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Hamstrings on Morphological Structure Characteristics, Stress Features, and Risk of Injuries: A Narrative Review


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Hamstring injury has been considered one of the most common exercise-induced injuries in sports. Hamstring injuries mostly occur proximal to the biceps femoris. However, the reasons and mechanisms remain unclear. To summarize hamstring morphological structure features and what the relationship is between their structure and risk of injury from the current literature, this review discussed the possible injury mechanism of hamstrings, from the morphological and connected pattern diversity, the mechanical properties, and the stress–strain performance, to probable changes in action control. Morphological and connected pattern diversity of hamstrings components show heterogeneous loads under muscle tension. Connections of gradient compliance between different tissues may lead to materials’ susceptibility to detachments near the tendon–bone junction sites under heterogeneous load conditions. The hamstrings muscle’s motor function insufficiency also brings the risk of injury when it performs multi-functional movements during exercise due to the span of multiple joints’ anatomical characteristics. These structural features may be the primary reason why most damage occurs near these sites. The role of these biomechanical characteristics should be appreciated by exercise specialists to effectively prevent hamstring injuries. Future work in this research should be aimed at exploring the most effective prevention programs based on the material structure and motor control to enhance the properties of hamstring muscle materials to minimize the risk of injury.
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Appl. Sci. 2022, 12, 12713.
Hamstrings on Morphological Structure Characteristics, Stress
Features, and Risk of Injuries: A Narrative Review
Yinbin Shi
, Gengsi Xi
, Mengzi Sun
, Yuliang Sun
and Li Li
College of Life Science, Shaanxi Normal University, Xi’an 710119, China
School of Physical Education, Shaanxi Normal University, Xi’an 710119, China
Biomechanics Laboratory, Beijing Sport University, Beijing 100084, China
Department of Health Sciences and Kinesiology, Georgia Southern University, Statesboro, GA 30460, USA
* Correspondence:
Abstract: Hamstring injury has been considered one of the most common exercise-induced injuries
in sports. Hamstring injuries mostly occur proximal to the biceps femoris. However, the reasons
and mechanisms remain unclear. To summarize hamstring morphological structure features and
what the relationship is between their structure and risk of injury from the current literature, this
review discussed the possible injury mechanism of hamstrings, from the morphological and con-
nected pattern diversity, the mechanical properties, and the stress–strain performance, to probable
changes in action control. Morphological and connected pattern diversity of hamstrings compo-
nents show heterogeneous loads under muscle tension. Connections of gradient compliance be-
tween different tissues may lead to materials’ susceptibility to detachments near the tendon–bone
junction sites under heterogeneous load conditions. The hamstrings muscle’s motor function insuf-
ficiency also brings the risk of injury when it performs multi-functional movements during exercise
due to the span of multiple joints’ anatomical characteristics. These structural features may be the
primary reason why most damage occurs near these sites. The role of these biomechanical charac-
teristics should be appreciated by exercise specialists to effectively prevent hamstring injuries. Fu-
ture work in this research should be aimed at exploring the most effective prevention programs
based on the material structure and motor control to enhance the properties of hamstring muscle
materials to minimize the risk of injury.
Keywords: hamstrings; morphological structure; morphological diversity; stress features;
manifestation of motor control
1. Introduction
Exercise-induced injuries frequently occur in sports. Hamstring injury is regarded as
one of the most common and destructive damages due to the high incidence of the initial
injury and frequent recurrence. Epidemiological investigations have shown that ham-
string injury represents 37% of all muscle injuries [1]. It is common in competitive sports
characterized by fast kicking and running [2,3], especially during non-contact sprinting
activities, including gallops in ball games and sprints in track and field [1,3]. It also occurs
in fitness exercises that involve extensive muscle lengthening-type maneuvers, such as
yoga, and dancing [3,4]. Male athletes are 64% more likely to sustain an acute hamstring
injury than female athletes [3,5]. These rates are similar in soccer, baseball, softball, and
indoor track [6]. Injuries seriously affect athletes’ performance in the arena, resulting in
losing playing time, suffering from pain, and even threatening their professional career
Hamstring injuries mostly occur proximal to the biceps femoris [3,11]. It has been
reported that 57–72% of all hamstring injuries occur during sprinting. Nearly 94% of
these injuries appear in the biceps femoris long head (BFLH) [12–16]. For example, in 17
Citation: Shi, Y.; Xi, G.; Sun, M.;
Sun, Y.; Li, L. Hamstrings on
Morphological Structure
Characteristics, Stress Features, and
Risk of Injuries: A Narrative Review.
ppl. Sci. 2022, 12, 12713. https://
Academic Editors: Agnese Magnani
and Alejandro Rodriguez Pascual
Received: 3 November 2022
Accepted: 9 December 2022
Published: 11 December 2022
Publisher’s Note: MDPI stays neu-
tral with regard to jurisdictional
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tional affiliations.
Copyright: © 2022 by the author. Li-
censee MDPI, Basel, Switzerland.
This article is an open access article
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ditions of the Creative Commons At-
tribution (CC BY) license (https://cre-
Appl. Sci. 2022, 12, 12713 2 of 13
subjects with single injuries or isolated re-injuries, 12 of these were injured on the proxi-
mal part [17]. This evidence supports the opinion that the biceps femoris is the major area
of hamstring injuries and the injuries mostly occur in the proximal part of BFLH.
Researchers explored the underlying mechanism of hamstring injury through ana-
tomical, kinematic, and kinetic analyses of sprinting [14,18–20]. The injury’s timing, loca-
tion and tissue involved are the discussions’ foci due to the discordance of muscle–tendon
length and force changes in leg swing progression throughout the gait cycle [15,21–24].
However, the reasons and mechanisms for why hamstring injuries tend to occur proxi-
mally remain unclear. In addition, injury details are equally unclear, e.g., parallel tearing,
tissue becoming separated, misalignment of myofibrils, and others [25]. Thus, it is essen-
tial to explore further hamstring injuries.
To understand the reason that hamstring injuries tend to occur in the proximal area
and explore the relationship between the part and injury from the current literature, this
review focused on discussing proximal structural features of the hamstrings, from mor-
phological and connected pattern diversity to the tissue’s mechanical properties under
different loadings. Are injuries related to the structural features of BFLH? What is the risk
factor of injury to the BFLH when hamstrings perform multi-functional movements? Alt-
hough the discussion is focused on injuries to the proximal part of hamstrings, we also
attempt to provide suggestions to prevent hamstring injury for sports participants.
2. The Morphological Structure of Hamstrings
Biceps femoris, semitendinosus and semimembranosus are colloquially termed the
“hamstrings” [26]. They are the main distributed muscle group in the posterior thigh (see
Figure 1) [26,27]. Like other normal skeletal muscles, hamstrings are composed of muscle
fibers, nerve and vessel networks, and extracellular matrix that connects the tissue [26,27].
Figure 1. The posterior thigh muscles (with gluteus maximus and medius partially removed). Re-
printed/adapted with permission from Ref [26]. Copyright 2016, Elsevier Health Sciences. (With a
slight modified).
2.1. Morphological Diversity of the Tendons of Hamstrings
Detailed descriptions of the starting and ending points of the various components of
the hamstrings can be learned in Standring’s works [26]. Tendons of the hamstring com-
ponents showed morphological diversity when they were attached to the upper and lower
attachments, and each one has a unique shape [28,29], ranging from thin fascia shapes to
Appl. Sci. 2022, 12, 12713 3 of 13
long cord structures (see Figure 2) [28,30]. The long cord-like tendons prove advantageous
in compact structures for mechanical properties, transmitting a force quickly, and tensile
properties [31]. The thin fascia-like structures of the tendons, also called aponeurosis, pro-
vide a roomy surface area. They is advantageous in dispersing force to quickly reduce the
stress on the unit area [31]. Researchers indicated that this difference in shape results in
different performances under muscle tension [32]. Using a three-dimensional model, they
suggested that the aponeurosis morphology may play a role in determining stretch distri-
bution on the hamstrings [32].
Figure 2. The morphological characteristics of the hamstrings (excluding semimembranosus). 1
Semitendinosus. 2 Raphe. 3 Length of the raphe (mean 9.0 cm). 4 Width of the raphe (3.0 cm
maximum). 5 Semitendinosus tendon. 6 BFLH. 7 Short head of biceps femoris. 8 Biceps femoris
tendon. 9 Ischial tuberosity. 10 Conjoint tendon (BFLH and semitendinosus). (Reprinted with
permission from Ref. [33]. Copyright 2013, Springer.
Amazingly, parts in which the semitendinosus and the BFLH muscles form the com-
mon tendon show an apparent angle, similar to a pinnate angle. Its width is approximately
divided into 2:1 [34], and a shift in the hip joint position changes the angle and length of
the fascicle [35]. It was reported that the angle makes the muscle especially vulnerable to
strain injuries during passive eccentric contractions (muscle contraction that occurs while
the muscle is lengthening as it develops tension and contracts to control motion by an external
force) [34]. In addition, the semitendinosus has a tendinous connective tissue in the spindle
muscle belly, called raphe, running proximal to distal (see Figure 2) [33,36]. Whether this ra-
phe strengthens the structure of the semitendinosus and thus protects it from injury is
unclear, but this view has been suggested [33]. With abundant short unipennate and
multi-pennate fibers, the number of muscle fibrils per unit area reaches its maximum in
the semimembranosus [18].
2.2. Connected Pattern Diversity of Hamstring
Efficient interleaved fusion connection of muscle–tendon and tendon–bone is neces-
sary to effectively transfer muscle force from muscle contraction to movement by pulling
on the bone [29]. It is generally known that muscle, tendon, and bone exhibit dramatically
distinct mechanical behavior [31,37,38]. At the level of tissue, tensile modulus of the ten-
don is on the order of 200 MPa in the direction of the muscle force. This results in easy
bending when it is compressed [39]. In addition, tendons are tough and more extensible
compared to bone and muscle [28,37]. Bone, however, has a modulus of 20 GPa in tension
and compression, and it is hard and brittle versus tendon and muscle [29,38]. Muscle has
approximately a modulus of 1 MPa in passive stretching, and it is more elastic and visco-
elastic relative to tendon and bone [40]. An overview of the material properties of three
kinds of tissue is presented in Table 1. Stress must be transferred among materials whose
Appl. Sci. 2022, 12, 12713 4 of 13
stiffness differs by two or more orders of magnitude [29], as shown in the elastic modulus
values in Table 1. Therefore, three kinds of different tissue elements among muscles, ten-
dons, and bones are connected in series; the connected structure has to show specific struc-
tural characteristics in connection to transmit muscle force effectively and facilitate move-
ment. Throughout musculoskeletal system assembly, both the muscle–tendon and ten-
don–bone attachment units form complex structures: the musculotendinous junction
(MTJ) and entheses, respectively [29].
Table 1. Material properties at the level of tissue.
Tissue Tensile Modulus Material Properties
Bone 20 GPa (20,000 MPa) Rigidity, hardness, toughness, extensible
Tendon 200 MPa Tough, slightly elastic, flexible
Muscle 1 MPa more elastic and viscoelastic
The musculotendinous junction, which connects muscles to the tendon, is mechani-
cally and chemically coupled in its formation [9]. Reports from histological observations
suggested that the muscle fibers are consecutively arranged, and end in a cone shape [41].
The membranes of terminal muscle fibers were deep invagination and formed finger-like
processes with rich rugae [41]. The study of the human MTJ reported that the endotenon
that wraps tendon collagen fibers is inserted into the deep recesses formed by muscle fiber
membranes [42]. The myotendinous junction is formed with tendinous collagen fibers
which are enclosed by the endotenon, which is bonded to muscle fibers near invaginated
membrane by integrin, cohesive proteins, and fibronectin [42]. The finger-like processes
with rich rugae between muscle fiber and tendon fiber implicate a greater surface area for
connection; it was deemed to reduce the stresses of the tissue to some extent, and increase
the load capacity of the MTJs [42,43].
In long cord-like tendons, distal of hamstrings, and proximal to the BFLH, each ten-
don fiber bundle is concentrated together by the endotenon and continues into the peri-
mysium at the MTJs [38,44]. After leaving the musculotendinous junction, the collagen
fibers in the endotenon align along the longitudinal axis of the tendon in a linear, spiral,
or crossed fashion [31]. In comparison, the aponeuroses are fused with the tendon sheath’s
vascular mesangial or tendinous mesangial and covered with the tendon sheath to form a
tendon with a dense structure [31]. At the concave side of the cross-joint action arc, there
is often a tendinous sheath fiber pulley in the tendon to prevent the tendon from deflecting
the center axis of motion when it contracts. It provides a greater range of motion and a
more favorable torque [45].
In thin fascia-like tendons, such as the semimembranosus and the proximal part of
biceps femoris short head, the proximal part of the tendon expands into an aponeurosis,
which is inserted into the surface of muscle to form a junction on the side of the muscle,
then directly enters the periosteum and attaches to the bone [26]. Therefore, these muscles
are more wider and flatter than others [28].
While kinds of insertions exist between tendons and bones, for example, described
as fibrous or fibrocartilaginous, indirect, or direct according to the character and mode of
the tissue at the bone-tendon interface, the most common anatomical structure is the in-
sertion of the tendon into the bone across a fibrocartilaginous transition by entheses
[30,38]; this is a typical structural region of adult tendons. The enthesis is a site where the
ligament, tendon, or muscle attaches to a bone. It is equipped with a unique and intricate
structure, including the end of the tendon next to the bone, the transitional zone across
which the tendon inserts into a bone, and the mineralized side of the attachment [29]. At
the tendon–osseous junction, the collagenous fibers of endotenon enter the bone as perfo-
rating fibers and attach to the periosteum [44].
The entheses connect the tendon to bone through gradations of composition, struc-
ture, and mechanical properties as a specialized organ on the tendon-to-bone interface
[46]. Starting at the subterminal of the tendon next to the bone, the zone shows tendon
Appl. Sci. 2022, 12, 12713 5 of 13
properties, such as aligned type I collagen fibers and proteoglycan decorin [46]. The tran-
sitional zone where the tendon inserts into a bone comprises fibrocartilage. The fibrocar-
tilage contains type II collagen, with a little bit of type I collagen. The extracellular matrix
of the part contains type III collagen, decorin, and aggrecan [47–49]. At the mineralized
zone of entheses, there is mineralized fibrocartilage, which includes type II and type X
collagen, and aggrecan [46–49]. The entheses exhibit numerous types of shapes and sizes
on bone surfaces under the effect of muscle tension [29]. This is important because the
adaptive entheses can absorb a greater amount of energy by increasing local deformation
at the interface. Related experiments exhibited measurable local strains above 7%. This
deformation usually ensures that it is not broken between dissimilar materials, thus main-
taining its integrity [39].
3. Transfer of Force among Muscles, Tendons, and Bone
3.1. The Transfer of Force between Muscle Fibers and Tendons
Muscle fibers and their innervation work together to achieve muscle contraction,
whereas the extracellular matrix comprises the frameworks which bind the individual
muscle fiber together [46]. The extracellular matrix forms a daedal and dynamic network
that consists of collagens, non-collagenous glycoproteins, elastin, and proteoglycans [47],
and laterally binds the individual element of muscle fibers together by three levels of
sheaths: the epimysium (enveloping muscle), perimysium (enveloping muscle bundle),
and endomysium (enveloping muscle fibers) [48,49]. At the end of the fibers, muscle is
attached to its tendon, and the collagen fibrils of muscle and tendon fuse or interdigitate
The endomysium structure by scanning electron microscopy and surface topology of
the perimysium suggests that the shape and structure of the extracellular matrix of muscle
are considerably intricate compared to the tendon [48]. A highly ordered extracellular ma-
trix network surrounds individual muscle fibers and forms a load-bearing network [48].
Perimysium collagen fibers extend along and across muscle fibers. The lengthwise exten-
sion perimysium collagen fibers form a dense series of bands along muscle fibers, and
transverse collagen fibers interconnect muscle fibers at discrete points. Epimysium is com-
posed of abundant collagen bundles with a similarity to the structure seen in tendon [48].
The structure of the epimysium no longer resembles the network of endomysium and
In addition, the muscle fibers are usually shorter than the bundle they are in, so the
distance between the tendons of the two ends of the bundle must be crossed by several
muscle fibers connected in series in sequence. In that case, a portion of the end of the
epimysium and/or perimysium would have appeared as an extension of the tendon. It
means that the extracellular matrix plays a key role in muscle fiber force conduction
[48,50,51], because it aggregates the contraction of individual muscle fibers into a collab-
orative effect. That is to say, the longitudinal force produced by the tandem fibers and the
transverse force between the bundles of fibers of different lengths is rapidly polymerized
via shear forces and transmitted into the tendon by the extracellular matrix [48].
Although each muscle is an anatomical entity, it is not always a functional unit [52],
especially in a complex such as the hamstrings. The different parts of muscles may have
significantly different functions because muscle is made up of individual motor units
comprised of muscle fibers [52]. The amount of muscle fiber controlled by a specific motor
unit is the same under normal circumstances, but the amount of muscle fiber controlled
by different motor units is different [31,52]. The muscle fibers in a particular motor unit
always contract in the same way when they are working as a unit, more or less inde-
pendently of the other motor units in the same muscle [52], such as the innervation of two
separate nerves in different parts of the biceps femoris.
Appl. Sci. 2022, 12, 12713 6 of 13
When different parts of a muscle perform different functions, or different motor units
that are involved in the same function fail to synchronously coordinate, significant differ-
ences in tensions among fibers/bundles will cause a change of the internal conformation,
i.e., tear damage will occur between the muscle fibers/bundles [31]. This type of injury
occurs along the direction of the muscle fibers/bundles, a lengthwise direction or verge
towards to tear in the clinical images [53].
Research had shown that the part which resembles a pinnate angle between the se-
mitendinosus and the tendon of the long head of the biceps femoris is more frequently
torn [54]. A laceration at the fusion of the short and long heads of the biceps femoris is
another injury of the proximal hamstrings during the eccentric contraction of muscles.
[18]. This type of injuries is thought to occur due to the differential contraction of the two
muscles, which increases susceptibility to tearing [18]. To be specific, the BFLH is inner-
vated by the tibial portion of the sciatic nerve, while the biceps femoris short head is in-
nervated by the peroneal division [26]. The duality of innervation of the biceps femoris
may lead it to be desynchronized in the coordination or intensity of stimulation of the two
heads. This is deemed as a reason for the muscle being torn a lot [55]. In addition, the
angles between the BFLH and the semitendinosus and between the BFLH and the short
head of biceps femoris are also postulated as a cause for their susceptibility to be torn. The
angles are formed due to the oblique trended distribution of the biceps femoris from the
inside out and from the top down [18].
The intricate architecture of MTJ buffers the tensile stress exerted on tendons when
muscles contract. However, MTJs are still regarded as the weakest part of the muscle–
tendon unit [41]. It is reported that acute muscle strain caused by sudden eccentric con-
traction usually occurs at or near MTJs [56]. This kind of injury occurs almost perpendic-
ular to the direction of the muscle fibers/bundles. Incomplete avulsion injury or root avul-
sion injury of the muscle from the tendon is exhibited in the clinical images [18]. In reality,
though, the myotendinous junction is rarely injured [57]. It is reported that the MTJ is not
a briefly involved area but a complex part of the transition from muscle to tendon, where
myofibrils widely intersect with the tendon collagen fibers or ligamentous collagen fibers
3.2. The Transfer of Force between Tendons and Bone
The attachment of tendons to the skeleton is unique to vertebrates [28]. The develop-
ment of entheses is essential for musculoskeletal functionality, because they provide flex-
ible, robust, and resilient anchor points for muscles, and transmit force by muscle gener-
ated to the skeleton [28]. Mineral content and collagen fiber orientation combine to pro-
vide the entheses a unique grading in mechanical properties [29]. The linear increase in
accumulation of minerals on collagen fibers causes significant stiffening of the partially
mineralized fibers, while expanding dispersion in the orientation distribution of collagen
fibers from tendon to bone is another major determinant of tissue stiffness. Combining the
two factors leads to be the non-monotonic variation of stiffness over the entheses [38].
When two materials with different physical properties and a compliant interface are ex-
posed to external loads, they will display non-uniform deformation [39]. The incongruity
in the deformation between the two parts will cause a stress singularity to arise locally;
this will increase the risk of crack propagation and failure [39].
When the tendon is acted upon by the muscle force, the directionally distributed col-
lagen fiber bundles show force compliance, rapidly following the direction of tension,
transferring tension along the extremely tenacious tendon to the entheses [58]. During this
process, tendons exhibit different rigidity due to the dependence on stress repetition and
stress rate [31]. Specifically, the stress–strain curve of tendon tissue material will be offset
to the right along the horizontal axis after being stretched repeatedly [59]. The tissues will
exhibit a greater elasticity due to the certain plastic deformation of the tendon tissue [58].
It means that the tendon can store more energy when under high stress, and be used to
buffer possible fractures by large strain [58]. Additionally, the slope of the linear part of
Appl. Sci. 2022, 12, 12713 7 of 13
the tendon stress–strain curve increases under an increasing load rate [59]. The tendon
tissue shows greater stiffness under a greater load rate [31]. This means that tendons will
exhibit brittleness rather than elasticity when they subjected to more sudden stresses. In
this case, the tendon tissue may be broken due to stress concentration [31]. A rupture of
the Achilles’s tendon is a typical case of this type of injury. However, this rupture rarely
occurs in hamstrings. This might be because of the flexibility of the muscle [6,60,61].
4. Possible Performance of Motor Control during Hamstrings Acute Strain
From the hamstring muscles’ starting and ending position, one can observe that the
semimembranosus, semitendinosus, and the biceps femoris long head are biarticular.
They go across the hip joint and knee joint, and the biceps femoris short head is monoar-
ticular; t spans just across the knee joint [9,62]. As essential muscles for hip joint extending
and knee joint flexing in the gait cycle, the biarticular muscle groups of hamstrings have
the weakness of insufficient movements, such as initiative power insufficiency or passive
stretch insufficiency [63]. More specifically, the BFLH becomes a weaker flexor of the knee
when the hip is extended and becomes a weaker extender of the hip when the knee is
During sprinting, athletes usually gain forward momentum by contacting the ground
with the ball of the foot while the ankles are kept rigid [64]. After a quick forward swing,
to gain stronger forward momentum, athletes commonly tend to be more proactive in
completing the movement of touching the ground by pressing down quickly and swing-
ing the leg backward [65]. The hamstring muscles are always active throughout the gait
cycle; because of the need to swing and contact the ground, they quickly switch between
eccentric and concentric contraction [66,67]. The sudden change in hamstrings’ function
between the rapid, steady flexion activity and extension has been postulated to cause
acute injury [67,68]. This is almost consistent with the view of Orchard’s team: they re-
ported that increased rates of hamstring injuries on batting and fielding in short form (50-
over and T20) in Australian Cricket may relate to changes in intensity of running speed
Flexion of the knee is largely passive and the crus is in the state of a dependent swing
as the open link of the movement chain of the lower limb during the thigh is swung for-
ward [69,70]. When the knee joint is partially flexed, the biceps femoris rotates the leg
slightly outward in consequence of its oblique direction, whereas the semitendinosus and
partly semimembranosus rotate the leg slightly inward. At the end phase of the swing
forward phase, the hamstrings incur the greatest stretch. They are actively, eccentrically
contracting to decelerate the lower limb and prepare for contacting the ground [19]. A
kinetic study indicates that peak lengths were significantly larger in the biceps femoris
than the semitendinosus and semimembranosus due to different moment arms in the
hamstrings muscles because of their diversified shape and position [71]. It may be easy to
cause hamstring strain if active knee extension occurs at this moment due to the situation
mentioned above. This kind of case commonly happens when a soccer player is running
at high speed with the ball or a tackle [72]. In a systematic review [24], three studies also
reported such injuries [73,74].
The backward swing process of the thigh, with the stance status as the critical point,
can be divided into two stages: before and after striking the ground [26]. At the end phase
of the swing forward, the hamstring muscles are passively elongated due to eccentric con-
traction, then begin to play the role of the hip extensor and perform concentric contraction
[75,76]. To actively contact the ground, the hamstrings actively contracted to pull down
the leg quickly, making it swing backward. At this phase, the arch of the foot, ankle joint,
and knee joint are highly tense and stable to prepare the body for coping with the impact
of the ball of the foot striking the ground [70,77]. In this process, a hamstring injury is
rarely seen due to the active shortening of the hamstring by concentric contraction, but
joint ligament injury caused by weak stability of the knee and ankle joints is common [78].
During a swing backward after striking the ground, the knee joint is flexed quickly, and
Appl. Sci. 2022, 12, 12713 8 of 13
the leg is folded rapidly close to the thigh and hip when the hamstrings, especially the
biceps femoris short head, continued to contract [79]. At the end phase of the swing back-
ward, the hip is fully extended and the knee is flexed in preparation for the swing forward
[70]. When hamstrings are quickly switched from concentric to eccentric contraction dur-
ing sprinting, if the knee is to be stretched and the thigh is to be started to swing forward,
it is very easy to cause the hamstrings to strain due to sudden eccentric contraction. This
kind of injury usually occurs during the sudden acceleration in the thrust phase [80].
The two cases of action mentioned above, such as significantly greater anterior pelvic
tilt or thoracic lateral flexion, both contain the sudden appearance or persistence of mis-
takes or abnormalities in action, and were deemed as an irrefutable factor contributing to
the injury and re-injury [81–83]. However, this viewpoint is subject to be supported by
experimental evidence. Differences in the functional demands of the hamstring muscles
during acceleration and swing at maximum speed are also risk factors [84–86].
5. Differential Diagnosis
5.1. Typical Symptoms of Hamstring Strain Injuries
At present, there is no completely unified consensus on the definitions and classifi-
cations for muscle injuries [87,88]. The Munich Consensus Statement may be known as a
highly comprehensive guideline for studying muscle injury [57,87]. The approach of the
Munich Consensus Statement deals with muscle injuries in a comprehensive manner,
which covers the incorporation of acute, overuse, direct, and indirect injury descriptors
[57]. Usually, it is not difficult to classify one as an acute injury or overuse injury based on
its onset process and pathological features. An acute muscle injury in sport is character-
ized as a closed trauma with a clearly defined cause or sudden occurrence. The stresses
and strains generated by force applied to tissue are greater than the tissue can withstand
[89]. Either internal forces cause the macro-trauma of the tissue as tensile ruptures
(strains/laceration) or by external forces from direct contacts, such as by contusions by
collision [9,89]. Non-contact acute muscle injury caused by excessive internal tension usu-
ally manifests as muscle fibers tear/rupture [90]. Injuries mostly occur in muscles that suf-
fer from high active and passive tensions, which cause muscle fibers and their surround-
ing connective tissue to be damaged. Researchers considered that explosive and paroxys-
mal active tension is produced by the muscle contracting strongly instantly. And passive
tension is caused by a stretch of the connective tissue elements between different muscle
fibers [91].
The most severe acute injury of hamstrings is avulsion, which usually involves the
entheses in adults. This injury pattern occurs more commonly at the ischial tuberosity
than at the distal ligamentous insertion [18]. In such a case, avulsion almost always in-
volves the conjoint tendon, such as the biceps femoris and semitendinosus muscles. It of-
ten results in either complete or incomplete tearing of the semimembranosus due to re-
traction of the avulsion parts. This is the most common form of proximal avulsion [18].
Oblique coronal magnetic resonance imaging demonstrates a large hematoma, usually
accompanied by retraction of the semitendinosus muscle and the long head of the biceps
femoris tendon [24]. It is reported that avulsion usually occurs in the setting of prior or
chronic injury, with abnormal entheses morphologic features or degeneration being the
most likely predisposing factors [18].
Overuse injuries are caused by repetitive microtrauma to tissue, usually accompa-
nied by progressively increasing pain [92]. An overuse injury is often associated with un-
derlying pathology which is accumulated by a long-term inappropriate load. Our study
will not elaborate on it in detail due to its complex pathology. As knowledge of hamstring
injuries continues to grow, researchers suggested that not all hamstring injuries are the
same [93]. For instance, the stretch type of muscle fibers injury and the injuries involving
the tendon are totally different in hamstrings.
5.2. Grade of Hamstring Strain Injury
Appl. Sci. 2022, 12, 12713 9 of 13
Appropriately classifying hamstring injuries is necessary to develop treatment strat-
egies, predicting prognosis, and determining the readiness for return to play, which is
maybe the most important, [7]. For classification of injury severity, the modification of
Peetrons classification [12,94] was used, and the grading system is as follows: Grade 0—
negative MRI without any visible pathology; Grade 1—edema, but no architectural struc-
ture distortion, and this kind of symptom commonly occurs in overuse hamstring injury;
Grade 2—architectural structure disruption indicating partial tear; Grade 3—total muscle
or tendon rupture, the two kinds of symptom commonly appearing in acute hamstring
strain injury, including hamstring tendon avulsions, ischial epiphyseal avulsions, proxi-
mal tendinopathies of hamstring, and associated severe pain in the back of the thigh [9,12].
For the severity of the injury, in the 180 cases with some muscle pathology visible on
MRI (Grades 1–3), 151 (84%) affected the biceps femoris, while 20 (11%) occurred in the
semimembranosus and 9 (5%) in the semitendinosus [12]. In another five-year follow-up
study with 207 injuries, 27 (13%) were of Grade 0, 118 (57%) were of Grade 1, 56 (27%)
were of Grade 2, and 6 (3%) were of Grade 3 [12]. These studies demonstrated that Grade
1 and 2 injuries are the most common, while Grade 3 injuries are relatively rare.
Therefore, studying the structure of hamstrings, especially BFLH, and its mechanics
during sprinting can help us to further understand the potential injury risk factors. It is
needed for the effective prevention of hamstring injury and the improvement of rehabili-
6. Risk Factors and Prevention for Hamstring Injury
Combined with the above description of the morphology and structure of the ham-
string autogenous tissue materials and the analysis of the performance of motor control
in sprinting, the sudden or persistent of mistakes or abnormal movements during sprints
are a major external risk factor for hamstring injuries; they lead to non-compliant changes
in the direction of the force and the destruction of the material structure. Differences in
the functional demands of the hamstring muscles during acceleration and swing at maxi-
mum speed are also risk factors. The differences may come from asynchrony in the coor-
dination or intensity of stimulation. Structural differences caused by morphological di-
versity and different tissue elements are also potential risk factors. This difference is due
to different loads during musculoskeletal phylogenetic assembly.
Prevention of a first-time hamstring injury is important due to the frequent recur-
rence and considerable impairment. Therefore, further research is needed to specifically
define the most effective prevention programs based on the material structure and motor
control of hamstrings in sprinting. For example, the results from the meta-analysis sug-
gested that the Nordic Hamstring Exercises were effective in reducing the incidence of
hamstring injury; the teams using the Nordic Hamstring Exercise in isolation or as part of
a larger injury prevention program reduced hamstring injury rates by up to 51% [86]. Re-
sults from another case series supported the incidence of hamstring injury decreased by
using of isokinetic strengthening exercises or adding agility and flexibility into strength
training [95].
In addition, the influence and interaction of hamstring strength, flexibility, fatigue,
and age are also etiological factors. They should be addressed to prevent hamstrings in-
jury [85].
7. Review Limitations
The major limitations of the present study include the following two aspects: First, it
only describes the diversity of the material morphology of the hamstrings, without ana-
lyzing the source of the morphological difference of the same structure among different
individuals and the possible influence on injury; Second, the performance of motor con-
trol in sprinting was only investigated during the swing phase, but not in the contact
stage. Although the detail obtained during the swing phase helps the understanding of
the performance of movement, investigation of limb’s work and power during the contact
Appl. Sci. 2022, 12, 12713 10 of 13
phase provides a deeper understanding of accelerated locomotion. Therefore, they are di-
rections for future investigation.
8. Conclusions
Hamstring injury is the most common and destructive damage due to the high inci-
dence of the initial injury and frequent recurrence. Mostly hamstring injuries are acute
injuries and appear in the biceps femoris. This is closely related to the diverse shapes of
the components in the hamstrings and their tendons. The difference in shape brings them
different stress statuses during rapid muscle contraction. The intricate connection of gra-
dient compliance between tissue materials with different mechanical properties leads to
the susceptibility of materials to detach near the junction sites under sudden stress condi-
tions, which is perhaps the primary reason most damage occurs near these sites. This
opinion holds truth for all materials after the elastic state. Due to the anatomical charac-
teristics that span multiple joints, the insufficiency of the motor function of the hamstring
muscle also brings the risk of injury when performing multi-functional movements dur-
ing exercise. It hints that the leg should strictly avoid unnecessary overextension during
the swing and make sure to maintain the proper hip angle and knee angle during the
swing. In future research, therefore, the more detailed understanding of the motor func-
tion performance, the exact location, and the specific morphology of hamstrings when the
injury occurs via detailed experiments, the more conducive to understanding the mecha-
nism of the injury; this will be helpful to propose targeted prevention strategies for ham-
string injuries.
Author Contributions: Conceptualization, Y.S. and L.L.; methodology, software, literatures col-
lected, Y.S. and M.S.; validation, G.X. and Y.S.; writing original draft preparation, Y.S.; writing re-
view and editing, M.S. and Y.S.; visualization, G.X.; supervision, L.L.; funding acquisition, Y.S. All
authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by [a state scholarship fund of the China Scholarship Council],
grant number [201810038]. It was also partly supported by [the sport regular subject fund of Shaanxi
Provincial Bureau], grant number [20180523]. And The APC was partly funded by [201810038].
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: We should choose to exclude this statement because the study did not
report any data.
Conflicts of Interest: The authors declare no conflict of interest.
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ResearchGate has not been able to resolve any citations for this publication.
Full-text available
Hamstring injuries are among the most common muscle injuries. They have been reported in many different sports, such as running, soccer, track and field, rugby, and waterskiing. However, they are also present among the general population. Most hamstring injuries are mild strains, but also moderate and severe injuries occur. Hamstring injuries usually occur in rapid movements involving eccentric demands of the posterior thigh. Sprinting has been found to mainly affect the isolated proximal biceps femoris, whereas stretching-type injuries most often involve an isolated proximal injury of the semimembranosus muscle. The main cause of severe 2- or 3-tendon avulsion is a rapid forceful hip flexion with the ipsilateral knee extended. Most hamstring injuries are treated non-surgically with good results. However, there are also clear indications for surgical treatment, such as severe 2- or 3-tendon avulsions. In athletes, more aggressive recommendations concerning surgical treatment can be found. For a professional athlete, a proximal isolated tendon avulsion with clear retraction should be treated operatively regardless of the injured tendon. Surgical treatment has been found to have good results in severe injuries, especially if the avulsion injury is repaired in acute phase. In chronic hamstring injuries and recurring ruptures, the anatomical apposition of the retracted muscles is more difficult to be achieved. This review article analyses the outcomes of surgical treatment of hamstring ruptures. The present study confirms the previous knowledge that surgical treatment of hamstring tendon injuries causes good results with high satisfaction rates, both in complete and partial avulsions. Early surgical repair leads to better functional results with lower complication rates, especially in complete avulsions. • KEY MESSAGEs • Surgical treatment of hamstring tendon ruptures leads to high satisfaction and return to sport rates. • Both complete and partial hamstring tendon ruptures have better results after acute surgical repair, when compared to cases treated surgically later. • Athletes with hamstring tendon ruptures should be treated more aggressively with operative methods.
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Hamstring strain injury (HSI) is a common and costly injury in many sports such as the various professional football codes. Most HSIs have been reported to occur during high intensity sprinting actions. This observation has led to the suggestion that a link between sprinting biomechanics and HSIs may exist. The aim of this literature review was to evaluate the available scientific evidence underpinning the potential link between sprinting biomechanics and HSIs. A structured search of the literature was completed followed by a risk of bias assessment. A total of eighteen studies were retrieved. Sixteen studies involved retrospective and/or prospective analyses, of which only three were judged to have a low risk of bias. Two other case studies captured data before and after an acute HSI. A range of biomechanical variables have been measured, including ground reaction forces, trunk and lower-limb joint angles, hip and knee joint moments and powers, hamstring muscle–tendon unit stretch, and surface electromyographic activity from various trunk and thigh muscles. Overall, current evidence was unable to provide a clear and nonconflicting perspective on the potential link between sprinting biomechanics and HSIs. Nevertheless, some interesting findings were revealed, which hopefully will stimulate future research on this topic.
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Limited information is available on site‐specific features of muscle stiffness and aponeurosis strain of the biceps femoris long head (BFlh) during contractions. Therefore, understanding of the mechanics and etiology of hamstring strain injuries remains difficult. As a first step to gain further insight into them, the present study aimed to identify whether active muscle stiffness and proximal aponeurosis strain during contractions are varied along the long‐axis of the BFlh. The BFlh muscle shear wave speed (proxy for stiffness) was measured in the proximal, central, and distal sites during 20%, 50%, 80% of maximal voluntary isometric contraction (MVC) of knee flexion exerted with the hip and knee joints flexed at 40° and 30°, respectively, using ultrasound shear wave elastography. Further, a segmental strain of the BFlh proximal aponeurosis was assessed in the proximal, central, and distal sites during isometric knee flexion, using B‐mode ultrasonography. The shear wave speed was significantly higher in the distal site than the proximal and central sites at 20% MVC (P ≤ 0.002, with a large effect size), whereas no significant difference was found between the three sites at 50% and 80% MVC. The BFlh proximal aponeurosis strain showed no significant difference between the proximal, central, and distal sites at any contraction intensity. These findings indicate that site‐specific differences in muscle stiffness and proximal aponeurosis strain are substantially small and that muscle stiffness and proximal aponeurosis strain of the BFlh at moderate‐to‐high contraction intensity is not exceptional in the site where a sprinting‐type hamstring strain typically occurs.
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The incidence of strain injuries continues to be high in many popular sports, especially hamstring strain injuries in football, despite a documented important effect of eccentric exercise to prevent strains. Studies investigating the anatomical properties of these injuries in humans are sparse. The majority of strains are seen at the interface between muscle fibers and tendon: the myotendinous junction (MTJ). It has a unique morphology with a highly folded muscle membrane filled with invaginations of collagen fibrils from the tendon, establishing an increased area of force transmission between muscle and tendon. There is a very high rate of remodeling of the muscle cells approaching the MTJ, but little is known about how the tissue adapts to exercise and which structural changes heavy eccentric exercise may introduce. This review summarizes the current knowledge about the anatomy, composition and adaptability of the MTJ, and discusses reasons why strain injuries can be prevented by eccentric exercise.
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Background: Injuries to the hamstring muscles are among the most common in sports and account for significant time loss. Despite being so common, the injury mechanism of hamstring injuries remains to be determined. Purpose: To investigate the hamstring injury mechanism by conducting a systematic review. Study design: A systematic review following the PRISMA statement. Methods: A systematic search was conducted using PubMed, EMBASE and the Cochrane Library. Studies 1) written in English and 2) deciding on the mechanism of hamstring injury were eligible for inclusion. Literature reviews, systematic reviews, meta-analyses, conference abstracts, book chapters and editorials were excluded, as well as studies where the full text could not be obtained. Results: Twenty-six of 2372 screened original studies were included and stratified to the mechanism or methods used to determine hamstring injury: stretch-related injuries, kinematic analysis, electromyography-based kinematic analysis and strength-related injuries. All studies that reported the stretch-type injury mechanism concluded that injury occurs due to extensive hip flexion with a hyperextended knee. The vast majority of studies on injuries during running proposed that these injuries occur during the late swing phase of the running gait cycle. Conclusion: A stretch-type injury to the hamstrings is caused by extensive hip flexion with an extended knee. Hamstring injuries during sprinting are most likely to occur due to excessive muscle strain caused by eccentric contraction during the late swing phase of the running gait cycle. Level of evidence: Level IV.
Injuries and overload to the hamstring muscles are frequently observed in recreational and high-performance athletes. The proximal hamstring muscle complex is in a highly vulnerable region that is likely to sustain injury in several disciplines. Proximal hamstring injuries (PHI) are responsible for a substantial loss of training and competition days. A precise and targeted diagnostic approach is essential to provide a correct assessment of injury severity. In general, a careful anamnesis and thorough clinical examination with inspection, palpation, and functional testing provide substantial information about the extent and severity of injury. Imaging of muscle tissue is essential to confirm precise diagnosis for providing further assessment of the degree of damage or differential diagnosis. Understanding the specific tasks in the diagnosis of PHI requires knowledge of functional anatomy, injury, and pathophysiological mechanisms as well as examination and imaging techniques. The present work provides a structured overview of the diagnostic work-up of PHI, emphasizing structured examination and imaging techniques to enable reliable diagnosis and rapid treatment decisions.
Context Hamstring muscle injury location using magnetic resonance imaging (MRI) is not so well described in the literature. Objective To describe the location of hamstring injuries using MRI. Data Sources PubMed, Web of Science, Scopus, SPORTDiscus, Cochrane Library. Study Selection The full text of studies, in English, had to be available. Case reports and reviews were excluded. Included studies must report the location of hamstring injuries using MRI within 8 days of the acute injury. Study Design Systematic review. Level of Evidence Level 4. Data Extraction A first screening was conducted based on title and abstract of the articles. In the second screening, the full text of the remaining articles was evaluated for the fulfillment of the inclusion criteria. Results From the 2788 references initially found in 5 databases, we included 34 studies, reporting a total of 2761 acute hamstring injuries. The most frequent muscle head involved was the long head of the biceps femoris (BFLH) (70%), followed by the semitendinosus (ST) (15%), generally associated with BFLH. The most frequent tissue affected was the myotendinous junction (MTJ) accounting for half the cases (52%). Among all lesions, the distribution between proximal, central, and distal lesions looked homogenous, with 34.0%, 33.4% and 32.6%, respectively. The stretching mechanism, while only reported in 2 articles, represented 3% of all reported mechanisms, appears responsible for a specific lesion involving the proximal tendon of the semimembranosus (SM), and leading to a longer time out from sport. Conclusion BFLH was the most often affected hamstring injuries and MTJ was the most affected tissue. In addition, the distal, central, and proximal locations were homogeneously distributed. We also noted that MRI descriptions of hamstring injuries are often poor and did not take full advantage of the MRI strengths. Systematic Review Registration Before study initiation, the study was registered in the PROSPERO International prospective register of systematic reviews (registration number CRD42018107580).
Objective: The aim of this study was to explore expert opinion to identify the components of sprinting technique they believed to be risk factors for hamstring strain injuries (HSI). Design: Mixed-method research design. Methods: The Concept Systems groupwisdom™ web platform was used to analyse and collect data. Participants brainstormed, sorted and rated the components of sprinting technique to consider in a HSI prevention strategy. Results: Twenty-three experts (academic/researcher, physiotherapist, strength and conditioning coaches and sprint coaches) brainstormed 66 statements that were synthesised and edited to 60 statements. Nineteen participants sorted the statements into clusters and rated them for relative importance and confidence they could be addressed in a hamstring injury prevention program. Multidimensional scaling and cluster analysis identified a 8-cluster solution modified to a 5-cluster solution by the research team: Training prescription (10 statements, mean importance: 3.79 out of 5 and mean confidence: 3.79); Neuromuscular and tendon properties (9, 3.09, 3.08); Kinematics parameters/Technical skills (27, 2.99, 2.98); Kinetics parameters (10, 2.85, 2.92); and Hip mechanics (4, 2.70, 2.63). The statement: "low exposure to maximal sprint running" located in the cluster "Training prescription" received the highest mean importance (4.55) and confidence ratings (4.42) of all statements. Conclusion: The five clusters of components of sprinting technique believed to be risk factors for HSIs in order of most to least important were: training prescription, neuromuscular and tendon properties, kinematics parameters/technical drills, kinetics parameters and hip mechanics.
Objective To systematically review risk factors for hamstring strain injury (HSI). Design Systematic review update. Data sources Database searches: (1) inception to 2011 (original), and (2) 2011 to December 2018 (update). Citation tracking, manual reference and ahead of press searches. Eligibility criteria for selecting studies Studies presenting prospective data evaluating factors associated with the risk of index and/or recurrent HSI. Method Search result screening and risk of bias assessment. A best evidence synthesis for each factor and meta-analysis, where possible, to determine the association with risk of HSI. Results The 78 studies captured 8,319 total HSIs, including 967 recurrences, in 71,324 athletes. Older age (standardised mean difference=1.6, p=0.002), any history of HSI (risk ratio (RR)=2.7, p<0.001), a recent HSI (RR=4.8, p<0.001), previous anterior cruciate ligament (ACL) injury (RR=1.7, p=0.002) and previous calf strain injury (RR=1.5, p<0.001) were significant risk factors for HSI. From the best evidence synthesis, factors relating to sports performance and match play, running and hamstring strength were most consistently associated with HSI risk. The risk of recurrent HSI is best evaluated using clinical data and not the MRI characteristics of the index injury. Summary/conclusion Older age and a history of HSI are the strongest risk factors for HSI. Future research may be directed towards exploring the interaction of risk factors and how these relationships fluctuate over time given the occurrence of index and recurrent HSI in sport is multifactorial.