To our knowledge, this is the first study that assessed the association between ankle and hamstring injuries and their predisposing role that each one of them has in a re-injury affecting the other location.

The present study opens the way to investigate the relationship between injuries occurring in different parts of the musculoskeletal system but in the same limb.

Athletes with a previous ankle injury face a higher risk of hamstring injury.

In clinical practice, there is a need for a holistic approach to musculoskeletal rehabilitation which should not be directed only to the region where a given injury occurred.

Ankle sprains and hamstring muscle injuries are among the most common sport injuries, and are a major cause of time lost from sport participation [1–5].

The main mechanisms of hamstring injury is eccentric contraction at high velocity and slow stretching at outer range of motion [6]. A hamstring injury is a major risk factor for future hamstring injury [7–10]. Lateral ankle sprains are usually provoked by excessive foot inversion with the foot in plantar flexion [11]. Certain somatometric factors such as higher longitudinal foot arch, wider foot, cavovarus deformities of the foot may be associated with this injury [12]. Additionally, there is an association of the incidence of lateral ankle injuries with reduced cardiorespiratory endurance, decreased muscle strength and range of motion of ankle dosriflexors and decreased movement coordination [13].

There are several extrinsic risk factors for lower limb injuries such as level of competition, skill level, shoe type, ankle bracing and playing surface [14]. Intrinsic risk factors include age, gender, limb dominance, limb flexibility, body size, foot morphology, anatomical alignment of the lower limb, muscle tightness, joint laxity, aerobic fitness, postural stability and inadequate rehabilitation after previous musculoskeletal injury [14].

In addition, a recent systematic review concluded that a single ankle sprain is the most common injury resulting in a secondary sprain of the ipsi- or contralateral ankle [15].

Furthermore, a growing body of evidence links proximal and distal contributors/risk factors to lower extremity injury [16]. A pathomechanical model has been proposed, linking the biomechanics of the ankle joint with development of patellofemoral pain syndrome [17, 18], altered position and function of the hip and pelvis [19, 20] and the development of low back pain [21]. According to this model, excessive foot pronation delays external rotation of the tibia and disrupts timing between knee extension and rearfoot supination [18, 22, 23].

A link has also been suggested in terms of the role of proximal structures in biomechanical function of the lower limb and the development of lower extremity injury [24–26].

The core muscles (lumbar-pelvic hip complex) are essential in controlling hip abduction, subsequent femoral internal rotation, and potentially more distal movement [17, 27, 28].

There is nevertheless a gap in the literature evaluating whether a prior injury in the lower limb could be a predisposing factor for an injury occurring in a different location in the same lower limb.

This study compared and evaluated the events of first time unilateral hamstring and ankle injuries in elite track & field athletes over a period of 17 years, and focused concurrently on both these injuries and their possible pathophysiological interdependence, while the majority of the studies reports on each of these injuries in isolation.

Initially, all track and field athletes included were grouped in two categories according to the first traumatic event occurring in one of the two sites under study (ankle injury or hamstring injury). The athletes were then grouped in two categories indicating the occurrence of a traumatic event in both sites or in one of the sites over the time of the study. Frequencies and proportions were calculated and correlated with age, gender, athletic discipline and grade of injury, in both cases and based on the total of the 367 athletes having been injured either in one or in both sites under study.

The risk of the elite track and field athletes under study to experience either a hamstring or an ankle injury was calculated as an incidence proportion. Consequently, the numerator includes only new cases of hamstring or ankle injuries, whereas the denominator is the number of elite track and field athletes having been recorded and followed up during the observation period.

Therefore, the incidence proportion was estimated as follows: [35].

[(Number of events per type of injury) / (total of athletes during a specified period and registration terrain)*100].

The Mann-Whitney U test was performed to assess possible associations between continuous and categorical data. The Chi-square test was performed to test possible associations between categorical variables. The Monte Carlo method was used in both cases to estimate the significance level. If there was a 2 × 2 table, exact results were provided instead of Monte Carlo. The basic assumption that 0 cells (0.0%) should have expected a count less than 5 was accepted. Statistical analysis was conducted with Stata 12.0 statistical software and confidence level was set at 0.05.

There was a higher incidence of hamstring injuries (16.3%, 245 incidents) compared with ankle injuries (8.1%,122 incidents). The proportion of athletes having experienced both injuries accounted for 3.3% (50 incidents out of 1500 athletes) of the total number of athletes with any type of injury.

Hamstring injuries experienced as first event accounted for 66.8% (245 athletes) and ankle injuries accounted for 33.2% (122 athletes), considering that there was no antecedent injury in these two areas. Most ankle injuries were clinical grade II (48.2%), followed by grade I (24%), and grade III (17.6%). Similarly, a grade II injury was recorded in 53.6% of the hamstring injuries, followed by grade I (28.3%) and grade III (15.1%) (Fig. 3).

Age was not statistically significantly different between ankle and hamstring injuries reported as first injury (U = 7674, p = 0.692). Additionally, gender was not statistically significantly associated with the type of first injury recorded (× 2 = 3.492, p = 0.324).

The athletic discipline was statistically significantly associated with the type of first injury (× 2 = 14.325, p = 0.002). Hamstring injuries were significantly more frequent than ankle injuries, especially in runners, jumpers, and combined sports events: 73.6% of the runners, 54.5% of the jumpers, and 72% of the combined events athletes experienced a hamstring injury as first injury (Fig. 4).

From a total of 367 elite track and field athletes who had experienced a first injury of the hamstring or of the ankle, 13.6% (50 athletes out of 367 athletes) experienced both injuries over the data collection period (Fig. 5). Athletes with a preceding ankle injury (23/122 = 19%) had a higher chance of experiencing a subsequent hamstring injury compared with athletes who had experienced a hamstring injury as the first traumatic event (27/245 = 11%; × 2 = 4.245, p = 0.039).

Athletic discipline (× 2 = 3.025, p = 0.388) and gender (× 2 = 3.120, p = 0.77) did not significantly affect the number of athletes experiencing both ankle and hamstring injuries. The proportion of athletes experiencing both ankle and hamstring injury events was 18% (25 of 142) in females and 11% (25 of 225) in male athletes. The clinical grade of ankle or hamstring injury did not influence the subsequent traumatic event to occur in the different area (× 2 = 7.633, p = 0.54).

To our knowledge, this is the first study that assessed the interdependence of acute traumatic ankle and hamstring injuries and their predisposing role in a re-injury affecting the other location. The most important finding of this study was the fact that athletes with a previous ankle injury had higher proportion of a subsequent first injury in the hamstrings compared with athletes with a previous hamstring injury and a subsequent ankle injury (p < 0.05). Also, athletes who first suffered from an ankle injury were at greater risk of further ankle injuries. This study does not answer the question, why a hamstring injury following ankle injury is more common than an ankle injury following hamstring injury. This interesting finding has to be investigated in further prospective research.

Generally, injury rates increase with the number of preceding injuries [36]. Furthermore, psychosocial factors such as risk-taking behaviour, life event stress and trait anxiety may contribute to an increased risk of re-injury even in a different location [37, 38].

Lower extremity injuries are multifactorial [39], and depend on several intrinsic (athlete-related) and extrinsic (environmental) factors. In some instances, intrinsic factors have proven more predictive of muscle strain injury than extrinsic factors [40]. In the present study, the frequency of both ankle and hamstring injury events was greater in female (18%) than in male (11%) athletes, although the difference was not statistically significantly. Interestingly, age did not influence the proportion of ankle and/or hamstring injury events.

Up to now, the literature has not associated directly ankle and hamstring injury, even though there is evidence to suggest that previous calf muscle injury is independently predictive of hamstring muscle injury. After injury, changes may occur in the functional biomechanics of the lower limb, predisposing the athletes to injury in different muscle groups. Accordingly, altered lower limb biomechanics have been reported in gait, unilateral stance and functional testing after ankle sprain injuries compared with controls [40].

A significant increase in ankle joint inversion and a reduction in joint plantar flexion during gait have been found in patients following lateral ankle sprain [41]. Limited ankle dorsiflexion has also been proposed as a predictor for lateral ankle sprain [42]. Kinetic and kinematic alterations were observed, and were attributed to an increased reliance on more proximal structures (knee and hip) to absorb impact forces [41]. Patients with a 6 month history of first-time lateral ankle sprain displayed a greater hip-dominant coordination strategy for static unilateral stance [43] and greater hip extensor dominance during drop jumps [44]. The observed flexor movement produced during the initial phase of the drop jumps may have led to a force attenuation strategy following initial contact [44]. A plausible explanation could be given for the findings of the present study by the fact that 32–74% of individuals with a history of ankle sprain report residual and chronic symptoms and aberrancy of sensorimotor variables of neuromuscular control [45–47]. This situation alters lower limb biomechanics in the injured ankle and affects proximal muscle groups such as hamstrings [48]. Previous injury in the lower extremity, especially when followed by inadequate rehabilitation, is a risk factor for further injuries to the ankle, knee, and to the ipsilateral lower extremity in general [14].

Regarding athletes with a prior history of acute ankle injury, neurophysiological changes have been reported in the ipsilateral posterior thigh muscles [49–51]. Additionally, mechanical instability of the ankle joint leads to an increased inversion, predisposing to further ankle sprain injury [52].

Evidence suggests at least an indirect biomechanical link between specific regions of the leg. Tight hamstring muscles have been associated with plantar fasciitis [45, 48, 53]. Increased hamstring tightness causes early contraction of the posterior leg muscles through the gait cycle, and decreases ankle dorsiflexion [54, 55] which in turn induces prolonged forefoot loading and the increased magnitude of tensile loading forces within the plantar fascia [45].

There is need for further studies on the interdependence of injuries in these specific anatomic sites. The outcome of the current study underlines the need for consequent rehabilitation of ankle and hamstring muscle injuries. Further research should address the interferences between hamstring and ankle injuries. In addition, it seems reasonable to elucidate, if the observed injury associations also occur in different sports (e.g. football). The question, if the demonstrated relations are also true for recreational athletes has also to be answered in future investigations. Probably, there is also an influence of a specific injury to induce a contralateral lesion.

The study population of elite track and field athletes showed a significantly higher frequency of hamstring injuries if an athlete had sustained a prior traumatic acute ankle injury compared with athletes who initially had posterior thigh muscle injury and secondarily suffered an ankle joint injury. Given the present findings, rehabilitation programmes should focus on improvement of proprioception and strength of the whole lower extremity after ankle injuries.

In addition, the association between injuries occurring in different parts of the musculoskeletal system needs to be investigated further.

One limitation of the study was the fact that 80 athletes did not report their sport discipline. However, their injuries were recorded in detail, and therefore it does not bear any consequence on the main study question.

Recall bias regarding correct reporting of first injury was also likely to be present during the follow up period. Also, the grading of severity of the two injuries is not homogeneous, so, for example, a grade II ankle injury is not comparable, in terms of severity and recovery time, to a grade II hamstring injury. Another potential confounder was the heterogeneity of the study population. Although all included subjects were elite track and field athletes, they were divided in different sport disciplines with different load demands to the lower extremities.