By Jack T. Hickey, PhD, AEP; David A. Opar, PhD; Leigh J. Weiss, DPT, PT, ATC; and Bryan C. Heiderscheit, PhD, PT
Impaired performance and reinjury rates are high for this common injury. This narrative review discusses the causes and mechanisms of hamstring strain injury and current clinical concepts related to the rehabilitation process, with the aim of helping practitioners improve athletes’ outcomes.
Rehabilitation practitioners (eg, athletic trainers and physical therapists) regularly manage athletes who have sustained acute hamstring strain injuries (HSIs). The aim of HSI rehabilitation is to prepare athletes for return to sport (RTS) performance as soon as possible while also mitigating their reinjury risk. Athletes typically complete rehabilitation and RTS within 3 weeks of HSI1; however, reinjuries frequently occur soon after RTS,2 and subsequent performance may be impaired.3 Therefore, rehabilitation practitioners need to be cognizant of current evidence-based practices so that athletes have the best opportunity for a full recovery.
This narrative review presents a brief overview of the causes and common mechanisms of HSI, the important features of the clinical examination, a detailed breakdown of different rehabilitation interventions and implementation considerations, and outcome measures to guide rehabilitation and RTS prognosis; it also identifies 2 key questions to inform future directions for research and practice. The Strength of Recommendation (SOR) Taxonomy4 was applied during open discussion among all authors to reach consensus on our recommendations related to clinical examination, rehabilitation interventions, and outcome measures. In this article, we aim to provide practitioners with the contemporary, evidence-based information necessary to deliver best-practice rehabilitation for athletes with HSIs, promoting expeditious RTS performance while minimizing the risk of recurrent injury.
Causes and Mechanisms
Whether HSI occurs after accumulated repetitive microscopic muscle damage or in response to a single aberrant event exceeding the limits of the muscle-tendon unit is debatable.5 Some HSIs may result from an ongoing decline in tissue integrity due to repetitive damage, leaving the athlete vulnerable to an innocuous inciting event (eg, submaximal velocity running). In other instances, HSI may be caused by a single macrotraumatic event (ie, forceful and rapid hip flexion), irrespective of underlying tissue integrity. Either way, HSI mechanisms likely involve a combination of (1) high muscle-tendon unit forces (active or passive), (2) muscle-tendon unit lengthening beyond moderate lengths, and (3) high-velocity movements.6,7 Whether all 3 factors are necessary for an athlete to sustain an HSI remains unclear. Nonetheless, these causes should be in the forefront of the practitioner’s mind when developing both HSI prevention and rehabilitation programs.
In a sporting context, the most common mechanism of HSI is high-speed running, followed by movements involving forceful and extensive hamstring lengthening, such as kicking.8 During high-speed running, the terminal swing phase is considered most injurious.7,9 In the second half of the swing, the hamstrings are active, rapidly lengthening, and absorbing energy to decelerate the limb in preparation for foot contact.6 Hamstring muscle force increases approximately 1.3-fold as running velocity increases from 80% to 100% of maximum and the greatest muscle-tendon unit stretch is incurred by the long head of the biceps femoris.10 These findings may explain why the long head of the biceps femoris is the most injured hamstring muscle,11 often during high-speed running.
- Mechanisms of hamstring strain injury likely involve a combination of high muscle-tendon unit forces (active or passive), extensive muscle-tendon unit lengthening beyond moderate lengths, and high-velocity movements.
- Returning to high-speed running is arguably the most important aspect of rehabilitation, given that it is fundamental to performance in many sports and a common mechanism for hamstring strain injury.
- Eccentric hamstring exercises and hip-extensor strengthening should also be implemented during rehabilitation to prepare athletes for the demands of high-speed running and address deficits in strength and muscle structure.
When athletes experience acute-onset posterior thigh pain in response to a common mechanism of HSI, the clinical examination is less about diagnosis and more about the rehabilitation needs or RTS prognosis.12,13 Athletes presenting with posterior thigh pain resulting from either a mechanism not typical of HSI or a more chronic onset require a differential diagnosis to either confirm or rule out the presence of other pathologies (Table 1). In this section, we highlight the important features of an initial clinical examination of HSIs in athletes.
In our collective clinical experiences, athletes with a suspected HSI typically report the sudden onset of posterior thigh pain, sometimes accompanied by an audible or sensory pop, causing the immediate cessation of activity. Athletes should be asked to rate their pain at the time of suspected HSI, which is associated with the RTS prognosis12 and may be used as a reference point when monitoring symptoms throughout rehabilitation. Recording a thorough history of the athlete’s injuries before this incident is important, as previous HSI increases the risk of future HIS by 2.7 times14 and recurrence at the site is common in the weeks after RTS.2 Concurrent or previous injuries to other areas, particularly the lower back, hip or groin, and knee, should also be noted, as these findings could alter the clinical examination or rehabilitation protocol. SOR: A
Palpation of the Injured Area
With the athlete lying prone and the knees in full extension, the practitioner can palpate the posterior thigh to assess defects in the muscle-tendon unit and identify the possible injury site by establishing the point of maximal pain provocation. Distance from the ischial tuberosity to the site of maximal pain provocation by palpation and the total length of palpable pain should be measured and monitored throughout rehabilitation. Palpable pain that is closer to the ischial tuberosity or of greater total length has some association with an increased duration of HSI rehabilitation.13,15 SOR: B
Hip-flexion and knee-extension range of motion (ROM) should be evaluated during the clinical examination to determine hamstring flexibility and tolerance to muscle lengthening. In our experience, pain may limit the accurate assessment of actual muscle-tendon unit extensibility, but ROM comparison with the contralateral uninjured limb may still provide an indication of HSI severity.8 Between limbs deficits in knee ROM and pain during the active knee-extension tests are useful measures in providing a prognosis for RTS16 and the progression of running intensity throughout HSI rehabilitation.13 The active knee extension test can be performed with the hip flexed to either 90° or the maximal angle of flexion possible for each athlete (Figure 1).13
Assessment of hip-flexor flexibility and ankle-dorsiflexion ROM may also be warranted, as these measures have some association with HSI risk.17,18 In a prospective study of Australian rules footballers, the HSI risk increased by 15% for every 1° increase in hip flexion during the modified Thomas test.17 The average dorsiflexion lunge test distance reported by van Dyk et al18 was less in soccer players who sustained HSIs (9.8 ± 3.1 cm) than in their uninjured counterparts (11.2 ± 3.1 cm). However, practitioners must be aware that these group-level associations are limited in their ability to predict HSI at the individual level. SOR: B
Hamstring strength is usually evaluated during isometric contractions at the initial clinical examination,19 and practitioners should ask athletes to rate their pain on a numeric rating scale (range = 0–10) during these tests.20 Strength can be objectively measured if practitioners have access to equipment such as a handheld dynamometer,21 load cells,22 or force plates.23 Practitioners without access to such equipment may consider using manual muscle testing to subjectively characterize strength, but we encourage exploration of relatively cheap alternatives, such as crane scales, which can objectively measure force.24
Given the biarticular nature of the hamstring, knee-flexion and hip-extension strength should be tested with the athlete lying both prone and supine (Figure 2), ideally with the hamstring in a lengthened position,19,21 which appears most useful for RTS prognosis.12,13 Internal and external rotation of the tibia can be added to knee-flexion strength tests to differentiate between medial and lateral hamstring muscle injury, respectively.25 Hip-extension strength can be assessed with the knee flexed to identify muscles other than the hamstring, such as the gluteus maximus, that require strengthening during rehabilitation.26 Practitioners may also consider testing the strength of movements not involving the hamstring based on the athlete’s injury history (eg, hip adduction in those with hip and groin pain27), which may inform exercise selection during rehabilitation. SOR: A
Magnetic Resonance Imaging
Beyond the subjective and physical clinical examinations mentioned, magnetic resonance imaging (MRI) may be used to confirm the HSI diagnosis by identifying the location and extent of tissue damage. Several MRI-based muscle-injury classification and grading systems have been proposed and applied to HSI to provide the RTS prognosis.28 Prolonged RTS after HSI may occur when MRI scans show signs of tissue damage compared with no damage or if the proximal tendon is disrupted compared with intact.29 However, further detailed classification or grading of HSI based on MRI findings appears to offer negligible prognostic value beyond that of routine clinical examination.12
An emerging recommendation is that HSI rehabilitation should be more conservative when MRI reveals disruption to the intramuscular tendon,30,31 which was originally based on retrospective observations of prolonged RTS and greater recurrence rates with this diagnosis.32 More recent prospective work31 has shown that when rehabilitation is informed by the MRI diagnosis, recurrence rates can be kept similarly low across all types of HSI, but RTS time is prolonged by at least 2 weeks in athletes with intramuscular tendon disruption. This prolonged RTS was likely the result of the 2-week delay in progression of eccentric loading and running intensity that was applied to HSIs with intramuscular tendon disruption in the study by Pollock et al.31 Yet it remains unclear if delayed progression of eccentric loading and running intensity is truly necessary in HSIs with intramuscular tendon disruption, as the rehabilitation practitioners were not blinded to the MRI findings.31
In a prospective study that did blind rehabilitation practitioners to the MRI findings, time to RTS and recurrence rates were not different when comparing HSIs with and those without intramuscular tendon disruption.33 However, RTS was prolonged in participants with full-thickness intramuscular tendon disruption (31.6 ± 10.9 days) versus those with no disruption (22.2 ± 7.4 days) as well as in participants with waviness of the intramuscular tendon (30.2 ± 10.8 days) versus those with no waviness (22.6 ± 7.5 days).33 Nonetheless, athletes can successfully RTS despite persistent signs of intramuscular tendon disruption on follow-up MRI scans without increasing their risk of reinjury.34
Based on current evidence, practitioners who can refer patients for MRI may be able to provide a more accurate prognosis for RTS by differentiating between HSIs with and those without visible tissue damage or proximal tendon involvement. Still, the need to alter rehabilitation and RTS decision making based purely on other MRI findings, such as intramuscular tendon disruption, requires further investigation before being recommended as standard practice. SOR: B
This article appears courtesy of the National Athletic Trainer’s Association (NATA.org). It first appeared in the February 2022 issue of the Journal of Athletic Training.
Part II of this article, which focuses on Rehabilitation, will appear in the April issue of Lower Extremity Review.
Jack T. Hickey, PhD, AEP, is a Lecturer in exercise physiology with the School of Behavioural and Health Sciences and the Sports Performance, Recovery, Injury and New Technologies (SPRINT) Research Centre at the Australian Catholic University in Melbourne.
David A. Opar, PhD, is the Director and Injury Program Lead at the Sports Performance, Recovery, Injury and New Technologies (SPRINT) Research Centre at the Australian Catholic University in Melbourne.
Leigh J. Weiss, DPT, PT, ATC, is the Director of Rehabilitation/Physical Therapy for the New York Football Giants, in East Rutherford, New Jersey.
Bryan C. Heiderscheit, PhD, PT, is a professor in the Department of Orthopedics and Rehabilitation at the School of Medicine and Public Health at the University of Wisconsin in Madison.
- Ekstrand J, Krutsch W, Spreco A, et al. Time before return to play for the most common injuries in professional football: a 16-year follow-up of the UEFA Elite Club Injury Study. Br J Sports Med. 2020;54(7):421–426.
- Wangensteen A, Tol JL, Witvrouw E, et al. Hamstring reinjuries occur at the same location and early after return to sport: a descriptive study of MRI-confirmed reinjuries. Am J Sports Med. 2016;44(8):2112–2121.
- Whiteley R, Massey A, Gabbett T, et al. Match high-speed running distances are often suppressed after return from hamstring strain injury in professional footballers. Sports Health. 2021;13(3):290–295.
- Ebell MH, Siwek J, Weiss BD, et al. Strength of Recommendation Taxonomy (SORT): a patient-centered approach to grading evidence in the medical literature. Am Fam Physician. 2004;69(3):548–556.
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- Chumanov ES, Heiderscheit BC, Thelen DG. Hamstring musculotendon dynamics during stance and swing phases of high-speed running. Med Sci Sports Exerc. 2011;43(3):525–532.
- Heiderscheit BC, Hoerth DM, Chumanov ES, Swanson SC, Thelen BJ, Thelen DG. Identifying the time of occurrence of a hamstring strain injury during treadmill running: a case study. Clin Biomech (Bristol, Avon). 2005;20(10):1072–1078.
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- Kenneally-Dabrowski CJB, Brown NAT, et al. Late swing or early stance? A narrative review of hamstring injury mechanisms during high-speed running. Scand J Med Sci Sports. 2019;29(8):1083–1091.
- Chumanov ES, Heiderscheit BC, Thelen DG. The effect of speed and influence of individual muscles on hamstring mechanics during the swing phase of sprinting. J Biomech. 2007;40(16):3555–3562.
- Ekstrand J, Healy JC, Waldén M, Lee JC, English B, Hagglund M. Hamstring muscle injuries in professional football: the correlation of MRI findings with return to play. Br J Sports Med. 2012;46(2):112–117.
- Jacobsen P, Witvrouw E, Muxart P, Tol JL, Whiteley R. A combination of initial and follow-up physiotherapist examination predicts physician-determined time to return to play after hamstring injury, with no added value of MRI. Br J Sports Med. 2016;50(7):431–439.
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