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Knee Surgery, Sports Traumatology, Arthroscopy https://doi.org/10.1007/s00167-018-5265-z

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Anatomy of proximal attachment, course, and innervation of hamstring muscles: a pictorial essay

Karolina Stępień1  · Robert Śmigielski2,3 · Caroline Mouton4 · Bogdan Ciszek5,6 · Martin Engelhardt7 · Romain Seil4,8

Received: 12 July 2018 / Accepted: 23 October 2018 © European Society of Sports Traumatology, Knee Surgery, Arthroscopy (ESSKA) 2018

AbstractHamstring injuries are very common in sports medicine. Knowing their anatomy, morphology, innervation, and function is important to provide a proper diagnosis, treatment as well as appropriate prevention strategies. In this pictorial essay, based on anatomical dissection, the detailed anatomy of muscle–tendon complex is reviewed, including their proximal attachment, muscle course, and innervation. To illustrate hamstrings’ role in the rotational control of the tibia, the essay also includes the analysis of their biomechanical function.Level of evidence V (expert opinion based on laboratory study).

Keywords Hamstring · Hamstring anatomy · Hamstring injury · Muscle · Biceps femoris · Semimembranosus · Semitendinosus · Posterior thigh · Sciatic nerve

Introduction

Hamstring injuries are one of the most common prob-lems in sports medicine. Their prevalence is estimated to reach 12–15% among professional football players [22, 28, 87] and increases by 4% annually in this group [31]. It is also a major problem of track and field sports, dancing

and skiing [3, 4], but can be observed in the general non-sporting population as well [50]. Males and professional football players are particularly at risk of hamstring inju-ries [4, 50], which cause on average more than 14 days of time loss from sports participation (range 1–128 days) [29]. Although different prevention programs have been developed, the reinjury risk remains unacceptably high (12–63%). The majority of reinjuries occur at the same location of the hamstrings complex than the primary

Electronic supplementary material The online version of this article (https ://doi.org/10.1007/s0016 7-018-5265-z) contains supplementary material, which is available to authorized users.

* Karolina Stępień Karolina.stepien@carolina.pl

Robert Śmigielski rsmigielski@gmail.com

Caroline Mouton mouton.caroline@chl.lu

Bogdan Ciszek bjciszek@o2.pl

Martin Engelhardt engelhardt@primomedico.com

Romain Seil rseil@yahoo.com

1 Department of Orthopedics, Carolina Medical Center, Pory 78, 02-757 Warsaw, Poland

2 Mirai Institute, Wolska 96, 01-126 Warsaw, Poland

3 MIBO Foundation-The International Institute for Orthopaedic Research, Twarda 4, 00-105 Warsaw, Poland

4 Department of Orthopaedic Surgery, Centre Hospitalier Luxembourg, Clinique d’Eich. 78, rue d’Eich, 1460 Luxembourg, Luxembourg

5 Department of Descriptive and Clinical Anatomy, Centre of Biostructure Research, Medical University of Warsaw, Chałubińskiego 5, 02-004 Warsaw, Poland

6 Department of Neurosurgery, Prof. Bogdanowicz Children Hospital, Niekłańska 4/24, 03-924 Warsaw, Poland

7 Department of Orthopaedics, Trauma and Hand Surgery, Osnabrück Clinic, Am Finkenhügel 1, 49076 Osnabrück, Germany

8 Sports Medicine Research Laboratory, Luxembourg Institute of Health, 78 rue d’Eich, 1460 Luxembourg, Luxembourg

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injury [33, 52, 81, 85]. The common mechanisms of injury are indirect trauma, running and stretching [34].

Most of the injuries in football and athletics occur in the midportion of the hamstring muscle–tendon complex [21, 30]. Avulsions of the proximal attachment area differ from the above. They have a different injury mechanism and are often caused by a forced hyperextension [7, 8, 16, 20].

Hamstring tendons are also among the most frequently harvested grafts for ligament reconstructions. Even though they have a high capacity for regeneration [73], functional deficits after hamstring tendon harvesting remain common. It may induce a reduction of knee flexion, extension and internal rotation [44, 74].

Recent biomechanical investigations have suggested that the hamstring muscle–tendon complex plays an important role in controlling the rotational stability of the knee. Ultimately, they also play a role in the prevention of the valgus collapse observed in severe knee injuries such as the anterior cruciate ligament (ACL) injuries. Indeed, hamstrings turn out to be the main ACL agonists and pro-tectors of the ACL [89, 90].

A better understanding of the anatomy of the hamstring muscle complex may help to provide a better diagnosis, treatment as well as appropriate prevention strategies. The aim of this article, based on anatomical dissection, was to provide a detailed review of the anatomy of muscle–ten-don complex, including their proximal attachment, muscle course, and innervation. This kind of essay with clear and useful photographic documentation has not been reported in the literature previously. The biomechanical function of

the complex was also analysed to bring a fresh perspective on this problematic anatomic entity of lower leg.

The proximal attachment of the hamstrings

The semitendinosus (ST), long head of the biceps femo-ris (long head, lhBF) and semimembranosus (SM) muscles originate from the ischial tuberosity (Fig. 1a, b). The ST and lhBF have a common origin on the posteromedial aspect of the ischial tuberosity, over its top. Tendons of the ST and lhBF are conjoined at a distance of 9.1–10 cm [35, 37, 58, 81]. The SM origin is separate from the previous one and it is located anterolaterally from the ST/lhBF attachment. Fibres of the proximal SM attachment are twisted before forming a proper tendon (Figs. 2a, b, 3a, b, 4).

A majority of authors agree with the presence of a con-joined tendon of the ST/lhBF, but the precise description of its attachment area varies amongst authors. Most of authors observed the attachment on the posteromedial aspect of the ischial tuberosity as in our dissection [61, 68, 82], whereas others indicated it to be directly medial [10, 58] or lateral on the ischial tuberosity [35, 60]. Consequently, the SM attach-ment is also described in different ways: on the anterolateral aspect of the ischial tuberosity as in our dissection [61, 68, 82, 84], but also anteriorly [35] or purely lateral [58].

We observed the shape of the ST/lhBF attachment as being oval and of the SM footprint as a crescent-shaped being wider than the ST/lhBF—similar to most of the authors [58, 61, 68, 82].

The ischial tuberosity is also the area of the distal attach-ment of a sacrotuberous ligament (STL)—an elastic and

Fig. 1 a, b Posterior view of the gluteal region and the proximal part of the posterior thigh of a right lower extremity. (1) Gluteus maximus muscle; (2) semitendinosus muscle; (3) ischial tuberosity; (4) sciatic nerve; (5) perineal branches of the posterior femoral cutaneous nerve

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dynamic structure [40, 83]. Fibres of the STL are descend-ing from the sacrum to the ischial tuberosity in continuity with fibres of the lhBF [12, 58, 68] (Fig. 5).

In the proximal hamstring attachment area, two main bur-sas can be identified. The bursa of the ischial tuberosity cov-ers a prominent part of the ischiatic bone [24, 38, 57]. The bursa of the proximal biceps femoris can be found between the common attachment of the ST/lhBF and the SM fill-ing the space between proximal tendons close to their bony attachment [14](Figs. 6, 7a–c).

The proximal attachment of short head of biceps femo-ris (shBF) arises on the middle third of femur. Its origin is located on the lateral lip of the linea aspera, descending distally and laterally [17].

Course of the muscles

Semitendinosus muscle

The semitendinosus muscle (ST) lies in the posteromedial area of the thigh. It runs distally and medially from its proxi-mal insertion on the ischial tuberosity and lies directly on the SM. From its origin, the ST creates a conjoined tendon with the lhBF forming an aponeurosis. The muscle belly of the ST is fusiform (from external aspect) and has a characteristic oblique or V-shaped raphe (tendinous inscription) [41, 43, 51, 82, 86]. The distal tendon starts below the mid-thigh and runs around the medial condyle of the tibia to its distal insertion as a part of pes anserinus (Fig. 8).

Semimembranosus muscle

The semimembranosus muscle (SM) lies posteromedially in the thigh and has a similar location as the ST. It starts on the

Fig. 2 a, b Posterolateral view of the area of the proximal attach-ment of the hamstring muscles (right lower extremity). (1) Area of the attachment of the conjoined tendon of the semitendinosus and the long head of the biceps femoris; (2) the proximal attachment area of the conjoined tendon; (3) conjoined tendon of the semitendinosus and the long head of the biceps femoris—cut and rotated 180°; (4) proxi-mal tendon of the semimembranosus muscle; (5) area of the attach-ment of the semimembranosus muscle; arrowheads—shape of the semimembranosus attachment

Fig. 3 Proximal tendons of the hamstring muscles—dorsal (a) and abdominal (b) view with indicated direction of the fibres (lines: ST—blue, lhBF—red, SM—white). (1) The conjoined tendon of the semitendinosus and the long head of the biceps femoris; (2) the proximal tendon of the semimembranosus muscle

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anterolateral part of the ischial tuberosity to the medial con-dyle of the proximal tibia to the pes anserinus and descends under the ST, from its wide proximal insertion [11]. The proximal and distal tendons of SM overlap. It means that the part of fibres in the middle of SM has a connection to both tendons: the proximal and the distal [82] (Figs. 9, 10a, b).

Biceps femoris muscle

The biceps femoris muscle (BF) forms the posterolateral part of the thigh. It consists of two heads: the long (lhBF) and the short head of the biceps femoris (shBF). The proxi-mal tendon of the lhBF runs laterally after division of the conjoined tendon with the ST. Like in the SM, the proximal

and distal tendons of the lhBF are overlapping [82]. The shBF originates in the posterolateral region of the femur. It fuses with the lhBF in the distal part of the thigh, forming an aponeurotic structure. The conjoined distal tendon of both heads attaches to the head of the fibula [76, 78].

Mean lengths of hamstring muscles according to different authors are shown in Table 1 [42, 82, 85].

Innervation

The sciatic nerve innervates the hamstring muscle complex. The ST, SM, lhBF are innervated through its tibial divi-sion, while the shBF is innervated by its fibular division. The extra-pelvic part of the sciatic nerve appears beyond the greater sciatic foramen just under the piriformis muscle. It runs caudally and medially to the ischial tuberosity [66].

The SM, lhBF, and shBF are supplied by one motor branch, while the ST receives two motor branches from the sciatic nerve, which is running directly to the popliteal fossa [2, 5, 64, 65, 71, 86].

In the proximal area of the posterior thigh, the sciatic nerve gives the motor branch to the lhBF. It contains a few terminal branches heading distally and one recurrent which goes directly to the area of the bone attachment of conjoined tendon lhBF/ST [54]. The primary motor branch for the ST is located at a similar level as the branch for the lhBF. The secondary motor branch for the ST for the part of the muscle under the raphe and motor branches for the SM and shBF are located in the distal area of the posterior thigh [5, 64, 65, 71, 80, 86] (Figs. 11, 12a–c, 13a, b).

The precise measurements of the entries of the motor branches to the hamstring muscles are represented in Table 2 [5, 64, 65, 86].

Fig. 4 Posterolateral view on the area of the proximal attachment of the hamstring muscles. (1) Area of the attachment of conjoined ten-don of the semitendinosus and the long head of the biceps femoris; (2) area of the attachment of the semimembranosus muscle; (3) quad-ratus femoris muscle

Fig. 5 Posterior view of the area of the ischial tuberosity (right lower extremity). (1) Sacrotuberous ligament; (2) ischial tuberosity; (3) gemellus superior, obturator internus, gemellus inferior muscles; (4) piriformis muscle; (5) sciatic nerve; (6) semitendinosus muscle

Fig. 6 Lateral view of the area of the ischial tuberosity (right lower extremity). (1) Sacrotuberous ligament; (2) superficial bursa of the ischial tuberosity; (3) tendon of the long head of the biceps femoris; (4) sciatic nerve; (5) perineal branches of the posterior femoral cuta-neous nerve; (6) nerve branch to the bursa of the ischial tuberosity

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Biomechanical function of hamstring muscles

The hamstring muscle complex has a major significance in the human kinematic chain, directly influencing the func-tion of the lower limb and supporting an upright body

posture. The particularity of the hamstrings resides in the fact that their function can be considered both as a

Fig. 7 a–c Lateral view of the area of the proximal attachment of the hamstring muscles (right lower extremity). (1) Ischial tuberosity; (2) conjoined tendon of the semitendinosus and the long head of the biceps femoris; (3) proximal tendon of the semimembranosus muscle; (4) bursa of the proximal biceps femoris between split tendons

Fig. 8 Posterolateral view of the posterior thigh of a right leg. (1) Ischial tuberosity; (2) conjoined tendon of the semitendinosus and the log head of the biceps femoris; (3) sciatic nerve; (4) semitendinosus muscle; (5) long head of the biceps femoris muscle

Fig. 9 The hamstring complex. (1) Proximal tendon of the semimem-branosus muscle; (2) distal tendon of the semimembranosus muscle (3) conjoined tendon of the semitendinosus and the long head of the biceps femoris; (4) tendinous inscription (raphe) of the semitendino-sus muscle; (5) distal tendon of the semitendinosus muscle; (6) com-mon distal tendon of the long and short head of the biceps femoris muscle

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synergistic work of the entire muscle group, but also indi-vidually for each muscle.

Hamstring muscles functioning as a group

The primary functions of the hamstring muscles, aris-ing from their biarticular arrangement, are knee flexion, hip extension and slight abduction of the lower limb. The

simultaneous contraction of hamstring muscles influences both the knee and the hip so their function cannot be limited to one joint only [46]. Most of hamstring activity as a group is eccentric.

During the gait cycle, the hamstrings play a main role in the swing phase. By contraction, they coordinate hip exten-sion and prevent excessive extension of the knee. In the ter-minal swing phase, they also performed a significant amount of negative work (energy absorption) [19, 69].

The hamstring muscles are the main antagonist for the quadriceps femoris muscle (QF). Their coactivation during the contraction of the QF balances the lower limb [32, 35, 88]. At the same time, the hamstring muscles work along-side with the anterior cruciate ligament (ACL) decelerating forward translation of the tibia during knee extension (ACL-agonist muscles) [13, 36, 45, 55, 75, 89, 90].

Through their proximal attachment on the ischial tuber-osity, the hamstrings muscles have a direct influence on the position of the pelvis. Posture can be influenced by the forces carried by the STB between the vertebral column and legs which may change the angle of the pelvic axis [83].

Individual function of hamstring muscles

The biomechanical analysis of hamstrings indicates some differences between each of the individual muscles. Based on their anatomy, each muscle generates contractions in a slightly different plane and direction. The main results—knee flexion and hip extension—are the net force of these components. Analysis prove that the biomechanical load, the metabolic activity, and the EMG activity of each hamstring muscle differ [66, 69, 70, 89].

The BF with its distal insertion on the lateral side of the proximal fibula and tibia influences stability of the poste-rolateral corner of the knee [77]. The contraction of the BF rotates the tibia and fibula externally [56, 75, 77]. Conse-quently, it prevents internal rotation of the tibia in relation to the femur [9]. The BF is the most effective hamstring muscle in reducing the ACL-loading component produced by the QF through decreasing anterior tibial translation [13, 25].

Due to their distal insertion on the medial part of the proximal tibia, the ST and SM contraction induce an internal

Fig. 10 a, b Posterolateral view of the posterior thigh of a right lower extremity. (1) Conjoined tendon of the semitendinosus and the long head of the biceps femoris; (2) ischial tuberosity; (3) proximal tendon of the semimembranosus muscle; (4) semimembranosus muscle; (5) semitendinosus muscle; (6) long head of the biceps femoris muscle; (7) short head of the biceps femoris muscle; (8) conjoined tendon of the long and the short head of the biceps femoris

Table 1 Mean lengths of hamstring muscles (cm) [41, 42, 82, 86]

Woodley and Mercer [86]

Kellis et al. [41, 42] Van der Made et al. [82]

Number of specimens 6 8 56Biceps femoris (long head) 43.8 38.93 ± 4.03 42.0 ± 3.4Biceps femoris (short head) 29.1 28.53 ± 1.92 29.8 ± 3.9Semitendinosus 43.8 47.03 ± 2.99 44.3 ± 3.9Semimembranosus 43.8 40.43 ± 2.52 38.7 ± 3.5

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rotation of the tibia [75]. These muscles are antagonists of the external rotation generated by the BF [56, 75, 77]. The understanding of their role in the prevention of the valgus collapse, which is the primary mechanism of noncontact ACL injuries [67], is under investigation [90].

The antagonism between the ST and lhBF becomes apparent during a cadaver dissection. The application of a proximally oriented traction force on the ST and lhBF nicely demonstrate their respective functions as internal and exter-nal rotators of the tibia. Interestingly, pulling the SM proxi-mally does not significantly affect internal rotation, which illustrates the muscle’s static function in internal rotation of the tibia through preventing external rotation [LINK to the movie].

The mechanical resistance of junctions in hamstrings seems to be lower than the structure of the actual tendon or muscle tissue. In the present investigation, an appar-ently thinner structure was noticed at two precise areas in the hamstring muscle complex: the conjoined tendon of the lhBF and ST and the conjunction between the lhBF and shBF(Figs. 14, 15a, b, 16).

Discussion

Most of the studies analysing hamstrings’ anatomical structure have shown little variability with respect to the musculotendinous pattern as well as the anatomy of their innervation. Nevertheless, information about the distribu-tion of hamstring injuries is very heterogeneous [6–8, 16, 20, 21, 23, 28, 30, 39, 45, 63, 72, 84]. This discrepancy may be explained both by the complexity of their musculoten-dinous structure and the heterogeneity of existing muscle

injury classification systems of which several are new and still under investigation [18, 59, 62, 79].

There seems to be a predominance of injuries to the lhBF and SM over the ST [6, 8, 20, 21, 23, 39, 45, 72]. It has been postulated that this may be related to the presence of overlapping tendons in the lhBF and SM. Some authors

Fig. 11 Lateral view on the innervation of the proximal hamstrings. (1) Ischial tuberosity; (2) sciatic nerve; (3) motor branches of sciatic nerve to long head of the biceps femoris muscle; (4) recurrent branch to the proximal attachment of conjoined tendon

Fig. 12 a–c Lateral view on the innervation of the hamstring muscle complex. (1) Ischial tuberosity; (2) sciatic nerve; (3) motor branch to the long head of the biceps femoris muscle; (4) recurrent branch to the proximal attachment of conjoined tendon; (5) motor branch to the semitendinosus muscle; (6) motor branch to the semimembranosus muscle; (7) motor branch to the short head of the biceps femoris mus-cle

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hypothesised that the raphe in the ST has a protective prop-erty [49, 82].

Morphologically, most injuries appear at the musculo-tendinous junction [21, 30]. The conjoined tendon of the lhBF and ST has a very delicate structure located where a relatively high prevalence of injuries can be observed [7, 16, 20]. Second, the conjunction between the lhBF and shBF, located in the distal part of the hamstring, has also a similar, delicate structure. The prevalence of injuries in this area seems to be reported less frequently [53].

Hamstring muscle injuries can involve the disruption of innervation by damaging motor nerve branches. The nerve conduction velocity in injured hamstrings is significantly lower than in uninjured muscles [47]. This occurrence may

be associated with high reinjury rate [85] and needs further investigation. The anatomic dissections revealed a recurrent branch of the motor branch of the lhBF. It is located in the area of the proximal conjoined tendon and may be dam-aged in case of an injury [54]. Theoretically, it could also be affected if an inflammatory process of the bursa occurs in this area. The damage of this branch may be responsible for symptoms of the muscle denervation after proximal tendon avulsion, which may remain even after surgical reconstruc-tion. This observation offers a new perspective on the proxi-mal hamstring injuries, but it needs to be further analysed.

The anatomical observations highlighted an individual-ised function of each hamstring muscle. There are pointed selective features of the medial hamstring muscles respon-sible for internal tibia rotation versus the lateral hamstring muscles responsible for external tibia rotation [13, 69, 89, 90]. Hamstring muscles have an essential role in the control of tibiofemoral rotation and consequently on the rotational stability of the knee, which has been illustrated by the pre-sent investigation. Although our test was not standardised, it may provide an opening for further investigation.

The ST tendon is one of the most frequently used auto-grafts in orthopaedic surgery. Harvesting this tendon nega-tively impacts its biomechanical function of an active inter-nal rotator, which allows it to prevent the external rotation of the tibia and reduces ST tendon’s direct antagonistic role to the lhBF. Likewise, this may have a consequence on the function and stability of the knee. A recent cadaveric study has indeed identified an increased valgus and external rota-tion of the knee after harvesting the ST and gracilis tendons [48]. Furthermore, while the hamstring tendons have a high capacity for regeneration [73], the remaining functional defi-cits—decreased knee flexion, extension and internal rota-tion, have been reported [44, 74]. A long-term effect of these disorders is yet to be established [1].

The use of hamstrings as graft material renders the matter of their internal structure a very important one. Recently, the relationships between tendon and muscle inside the mus-cle belly were presented for sartorius, gracilis and peroneus

Fig. 13 a, b Entry points of motor branches to the hamstring muscles. (1) Motor branch to the long head of the biceps femoris muscle; (2) two motor branches to the semitendinosus muscle; (3) motor branch to the semimembranosus muscle

Table 2 The placement of motor branch entries to hamstring muscles established by measuring a distance from the ischial tuberosity [5, 64, 65, 86]

a In the original scale the authors used 0% for the line crossing the medial and lateral tibial condyles and 100% for the ischial tuberosity. For this table, the scale was reversed

Rab et al. [64] Woodley and Mercer [86] An et al. [5] Rha et al. [65] a

Number of specimens 30 6 50 32lhBF 15.1 ± 3.4 cm − 2.8 to − 4 cm (proximally)

One specimen 3.6 cm14.1 ± 3.3 cm 41.5 ± 9.4% 40–50%

shBF No information 20 cm 19.1 ± 2.3 cm 56.2 ± 6.0% 70–85%Primary ST 4.75 ± 1.4 cm 4.2–12 cm 7.0 ± 2,2 cm 20.3 ± 5.7% 20–40%Secondary ST 14.47 ± 2.6 cm 7.5–19 cm 20.3 ± 2.9 cm 59.9 ± 6.6% 60–75%SM No information 14.6–21.2 cm 21.1 ± 3.3 cm 62.2 ± 9.2% 60–80%

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longus [15, 26, 27]. Such a relationship in case of hamstrings is under the investigation by our group.

The direct connection between the hamstring muscle complex and the pelvis through the STL is yet another find-ing worth further research, since the proper tension of ham-string muscles is required to achieve the proper position of the pelvis and consequently of the sacrolumbar part of the spine [83].

All discussed issues are relevant in daily practice of ortho-paedic surgeons, sports medicine specialist, physiotherapists,

etc. Detailed photographs can be a very useful instrument during preparation for surgical and non-surgical treatment of hamstring injuries.

Conclusion

Knowledge of the anatomy of the hamstrings—their mor-phology, innervation, and function, provides valuable insight concerning clinical implications. It helps to improve the

Fig. 14 Location of potential areas of the decreased resist-ance in the hamstring muscle tendon. (1) Conjoined tendon of the semitendinosus and the long head of the biceps femoris; (2) common distal tendon of the long and short head of the biceps femoris

Fig. 15 Posterior view on the proximal posterior thigh with marked potential area of the decreased resistance of conjoined tendon (arrows). (1) Sacrotuberous ligament; (2) ischial tuberosity; (3) gemellus superior, obturator internus, gemellus inferior muscles; (4) sciatic nerve; (5) piriformis muscle; (6) conjoined tendon of the semitendinosus and the long head of the biceps femoris

Fig. 16 Lateral view of the thigh with marked areas of potentially decreased resistance of distal biceps femoris tendon (arrowheads). (1) Long head of the biceps femoris muscle; (2) short head of the biceps femoris; (3) quadriceps muscle; (4) patella; (5) intermuscular septum

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understanding of their frequent involvement in pathologic conditions, and the significant amount of sports-related mus-cle injuries and reinjuries.

Acknowledgements We thank Maciej Śmiarowski (https ://www.artla borat ory.eu) who provided photographic documentation. Dr. Robert Śmigielski wishes to dedicate the article to his mentor—Dr. Bernhard Segesser. The project was co-funded by the Luxembourg Institute of Research in Orthopaedics, Sports Medicine and Science (LIROMS).

Funding The project was co-funded by the Luxembourg Institute of Research in Orthopaedics, Sports Medicine and Science (LIROMS).

Compliance with ethical standards

Conflict of interest The authors declare that they have no conflict of interest.

Ethical approval All procedures performed in studies involving human participants were in accordance with the ethical standards of the insti-tutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

Informed consent For this type of study formal consent is not required.

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