To address our research question, a study population comprised of two different study cohorts was used (50 Catalan cohort and 58 Qatari cohort). Patients with acute hamstring injuries were either recruited at the multi-sports club FC Barcelona, or were part of a double-blind randomized controlled trial (RCT) investigating the effect of platelet-rich plasma (PRP) in acute hamstring injury on time to RTP.13 This RCT did not show a benefit of PRP over placebo. Eligibility criteria for both study cohorts are shown in Box 1.

Ethical approval was obtained from the Ethical Committee of ASPETAR (Orthopaedic and Sports Medicine Hospital) and the Consell Català de l’Esport (Generalitat de Catalunya). Informed consent was obtained from all included subjects.

At FC Barcelona, images were acquired with a 3.0T magnet system (Siemens, Erlagen, Germany). Subjects were positioned inside the scanner with both legs parallel to the MRI table, and the thighs were covered with multichannel coils. Coronal and axial T1-weighted images (TR/TE 900/15ms; FOV 330mm×300mm; slice thickness 3.5mm; matrix 384×512) were obtained. Coronal and axial T2 fat sat (TR/TE 4000/35ms; FOV 330mm×300mm; slice thickness 3.5mm; matrix 320×384) as well as sagittal T2 (TR/TE 4000/32ms; FOV 330mm×300mm; slice thickness 2.0mm; matrix 320×384) was obtained.

In the ASPETAR cohort, images were acquired with a 1.5T magnet system (Magnetom Espree, Siemens) and a body matrix coil. Coronal and axial proton density (PD) weighted images (TR/TE 3000/32ms; FOV 240mm; slice thickness 5mm; matrix, 333×512) were obtained. Subsequently, coronal and axial proton density weighted images with fat saturation (PD-FS) were obtained (TR/TE 3000/32ms; FOV 240mm; slice thickness 3.5mm; matrix 326×512 for coronal and TR/TE 3490/27ms; FOV 320mm; slice thickness 3.5mm; matrix 333×512 for axial).

Each MRI was assessed by one of the two musculoskeletal radiologists, who were blinded to clinical outcome. Each radiologist scored the MRIs in a random order between July 2009 and July 2016. The free proximal BFLH tendon measurement is shown in Figs. 1 and 2.

Figure 1.

Proximal BFLH free tendon length is measured (in cm) from the most inferior margin of the ischial tuberosity to the point where the first muscle fibers start to insert onto the tendon. IT: ischial tuberosity; ST: semitendinosus; BFLH: long head of the biceps femoris; *: free tendon. (a) Short free tendon, (b) medium free tendon and (c) long free tendon.

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Figure 2.

T1 coronal MRI of a short free tendon (a) and long free tendon (b) are shown. Arrowheads show the most inferior margin of the ischial tuberosity and arrows show the point where the first muscle fibers started to insert onto the tendon.

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The rehabilitation protocols used in both cohorts are described in detail in previous studies.13,14

At FC Barcelona, the RTP decision is made based on clinical assessment, sport-specific field tests and imaging.15,16 The field test includes a comparison of data such as workload parameters (including GPS) with athlete-specific pre-injury data. No specific pre-defined criteria were used. Time to RTP was defined as the number of days from initial injury until the athlete was cleared by one of the team physicians to resume full, unrestricted training.

In the ASPETAR cohort, the guidelines for making the final RTP decision included successful and asymptomatic completion of the progressive criteria-based rehabilitation program including three sport-specific phases, clinical evaluation by an experienced sports medicine physician, and interpretation of the results of an isokinetic assessment. The physician's final decision was guided, but not determined by these medical factors, and also included consideration of sport risk modifiers and decision modifiers.17,18

Statistical analysis was carried out using IBM SPSS Statistics (version 21, IBM Corp.). Continuous variables were tested for normality and presented as mean (±SD) unless otherwise stated. To evaluate intra-observer reliability, one of two specialized musculoskeletal radiologists recorded free tendon length bilaterally in 30 subjects. The same radiologist repeated the measurements one week later. To evaluate inter-observer reliability, both radiologists performed the measurements independently. Intraclass correlation coefficient (ICC) analysis was carried out to determine intra- and inter-observer reliability. We used Cronbach's α and the model for the computation of ICC was a two-way mixed (people effects are random, and the item effects are fixed). Reliability was considered excellent if the ICC is >0.75, fair to good if 0.4<ICC<0.75 and poor if ICC is <0.4. The Pearson correlation coefficient was used to analyze the relationship between tendon length (in cm) and time to RTP (in days). Statistical significance was set at p<0.05.

ICCs for intra-observer and inter-observer reliability of the free tendon length measurement (cm) are presented in Table 2.

Average length of the free proximal BFLH tendon was 4.8±2.4cm on the right side, and 5.0±2.2cm on the left side. Average time to RTP was 29±15 days. There was no statistically significant correlation between free proximal BFLH tendon length in cm and time to RTP in days (Pearson correlation coefficient=−0.037; p=0.712). Fig. 4 shows a scatter plot for these parameters.

Figure 4.

Scatter plot for RTP (days) and free proximal BFLH tendon length (cm).

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Athletes in the current study are predominantly football players and track and field athletes with a mean time to RTP of approximately 4 weeks. For comparison: a large study on RTP in (elite) football reported that 84% of injuries involved the biceps femoris, with a mean time to RTP of 20±15 days.19 Askling et al.20 studied 18 sprinters who all had the primary injury in the biceps femoris with a median time to return to pre-injury activity level of around 16 weeks (range 6–50 weeks).

According to our measurements, average proximal length of BFLH free tendon was 4.8±2.4cm on the right side and 5.0±2.2cm on the left side. A recent anatomical study of the hamstring muscle complex included muscle and tendon lengths and reported an average proximal BFLH free tendon length of 5.0±3.4cm.5 Although this study involved measurements upon dissection, the average of this tendon length corresponds well with our measurements on MRI.

The clinical suspicion that proximal BFLH free tendon length might impact recovery was partly due to observations in clinical practice, and partly due to recent attention for the effect of musculotendinous architecture on injury risk.21–23 Hypothetically, if certain architectural characteristics result in conditions that may predispose to muscle injury, they could also potentially result in less favorable conditions for muscle recovery. We sought to investigate whether this was the case for proximal free tendon length. Nonetheless, our data do not indicate that free proximal BFLH tendon length influences time to RTP following acute hamstring injury in athletes.

In any case, is interesting to note that tendinous injuries (for example proximal BFLH free tendon tears) could need a surgical intervention for a whole healing because in case of perform a conservative treatment the RTP is much longer and the risk of reinjury are high.24 However, a recent work25 has pointed that RTP in intramuscular hamstring tendon injuries is only significantly longer compared with injuries without intramuscular tendon disruption.

The subjects in this study were either participants in a randomized controlled trial (Qatari cohort) or athletes of FC Barcelona (Catalan cohort) with some differences in eligibility criteria, MRI protocols and RTP criteria. Although this may reflect clinical practice, it can be considered as a limitation. In addition, a univariate analysis was used. Therefore, we have not controlled for potential confounders.

Aside from impact on recovery, in accordance with recent studies on architectural characteristics it would be interesting to investigate whether free proximal BFLH tendon length is a risk factor for injury and re-injury. This requires prospective study designs.