Do athletes alter their running mechanics after an Achilles tendon rupture?

The Achilles tendon (AT) played a crucial role in the evolution of early humans enabling them to run faster [1]. The AT’s important role is to provide sufficient plantar flexor power in running activities. Over the past thirty years, there has been dramatic increase in incidence of AT rupture (from 2 to 22 per 100,000 person-years) primarily in the athletic population [2, 3]. Despite all medical efforts, athletes with history of AT rupture have been shown to have a substantially decreased performance in sports with running and jumping activities [4, 5]. Although up to 30% of these athletes end their sporting career after rupturing their AT, many manage to return to a physically active lifestyle [4, 5].

Evidence has been reported that individuals with a history of AT rupture have decreased ankle joint proprioception, decreased plantar flexor muscle volume, increased AT length and affected AT stiffness [6‐8]. The changes in mechanical, anatomical and/or neuromuscular properties of the triceps surae lead to Achilles tendon weakness in a plantar flexed position [9], an increase plantar flexor muscle activity during locomotion [10] and reduced plantar flexor endurance even several years after rupture [11]. In addition, athletes with a previous AT rupture were 176 times more likely to suffer a contralateral Achilles tendon rupture compared to an individual without a previous AT rupture [12]. In this study, the authors hypothesized that the increased injury risk is a result of a genetic predisposition or degeneration and atrophy of the contralateral tendon. However, they did not consider the possibility of mechanical overload as a possible cause of injury.

There have been a number of published studies in the literature that reported biomechanical deficiencies in individual subsequent to an AT injury. Using the Achilles tendon total rupture score, it was reported that 50 % of individuals with a history of AT rupture suffered from post-injury problems (decreased strength, fatigue, stiffness or pain in the calf) and functional declines during physical activity at long-term follow-up [12, 13]. In a previous a case study, it was reported that a four-year running training program did not reduce the biomechanical consequences of the Achilles tendon rupture such as loading behavior and position of the ankle during the stance phase of running [14, 15]. In their case study of shod running, Jandacka et al. [14] suggested an association between AT elongation and increased dorsiflexion during initial contact of stance phase and reduced plantarflexion on the AT affected limb during toe off [14]. Only one study investigated the biomechanics of walking, light jogging and hopping of AT ruptured participants [16]. They found increased knee joint loads on the affected side during the jogging and hopping when comparing the affected and unaffected sides. Willy et al. [16] concluded that patients after an AT rupture may be at a greater risk of overuse injuries to the patellofemoral joint and knee extensors during light running. However, to our knowledge, there have been no studies that compared differences between AT ruptured athletes and healthy control group.

Therefore, the purpose of this study was to compare the lower extremity mechanics of AT ruptured runners with healthy controls (CTRL). Based on previous case reports and side to side comparisons [14‐16] we hypothesized that, compared to the healthy control group the Achilles tendon group would have: 1) increased dorsiflexion at initial contact (IC); 2) reduced ankle angle range of motion on the AT affected limb during the second half of the stance phase; and 3) reduced maximum ankle and increased knee joint moments.

The AT group reported significantly greater side-to-side differences in AT length and the Silfverskiöld test indicated an elongated gastro-soleus complex. Further analysis showed that eight participants from the AT group had an elongated Achilles tendon (Table 1). In addition, the AT group reported significantly lower plantarflexion strength and lower circumference of the shank on affected side (Table 1). There was no maximal isometric plantarflexion strength difference between the groups on unaffected side (P ≥ 0.05; ES ≤ 0.5). Additionally, the AT group reported lower self-reported outcome (ATRS 73/100) indicates some limitation/difficulty with various activities including running.

During the initial contact of the stance phase, the affected lower extremity of the AT group exhibited lesser knee flexion by 5.2°, 95% CI [0.5, 10.3] compared to the matched lower extremity of the CTRL group (Fig. 2; P ≤ 0.05; ES ≥ 0.5). There was a statistically significant greater range of motion at the knee of the AT group on their affected limb by 4.4°, 95% CI [0.4, 8.8] during the weight acceptance phase (Table 2; P ≤ 0.05; ES ≥ 0.5). In addition, there was a less range of motion for the ankles of AT group (ES ≥ 0.5), however, it was significantly less (7.6 °, 95% CI [0.6, 15.7]) only on the affected limb during the push-off phase (Table 2; P ≤ 0.05; ES ≥ 0.5).

There was a greater maximal net hip extension moment on the unaffected lower extremity of the AT group by 0.58 Nm/kg, 95% CI [−0.1, 1.3] compared to the matched lower extremity of the CTRL group (P ≤ 0.05; ES ≥ 0.5). Additionally, there was greater change in the net hip moment of the AT group for both lower extremities compared to the healthy control group during whole stance phase (Table 3, ES ≥ 0.5). There was a shorter stance time of the AT group by 8 ms, 95% CI [0.0, 20.0] compared to the control group (P ≤ 0.05; ES ≥ 0.5). Additionally, there was no maximal net ankle, knee or hip power difference between the groups (P ≥ 0.05; ES ≤ 0.5). The AT group exhibited greater asymmetry than the healthy control group for the ankle angle RoM from MID to TO, knee angle at IC, hip angle at IC, hip angle RoM from IC to MID and time to VGRF max (P ≤ 0.05) (see Table 3). Side to side asymmetries for all kinetic dependent measures exhibited no statistically significant differences between groups (P ≥ 0.05).

The purpose of this study was to compare lower extremity mechanics of both the affected and unaffected limbs of AT ruptured runners with healthy controls. We found that participants with a previous AT rupture, compared to the healthy control group, did have: 1) a reduced ankle angle range of motion on the affected limb during push-off of the stance phase; 2) a reduced knee angle at initial contact and a greater knee range of motion.

Similar to previous reports [31], we found that participants from the AT group had an elongated Achilles tendon and an affected gastro-soleus complex and lower plantarflexion strength on affected side. We can speculate that an imbalance of the triceps surae tendon-muscular complex may influence the sagittal plane mechanics of the knee and ankle. To ensure sufficient tension of the elongated triceps surae complex during the initial ground contact, we can suggest two possible compensation mechanisms: 1) increased ankle dorsiflexion or 2) reduced knee flexion. Based on the results of this study, it appears that reduced knee flexion might be the preferred movement strategy used by the nine of the 11 AT individuals compared to the healthy controls (see Fig. 2). Although it was described in the case study of an athlete with a history of AT rupture [14], it appears that increased dorsiflexion is not preferred compensation mechanism in the AT individuals in this study. In the case of increased dorsiflexion, increased maximal vertical ground reaction forces during loading phase could occur. However, in this study there were no differences in the maximal vertical ground reaction force components of the AT group with reduced knee flexion.

Although compensation throughout reduced knee flexion might be advantageous to avoid a footfall pattern with a greater vertical ground reaction force component, this might not be a strategy without consequences [14]. Cooper and colleagues [32] suggested that knee hyperextension may provide a mechanism to control an unstable limb during the stance period of the gait cycle. However, hyperextension may place the knee at risk for injury of the capsular and ligamentous structures of the posterior aspect of the knee [32]. The gastrocnemius plays important role as stabilizer against overextension of the knee and anterior knee laxity [32, 33]. Overextension generally indicates the presence of an abnormal extension pattern following initial contact rather than the knee flexion pattern typical in healthy subjects [32]. The findings of the present study suggest that the AT repair results in an adaptation of the knee kinematics (i.e. reduced knee flexion). Whilst participants did not demonstrate hyperextension, there was less knee flexion during initial contact and thus a relatively extended knee that may predispose the runner to other injuries. An overextended knee might be a significant risk factor for an anterior cruciate ligament rupture particularly when landing from a jump or when transitioning from running to a cutting maneuver [34, 35]. However, this current research did not provide evidence that the participants with the AT repair demonstrated reduced knee flexion during landings or cutting maneuvers and this would need further evaluation.

The current study found that there was a 22% greater maximal hip joint moment on contralateral side of the AT group compared to the healthy controls. However, the 95% CI [−0.1, 1.3] suggests that difference 0.58 Nm/kg might not be significant. Increased joint moments on contralateral lower extremity may theoretically indicate increased load on contralateral side compared to the healthy controls. A study by Årøen et al. [12] found increased risk of contralateral AT rupture in AT ruptured population. In this study, Prilutsky et al. [28] demonstrated how proximal muscles transfer power to distal muscles via two joint muscles. Taking into account the two joint muscle transfer of power from hip to ankle via the gastrocnemius, we can speculate that an increased load on the AT of the contralateral side of the affected group would result [36]. However, with a non-significant increase in the ankle joint moments on contralateral side of AT group (0.23 Nm/kg; ES = 0.4), it seems that, instead of AT loading, there may be a strategy of increased contralateral hip loading during running of individuals with a history of AT rupture. This strategy of increased hip moments might be useful during low intensity activities. In this line, less ankle range of motion may actually lead to reduced affected limb ankle plantar flexion strength since this occurs during push off. A greater change in the net hip moment of the AT group compared to the healthy control group during the whole stance phase may be explained by reduced ankle plantar flexion strength on affected lower extremity.

The greater asymmetry of the AT group supports the results of the kinematics comparison between the AT affected and matched control limbs. The most interesting finding was that AT group exhibited higher asymmetry in the knee angle during initial contact. The present findings seem to be consistent with prospective studies that reported that previous injury is most the reported risk factor for running related injuries [37]. Nine of eleven AT individuals exhibited relative increased knee extension on injured side. In contrast, the healthy control group exhibited less asymmetry. It would seem that higher knee kinematic asymmetry of AT group may be related to a risk factor for injury of musculoskeletal system such as overextension.

This study has several imitations. First, the present study was designed as cross-sectional study and, as a result, we cannot determine if the findings of this study were the ‘cause’or the ‘result’ of the injury. A second limitation of this study concerns the Achilles tendon surgery performed on the individuals in the AT group. These individuals had their surgery performed by different surgeons who may have used different surgical techniques. A third limitation of this study concerns the length of time between the surgery and testing the participants. It is possible that the resulting altered running mechanics may have been an accommodation of the participants developing a reasonable strategy over the six years’ post-surgery to minimize the risk of injury.

This study indicated that the individuals with history of AT rupture have reduced ankle range of motion during push-off phase of stance, reduced knee flexion during initial contact and an increased knee range of motion during the weight acceptance phase of stance on their affected limb compared to the healthy control group. These results suggest a compensation mechanism, overextending the knee at initial ground contact, against the deficit in the muscle-tendon complex of the triceps surae (i.e. elongation of AT). However, this compensation during sporting activities may place the knee at risk for further injury. Avoidance of AT lengthening and plantar-flexors structural and strength deficit after surgery or during the subsequent rehabilitation might help to manage AT rupture since that seems to be possible reasons for altered running kinematics. The results of this study indicate that individuals who have had an AT rupture should re-consider their participation in sports with high risk of AT injury and/or knee injury. In future studies, assessing the effect of AT rupture on the mechanics of the lower extremities during higher intensity movements than those presented in this study would be insightful.