Joint contact loading in forefoot and rearfoot strike patterns during running
Introduction
Running is associated with improved cardiovascular health but also increased risk of injury to the lower limbs. The incidence of lower extremity injury from running has been reported to range from 20% to 79% (van Gent et al., 2007). Many factors have been identified for increased risk of running injury, ranging from innate factors such as age and gender to modifiable factors such as footwear and running mileage. An additional risk factor may be foot strike pattern. Foot strike pattern is defined by Cavanagh and Lafortune (1980) as the point of initial contact of the foot with the supporting surface. A foot strike in the posterior third is considered a rearfoot strike (RFS), in the middle third it is considered a midfoot strike and in the anterior third it is a forefoot strike.
RFS patterns were the dominant running style seen in a total of 283 runners at the 15 km point of an elite level half marathon race (Hasegawa et al., 2007). Nearly 75% were categorized as RFS with most of the remaining runners utilizing a midfoot strike and less than 1.5% with a forefoot strike pattern. However, there is a growing trend to run in minimalist footwear or even barefoot. These trends have been encouraged by anthropological studies such as Lieberman et al. (2010), popular books such as “Born to Run” by Christopher McDougall and the promotion of running techniques such as Chi running, Pose running and Good Form running and the advent of minimalist footwear that provide minimal control of the foot.
The minimalist approach does not provide the heel cushion of a typical running shoe therefore, runners are encouraged to run with a midfoot or forefoot strike pattern in order to distribute the impact force over a greater area and allow greater absorption of the impact by the plantar flexor muscles (De Wit et al., 2000, Lieberman et al., 2010, Divert et al., 2003). Cavanagh and Lafortune (1980) described the midfoot strike as a distribution of the pressure between the heel and metatarsal–phalangeal joint creating a center of pressure near the middle of the foot. De Wit et al. (2000) also observed an effect on the kinematics of the knee and ankle during barefoot running. The knee was more flexed and the ankle was more plantar flexed in preparation for foot contact. This kinematic change places the body in a more compliant position that can better absorb the impact.
The kinetics of RFS and FFS running has been characterized but their association with injury is not well understood. RFS running has a large vertical ground reaction force (GRF) impact peak (Cavanagh and Lafortune, 1980, Lieberman et al., 2010). FFS running is characterized by an attenuated or absent impact peak, but an increased active peak (Laughton et al., 2003, Lieberman et al., 2010). Even though the impact peak has often been measured, its relationship to injury has not been established as fact. Scott and Winter (1990) suggested that impact peaks and the ground reaction forces in general are not as important as the forces acting at the site of chronic running injuries. The internal loads that damage the biological tissues typically have much greater magnitudes than the external ground reaction forces. While GRFs reach magnitudes of approximately 2.5 times body weight during running, the joint contact forces are estimated to reach 8–15 body weights (Edwards et al., 2008, Sasimontonkul et al., 2007). Therefore, external GRFs are only a surrogate measure for the internal loading and will underestimate the actual loads experienced by the body. The internal loads take into account the reaction forces as well as the muscle forces and muscle moment arms. For example, a typical peak plantar flexor moment of 0.30 body weights ⁎ meter (BWm) and a moment arm of 0.05 m for the triceps surae muscle group will produce a compressive force of 6 BWs at the ankle (Stief et al., 2008, Maganaris et al., 2000). Assuming minimal ligament, joint capsule and friction forces, the internal joint load is composed of the vector sum of the joint reaction forces (JRF) and the muscle forces acting across the joint (Sasimontonkul et al., 2007, Winter, 2009). Therefore, a typical joint reaction force of 2.5 BWs and muscle forces of 6 BWs, would produce a total compressive load at the ankle of approximately 8.5 BWs. Although these internal loads are difficult to measure without using invasive techniques, there has been success using musculoskeletal modeling to estimate the loading using optimization or reductionist algorithms (Sasimontonkul et al., 2007, Scott and Winter, 1990, Glitsch and Baumann, 1997, Edwards et al., 2008). As part of this process the internal torques are balanced with the torques caused by the external forces and then used to estimate the muscle forces. These estimations have shown good agreement with measured forces when instrumented implants have been compared to modeled results in normal subjects (within 0.05–0.45 BW in walking knee forces, Lundberg et al., 2012).
There has yet to be a study investigating how strike patterns affect the internal loading of the lower extremity. Therefore, the main purpose of this study was to use musculoskeletal modeling and optimization techniques to compare the internal joint loading at the ankle, knee and hip of habitual RFS runners and habitual FFS runners. A secondary purpose was to determine if converted strike pattern runners adequately represent the internal joint loading of habitual runners. The FFS is expected to have greater internal joint loading at the ankle because of the increased activation of the gastrocnemius and soleus. However, the internal joint loading at the knee and hip are expected to be greater for the RFS pattern due to greater impact energy absorption demands. With respect to internal joint loading, we also hypothesize that the converted runners will provide an adequate model for habitual FFS and RFS running.
Section snippets
Subjects
Fifteen forefoot and 15 rearfoot strike runners were recruited from a population of competitive runners. They were required to be free from injuries that could affect their mechanics during running. The study was approved by the Iowa State Institutional Review Board, and upon arrival all participants read and signed the informed consent.
Protocol
Participants were provided with the same brand and model of running shoes. Anthropometrics were measured and recorded for each participant. These included body
Results
The running velocities of the FFS and RFS groups were 4.36 (0.23) and 4.25 (0.26) m/s respectively (Table 1). The groups were not significantly different in age, height, body mass, mileage or running speed. The HSI for habitual and converted FFS conditions were 63.3 (7.1)% and 69.1 (6.1)%; there was not a significant difference (p>0.05). The habitual and converted RFS conditions had similar HSI at 22.9 (2.0) % and 20.7 (4.4) % (p>0.05).
Hip, knee and ankle sagittal and frontal plane joint
Discussion
There has been little research comparing the internal joint loading of FFS and RFS runners. With trends toward minimalist running, there is an increasing need for an understanding of the potential for injury in these different running styles. The purpose of this study was to compare internal joint loading in RFS and FFS running. We were also interested in the ability of runners to convert from one strike pattern to another. A sample of competitive runners was used for this study because of the
Conflict of interest
There was no conflict of interest involved with this study. There was no funding source. All of the work was done as a part of a Master's thesis project.
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