamstrings. This has been observed in soccer players towards the end of a match