Hamstring strains are common across many sports. They are one of the most common injuries in soccer, football, track and field, and baseball to name a few (Erickson 2017, Camp 2018). The best way to deal with hamstring injuries is to prevent them from happening in the first place. Specific hamstring muscle training programs have been shown to cut the risk of hamstring injury in half for athletes.
Unfortunately, all hamstring injuries cannot be prevented, so what are important factors to consider for rehabilitation? With most athletic injuries, the goal is to return to sport as quickly and safely as possible. This means:
- Reducing the time to return to play.
- Reducing the rate of injury recurrence.
Often these two concepts are at odds. Faster return to play time may mean increased risk for reinjury, while lower reinjury risk may require longer rehabilitation and delayed return to play. Balancing these two goals will depend on many factors, e.g., age, injury severity, competition level, competitive season, athlete’s goal, etc. Ultimately, rehabilitation after hamstring strain will depend on many factors, but these five factors should be considered to reduce return to play times and reinjury after a hamstring strain.
Specificity of Exercise
The most common mechanism for hamstring strain injury is sprinting (Elliot 2011, Duhig 2016, Camp 2018). Sprinting results in high long head of biceps femoris muscle activation during late swing phase and early stance phase–the long head of biceps femoris being the primary site of injury for sprinting-related hamstring strains. Late swing phase (pictured below) is also the point of peak musculotendon length as well as force for the biceps femoris (Kenneally-Dabrowski 2019).
Source: William Warby on Flickr, Mens 100m Final, edits and diagrams added by Brooks Klein, CC BY 2.0
The red lines on the sprinter in the image above approximate the joint angles of the hip and knee in late swing phase.
The high level of hamstring muscle activation, speed of contraction, and muscle length produced during the late swing phase of sprinting are difficult to reproduce with other exercises. Nordic hamstring curls are typically considered a high biceps femoris muscle activation exercise with EMG activity over 70% MVIC (Lovell 2016, Bourne 2017). However, when the peak biceps femoris muscle activity of various exercises is compared to peak muscle activity during sprinting, most exercises fall short. Nordic hamstring curls are only about 40% of the peak muscle activity observed during sprinting (van den Tillaar 2017). Note that these comparisons are to hamstring activity at maximal sprint speeds.
Athletes can often run soon after hamstring strain injuries because the hamstring muscle force required for submaximal running is not nearly as high as the force required for maximal sprint speed. The chart below outlines the peak hamstring force across various running speeds when compared to all-out sprints.
Source: Chumanov 2007
Running at 80% of maximal sprint speed only requires 15.1 N/kg of biceps femoris force, and running at 100% of maximal sprint speed requires 21.4 N/kg of force. While running at 80% speed may seem challenging for the hamstring, it is still only 70.6% of the biceps femoris force experienced at 100% sprint speed. It makes sense that one of the best exercises to rehab a sprinting-related hamstring strain is to gradually progress to sprinting.
Do Eccentric Hamstring Exercises
Hamstring strength deficits are common after hamstring strains. Strength deficits that persist at time of return to play have been shown to increase risk of re-injury (de Vos 2014). These strength deficits are often more pronounced in eccentric strength than concentric (Fyfe 2013). A well-studied hamstring exercise to improve eccentric strength is Nordic hamstring curls. Nordics increase eccentric hamstring strength and increase hamstring fascicle length–two modifiable risk factors for hamstring injury (Presland 2018). Not only do Nordics effectively target risk factors for hamstring injury, they have also been shown to be effective in practical application. In a study of over 900 soccer players, a Nordic hamstring exercise program reduced re-injury rates by 85% (Peterson 2011). Nordics are safe, effective, and require no equipment. It’s hard to find a reason not to use Nordics in hamstring training and rehabilitation.
Nordic Hamstring Curls
Some Pain During Exercises Is OK and Probably Beneficial
As stated previously, hamstring strength deficits often persist after hamstring injury. However, hamstring strain rehab protocols may delay hamstring strengthening due to pain when performing the exercises.
A study by Hickey et al. (2019) sought to determine whether pain-free or pain-threshold was more beneficial for hamstring strain rehab. They performed a randomized controlled trial comparing two groups after acute hamstring strains. One group only performed and progressed exercises when pain-free. The second group was permitted to perform and progress exercises up to 4/10 pain (0 = absolutely no pain, 10 = worst pain imaginable).
There was no difference between the groups in time to return to play. Both rehab groups improved biceps femoris long head fascicle length at time of return to play, but there was no difference between groups. Both groups improved hamstring strength at time of return to play, but the pain-threshold group had significantly greater strength gains (on average 15% more than pain-free group). These differences in strength were still present two months after return to play. At six-month follow-up, there was no difference in re-injury rate between the groups.
Given the larger strength gains and similar return to play times for athletes allowed to rehab with 4/10 pain, some pain seems to be beneficial during the rehab of hamstring strains. Earlier inclusion of strengthening exercises, especially eccentric exercises, may reduce hamstring neuromuscular inhibition after injury.
Include Exercises at Lengthened Hamstring Positions
Another factor that needs to be considered when performing hamstring exercises is the length of the muscle. Strength loss after hamstring injuries is usually greater at increased hamstring lengths–positions of combined hip flexion and knee extension. In a study on 131 athletes after hamstring injury, Whiteley et al.(2017) tested athletes’ hamstring strength everyday during the rehabilitation process. Outer range strength was tested in supine using a break test with the hip and knee starting at 90 degrees of flexion. Mid and inner range strength were tested with the athletes in prone, shortening the length of hamstrings. They found that outer range hamstring strength had better association with rehabilitation progression than mid or inner range strength.
Exercises specifically addressing strength deficits at greater hamstring lengths have been shown to reduce return to play times (Askling 2013, Askling 2014). The lengthening protocol used in those studies consisted of three exercises: the Extender, Diver, and Glider. The Extender is basically a supine hamstring stretch with the athlete holding the back of their knee and actively extending the knee. The Diver is a single-leg Romanian deadlift (RDL). The Glider is easiest to explain through video. See the exercises demonstrated below.
The Extender
The Diver
The Glider
I like to include manually-resisted hamstring curls as well. Manually-resisted exercises allow me to informally assess the athlete’s hamstring strength and strength-endurance throughout a set of curls. Curls can be progressed from prone (inner and mid-range hamstring strengthening) to side-lying with the hip flexed to 90 degrees (outer range).
To perform side-lying manually-resisted hamstring curls, the athlete is positioned in side-lying with the involved leg up. The hip is flexed to 90 degrees with the knee resting on a foam roller. The athlete then performs hamstring curls in this position while the clinician resists the movement. This can be performed concentrically or eccentrically depending on the athlete’s tolerance and ability.
Askling H-Test
About one-third of hamstring strains will recur, and many of these strains recur within the first couple weeks after return to play (Erickson 2017). Inadequate rehabilitation is likely one reason for reinjury. Return-to-play should be based on multiple criteria and consideration of the demands of an athlete’s sport and position. No return-to-play decision should be based on one test, but there is one specific hamstring test that seems to significantly reduce reinjury. The Askling H-Test was implemented in the two studies using the lengthened hamstring protocols mentioned previously (Askling 2013, Askling 2014). While typical re-injury rates are greater than 30% in most studies, the reinjury rates were 3.6% and 1.3% in those two studies.
The Askling H-Test is a quick and easy test to help determine an athlete’s readiness to return to sport after a hamstring injury. To perform the test, the athlete lies supine. The original test requires a knee brace to lockout the injured side in full knee extension, and the athlete’s trunk and uninjured leg are strapped to the table to stabilize. The examiner then performs a passive straight leg raise on the injured side to a point of strong but tolerable stretch. The athlete must reach the same point during testing. The athlete then performs a single active straight leg with speed to warm-up followed by three repetitions performed as fast as possible (reaching the same degree of motion achieved during the passive stretch). The athlete passes if the test can be performed as fast as possible without pain or apprehension.
I will typically have athletes perform the test without equipment. The compensations in knee flexion are easily observed by the examiner, and the brace does not always maintain full knee extension. The trunk is mostly stabilized by the exam table. The belt around the contralateral leg may be the most useful because it maintains pelvic position. While I’m not sure all the equipment is necessary, I’m also not aware of any research performing the test without the equipment listed. I’ve found the test to be sensitive to insecurity in the late stages of rehab with or without equipment. I’ll let you decide whether you use all the equipment.
Summary
Many factors influence rehabilitation after hamstring strain, but these five factors should be considered for most hamstring strains, especially running-related injuries. These five factors are:
- Specificity of exercise–most hamstring strains are running-related.
- Do eccentric hamstring exercises.
- Some pain during exercises is OK and probably beneficial.
- Include exercises at lengthened hamstring positions.
- Use the Askling H-Test as part of return to play testing.
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