Are knee mechanics during early stance related to tibial stress fracture in runners?
Introduction
Tibial stress fractures are a serious overuse injury to runners that can lead to significant loss of training time. This bony injury typically requires up to 6–12 weeks of functional rehabilitation for full recovery (Harmon, 2003, Tuan et al., 2004). Tibial stress fracture is typically the most common stress fracture in runners, accounting for 26–45% of stress fractures (Bennell et al., 1996, Brukner et al., 1996). Recent evidence from a comparison of runners with and without previous tibial stress fracture suggests a predictive relationship between high tibial shock and tibial stress fracture (Milner et al., 2006). Additionally, it has been suggested that loading rate is important in the occurrence of fatigue fracture of long bones. In particular, repeated loading at higher physiological loading rates, such as those occurring during running, is more damaging than repeated loading at lower loading rates (Schaffler et al., 1989). Hennig et al. (1993) reported that tibial shock was related to vertical ground reaction force loading rates. In further support of this relationship, Milner et al. (2006) found higher vertical loading rates and higher tibial shock in runners with previous tibial stress fracture compared to controls.
There appears to be some controversy in the literature regarding the role of knee flexion on tibial shock and loading rates. In general, increased knee flexion excursion during landing reduces peak vertical forces that the lower extremity experiences by increasing the time over which the vertical velocity of the center of mass is brought to zero. Both DeVita and Skelly, 1992, McNitt-Gray et al., 1993, McNitt-Gray et al., 1994 reported reductions in peak loading when individuals produced “soft landings”. Softer landings were accomplished through increased knee flexion excursion, believed to be a shock attenuating mechanism. However, “Groucho running”, characterized by exaggerated knee flexion throughout stance, has been reported to be associated with an increase in tibial shock compared to normal running (McMahon et al., 1987). These authors also reported that lower extremity stiffness calculated from footstrike to peak knee flexion (McMahon and Cheng, 1990) was less during “Groucho running”. However, they did not provide information regarding knee joint excursion or knee joint stiffness during early stance when peak tibial shock occurs. It is quite possible that, while the knee was more flexed at landing, the excursion was reduced resulting in increased knee joint stiffness resulting in greater tibial shock.
Derrick (2004) reported that environmental perturbations (reduced light intensity, long grass, uneven surfaces) resulted in small increases (1–4°) in knee flexion angle at footstrike. These increases were associated with small increases (0–1.8 g) in tibial shock. The author interpreted this as providing support for an effective mass model, whereby as knee joint stiffness decreases, tibial shock increases. However, only the angle at footstrike was assessed and not excursion or knee joint stiffness. Thus, while shock increased with increased knee flexion at footstrike, this again might be due to a stiffer knee. In addition, to obtain an increased knee flexion posture at footstrike, one might reduce the stride length to bring the foot under the knee. This would be associated with a more vertical orientation of the shank. A more vertical shank would more closely align the long axis of the tibia with the large vertical component of the ground reaction force, thus potentially increasing the magnitude of tibial shock.
During the period of initial loading from footstrike to the vertical impact peak, vertical loading rates are at their highest. Peak tibial shock also occurs around this time. Therefore, lower extremity mechanics during initial loading may be related to peak tibial shock. If so, they may be associated with an increased risk of tibial stress fracture. Identification of these mechanics is the first step in the development of strategies to reduce tibial shock and, potentially, the risk of tibial stress fracture in runners. This is important clinically because once the relationship between tibial shock and running mechanics during initial loading is clear, gait retraining methods to alter running mechanics and reduce the risk of tibial stress fracture can be developed.
The purpose of this cross-sectional study was, therefore, to identify lower extremity biomechanics that may contribute to high tibial shock. In particular, the aim was to determine whether differences existed in initial loading mechanics between distance runners with a history of tibial stress fracture and those with no previous lower extremity bony injuries. We hypothesized that runners with a previous tibial stress fracture would have higher sagittal plane knee joint stiffness and lower knee flexion excursion during the initial loading phase and greater knee flexion and a more vertical shank at footstrike than runners who had not sustained a fracture. A second aim was to determine whether the magnitude of tibial shock was correlated with the variables of interest. Specifically, that there would be a positive correlation between knee stiffness and peak tibial shock and negative correlations between knee flexion excursion and tibial shock and shank angle at footstrike and tibial shock across the two groups.
Section snippets
Methods
Approval for all procedures was obtained from the Institution’s Human Subjects Review Board prior to commencing this study. All participants gave their written informed consent prior to participating. Female runners aged between 18 and 45 years and running at least 32 km per week on average were recruited from the local running population. Subjects were excluded if they had any current injuries or had not yet returned to at least 50% of their pre-injury mileage, had a history of cardiovascular
Results
The comparison of variables of interest is presented in Table 1. Tibial shock for each group is also presented. As expected, runners in the tibial stress fracture group had significantly higher knee stiffness, associated with a moderate effect size, during initial loading compared with the control group. However, knee flexion excursion, knee flexion and shank angle at footstrike were not significantly different between the groups (Fig. 2). The small effect sizes for these variables further
Discussion
The purpose of this study was twofold. The first was to compare lower extremity mechanics during initial loading in runners with a history of tibial stress fractures to a group without history of any lower extremity stress fractures. Results suggest that the tibial stress fracture group runs with a stiffer knee. Stiffness is determined by the changes in knee excursion and knee moment. Knee flexion excursion was lower in the tibial stress fracture group, although not significantly so, but the
Conclusions
In conclusion, sagittal plane knee joint stiffness during initial loading was greater in runners with a previous tibial stress fracture compared to runners with no previous lower extremity bony injuries. These findings, along with those reported previously (Milner et al., 2006), provide support for the notion that understanding lower extremity mechanics during initial loading is critical to the understanding of tibial stress fracture in runners.
Acknowledgement
This study was supported by Department of Defense grant DAMD17-00-1-0515.
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