The “Bunny Tail” Lesion: An MRI Pattern of Proximal Biceps Femoris Tendon Injury With Rapid Return to Play in Elite Soccer Players—a Retrospective Observational Case Series

Abstract

Background:

Hamstring injuries are the leading cause of time lost in professional soccer. Among these, tendon-related lesions of the biceps femoris (BF) are associated with longer recovery and higher reinjury risk. Emerging magnetic resonance imaging (MRI) findings suggest the existence of morphological subtypes with different prognostic implications.

Purpose:

To describe an MRI pattern of proximal BF tendon injury—the “Bunny Tail lesion”—and report its management and outcomes in elite soccer players.

Study Design:

Case series; Level of evidence, 4.

Methods:

Over 2 competitive seasons (July 2023–June 2024), all acute hamstring injuries in a Serie A club were prospectively recorded and retrospectively reviewed. Players were eligible if MRI showed focal peritendinous edema at the distal tip of the proximal tendon of the BF long head, with increased intratendinous signal and preserved tendon continuity. Three musculoskeletal radiologists independently classified lesions using the British Athletics Muscle Injury Classification (BAMIC). Interobserver agreement was assessed with Fleiss κ. Rehabilitation followed a structured, football-specific program with standardized return-to-training and return-to-play (RTP) criteria.

Results:

Eight professional players met inclusion criteria. All lesions showed a well-circumscribed ovoid peritendinous edema surrounding the distal extremity of the proximal BF tendon, with preserved structural continuity. Lesions were variably categorized as BAMIC 2b to 2c, with one graded 3c by a single radiologist. Interobserver agreement for combined grade and tissue classification was very low [Fleiss κ close to 0]. All athletes were managed conservatively and returned to training after 14.6 ± 3.0 days and to play after 26 ± 6 days. No reinjuries occurred during a mean follow-up of 395 ± 202 days.

Conclusion:

This small series describes a consistent MRI pattern of proximal BF tendon injury characterized by focal ovoid peritendinous edema at the distal tendon tip, preserved continuity, and relatively rapid RTP despite predominantly tendinous BAMIC classification. This pattern may represent a less severe variant within the spectrum of proximal BF tendon injuries.

Keywords

biceps femoris injury Bunny Tail lesion MRI return to play muscle injury classification BAMIC classification

Muscle injuries are a leading cause of time lost from training and competition in professional soccer, with hamstring strains being the most frequent and recurrent.^12,18 Their incidence has steadily increased over the past 2 decades,¹¹ with a documented 4% annual rise since 2001,¹³ making them a growing clinical and economic burden in elite sport.

Within the hamstring muscle group, the long head of the biceps femoris (BF) is the most frequently affected component.²⁷ Its complex anatomical configuration, biarticular function, and exposure to high eccentric demands during sprinting and deceleration render it particularly susceptible to injury.³² A distinctive anatomical feature of the BF is its intramuscular tendon, which extends longitudinally throughout the muscle belly to the distal part of the muscle.^26,27 This central tendinous core plays a pivotal role in force transmission during dynamic movements⁸ and is enveloped by muscle fibers arranged in a nonuniform, interdigitating pattern.³

Structural factors such as tendon thickness, fiber orientation, and pennation angle vary across individuals and may further influence local strain distribution and, consequently, the site and severity of injury.¹⁷

These considerations highlight the importance of adopting a muscle-specific and architecture-based approach to risk assessment, rehabilitation, and refinement of current injury classification systems.⁵

Magnetic resonance imaging (MRI) has therefore become the reference standard for diagnosing and classifying hamstring injuries.⁴ It provides detailed information on injury location, connective tissue involvement, and edema morphology, which are essential for prognosis and for estimating return-to-play (RTP) timelines. Furthermore, MRI findings obtained before RTP have demonstrated predictive value in assessing reinjury risk.¹⁶

Among the available systems, the British Athletics Muscle Injury Classification (BAMIC) introduced a tissue-specific nomenclature,²³ distinguishing lesions that predominantly involve muscle fibers (a), the myotendinous junction (b), or tendinous structures (c).

Tendinous lesions, in particular, are typically associated with prolonged RTP and higher recurrence rates, likely reflecting their limited vascularity and slower healing potential. More recently, researchers have emphasized the need for lesion-specific descriptors and histoarchitectural perspectives to refine prognostic accuracy beyond these categorical definitions.³

In this context, new lesion patterns continue to emerge, challenging existing frameworks for diagnosis, rehabilitation, and RTP decision-making.²⁰ Accurate morphological characterization is therefore critical to ensure precise classification and appropriate clinical management. This study aimed to describe and characterize a consistent MRI pattern of proximal BF tendon injury in professional soccer players. We hypothesized that this newly described lesion would be associated, in this small series, with a more favorable clinical course and a shorter RTP timeline than typically reported for classic BAMIC “c” tendon injuries

Methods

Study Design

This study was conducted within the first team of a professional Italian soccer club competing in the Serie A league. The investigation covered a 2-season period (July 2023–June 2024), during which all acute hamstring injuries were prospectively recorded in the club's medical database and subsequently reviewed for eligibility.

All clinical assessments and imaging were performed as part of routine medical care and data were analyzed retrospectively. The study followed the principles of the Declaration of Helsinki (2024 revision) and complied with the EU General Data Protection Regulation. Written informed consent for data use and publication was obtained from all participants.

The manuscript was prepared according to the CAse REport¹⁵ and Strengthening the Reporting of Observational Studies in Epidemiology²⁹ reporting guidelines.

Participants

During the study period, 42 hamstring injuries were recorded in the first team of a professional Italian soccer club participating in the Serie A. Each player was examined by the same experienced sports medicine physician (A.C.) both immediately after injury and 48 hours later, following standardized clinical procedures. MRI was performed within 24 to 48 hours of symptom onset to confirm the diagnosis and define the lesion morphology. Of these, 35 injuries (83.3%) involved the BF (Table 1). Most (n = 27; 77%) of these BF injuries were excluded because they did not involve the distal portion of the proximal tendon of the long head or were associated with additional muscle or tendon lesions. Therefore, 8 cases met all inclusion criteria and were included in the present case series

Table 1

Anatomical Distribution of Injuries Observed Over 2 Competitive Seasons (n = 42 hamstring injuries) ^a

Season	BF	ST	SM
2023/2024	14	0	3
2024/2025	21	1	3
Total	35 (83.3)	1 (2.4)	6 (14.3)

Data are presented as n or n (%). BF, biceps femoris; SM, semimembranosus; ST, semitendinosus.

Athletes were retrospectively included in the present case series if they met the following inclusion criteria:

• Acute onset of posterior thigh pain during sports activity

• MRI confirmation of a focal lesion involving the distal portion of the proximal tendon of the long head of the BF

Exclusion criteria included the following:

• Complete discontinuity of the tendon or tendon retraction

• The presence of additional muscle or tendon injuries in either lower limb

For each case, demographic and clinical information were collected, including age, date of injury, mechanism of lesion, MRI findings, time to return to training (RTT) and RTP.

MRI Anatomical Assessment

Three musculoskeletal radiologists (E.V., D.O. and F.P.) with expertise in sports-related injuries independently reviewed the MRI scans. Imaging was performed using a 1.5-T Siemens Magnetom Avanto scanner (Siemens Healthineers) with a 16-channel body coil. Protocols included axial, sagittal, and coronal fluid-sensitive sequences and axial T1-weighted turbo spin-echo sequences with and without fat suppression. The radiologists reviewed all MRI scans independently, without access to each other's ratings. After the individual readings, classifications were compared to calculate multirater agreement (Fleiss κ) for the combined BAMIC grade and tissue component and for each element separately. Discrepancies were not resolved by consensus, as the aim was to assess the diagnostic reproducibility of routine, independent readings. Kappa values were interpreted according to commonly accepted thresholds: values <0.20 were considered poor agreement, 0.21 to 0.40 fair, 0.41 to 0.60 moderate, 0.61 to 0.80 good, and >0.80 excellent agreement.

Lesions were classified according to the BAMIC system,²² which provides a standardized framework for describing the site and extent of muscle injuries based on MRI findings. According to this system, injuries are categorized by the predominant tissue involved:

• type a, involving the muscle fibers themselves

• type b, located at the myotendinous junction

• type c, affecting the tendon, often associated with longer RTP times and an increased risk of reinjury

Each injury is further graded on a 0 to 4 scale reflecting the severity of structural disruption, with grade 1 representing minimal damage and grade 4 indicating complete discontinuity of the affected structure. This classification allows for a more precise characterization of the anatomical site and severity of muscle injuries, facilitating comparison across studies and clinical contexts.

A recurrence was defined as a subsequent injury of the same type and at the same site as the index injury after returning to full training.¹⁴ If occurring within 2 months, it is referred to as an “early recurrence”; between 2 and 12 months, a “late recurrence”; and >12 months, a “delayed recurrence.”²⁴

Treatment, RTP Time, and Reinjury Evaluation

Rehabilitation was based on the soccer-centered model,³³ which consists of 4 main phases: regeneration, functional rehabilitation (with specific subphases,) RTT, and RTP evaluation. Each program was individualized according to the athlete's locomotor profile and tactical role. Progression was guided by a combination of clinical, functional, and psychological criteria, with shared decision-making among medical staff, physical therapists, and performance coaches. A distinctive feature of this approach was the early introduction of ball drills and submaximal running (<70% of maximal sprint speed) to facilitate neurocognitive adaptation and maintain player engagement.

In line with established RTP frameworks,⁹ the rehabilitation process is structured to ensure the athlete meets the following RTP criteria:

• Complete absence of pain and clinically relevant asymmetry during region-specific clinical testing and throughout the execution of high-speed running and sport-specific tasks

• Restoration of jump performance, including double- and single-leg countermovement jump and drop-jump assessments, to values ≤10% of the individual's preinjury baseline

• Progressive reexposure to sprinting and soccer-specific external loads, objectively monitored via GPS. The athlete must demonstrate recovery of individual preinjury performance thresholds, defined as (A) ≥90% of preinjury peak sprint speed achieved in ≥2 separate sessions; and (B) ≥90% of the typical match high-speed running distance accumulated across the weekly rehabilitation microcycle

• Successful completion of ≥1 full, unrestricted team-training microcycle without symptom provocation or performance deterioration

• Reestablishment of the strength profile and neuromuscular function of the injured muscle group—and its synergists—to preinjury benchmarks, as confirmed through isokinetic, dynamometric, or validated field-based testing

Results

Participants

Eight professional soccer players sustained an acute injury involving the distal portion of the proximal tendon of the long head of the BF and met the study inclusion criteria. All athletes competed at the highest Italian national level (Serie A) and were actively engaged in regular training and match play at the time of injury. The mean age was 29.9 years (range, 25-34) (Table 2).

Table 2

Demographic, Clinical, and Imaging Features of the 8 Professional Soccer Players With “Bunny Tail” Lesion ^a

Player	Age, y	BAMIC Classification			RTT, d	RTP, d	Mechanism of Lesion
Player	Age, y	Radiologist 1	Radiologist 2	Radiologist 3	RTT, d	RTP, d	Mechanism of Lesion
1	34	2c	2c	2c	12	19	Acceleration phase
2	26	2c	2c	2b	14	29	Acceleration phase
3	30	2c	2b	2c	11	18	Curvilinear acceleration + opponent-induced trunk destabilization
4	30	3c	2c	2c	16	31	Curvilinear acceleration + opponent-induced trunk destabilization
5	30	2b	2c	2b	15	23	Curvilinear acceleration + opponent-induced trunk destabilization
6	33	2c	2c	2c	15	22	Ball protection + trunk destabilization
7	25	2b	2c	2c	12	35	Passing action
8	31	2c	2c	2c	22	31	Tackle (stretch type)

BAMIC, British Athletics Muscle Imaging Classification; RTP, return to play; RTT, return to training.

The injury mechanism was acute in all cases. No predominant injury mechanism could be identified. Through the combined use of video analysis and GPS-derived data, and by adopting the classification framework proposed by Aiello et al,¹ the injuries identified were as follows: 2 injuries during pure acceleration, 3 during curvilinear acceleration with opponent-induced trunk destabilization, 1 during ball protection with destabilization, 1 passing action, and 1 tackle (stretch-type).

None of the players reported an audible “pop” or immediate functional collapse. Instead, athletes described a localized pulling sensation in the distal posterior thigh, followed by pain and functional limitation that prevented continuation of play. Clinical examination consistently revealed localized tenderness at the distal posterior thigh, without swelling, ecchymosis, or palpable defects. Passive stretching reproduced mild to moderate discomfort with slightly reduced range of motion compared with the contralateral side. Resisted knee flexion elicited focal pain—particularly with external foot rotation—but without substantial strength loss or motor control deficit. Isometric and eccentric strength testing induced pain yet preserved performance. Importantly, no signs typical of high-grade tendon injury (eg, pronounced weakness, positive take-off or bent-knee stretch tests) were present. Collectively, these findings differentiated the observed lesions from conventional proximal hamstring tendon tears, indicating a milder clinical presentation and course.

MRI Findings

MRI performed within 48 hours of injury consistently demonstrated a focal lesion at the distal extremity of the proximal BF tendon (Figure 1). The lesion was best visualized on axial and sagittal T2-weighted fat-suppressed sequences, while T1-weighted images confirmed tendon integrity.

Figure 1.

Anatomical illustration of the biceps femoris, showing the proximal tendon, long head, distal tendon, and short head (cut at the linea aspera). The distal portion of the proximal tendon of the long head corresponds to the site of the “Bunny Tail” lesion described in this study (anatomical illustration was adapted from Brukner and Connell⁴ and is licensed under a Creative Commons Attribution–NonCommercial 4.0 license).

Across all cases, a reproducible morphological signature was observed. The hallmark feature was a well-circumscribed, ovoid peritendinous edema surrounding the terminal portion of the tendon, with high signal change into the tendon, without discontinuity or loss of tension.

On axial images, this appeared as a small, spherical or ovoid hyperintense area on fluid-sensitive sequences, tightly confined around the tendon tip. The distinctive rounded shape of the edema—resembling the tufted appearance of a rabbit's tail—inspired the term “Bunny Tail lesion.”

On sagittal and coronal views, edema diffusely encircled the distal tendon extremity, forming an image configuration reminiscent of Lagurus ovatus [“bunny tail” grass], where the tendinous stem corresponds to the stalk and the rounded peritendinous edema to the fluffy flower (Figure 2).

Figure 2.

Illustrative analogy and magnetic resonance imaging (MRI) appearance of the “Bunny Tail” lesion. The images of a rabbit's tail (top left) and of Lagurus ovatus [“bunny tail grass,” bottom left] are presented solely as visual analogies to illustrate the characteristic focal, ovoid configuration observed on MRI and are not intended as diagnostic material. In the MRI scans, the “Bunny Tail” lesion is characterized by a well-circumscribed, rounded peritendinous hyperintensity located at the distal tip of the proximal biceps femoris tendon, with increased intratendinous signal but preserved structural continuity. The T2–weighted imaging (T2) axial fluid-sensitive sequence (top right) shows a focal ovoid area of edema closely surrounding the tendon tip, while the T2 coronal view (bottom right) confirms the same ovoid peritendinous morphology. The analogy with a rabbit's tail and with Lagurus ovatus is intended solely as a visual descriptor of the peritendinous edema pattern and should not be interpreted as a diagnostic criterion.

Lesions were variably categorized as BAMIC 2b or 2c, with 1 case labeled 3c by a single radiologist (Table 2). Complete interobserver consensus for the combined grade and tissue classification was achieved in 3 of 8 cases (37.5%). When analyzed separately, consensus was 87.5% for injury grade and 50% for tissue component. Interobserver agreement among the 3 radiologists was assessed exclusively using Fleiss κ, reflecting agreement between independent raters. No intraobserver agreement analysis was performed, as each radiologist reviewed each MRI scan once. Multirater interobserver agreement was very low for the combined classification and for individual components (Fleiss κ = −0.062, P = .73, for combined grade + tissue; κ = −0.0435, P = .83, for tissue component; κ = −0.0105, P = .96, for grade).

In all cases, follow-up MRI was performed before RTP. Although residual peritendinous edema and incomplete signal normalization persisted on fluid-sensitive sequences, athletes were cleared for RTP once clinical and functional recovery criteria were fulfilled.

Treatment, RTP Time, and Reinjury Evaluation

All players were managed nonoperatively through a structured rehabilitation program delivered by the same physical therapist (L.V.). No invasive procedures were required, no transient symptom exacerbations occurred and no reinjuries were recorded during follow-up (mean ± SD, 395 ± 202 days).

The mean ± SD time to RTT was 14.6 ± 3 days and to RTP was 26 ± 6 days (Table 1). Notably, despite their predominant BAMIC c classification, most players resumed competition within 3 to 4 weeks.

Discussion

In this retrospective observational case series of elite professional soccer players, we identified a recurrent MRI pattern of proximal BF tendon injury characterized by focal, ovoid peritendinous edema at the distal tip of the proximal tendon, with increased intratendinous signal but preserved structural continuity. Despite being predominantly classified as tendinous (BAMIC c) lesions by independent radiologists, these injuries were associated with a relatively rapid RTT and RTP, and no reinjuries were observed during extended follow-up. Collectively, these findings suggest that this specific MRI configuration, within this small series, may be associated with a more favorable clinical course than typically reported for classical intratendinous BF injuries.

Hamstring injuries remain a major cause of time loss in professional soccer.¹⁰ Among them, the long head of the BF is particularly prone to injury because of its biarticular function, complex internal architecture, and exposure to high eccentric loads during sprinting and deceleration. Tendon-related injuries are especially problematic, as they are typically associated with prolonged rehabilitation and higher recurrence rates due to limited vascularity and slower healing compared with muscle tissue.⁶ Moreover, premature RTP after tendon injuries increases the risk of reinjury, emphasizing the need for accurate lesion characterization and individualized rehabilitation.¹⁹

In the present study, we describe an MRI pattern involving the distal tip of the proximal BF tendon that, to our knowledge, has not been specifically characterized as such in previous literature. which we termed the Bunny Tail lesion. The analogy with a rabbit's tail and Lagurus ovatus is used solely as an illustrative tool to depict the focal ovoid morphology and is not intended as a diagnostic criterion. Although most cases were radiologically categorized as BAMIC type c, a group typically linked to extended RTP durations (6-8 weeks) and elevated reinjury risk,^2,10,23 all players in this series returned significantly earlier (mean RTP, 26 days). Clinically, these lesions behaved less severely compared with the classical c classification.³

Previous literature reports that BAMIC c injuries often require prolonged rehabilitation and may entail a reinjury risk up to 30%.^25,28 In contrast, none of the athletes in this series sustained reinjury over a mean follow-up exceeding 1 year, supporting the hypothesis that the Bunny Tail lesion may represent a specific subtype with a potentially different prognosis of tendon-related injuries. We propose that this pattern involves microtrauma confined to the distal intramuscular tendon tip, an area subject to relatively lower mechanical stress during peak eccentric load, thereby explaining the preserved function and rapid, uncomplicated recovery observed clinically.

Structurally, the MRI signature of the Bunny Tail lesion—focal, ovoid peritendinous edema with partial tendon involvement—differs markedly from classical tendinous injuries, which usually demonstrate fiber disruption, retraction, and diffuse edema.⁷ This appearance likely reflects a localized peritendinous overload and inflammatory response rather than a full-thickness structural failure. The limited interobserver agreement observed among radiologists underscores the anatomical complexity of this region and highlights the difficulty in defining tissue involvement within the current categorical system. Such variability further supports the need for standardized imaging descriptors and closer clinicoradiological correlation.

Importantly, RTP was granted despite residual edema on follow-up MRI, reinforcing evidence that functional recovery may be a more relevant indicator of readiness than complete radiological normalization.^21,30,31

The non–full thickness nature of the lesion and the preservation of tendon integrity likely explain its favorable prognosis and low reinjury rate.

Although the present data set was limited, no predominant injury mechanism could be identified. Traditionally, hamstring injury mechanisms have been dichotomized into stretch-type and sprint-type categories¹³; however, with the integration of GPS-based load monitoring and video analysis, a more nuanced view has emerged. Evidence now suggests that substantial mechanical stress may also occur during accelerative phases, changes of direction, and sport-specific movements involving external perturbations and trunk flexion.

The heterogeneity observed in our series precludes definitive biomechanical conclusions but highlights the multifactorial nature of hamstring injuries and supports the need for integrated approaches combining biomechanical assessment, contextual load analysis, and individual anatomical profiling to optimize prevention and rehabilitation strategies. In the BAMIC system,²² these lesions are most often classified as type 2c because of their tendinous proximity, yet their morphological and prognostic characteristics differ substantially from typical intratendinous tears.

We tentatively suggest the descriptive term “BAMIC type 2c distal tip variant” to describe lesions that meet the following criteria:

1. focal, well-defined peritendinous edema surrounding the distal extremity of the tendon;

2. high signal change into the tendon without evidence of traction or discontinuity; and

3. absence of diffuse edema propagation along fascial planes

This introduction requires further validation before being incorporated into formal classification systems.

Recognition of this variant would improve the precision of radiological reporting and refine prognostic stratification, helping clinicians distinguish confined, self-limiting lesions from higher-risk intratendinous tears. Future studies should validate this proposed subtype through larger, multicenter data sets and interobserver reliability analyses. Moreover, it would be valuable to investigate whether similar peritendinous patterns occur in other intramuscular tendons—such as the rectus femoris conjoint tendon—to determine whether the “distal tip variant” reflects a broader histoarchitectural phenomenon rather than an isolated feature of the BF.

Limitations and Future Directions

This study has several limitations that may affect the generalizability of its findings. First, the analysis was conducted within a single elite professional soccer team, with all injuries managed by the same medical and physical therapy staff, potentially limiting external validity. Second, although injuries were prospectively documented in the club's medical database, the retrospective study design may still introduce selection and information bias. Third, MRI interpretation was performed by radiologists working in a highly specialized sports medicine environment, which may not reflect diagnostic performance in broader clinical settings.

A key limitation is the markedly low interobserver agreement for BAMIC classification, with Fleiss κ values approaching zero. This finding challenges the reproducibility of both the overarching pattern and its tentative subclassification when applied independently by experienced musculoskeletal radiologists. Moreover, the absence of a comparison cohort of classical BAMIC c lesions restricts the ability to determine whether the observed clinical course or RTP timelines meaningfully differ from traditionally described injury profiles; any such interpretations should therefore be considered hypothesis generating.

Future research should aim to validate these findings in larger, multicenter cohorts using standardized MRI protocols and formal interobserver reliability assessments. Such work is needed to establish whether the proposed MRI configuration—provisionally described as a BAMIC type 2c distal tip variant—represents a consistently recognizable entity with distinct prognostic implications or simply reflects expected variability within intratendinous injuries.

Advanced imaging modalities, including diffusion tensor imaging and ultrashort echo time sequences, may offer further insight into tendon microstructure and peritendinous changes. Longitudinal studies across different competitive contexts could clarify the stability of the observed clinical trajectory. Finally, integrating biomechanical modeling with contextual load analysis may help elucidate stress distribution in the distal portion of the proximal tendon tip, advancing understanding of the mechanisms underlying this focal injury phenotype and supporting the development of targeted prevention and rehabilitation strategies.

Conclusion

This study describes a consistent MRI pattern of proximal BF tendon injury in professional soccer players. Recognition of this imaging appearance may facilitate more accurate lesion classification and help distinguish it from classical BAMIC c tendon injuries. In this small series, the described lesion was associated with a favorable clinical course and a relatively short RTP timeline. Awareness of this pattern may therefore support more appropriate clinical decision-making and expectations regarding recovery.

Footnotes

Final revision submitted January 6, 2026; accepted February 1, 2026.

The authors declared that there are no conflicts of interest in the authorship and publication of this contribution. AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

The requirement for ethical approval was waived by the Medical Directorate of the club, as this retrospective study analyzed anonymized data collected during routine clinical care.

ORCID iD

Alice Patrignani

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