Muscle Imbalances: Testing and Training Functional Eccentric Hamstring Strength in Athletic Populations

Summary

The hamstrings are a group of muscles that are sometimes problematic for athletes, resulting in soft tissue injury in the lower limbs. To prevent such injuries, functional training of the hamstrings requires intensive eccentric contractions. Additionally, hamstring function should be tested in relation to quadricep function at different contraction speeds.

Abstract

Many hamstring injuries that occur during physical activity occur while the muscles are lengthening, during eccentric hamstring muscle actions. Opposite of these eccentric hamstring actions are concentric quadriceps actions, where the larger and likely stronger quadriceps straighten the knee. Therefore, to stabilize the lower limbs during movement, the hamstrings must eccentrically combat against the strong knee-straightening torque of the quadriceps. As such, eccentric hamstring strength expressed relative to concentric quadricep strength is commonly referred to as the "functional ratio" as most movements in sports require simultaneous concentric knee extension and eccentric knee flexion. To increase the strength, resiliency, and functional performance of the hamstrings, it is necessary to test and train the hamstrings at different eccentric speeds. The main purpose of this work is to provide instructions for measuring and interpreting eccentric hamstring strength. Techniques for measuring the functional ratio using isokinetic dynamometry are provided and sample data will be compared. Additionally, we briefly describe how to address hamstring strength deficiencies or unilateral strength differences using exercises that specifically focus on increasing eccentric hamstring strength.

Introduction

The relationship between knee flexor and extensor strength has been identified as an important parameter in assessing a person's risk of incurring a lower limb injury¹. Specifically, there is an increased probability of hamstring injury when ipsilateral or bilateral imbalances in hamstring strength are present when compared to quadricep strength². Therefore, many sport scientists and practitioners test knee flexor and extensor strength to determine whether an athlete is at risk of incurring a hamstring injury. However, various testing methods are used that do not allow for direct comparisons to be made between methods (e.g., different contraction speeds, different muscle actions, and field testing vs. laboratory testing)^{3,4,5,6,7,8,9}. Although different testing methods provide different bits of valuable information regarding strength levels, the methodological approach for thigh muscle isokinetic strength testing should be unified to enable comparisons across individuals, populations, and time.

Although the evaluation of ipsilateral imbalances between knee flexors and extensors have been often described using the conventional concentric hamstring to concentric quadriceps ratio (H/Q_CONV)^10,11, co-activation of the knee flexors and extensors is known to occur during all movements and takes place through opposing contraction modes. To explain, the knee extensors are primarily involved in propulsion during jumping and running, whereas the knee flexors primarily stabilize the knee during landing and running by decelerating the lower limb and counteracting the rapid and forceful concentric contractions of the extensors. As most movements in sports require simultaneous concentric knee extension and eccentric knee flexion, a relative strength comparison between the two would be appropriate. Therefore, eccentric knee flexor strength relative to concentric knee extensor strength is commonly tested and is known as the "functional ratio" (H/Q_FUNC)¹².

Compared to the H/Q_CONV ratio where values can range from 0.43 to 0.90¹², the H/Q_FUNC ratio can range from 0.4 to 1.4¹³, indicating that data from different protocols should not be compared to each other. Although maximal concentric torque decreases as concentric speed increases^14,15,16, eccentric torque is greater than concentric torque as speed increases^16,17. As such, the H/Q_FUNC ratio can approach a value of 1.0 as the speed of testing contraction increases^13,18. Since most sport movements occur at high velocities, knee extensor and flexor strength testing are likely more ecologically valid at higher speeds. Therefore, such strength testing protocols should include progressively increased speeds in a stepwise progression.

If isokinetic testing reveals a large discrepancy between eccentric hamstring and concentric quadricep strength, the discrepancy should be narrowed through training. For this purpose, decreasing knee extensor strength should never compensate for weak knee flexors at the expense of a more favorable H/Q_FUNC ratios, especially in sporting environments. The other option would be to progressively and intensively increase knee flexor strength so that the hamstrings become stronger, especially in relation to the quadriceps, at higher speeds. Therefore, if isokinetic testing reveals some degree of hamstring weakness, a training intervention will likely be necessary to increase hamstring strength, especially during eccentric muscle actions. As with all training interventions, follow-up testing should be performed to determine the efficacy of the eccentrically-focused hamstring strength training program, and further adjustments may need to be made. The objective of this paper is to describe how to test isokinetic functional eccentric hamstring strength, reveal potential hamstring weakness, and suggest how to resolve a functional hamstring weakness.

Protocol

The presented protocol follows the guidelines of human research ethics committee at Charles University, Faculty of Physical Education in Sport and has been previously approved as part of research.

1. Familiarize All Subjects Before Isokinetic Testing by Following Steps

1. Ensure that the subjects have not had any recent musculoskeletal injuries or pain in the lower limbs in the previous 6 months. If a subject reports recent knee pain, or has knee pain during testing, exclude the subject.
2. As eccentric isokinetic testing is likely a novel stimulus for many individuals, familiarize the subject with the protocols on a valid isokinetic dynamometer^19,20 (steps 1.3 to 1.7.6, below) at least twice before participating in official testing. Instruct the subjects to not perform any lower body resistance training, or other strenuous exercises, 72 h before testing.
3. To begin, guide the subjects through a general warm up.
   1. Instruct the subjects to jog for 5-10 min or cycle for 5-10 min on an ergometer with a resistance of 1.5-2 W/kg of body mass with a cadence between 60-90 rpm.
   2. After cycling, instruct the subjects to perform two sets of 8-10 body weight lunges and 8-10 hamstring curls on a Swiss ball with each leg with 1 min of rest between sets.
   3. Next, guide the subjects through dynamic stretching of the lower limbs including the quadriceps and hamstrings²¹.
4. Show the subject an example of the isokinetic torque-angle curve and explain that live visual feedback will be provided during the test.
5. Explain that the subject should "kick out as hard and fast as possible" for concentric knee extension and "pull back as hard as fast as possible" for concentric knee flexion. Also explain that the machine will move on its own during eccentric actions, but that the subject should try to "push as hard as possible" during eccentric knee flexion (eccentric action of the quadriceps) and "pull as hard as possible" during eccentric knee extension (eccentric actions of the hamstrings).
6. Allow the subject to ask any questions and make sure they understand what will happen during the test. Clearly state that if the subject experiences any pain or discomfort during the test that makes the subject wish to terminate the test at any time, the subject should inform the researcher immediately, and the test can be safely aborted.
7. Start the pre-set protocol listed in Table 1, and continually guide the subject through the protocol.
   1. Using the recommendations of Brown²², position the subject on the dynamometer in a sitting position with a hip angle of 100° of extension. Adjust the settings of the dynamometer to ensure that the subject's hips are all the way back and in contact with the chair and the dynamometer's axis of rotation axis is in-line with the axis of rotation of the subject's tested knee.
   2. Instruct the subject to hold a deep breath while fixing the shoulders, pelvis, and thigh of the tested leg using the pads and straps on the dynamometer. Fix the lever arm of the dynamometer to the distal part of the shin with the pad placed 2.5 cm over the apex of the medial malleolus, but do not support the non-exercised lower limb.
   3. Allow the subject to passively and actively go through the full extension and flexion range of motion while readjusting the straps, dynamometer settings, or both if needed.
   4. Ensure that the subjects can see a screen that shows the torque-angle curve and provide a verbal countdown to begin the test. Instruct the subjects to hold the handgrips located at the side of the seat during all testing efforts.
   5. Start the test and verbally encourage the subject by using phrases such as "go", "push harder", "pull, pull, pull", etc. During the rest intervals, provide the subject with short instructions about the upcoming task.
   6. After completing the protocol, allow the subject to get out of the dynamometer chair, and adjust the dynamometer to test the other limb.
   7. After repositioning the subject and adjusting the machine accordingly, perform the gravity correction measurement again and start the test for the untested lower limb.
8. Open the test results that show the angle-torque curve and check whether the subject achieved the selected speed of contraction for the whole movement.
   1. To determine if the desired speed was accomplished, ensure that the angle-torque curve does not appear to be interrupted (Figure 1).
   2. If the curve looks interrupted (Figure 2), it is likely that the subject did not push or pull against the lever arm fast enough for the dynamometer to register torque. If the subject was not able to reach the required angular velocity and register torque, continue with additional familiarization or exclude the subject from the study and check the possibility of an articular knee lesion²³.

2. Isokinetic Strength Measurement After Two Familiarization Visits

1. Set up the dynamometer's software to perform the tests according to Table 1, and complete the protocol as described in steps 1.3 to 1.7.6.
2. After the end of the protocol, allow the subject out of the chair and begin analyzing the data.

3. Hamstrings to Quadriceps Functional Ratio Calculation

1. Use the best peak torque values from all three trials at each given speed and type of muscle action. Insert the peak torque data and resultant ratios into a data organizing software that can graphically depict data such as Microsoft Excel.
2. Calculate the H/Q_FUNC60 ratio by dividing the hamstring eccentric peak torque at 60°·s^-1 by the quadriceps concentric peak torque at 60°·s^-1.
3. Calculate the H/Q_FUNC180 ratio by dividing the hamstring eccentric peak torque at 180°·s^-1 by the quadriceps concentric peak torque at 180°·s^-1.
4. Calculate the H/Q_FUNC240 ratio by dividing the hamstring eccentric peak torque at 240°·s^-1 by the quadriceps concentric peak torque at 240°·s^-1.
5. After creating a table similar to Table 2, compare the H/Q_FUNC ratios across different speeds and between the right and left limbs.
   1. Compare the measured peak values with normative data of a similar athletic group of the same age and gender.
   2. Determine if bilateral imbalances are present by comparing the right and left limbs at each tested speed.
   3. Determine if the ipsilateral H/Q_conv ratio at the same speed is above or below 0.6²⁴. If the values are below 0.6, the ipsilateral hamstring weakness is present compared to the quadriceps; design a specific hamstring strengthening intervention (Section 4).
   4. Determine if the ipsilateral H/Q_func ratio increases along with increased speed and reaches the desired value of 1.0^12,18, preferably in the speed of 180°·s^-1. If the HQ_func does not increase with the increased speed, implement hamstring training to resolve the reciprocal function of the hamstrings (Section 4).

4. Eccentric Hamstring Strength Training Examples

1. Consult with a trained exercise professional²⁵, such as Certified Strength and Conditioning Specialist, to select various exercises that target the hamstrings across a variety of muscle lengths, speeds, and movement patterns.
   1. Consult the exercise professional for advice regarding exercises that improve neuromuscular control during landing and jumping in addition to exercises reported to decrease hamstring injury risk.
   2. Under the professional's guidance, use the Nordic curl (Russian curl) exercise which can strengthen the hamstrings and reduce the risk of injury^26,27, as this exercise focuses on eccentric hamstring strengthening.
   3. Under the professional's guidance, use unilateral knee flexions on a Swiss ball to strengthen the hamstrings and possibly reduce a bilateral strength deficit^28,29.
   4. Under the professional's guidance, use unilateral or bilateral Romanian deadlifts, the good morning exercise, or both to strengthen the hip extension function of hamstrings^28,30,31.
   5. Under the professional's guidance, use complex exercises to strengthen both the hamstrings and quadriceps during "triple extension" exercises where the hips, knees, and ankle simultaneously flex and extend such as the squat, deadlift, and lunge.
   6. Under the professional's guidance, use exercises such as drop jumps or other repeated jumps to train proprioception in the lower limbs.
2. Under the professional's guidance, progressively increase the number of sets and repetitions in bodyweight exercises such as the Nordic curl and unilateral hamstring curl on the Swiss ball³², while also progressively increasing the external resistance and decreasing the number of repetitions in complex exercises (for an example, see Table 3).

Results

The examples below show the differences between young elite soccer athletes (age 15.4 ± 0.5 years, body mass 62.7 ± 8.2 kg, height 175 ± 9.1, training experience more than 8 years) performing eccentric hamstring training (EHT, n = 18) and without EHT (n = 15) for 12 weeks (Figure 3). The group performing EHT included this exercise two times per week, while the group without EHT performed core training and a general lower limb program instead. B...

Discussion

The first critical step in the aforementioned protocol is the athlete's familiarization, especially for the eccentric tests. Subjects may have to be familiarized two or three times to ensure reliable data during such isokinetic testing. Furthermore, it may be a good idea to re-familiarize subjects if testing sessions are more than two months apart. The second critical step is properly setting up the athlete in the dynamometer, ensuring that the knee axis is in-line with the axis of the dynamometer; it is also importa...

Materials

Name	Company	Catalog Number	Comments
HumacNorm	CSMI, Stoughton, MA, USA	021-54412236 (model 502140)	Standard Dynamometr
SoftwareHumac 2015	Computer Sports Medicine Inc. Stoughton, MA, USA	Version155	Software for dynamometr