Curr Rev Musculoskelet Med
DOI 10.1007/s12178-015-9262-2

MUSCLE INJURIES (SJ MCNEILL INGHAM, SECTION EDITOR)

Return to sport after muscle injury
Stephanie Wong & Anne Ning & Carlin Lee &Brian T. Feeley# Springer Science+Business Media New York 2015

Abstract Skeletal muscle injuries are among the most common sports-related injuries that result in time lost from practice and competition. The cellular response to muscle injury
can often result in changes made to the muscle fibers as well as
the surrounding extracellular matrix during repair. This cannegatively affect the force and range of the injured muscle
even after the patient’s return to play. Diagnosis of skeletal
muscle injury involves both history and physical examinations; imaging modalities including ultrasound and magnetic
resonance imaging (MRI) can also be used to assess the extent
of injury. Current research is investigating potential methods,
including clinical factors and MRI, by which to predict a patient’s return to sports. Overall, function of acutely injured
muscles seems to improve with time. Current treatment
methods for skeletal muscle injuries include injections of steroids, anesthetics, and platelet-rich plasma (PRP). Other proposed methods involve inhibitors of key players in fibrotic
pathways, such as transforming growth factor (TGF)-ß andangiotensin II, as well as muscle-derived stem cells.
Keywords Sports-related injuries . Skeletal muscle injury .
Platelet-rich plasma

Introduction
Acute skeletal muscle injuries are frequent injuries sustainedby athletes and are a significant cause of disability. Diagnosisof these injuries is primarily clinical, with additional

This article is part of the Topical Collection on Muscle Injuries
S. Wong : A. Ning : C. Lee : B. T. Feeley (*)
Department of Orthopaedic Surgery, University of California, 1500
Owens Ave, Box 3004, San Francisco, CA 94127, USAe-mail: feeleyb@orthosurg.ucsf.edu

information gained with ultrasound and MRI. Following theacute injury, predicting time to return to play is important toboth the athlete and clinician. Improvements in understandingof the physiology of muscle injury and resulting fibrosis have
led to recent advancements in development of adjuvant treatments, which have the goals of shortening time to return toplay and improving function during sporting activity.

Epidemiology of muscle injury
Muscle injuries are among the most common injuries
sustained during sporting activity. Across several professional
sports, acute muscle injury makes up between 23 and 46 % of
all injuries [1, 2•]. There have been many studies that focus on
epidemiology of athletic injuries utilizing the professional andcollegiate athletic injury databases.
In a 12-year study of European professional soccer leagues,
95 % of all muscle injuries occurred in the following fourgroups: hamstrings, adductors, quadriceps, and calf [2•].
Among these, the hamstrings are consistently the most frequently injured. Similarly, in the National Football League(NFL) training camps from 1997 to 2007, hamstring strains
were reported to be the second most common injury, after
knee sprain [1]. Similarly, UEFA professional soccer datafrom 2001 to 2013 demonstrated that the hamstrings are themost frequently injured muscle group, accounting for 34 % of
all muscle strains [2•]. The upper leg muscles were almost
always the most commonly injured muscle group in collegiate
athletes, as well (Tables 1 and 2).
Males are more likely to have muscle injuries compared tofemales across multiple sports, but the data is most clear insoccer. In professional soccer leagues in the USA and Europe,
the incidence of hamstring strains was reported to be between
0.35 and 1.5/1,000 h of soccer in male professional soccer
players [8, 9]. The rate of hamstring strains in US female

Curr Rev Musculoskelet Med
Table 1 Muscle strain injuries occurring during collegiate games[3–7]. Injury rates reported as injuries per athlete-exposure (A-E). An
athlete-exposure was defined as an athlete participating in a game orpractice and the injury rate was calculated per 1,000 athlete-exposures
Sport

Musclegroup

FootballPelvis/hip 581

1.9

0.65

FootballBasketball (men’s)
Basketball (men’s)
Basketball (men’s)
Basketball(women’s)
Soccer (men’s)
Soccer (men’s)
Soccer (men’s)
Soccer (women’s)
Soccer (women’s)

Upper leg
Pelvis/hipUpper leg
Lower leg
Upper leg

1,103
86
79
43
45

3.6
2.0
1.9
1.0
1.3

1.24
0.18
0.17
0.09
0.10

Pelvis/hipUpper leg
Lower leg
Pelvis/hipUpper leg

264
548
73
120
374

3.9
8.2
1.1
2.2
7.0

0.74
1.54
0.20
0.37
1.14

Soccer (women’s)

Lower leg 69

1.3

0.21

Frequency Percentage Injury rate per
1,000 A-E

professional soccer players was much lower, 0.02/1,000 h ofsoccer played [10]. Data from the National Collegiate Athletic
Association (NCAA) demonstrated that male collegiate soccer
players were 64 % more likely to suffer hamstring strains
compared to female soccer players [11]. Additionally, thosemale athletes had a higher frequency of recurrent hamstringstrains, 20 %, compared to 10 % of female players [11]. This
was thought to be due to more high-intensity running or
sprinting in male players compared to female and, perhaps,
inadequate rehabilitation aimed at these high-intensity activities in male players.
Table 2
[3–7]

Rates of muscle injury also vary across sports, as shown in
Tables 1 and 2 for collegiate sports and Table 3 for professional sports. Collegiate athletes sustained muscle injuries morecommonly during games than practices (Tables 1 and 2).

Muscle strain injuries occurring during collegiate practices

Sport

Musclegroup

Frequency Percentage Injury rate
per 1,000 A-E

FootballFootballPelvis/hip 2,196
Upper leg 4,518

3.9
10.7

0.15
0.41

Basketball (men’s)
Basketball (men’s)
Basketball (men’s)
Basketball(women’s)
Basketball(women’s)
Soccer (men’s)
Soccer (men’s)
Soccer (men’s)
Soccer (women’s)
Soccer (women’s)
Soccer (women’s)

Pelvis/hipUpper leg
Lower leg
Pelvis/hip

348
283
93
213

4.4
3.6
1.2
3.2

0.18
0.14
0.05
0.13

Lower leg 107

1.6

0.06

Pelvis/hipUpper leg
Lower leg
Pelvis/hipUpper leg
Lower leg

7.8
16.6
2.0
7.6
21.3
2.2

0.34
0.72
0.09
0.40
1.11
0.12

487
1,042
128
444
1,243
129

Physiology of muscle injury
The cellular response to injury provides a basis for evaluation of the broad range of skeletal muscle injuries. This
response occurs in two stages, the first marked by degeneration of the damaged muscle cells and the resulting tissue inflammation. The second stage involves regeneration
and renewal of muscle fibers and growth of the muscle
tissue. In normal, non-injured states, muscle stem cells
show a low rate of turnover and regeneration. After injury,
the process of muscle renewal occurs with proliferationand differentiation of satellite cells and generation of
new myofibers to replace those that were injured; however, the timeline for this process is variable and is often
complicated by development of fibrosis.
In the first phase, degeneration, the impact of injury toskeletal muscle leads to death on a cellular level. Injury damages and increases the permeability of the sarcolemma of affected myofibers. This causes the muscle cells to break down,
releasing intracellular proteins that can act as cytokines. As theserum concentration of these proteins increases, immune cells
are activated and attracted to the site of injury, causing inflammation of the region. The first wave of inflammatory cells to
infiltrate the muscle tissue is composed mainly of neutrophils.
After arriving at the site of injury, neutrophils have been suggested to promote secondary muscle cell damage for reasons
still unclear [13]. This is followed by a macrophage-mediatedresponse about 48 h after injury [14]. Macrophages phagocytose debris from the necrotic muscle cells, removing necrotic
material, and suppress inflammation [15].
Macrophages also play an active role in the second phase of
muscle injury repair by secreting cytokines that activate muscle
cell precursors. These cytokines, which include basic fibroblast
growth factor (FGF), IL-6, and transforming growth factor(TGF)-ß, help with both growth and differentiation of muscle

Table 312]

Muscle injury rates for professional baseball and football [1,Sport

Muscle group

Injury rate/
1,000 A-E

Major League Baseball
Minor League BaseballNational Football League

Hamstring
Hamstring
Hamstring
Hip flexor

0.7
0.7
2.2
2.2

Curr Rev Musculoskelet Med

stem cells. Regeneration begins with the proliferation of these
activated myogenic cells [16]. This population of muscle precursor cells includes both satellite cells and non-muscle stem
cells. Satellite cells are found surrounding mature muscle cells.
Once activated, they proliferate and terminally differentiate into
myoblasts. The resulting myoblasts can fuse either to one another, replacing muscle fibers that were lost in injury, or withexisting fibers in the injured muscle, repairing damage frominjury [17]. A portion of the satellite cell population then returns
to a reversible state of quiescence, allowing the cells to selfrenew with each round of regeneration [18]. Other non-muscle
stem cells are also capable of myogenesis and, after activation,
are able to regenerate muscle fibers [14]. After this process has
been completed, the newly generated myofibers are physiologically indistinguishable from the old muscle cells.

Function of muscle after injury
Though the individual muscle cells regenerated after injury are identical to pre-existing muscle cells, fibrosis often
alters the physiology of the muscle tissue as a whole.
Fibrosis is a process that involves the deposition of connective tissue and abnormal extracellular matrix within
regenerated muscle tissue. The occurrence of fibrosis
causes pain and mechanical stiffness, limiting the contractile force of the muscle tissue and decreasing the patient’s
range of motion [19]. Recently, the cytokine TGF-ß hasbeen implicated in the accumulation of fibrotic tissue within muscle after skeletal muscle injuries [20]. It is believedthat TGF-ß inhibitors may help to prevent fibrosis in patients recovering from relevant injuries. Thus far, fibrosis
has been difficult to quantify due to the variation in muscle
fiber size after injury is sustained. While it is generally
believed that increased fibrosis correlates with increased
stiffness, the relationship has not been quantitatively
established [21].

Diagnosis of muscle injury
Diagnosis of muscle injury should rely on a detailed history and physical examination. Injuries to the lower extremities are typically due to jumping, kicking, high-speed running, and sudden change in direction while running. In
general, patients often feel sharp pain and experience a loss
in function of the muscle post-injury. Pain may be associated with localized swelling and loss of motion. Individuals may also describe the occurrence of an audible pop[22]. Due to the high recurrence of hamstring injuries, clinicians should be mindful of a previous injury. Current
injury usually presents at or near location of previous injury [23]. Physical examination of the lower extremities

should consist of inspection, palpation, range of motion,and strength testing. The purpose of physical examination
is to determine the location and severity of the injury, moreso than the presence. An assessment shortly after injury is
recommended as this allows for quicker intervention and amore reliable medical history [24]. Physical examination
may reveal swelling and ecchymosis. Palpation serves todetect a discrete defect, edema, and increased muscle tone
due to injury [25]. Strength testing is best done with bilateral comparison to identify decreased strength of the injured muscle or post-injury changes [24]. Although many
patients will not have significant strength deficits withmanual testing, side to side pain or difficulty withexpending full effort should suggest that a patient has amuscle injury. Active range of motion tests should be conducted 48 h after injury as significant pain and disability ispresent immediately following injury and would provide
for an unreliable ROM assessment [24, 26].
Imaging can help confirm injury. X-rays are of little tono benefit in diagnosing pure muscle injuries, but it can be
helpful in visualizing chronic muscle injuries such as myositis ossificans or avulsion injuries including rectus femoris
avulsions and should be the first line of imaging obtainedin a majority of cases. MRI is arguably the gold standard
following muscle injury to assess the degree of injury.
Muscle strains are best visualized via MRI with T2weighted images, which optimize contrast between injured
muscles with edema and uninjured muscles [27]. Ultrasound is being increasingly used because it is readily accessible, low cost, and can acquire dynamic images [28].
Dynamic ultrasound can be used to differentiate between
partial and complete thickness tears as muscle retraction ismore evident through passive movement or active muscle
contraction [29]. However, ultrasound imaging is operator
dependent, cannot assess the extent of injury, or differentiate between old and new injuries [30]. Ultrasound is alsoless effective in diagnosing muscle tears within the first
few hours of injury because new hemorrhage may havethe same echogenicity as normal unaffected muscle [31•],
whereas MRI has high sensitivity to detect early hemorrhage and edema following muscle injury [27]. With costreservations, ultrasound may be used to diagnose 24 h after
injury as this is when hematomas become distinguishable
from surrounding muscle [31•]. It is important to notethough that the majority of muscle injuries are a clinicaldiagnosis. Advanced MRI techniques such as diffusion
tensor imaging (DTI) have been shown to provide a reliable assessment of skeletal muscle injury in mice [32].
Normal skeletal muscle displays orderly arrangements on
DTI which are disrupted following injury. This imaging
modality has yet to reach clinical acceptance but may become useful in future clinical practice and shows promise
to assess muscle fibrosis as well.

Curr Rev Musculoskelet Med

Return to sports after muscle injury
The timeline for return to sports following acute muscle injury
depends on several factors, including mechanism of injury,
severity of muscle injury, and what muscle group was injured.
In a study of American football players, number of training
camp days missed varied depending on which muscle group
was injured [1]. Those with hamstring injuries missed an average of 8.3 days, compared to 4.8 days for those with adductor injuries and 5.4 days for quadriceps injuries. Hamstringstrains were similarly associated with a lengthier time to returnto play compared to other muscle groups in European soccer
leagues [2•]. Those with hamstring strains missed 13 days,
compared to 9 days missed for adductor strains, 12 days for
quadriceps strains, and 13 days for calf strains. Return to playafter hamstring strains also differed in major compared tominor league baseball teams, with major league players missing 24 days and minor league players missing 27 days [12].
Recurrent muscle strains in the hamstring group appear tooccur more commonly in the biceps femoris compared to thesemitendiosis and semimembranosus [2•, 33].
The use of MRI to predict time to return to play following
acute muscle injury is controversial. Many studies have investigated MRI as a tool to prognosticate time to return to play,
given variability in return to play between different sports,
level at which the sport is played, and muscle groups injured.
In one such study by Ekstrand et al., MRI classification ofmuscle injuries [34] was utilized [35]. Grade 0 (negative
MRI) and 1 (MRI with edema but no architectural disruption)
injuries, while they were the most mild injuries, accounted forthe majority of the days missed (56 %, 2,141/3,830 injuries).
Grade 2 injuries were described as architectural disruption
indicating partial tear, and grade 3 injuries were complete
muscle tears. The authors of the study concluded that MRI
grading of muscle injury is highly correlated with amount oftime taken off from soccer, with increase of MRI grade correlated with increase in number of days off.
Reurink et al. recently conducted a systematic review todetermine the prognostic value of MRI in predicting returnto play in athletes who sustained a hamstring strain [36•]. Of
the 12 studies included in their review, 11 were found to havehigh risk of bias, and only 1 study was deemed to be at low
risk of bias. These biases were study sample, study attrition,
prognostic MRI measures, outcome measures, confounders,and statistical analysis. Based on these findings, the authors
concluded there was lack of strong evidence to use MRI as atool to predict return to play secondary to high risk of bias.
However, the authors were able to conclude that there is moderate evidence that hamstring injuries without hyperintensity
on MRI fluid-sensitive sequences are associated with shorter
return to play time. In contrast, Moen et al. suggested that
return to sports should be determined by clinical factors, suchas athlete-predicted time to return to play and passive straightleg raise deficit [23]. Furthermore, their study concluded thattime to return to play was not associated with findings on
MRI. It is therefore likely that MRI is partially helpful ingrading symptoms and severity of muscle injury, but the demands of individual athletes and types of sport must also betaken into account as well when assessing a proper return toplay.
Importantly, even after an athlete is able to return to sport,
their function after acute muscle injury is still in question.
Cloke et al. studied a group of youth soccer players and foundthat poor prognostic factors for prolonged symptoms of acutemuscle injury were hamstring strains, contact injuries, andolder age of the athlete [37]. A study of male semiprofessionalsoccer players found that function after return to sport improved with time after return to play [38]. At the time of returnto play, injured athletes were slower at sprinting, compared totheir non-injured counterparts. At 2 months after return toplay, the injured group of athletes demonstrated increases inhorizontal force and horizontal power, which was associatedwith improved early acceleration during sprinting with nodifference between injured and non-injured athletes.
In summary, hamstring injuries are associated with the longest times to return to play. MRI remains a controversial tool
for predicting return to play, and function after returning toplay from acute muscle injury appears to improve over time.

Treatments to enhance return to sports after muscle injury
Typically, conservative treatment of acute muscle injuries includes rest, ice, elevation, compression, rehabilitation exercises, and non-steroidal anti-inflammatory medications [39].
In recent years, advances in understanding of muscle injury
physiology and healing have led to proposed adjuvant
treatments.
Steroid injections in or around tendons and ligaments arecontroversial due to concern for the risk of rupture of thosestructures. However, intramuscular or myotendinous junction
injections may not carry the same risk and have thus beensuggested as treatment for muscle strains. A group of 58 professional football players with severe hamstring injuries (defined clinically as a palpable defect) were treated with an
intramuscular steroid and anesthetic injection [40]. Averagetime to return to full practice was 7.6 days. There were noreported complications, no recurrences during their NFL careers, and no functional deficits noted after recovery, though
no isokinetic testing was performed. Limitations of this study
include the lack of control group and its retrospective nature.
A case study of three professional baseball players who
sustained acute internal oblique muscle strains was also performed [41]. The players received intramuscular steroid andanesthetic injections. All three athletes had significant pain
relief within several days, and average return to play wasCurr Rev Musculoskelet Med

30.7 days. Steroid injections appear to be safe, though larger,
randomized studies should be conducted to better understandtheir efficacy.
Autologous platelet-rich plasma (PRP) injections havebeen studied for various applications, including as an adjuvant
treatment for muscle injury. Platelets release growth factors
including FGF-2, TGF-β1, PDGF, and IGF-1, and whenplatelets are highly concentrated by centrifugation, theresulting PRP solution is postulated to improve tissue healing.
The effect of PRP injections on acute muscle injury has been
best studied in rodent models. Several studies have investigated the effect of a series of PRP injections on isometric torquemuscle strength in rats. Delos et al. found no difference inmuscle strength at any time point and no difference in thenumber centronucleated fibers or inflammatory cells compared to control rodents injected with saline [42]. Hammondet al. found no difference in strength for single muscle contraction injuries treated with PRP [43]. However, they did
show increased muscle regeneration, improved contractile
function of multiple muscle contraction injured muscles, anddecreased recovery time from 21 to 14 days compared tocontrols. Other rodent models used immunohistochemical
staining after PRP injection as an outcome for muscle regeneration. Wright et al. showed an increase in satellite cell regeneration, and both Wright et al. and Gigante et al. foundgreater muscle regeneration in PRP-injected rodents as measured by the number of centrally nucleated fibers in the injured
area [44, 45], consistent with faster histological muscle
healing.
Several clinical studies on PRP use for muscle injury havealso been performed. A randomized controlled trial was conducted on 28 athletes who sustained hamstring strains [46].
Athletes were randomized to PRP injection and rehabilitation
versus rehabilitation alone. The PRP group had faster return tosport compared to the control group (26.7±7.0 vs. 42.5±
20.6 days) and had lower pain severity scores, though there
was no difference in pain interference scores. Another randomized trial studied ultrasound-guided PRP injections in athletes who sustained acute muscle injuries [47]. The players
were randomized to a single PRP injection or conservative
therapy (physical therapy, anti-inflammatory medications).
The PRP group in this study had decreased pain, increased
early strength, and early range of motion (days 7 to 14 after
injury). However, there was no difference in pain or strength
by day 28. The PRP group had decreased time to return tosport (10 compared to 22 days). The authors concluded that
PRP is helpful for decreasing pain in the immediate postinjury phase and may be helpful in facilitating early rehabilitation. Wright-Carpenter et al. conducted a controlled trial
where professional athletes with grade 2 or moderate muscle
strains were treated with a series of autologous conditioned
serum (ACS) [48]. Those in the ACS group had shorter time
to return to sport (16.6 days compared to 22.3 days in control

group), though this study is limited by lack of randomizationand lack of blinding. PRP may be useful for adjuvant treatment of muscle injury, especially acutely, though may nothave long-term benefits. However, large, randomized clinical
trials have yet to be conducted.
As discussed previously, fibrosis occurs after injury whenconnective tissue fibers are deposited between skeletal muscle
fibers. Fibrosis typically occurs during the second to third
week after the initial injury [49] and impairs the function ofskeletal muscle. TGF-ß has been implicated in fibrosis formation after muscle injury [49, 50], and thus, inhibition of TGF-ß
with antifibrotic agents is an attractive target to reduce muscle
fibrosis and improve function after injury. Various
antifibrotics have been studied including gamma interferon,alpha interferon, suramin, decorin, and relaxin, though thesehave yet to be used clinically.
Losartan, an angiotensin II receptor blocker, is of particularinterest given its established safety profile in humans. Losartan
use has been associated with decreased fibrosis in various settings, including neuromuscular disorders [51]. The use oflosartan in a mouse model with partial gastrocnemius lacerations improved muscle regeneration and decreased fibrosis formation [52]. In a clinical case study [53], two college athleteswith partial thickness tears of the biceps femoris were given
losartan, starting 5 to 10 days after initial injury, for 30 days (ata lower dose than would typically be used for treatment ofhypertension). One athlete had normal hamstring strength at
9 weeks, and the other athlete had normal hamstring flexibility
and strength by 3 weeks. Neither sustained recurrent muscle
injury at 1-year follow-up, there were no reported side effects,
and both remained normotensive. Losartan and other angiotensin II receptor blockers have potential to decrease fibrosis andimprove muscle healing, but larger scale, controlled studies areneeded to determine its efficacy.
A recent study explored the effect of combining anantifibrotic agent with PRP [54]. Mice were treated with either
PRP alone or PRP plus losartan. The group treated with boththe antifibrotic and PRP had improved skeletal muscle healingon histological analysis and decreased expression of Smad2/3(transcription factors that induce TGF-ß signaling). This suggests that using losartan to block the expression of TGF-ß
improves the PRP effect on acute muscle injury.
The use of stem cells for treatment of muscle injury has
been of great interest in multiple medical fields. Musclederived stem cells (MDSC) originate from the walls of bloodvessels and can differentiate into multiple cell lines including
myogenic and endothelial cell lines. Thus far, most studies
have been limited to animal models. Intraarterial stem celldelivery restored the dystrophin protein in a Duchenne muscular dystrophy model [55]. Interestingly, sex differences havebeen observed in Duchenne muscular dystrophy where female
MDSC had a superior ability to regenerate skeletal muscle
compared to male MDSC [56].

Curr Rev Musculoskelet Med

Muscle-derived stem cells have been proposed as a treatment for skeletal muscle injury [19]. In a controlled laboratory
study, MDSC were injected into injured tibialis anterior muscles of mice at several time points after muscle contusion
injury [57]. The mice injected with the muscle-derived stem
cells 4 days after injury had increased levels of vascular endothelial growth factor (VEGF) at week 1 and increased muscle regeneration and strength at week 2. The mice injected at 4
and 7 days after injury had decreased fibrosis 4 weeks after
injury. Intramuscular injection of MDSC may promote angiogenesis and decrease fibrosis. Recently, the development ofbiomaterials that function as delivery vehicles has been
discussed as an important factor for effective stem cell therapy[3]. Overall, stem cell therapy for muscle injury has beenshown to be effective in other skeletal muscles of the humanbody. Clinical studies are needed to further characterize theeffect of stem cell therapy on acutely injured muscles resulting
from sports.

The conventional treatment for muscle injuries involves
rest, ice, elevation, and compression. Recent studies have given rise to proposed adjuvant treatments that may enhancemuscle healing, decrease fibrosis, and provide a smoothertransition back to competition. Proposed treatments include
intramuscular or myotendinous junction steroid injections,
PRP and ACS injections, angiotensin II receptor blockers suchas losartan, and muscle-derived stem cells. For now, the results of studies proposing these treatments appear promising.
However, more studies are required in order to better determine their efficacy and allow for their use in clinical practice.

Compliance with Ethics Guidelines
Conflict of Interest Stephanie Wong, Anne Ning, Carlin Lee, andBrian T. Feeley declare that they have no conflict of interest.
Human and Animal Rights and Informed Consent This article does
not contain any studies with human or animal subjects performed by anyof the authors.

Summary and conclusions
Skeletal muscle injuries are among the most common injuries
in sport. Most muscle injuries occur in the hamstrings, abductors, quadriceps, and calf. Across sports, upper leg injuries
appear to be the most common. Due to lost time of play anddecreased muscle function and range of motion following
muscle injury due to fibrosis, muscle injuries pose a problem
to athletes.
Studies of muscle injury at the cellular level highlight thebiological basis of the physiological response to muscle injury. This information can guide future development of therapies that may enhance recovery from injury.
Current methods of diagnosing muscle injuries include a
detailed medical history, imaging, and a physical exam involving palpations, strength testing, and range of motion testing.
X-rays can be help rule out injuries involving bone such as
fractures and avulsion injuries. However, injuries restricted
solely to the muscle cannot be visualized through X-ray. In
these instances, MRI and ultrasound can be used to visualize
pure muscle injuries. In a majority of cases, MRI is the recommended imaging technique in assessing the extent of
injury.
Time to return to sport after injury may depend on themuscle that is injured, with hamstring injuries resulting in
the longest times to return to play. The use of MRI as a prognostic tool remains questionable. As such, more studies are
required in order to gauge its effectiveness in predicting time
to return to play. Even with tools that can potentially predict
return to competition, muscle function may not be 100 % upon
return to play, though it can improve with time. This must be
taken into account when determining time to return to preinjury level of sport.

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