T-Junction injuries of the distal biceps femoris – a more severe variant of hamstring strain injury

Although there are signs of a drop in the number recurrences of hamstring strain injuries (HSIs) in sport[1], it has been long evident both epidemiologically and experientially that these are amongst the most challenging in sport. In fact, since re-injuries have been reported to occur in up to 63% of HSIs[2], it is obvious that it is this recurrence risk underpins the high rates of HSI in sport. If these recurrences could be reduced, it would have an exponential effect on reducing the burden of this injury. What’s more, as these reinjuries typically present having worsened radiologically, they have a significant effect on a club’s injury burden [3].

An important development thus, has been the evolution in imaging classifications that have allowed for bespoke diagnosis, management and rehabilitation. These are particularly important in professional sport where rapid access to MRI common, to meet the pressurized demands of competition and contracts. The widespread adoption of classification systems such as the British Athletics Muscle Injury Classification (BAMIC)[4] mean that modern sports medicine now gives consideration to anatomical location in addition to grading, and can modify expectations accordingly. Although the research so far has been mixed [5, 6], some studies have demonstrated prolonged time to return to play and increased recurrence risk following intramuscular tendon injury in athletes [7-10].

This development has now widely expanded the considerations for clinicians in rehabilitation. Macdonald, McAleer [11] have described a tailored exercise progression with consideration of load, speed and contraction type dependant on BAMIC classification, while Askling, Tengvar [12] has discussed the variation in outcome following stretch and sprint type HSI. Hence, it’s now clear that all HSIs should not be thought to be the same, and they should not be homogenised from a prognostic or management perspective. In order to best mitigate against recurrence, it is necessary to understand the characteristics of each sub-type.

The most frequently injured of the hamstring muscles is the biceps femoris (BF) in running sports [13-15]. While these typically occur at the proximal musculotendinous junction (MTJ), a single retrospective analysis by a Super 15 rugby union team noted higher incidence of distal injuries than had otherwise been previously been reported [14]. Huygaerts, Cos [16] have extensively reviewed the anatomical and morphological features of BF that may predispose it to greater injury risk. As it’s name suggests, BF has two heads – the biarticular long head (LH) forming a conjoint tendon with semitendinosus proximally, and a second monoarticular short head (SH) which functions primarily as a knee flexor. These heads are innervated separately by the common peroneal and tibial nerves respectively.

Importantly, Entwisle, Ling [13] has recently descripted the distal MTJ ‘T-Junction’ of the biceps femoris, formed by a complex confluence of the epimyseal surface of the LH and SH in the distal posterior thigh (Image 1). Injuries to this location have been suggested to be a more severe variant of HSI that should, like intramuscular tendon injuries, be treated as a distinct clinical entity [13, 17].

The greatest insight into these injuries is derived from a retrospective analysis of 1850 MRIs in a predominantly professional cohort of athletes at Olympic Park in Australia following acute HSI. The authors detected 106 injuries involving the T Junction of the distal MTJ of the BH. While injuries to the LH component in isolation occurred in 50% of cases, injuries to the SH alone were infrequent. Critically, the recurrence rates, reported at 53.8% HSI (68% of grade 2s, 71% of grade 3s), are towards the very higher end of what has been reported for any sub-type of HSI. These findings were almost identical to those of Kayani, Ayuob [17], who found recurrences in 55% of conservatively managed high grade injuries.

While it is clear that these are a distinct clinical entity, and are more likely to recur, developing an optimal management strategy to achieve best outcomes remains a challenge. Kayani, Ayuob [17] reported good outcomes (function, satisfaction and recurrences) following surgical repairs of 34 professional athletes after Grade 3 T Junction injuries (involving either the MTJ or intramuscular tendon) and described a four stage rehabilitation strategy. The next stage however, in the same way that principles for management for other locations in the musculotendinous unit have been established, is to establish why these are injuries are recurring, and subsequently, how this should influence rehabilitation and the return to play process [11]. There must be anatomical or physiological reasons that injuries to the T Junction are recalcitrant, and have poorer outcomes than neighbouring muscles and locations, and ultimately it is these factors that should influence management.

Presently, however, there is a vacuum of information to guide us. Firstly, no characteristic mechanism of injury has been described that is associated with T Junction strain. While semimembranosus and proximal free tendon injuries have been shown to occur during overstretch, there is no distinct pattern of T Junction injury. Table 1 outlines the mechanisms of injury of a small subset of athletes who incurred a T Junction injury, but this is by no means an exhaustive analysis of the biomechanical factors involved. Some muscle injury classification systems have been proposed that include the mechanism of injury[18], and these would add value in this case in rising suspicion of such an injury that can only be determined using imaging. With further longitudinal analysis to better understand injury mechanism, preventative strategies – via exercise selection and skill execution – could be designed and trialled.

In addition, no study has described the clinical characteristics of such injuries. There are no proven pathognomic signs that should alert the clinician to the suspected presence of an injury in this location, other than palpation around the distal MTJ. So far, as these injuries can only be identified using MRI and thus, they are most frequently detected in elite sports people. In fact, it is suspected that in many cases these are never identified. This may result in frequent recurrences of BF strains over prolonged periods of time in sub-elite athlete athletes. It has been reported that these injuries become ‘relatively symptom free’ 3-4 weeks following injury, which may lull clinicians into under-estimating the prognosis of such an injury and progressing too rapidly through the return to play process

This early resolution of symptoms may cause the frequency of early recurrences that has been described for this sub-type, with 76% of recurrences occurring within the first 3 months [13]. Incomplete aponeurotic healing remains at 6 weeks following injury [19], so perhaps one principle of management would be a more cautious approach with consideration of the stages of healing.

Periodisation of rehabilitation should give consideration to three keystones – (1) anatomical diagnosis, (2) functional diagnosis or limitations and (3) the demands of ‘return to sport’. Given the nature of the anatomical diagnosis in this case, and the risk of early recurrence, it is proposed that a gentler cadence through the process is best. As with the recommendations for intramuscular tendon injury [11], sprinting activities and exercises involving maximal length, high force and rate of loading should be programmed in a manner that respect immature histological healing at the location of injury.

Consideration should also be given to the distinct characteristics of the distal BF that could be implicated in the poor recovery from injuries in this location as identification of these directs the strength training interventions[20]. The proximal and middle regions of the BFlh have larger pennation angles and greater fascicle lengths than the distal portion [16]. Although this has not prevented the proximal hamstring from being more frequently injured, perhaps the shorter fascicle lengths of the distal hamstring are implicated in poor recovery when injury does occur. Similarly, the distribution of fibre types is not uniform throughout the muscle, with a greater proportion of type 2 muscle fibres existing proximally. This could impact upon the type of training that is deemed optimal for this region.

The duel innervation of the BF is consistently suggested as a potential reason for the high proportion of injuries to this muscle. Given T-junction injuries occur at the confluence of the two heads, it is logical that an asynchrony in co-ordination of the heads could contribute. However, how this can be quantified, and resolved, is unclear and it is only speculative as to what approach could be taken. Macdonald, McAleer [11] suggest isometric exercises as a tool to overcome the neuromuscular inhibition that exists during eccentric exercise following injury[21], and a similar approach may yield benefit in injuries at this confluence.

Recent research has demonstrated the differences in inter-muscular activation during various exercises [20], and has allowed for increasing specificity in rehabilitation. The ‘Nordic hamstring exercise’ and ‘single-leg hip extension’ elicit high activity in the biceps femoris muscle, and thus exercises with these characteristics should be considered important for eliciting an adaptation, while also treated with caution given they are likely to stress this area. Subsequently, Hegyi, Csala [22] has expanded upon this, by demonstrating region specific differences within individual muscles (Figure 3). The eccentric component of the straight knee bridge, upright hip extension conic-pulley and slide leg curl may be most appropriate if attempting to elicit a location specific response in this region, with contraction type adapted dependant on whether the goal is to overcome inhibition (isometric), develop fascicle length or local strength and hypertrophy (eccentric), or simply to induce healing through mechanotherapy. However, again, it is not known whether these injuries occur due to compensation for proximal dysfunction – at which point offloading distally would be the targeted outcome. In the absence of this information, a balanced programme is advised, giving cognisance to inter-muscular and proximal-distal biases, across multiple contraction types.

In addition, the same group were able to demonstrate highest EMG activity of the distal BF during late swing phase of fast running [23]. This should influence running rehabilitation progression, and suggests that exercises targeting front-side mechanics and over-stride tolerance or reduction could be advocated.

Given that early recurrence is a key feature is this injury, the pace of progression is critical. If attempting to offload the distal BF, exercises that preferentially activate the medial hamstring or proximal BF may be indicated. In addition, initially restricting range of motion and rate of force development, as well as muscle length across the two involved joints are characteristics of exercise selections that can be manipulated and progressed, while still giving consideration to prioritising the adaptations that might be necessary to prevent recurrence (Table 4).

In conclusion, it’s clear that an even more complete approach to rehabilitation is required following T-Junction than for a typical HSI, giving consideration to the structural diagnosis and characteristics of the distal BF. At present however, it remains unclear which anatomical and morphological traits have the greatest influence on injury risk, nor which approaches will generate the most specific and appropriate adaptation. In addition, strategies that help with the identification of these injuries and raise suspicion amongst clinicians would likely improve management of this complex variant of HSI.

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