TABLE 1.
Summary of studies cited in Section “Anatomic and Physiologic Variations of the Hamstrings.”
| References | Population | Anatomic part assessed | Assessment method | Main findings |
| An et al. (2010) | 50 lower extremity cadavers from 15 males and 12 females (age: 71 years) | Innervation patterns of HT | Dissection | BFsh and SM show one innervation entry, and BFlh and ST show two innervation entries. In those with two entries, for BFlh, the incidence of type I innervation was 82% (18% of type II) while for ST, the incidence of type I innervation was 14% (86% of type II). |
| Blazevich et al. (2006) | 16 women and 15 men physically active (age: 19.9–20.6 years) | Architecture of quadriceps | Ultrasound | Description of the architecture of the quadriceps. Changes in the relative activation of individual muscles determines alterations in force, velocity movement range and contraction mode. Intramuscular activation changes with the movement requirements. Training should alter the pattern of activations to improve force transmission between muscles with different architecture, make these variations in activations efficient, and promote region-specific hypertrophic responses. There are little differences between sex in these adaptations. |
| Burkett (1975) | Review | Anatomical variation of HT | – | Extensive biceps femoris-femur attachment coupled with strength imbalances between HT increase the risk of strains. |
| Dahmane et al. (2006) | 15 healthy sedentary males (age: 17–40 years) and 15 male sprinters (age: 23.2 ± 3.1 years) | Muscle fiber types of BF | Mechanomyography | Strong potential for the BF muscle to transform from slow to faster contracting muscle fibers after long-term sprint training. |
| Evangelidis et al. (2015) | 30 healthy recreationally active individuals (age: 20.7 ± 2.6 years) | Aponeurosis of BFlh | MRI | The proximal aponeurosis size is highly variable between individuals, and it is not associated with to muscle size or knee flexor maximal isometric or eccentric strength. This disproportion may predispose those individuals with relatively small aponeurosis to hamstring strain injuries, as they could be subjected to greater mechanical strain in the muscle tissue surrounding the aponeurosis. |
| Evangelidis et al. (2017) | 31 healthy, recreationally active participants (age: 21 ± 3 years) | Muscle fiber types of HT | MRI | HT muscles exhibited a balanced myosin heavy chain isoform distribution comparable to that of vastus lateralis, so that the predominance of fast-twitch fibers in HT muscle increasing the risk of strains is not supported. |
| Fiorentino and Blemker (2014) | 12 male track and field athletes | Musculotendon of BFlh | MRI | A larger muscle and/or narrower proximal aponeurosis of the BFlh could predispose an athlete to an increased risk of injury by increasing peak local muscle tissue strain. |
| Fraser et al. (2013) | 1 female cadaver (age: 87 years) | Anatomical variation of ST | Dissection | Two-part origin of the ST, with the variant portion being originated along the medial border of the ischial tuberosity. Could predispose to HT injury, chronic pain and pelvic floor discomfort. |
| Freitas et al. (2020) | 40 male professional football players (age: 24.5 ± 4.9 years) | Aponeurosis of BFlh | MRI | There were no significant differences for size aponeurosis of the BFlh between players with and without previous BFlh injury. |
| Fridén et al. (1983) | 12 male physical education students (age: 25 ± 7 years) | Muscle fiber types of vastus lateralis | Muscle biopsy | Type II fibers were more extensively damaged than type I fibers after 30 min of pedaling at a frequency of 60 rpm, at an intensity of 80–100% of VO2max, in a bicycle ergometer modified for use in eccentric work. |
| Galpin et al. (2012) | Description muscle fiber types | – | – | – |
| Garrett et al. (1984) | 7 male and 3 female cadavers (age: 60 years) | Muscle fiber types of lower limb muscles | ATPase histochemical reaction | Relatively higher percentage of type II fibers in HT compared to other thigh and leg muscles. |
| Gérard et al. (2020) | SR: 10 RCTs evaluating 346 healthy adults (age: 18.3–29.6 years) | Architecture of BF | MRI and ultrasound | Eccentric strength training associated with increased fascicle length and muscle thickness, and decreased pennation angle, as well as eccentric strength of the HT. |
| Gray (1945) | 2 male cadavers (age: 44 and 84 years) | Anatomical variation of HT | Dissection | All HT muscles originated from a common tendon in case 1. A muscle from the linea aspera and passing medially to the capsule of the knee joint, homologous with the ST, is presented in case 2. |
| Huygaerts et al. (2021) | Critical review | Muscle-tendon unit of HT | – | Fiber-fascicle lengthening is greater, and architectural structure non-uniform in the BFlh, with pennation being greater in the proximal-middle section compared to the distal and extreme proximal sections, that in addition to the inter-individual differences in BFlh structural features may predispose to this muscle to higher risk of HT injury. |
| Julien et al. (2018) | Review | Muscle fiber types | – | The changing environment could elucidate changes in muscle fiber characteristics, which could have implications in metabolism-related muscular atrophies. |
| Kellis (2018) | Narrative review | Architecture of HT | – | BFlh, compared to other HT, may be at higher increase of injury due to inter-muscular differences in HT architecture. Targeting the specific muscle-tendon region, instead of HT as a whole, could be beneficial in rehabilitation programs. |
| Kellis et al. (2009) | 3 cadavers (age: 68.3 years) | Architectural parameters of BFlh and ST | Ultrasound dissection | High level of agreement in BFlh and ST architectural parameters measured through ultrasound compared to direct dissection. |
| Kellis et al. (2012) | 8 cadavers (age: 67.8 years) | Architecture of HT | Dissection | The four hamstring components showed low to moderate architectural dissimilarity. Pennation angles were similar between BFlh, BFsh and SM, but higher than for ST. |
| Lievens et al. (2020) | 32 male recreational athletes | Muscle fiber types of gastrocnemius | Magnetic resonance spectroscopy | Participants with predominantly fast typology fibers fatigues more markedly in repeated Wingate tests than participants with slow typology fibers. |
| Macaluso et al. (2012) | 8 healthy, untrained individuals (age: 22 ± 1 years) | Muscle fiber types of vastus lateralis | Muscle biopsy | An acute bout of plyometric exercise preferentially affects type II fibers |
| Marušič et al. (2020) | 40 healthy adults (age: 23.7 ± 2.5 years) | Architecture of BFlh | Ultrasound | A 6-week progressive eccentric HT training in a lengthened position showed positive effects for fascicle length (increase) and pennation angle (decrease), but not for muscle thickness. |
| Medeiros et al. (2020) | 32 adult football players (age: 18–23 years) | Architecture of BFlh | Ultrasonography | An 8-week Nordic hamstring exercise training program was not effective at elucidating any improvement in muscle thickness, pennation angle, or fascicle length. |
| Mendiguchia et al. (2020) | 32 adult football players | Architecture of BFlh | Ultrasound | The sprint training group showed moderate increase in fascicle length, and the Nordic group a small increase. The Nordic group presented a small increase at pennation angle. |
| Methenitis et al. (2019) | 10 sedentary, 9 endurance runners, 10 power-trained and 9 strength-trained individuals | Muscle fiber composition of vastus lateralis | Muscle biopsy | Muscle fiber composition and rate of force development is affected by systematic training among different athletes. Type IIx fibers better correlated to rate of force development. |
| Rehorn and Blemker (2010) | 1 healthy male individual | Aponeurosis of BFlh | MRI and 3D modeling | The fact that proximal aponeurosis is narrower than distal aponeurosis in BFlh could explain the prevalence of injuries near the proximal myotendinous junction in this muscle, and relative aponeurosis dimensions could also explain the more prevalence of injuries in BFlh compared to other HT muscles. |
| Schmuter et al. (2021) | 1 female cadaver (age: 59 years) | Anatomical variations of BFlh and ST | Dissection | BFlh and ST were fused near their origin at the ischial tuberosity. |
| Shalabi et al. (2002) | 14 males and 2 females (age: 26 years) | Muscle fiber types of ST | Muscle biopsy (percutaneous ultrasound-guided biopsy) | Patients following ACL reconstruction show a composition of 50 ± 13% type 1 fibers, 26 ± 8% type 2A, 23 ± 19% type 2B and 1 ± 1% type 2C. Muscle biopsy technique is adequate to identify muscle composition. |
| Solomon and Stevenson (2008) | 1 male (age: 40 years) | Anatomical variation of BF tendon insertion | MRI | Case of a bilateral tibial insertion of the BF tendon in a previously asymptomatic patient, which could have implications on lateral knee stability. |
| Trappe et al. (2015) | 1 world champion sprinter | Muscle fiber types of vastus lateralis | Muscle biopsy | Large proportion of vastus lateralis type IIx fibers in the sprinter, and power output from type IIa and IIx fibers was higher than any human values reported to date. |
| van der Made et al. (2015) | 29 human cadaveric specimens (age: 71.5 years) | Anatomical variations of HT (origin dimensions, muscle length, tendon length, MTJ length and width, length of tendinous of ST (raphe). | Dissection | Overlapping proximal and distal tendons and muscle architecture may lead to a force not in line with the tendon and predispose to muscle injury. Protective effect of the presence of a raphe. |
| Vieira et al. (2007) | 41 male and 56 female asymptomatic patients (age: 52.8 years) | Innervation patterns of BF | MRI | Description of the normal anatomy of the distal BF and the relationship with the peroneal nerve. The peroneal nerve can pass downward posterior to the BFsh and superficial to the lateral head of the gastrocnemius, but also in a tunnel between the two muscles (23%). An unusual relationship between the nerve and the distal BF could predispose to peroneal neuropathy. |
| Woodley and Mercer (2005) | 3 female and 3 male cadavers (age: 69–88 years) | Architecture of HT | Dissection | ST, SM and BF showed anatomical partitioning defined by architecture and/or pattern of innervation. There was high degree of variation from subject to subject in most of the architectural features of the HT, such as, fascicular length, volume, physiological cross-sectional area or tendon characteristics. |
| Yang et al. (2018) | 93 patients (age: 40.2 years) | Innervation patterns of BF | MRI | There were less participants with type I (38.7%) than type II innervation (61.3%), in which the thickness was lower. The course of common peroneal neuropathy through the “popliteal tunnel” formed between the BFsh and the lateral gastrocnemius muscle was approximately 40%. |
| Yasin et al. (2010) | 25 patients undergoing ACL reconstruction with HT tendon autograph (age: 28 years) | Anatomy of accessory bands | Dissection | Gracilis and ST tendons showed a variable pattern of accessory bands, all of them occurring more than 11-cm proximal to the insertion of the tendons onto the tibial crest. |
HT, hamstrings; MTJ, musculotendinous junction; ST, semitendinosus; BFlh, biceps femoris long head; VO2max, maximal oxygen consumption; ATPase, adenosine triphosphatase; BF, biceps femoris; MRI, magnetic resonance imaging; ACL, anterior cruciate ligament; BFsh, biceps femoris short head; SM, semimembranosus; RCT, randomized controlled trial.