Jan 29, 2015
Handling Hamstring Injuries

By James A. Onate, PhD, ATC; Zahra Ismaeli, MS, ATC; and Bonnie Van Lunen, PhD, ATC

Muscle strains are one of the most common injuries in sports, and one of the most frequent strains occurs in the hamstring group. Incidence of hamstring injuries ranges from 6 percent to 36 percent of all injuries in sport activities. Hamstring injuries often result in prolonged convalescence and time away from activity. Especially frustrating for athletes and the clinicians treating them is the high rate of recurrence. One-third of athletes who sustain an initial hamstring injury will reinjure their hamstring within a year of returning to play. This high prevalence has challenged clinicians and researchers to recognize and identify factors to prevent initial injury and recognizing those susceptible for reinjury.

The most common hamstring injuries occur in muscles where the forces that the muscle attempts to absorb exceed the forces that it is able to generate. The hamstrings function primarily by eccentric contraction to decelerate forward progression of the tibia during the swing phase of gait. It is the eccentric component of the hamstring that the majority of investigators attribute to the strain. Although these factors have been identified as originating from an intrinsic and extrinsic source, they are believed to function in a multi-factorial capacity.

The intrinsic factors are identified as previous injury, age, race, height, weight, body mass index, inadequate flexibility, insufficient warm-up, muscle fatigue, muscle weakness/imbalance, and early return to play before complete healing has occurred. Although these factors have been identified and subsequent prescreening and rehabilitation components have been changed, the rate of recurrence remains high.

One explanation for the reoccurrence of hamstring injury is the lack of research that supports the use of current objective assessments and intervention strategies. Methodological assessments of previous history of hamstring injury, isolated strength, functional movement patterns, and intrinsic physical structural properties of the muscle are sorely lacking in the clinical field.

Pre-Participation Assessment

The anatomic structure and composition of the hamstring may be one of the predisposing causes for this strain. The hamstrings are one of a few groups of muscles that are biarticular, crossing both the powerful hip and knee joints. The fact that the hamstring is a biarticular muscle has lead to speculation about its susceptibility to injury. Being biarticular, the hamstrings are subjected to large length changes, particularly during activities when the knee is extended and the hip is flexed (running and kicking), bringing the hamstrings to long lengths where risk of muscle tears become significant.

The hamstrings act as principal agonists of the anterior cruciate ligament, and prevent anterior translation during running and pivoting. Pre-participation strength assessments, along with information concerning previous history of injury, should be undertaken in both isolation and functional capacity across various levels of evaluation.

We propose an objective pre-participation approach utilizing a primary, secondary, and tertiary mode of assessment. A primary mode of assessing muscular strength (e.g., manual muscle testing) utilizes very low-end technology and is cost-effective, time-efficient, and easily learned. Manual muscle testing (MMT) has been shown to be valid and reliable across a variety of types of populations, yet the ability to discriminate across the population of healthy elite athletes has not been extensively analyzed. Nevertheless, MMT can provide a cursory assessment of hamstring muscle strength to provide information for subsequent in-depth analysis.

A secondary mode of assessing muscular strength (e.g., isotonic hamstring curl) utilizes various levels of technology (e.g., hand-held dynamometry to isotonic weight machines), yet maintains moderate investments in cost, time, and training. Secondary analysis of hamstring strength can include one-repetition maximum types of tests performed in either isotonic or isometric fashion.

A tertiary mode of assessing muscular strength requires high-end technology, with high demands of time and training levels. Sophisticated dynamometry assessments of isotonic or isokinetic movements with precisely assessed loads and speeds can provide detailed descriptive curve analysis of power, work, force, and torque at various points in the range of motion. Tertiary analyses provide in-depth information on the specifics of hamstring strength, but the functionality of such tests are constantly called into question. A combination of functional and isolated assessments of hamstring strength across various types of movements and modes of assessment should be considered per individual/group.

Since the relationship between the ability of the quadriceps to generate speed and the capacity of the hamstring to resist the resulting forces is critical, hamstring strength is often expressed relative to quadriceps strength as the hamstring:quadriceps ratio. In assessing this ratio, concentric hamstring and quadriceps muscle are compared. However, this may not be a true representation of the relationship.

The relationship for knee extension and flexion may be better described by the more functional ratio of eccentric hamstring to concentric quadriceps. However, it is not readily apparent that attempting to reverse hamstring:quadriceps deficits can reduce the incidence of hamstring strain. Although an association with injury has yet to be fully established, current research has identified hamstring muscle weakness as a predictor of hamstring reinjury. Another suggestion is that athletes have not fully rehabilitated when resuming practice, and thus train or compete in a position of weakness and agonist-antagonists imbalances.

A decrease in the amount of optimum torque of the initial injured hamstring can be produced in comparison to the non-injured hamstring. This suggests that there is a weakness in the muscle either due to incomplete or improper healing (scar tissue), and this weakness may predispose the athlete to re-injury.

The last phase of rehabilitation which constituents sports specific training and eccentric strengthening is usually neglected. Investigators have also identified the eccentric force that the hamstring can produce as the predictor of risk. In a 2002 study of athletes with a history of hamstring injury, Croisier et al reported that 69 percent had significant peak torque deficits.

Each athlete was trained for 10-30 sessions using isokinetic testing until they were within 5 percent of unaffected side. After 12 months, none of them sustained a clinically diagnosed hamstring muscle reinjury. The evidence of this has a dual outcome. First, if you train the hamstring to absorb the eccentric force, you may prevent initial injury occurring via eccentric deceleration. The second outcome is that you can incorporate eccentric hamstring strengthening into a rehabilitation program in an attempt to return the hamstring to the pre-injury strength.

Residual effects may be identified in a previously injured muscle by eccentric testing. Even though muscle has the ability to regenerate following injury, muscle healing has been found to be very slow, depending on the severity of injury. Early return to activity before complete healing is a risk factor for more severe injury.

Return to Play

In addition to assessing muscular strength and function, evaluating physical properties of the muscle has come to the forefront of determining return to play status. Scar tissue is one the structures that recently has been shown to impede many physiologic and physical properties of the muscle.

The role that scar tissue contributes to the rate of recurrence has yet to be identified. In a majority of studies evaluating risk factors, all have mentioned components of scar tissue contribution, yet they have neglected to remark on the significant role of scar tissue on healing muscle. Scar tissue affects all previously mentioned risk factors for hamstring injuries.

In the past, there was no objective clinical tool to assess the healing rate of muscle and the effects of scar tissue. Recently, the use of magnetic resonance imagining (MRI) has been beneficial in assessing components of hamstring muscle injury. MRI can clearly delineate severity of muscle injury through identification of the deformity of the muscle through the presence of abnormal signal reflecting hemorrhage and edema.

Imaging of the hamstrings was initially limitedly to assessing severe clinical situations, chronic conditions, and occasions when surgical intervention was needed; however is it is now routinely used with elite athletes in determining the severity of injury and predicting the amount of time for convalescence before returning to competition. Most imaging is done to identify general damage to muscle, not the specific type of damage (fat deposition, edema, hemorrhage).

More imaging needs to be done looking specifically at the designated type of muscle damage, particularly the impact of scar tissue on muscle healing, and a more detailed histological analysis of the injury profile during the healing phase is needed. The information obtained from the MRI will help formulate criteria for extent and specific type of damage, rehabilitation options, and risk for recurrence by allowing identification features of injuries that may predict recurrence of strains and/or provide some prognostic information about the convalescence period required before a return to function can be anticipate.

Evidence-based approaches to hamstring injury, strength, and physical structural properties will not only help in determining the initial risk for injury, but will also identify the risk for recurrence and return to play criteria. A comprehensive approach utilizing a global model of training and conditioning assessment with an evidence-based medicine approach is best utilized when determining return to play for any injury, especially hamstring strain.

James Onate, PHD, ATC, is Director of the Sports Medicine Research Laboratory at Old Dominion University. He is a Certified Athletic Trainer and is a member of the National Athletic Trainers’ Association and the American College of Sports Medicine. He is currently an editorial board member for the Journal of Athletic Training and serves as a clinical consultant for Baltimore Therapeutic Equipment (BTE) Technologies, Inc. His primary research interests focus on lower extremity injury and factors to aid in the development of lower extremity injury prevention programs.

Zahra Ismaeli, MS, ATC, is a doctoral student in the Human Movement Science program at Old Dominion University. She is a Certified Athletic Trainer and a member of the National Athletic Trainers’ Association and the American College of Sports Medicine. She is currently working as a research assistant at Naval Medical Center Portsmouth with her primary research interests focused on hamstring injury assessment, treatment, and return to play considerations.

Bonnie Van Lunen, PHD ATC, is an Associate Professor at Old Dominion University and is Director of the Human Movement Science Doctoral Program and the Post Professional Graduate Athletic Training Program. She is a Certified Athletic Trainer and is a member of the National Athletic Trainers’ Association. Her primary research interests focus on education, manual therapies, evidence based practice, and lower extremity injury management and rehabilitation.

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