Differences in muscle activity between natural forefoot and rearfoot strikers during running
Introduction
Running is a popular activity with annual injury rates as high as 56% among long distance runners (Van Mechelen, 1992). Runners who experience overuse injuries are occasionally advised to transition from a rearfoot striking (RFS) to a forefoot striking (FFS) running pattern because FFS is thought to reduce the chance of injury. One retrospective study found that, compared with RFS, certain types of injury rates are reduced in FFS runners (Daoud et al., 2012). More research is needed to identify and interpret the differences between foot strike patterns before recommending an optimal running style.
FFS runners and RFS runners have different vertical ground reaction profiles. RFS runners show an impact peak and a higher loading rate after foot contact, whereas FFS runners often demonstrate no initial impact peak and a lower loading rate (Cavanagh and Lafortune, 1980, Laughton and Davis, 2003, Lieberman et al., 2010). Since muscle forces are largely responsible for generating ground reaction forces during running (Hamner et al., 2010), it is likely that changes in muscle activity play a role in the differences in ground reaction forces between foot strike patterns. For example, Schmitz et al. (2014) reported that increasing hip flexor activity during RFS running can decrease the loading rate.
RFS runners have a dorsiflexed ankle during terminal swing phase (Arendse et al., 2004) and early stance phase (Lieberman et al., 2010), whereas FFS runners keep their ankles in a more neutral position during late swing phase (Arendse et al., 2004) and land with a plantarflexed ankle (Lieberman et al., 2010). These differences may be related to the larger ankle plantarflexion moments measured in FFS runners during early stance (Rooney and Derrick, 2013) and greater peak ankle plantarflexion moments and stance phase Achilles tendon forces (Kulmala et al., 2013). Additionally, FFS runners land with a more flexed knee (Arendse et al., 2004, Laughton and Davis, 2003, Lieberman et al., 2010) compared to RFS runners. Although sagittal plane kinematics can be replicated by a RFS runner running with a FFS pattern (Rooney and Derrick, 2013, Stearne et al., 2014), natural RFS runners running with a FFS pattern have longer stride lengths compared to natural FFS runners (Shih et al., 2013), a reduced peak ankle plantarflexion moment (Williams et al., 2000), and increased peak ankle external rotation moment during stance (Stearne et al., 2014). It is therefore important to identify differences between natural FFS and natural RFS runners.
Muscle forces affect foot position and limb kinematics during swing phase (Piazza and Delp, 1996, Schmitz et al., 2014); thus, it is important to understand the relationship between swing phase kinematics and muscle activity. Muscle activities during running have been examined to study the effects of speed (e.g. Gazendam and Hof, 2007) and gait modifications (e.g. Giandolini et al., 2013) on muscle activity. Muscle activities have also been recorded to evaluate muscle function during running (Bartlett et al., 2014, Modica and Kram, 2005, Novacheck, 1998), test the accuracy of running simulations (Hamner et al., 2010), and estimate muscle fiber lengths and velocities (Arnold et al., 2013). Studies have reported differences in muscle activity when RFS runners ran with both their natural RFS pattern and a FFS pattern (Olin and Gutierrez, 2013, Shih et al., 2013). Just prior to foot contact, activity of the tibialis anterior was found to be greater when RFS runners ran with their natural RFS pattern, compared to a FFS pattern. When these same runners ran with a FFS pattern, the gastrocnemius had greater activity compared to the runners׳ natural RFS pattern (Shih et al., 2013). During stance phase, Olin and Gutierrez (2013) reported that RFS runners using their natural pattern had greater average and peak activity in the tibialis anterior, and greater average activity in the medial gastrocnemius when these natural RFS runners used a FFS pattern. It is unknown if the same differences in muscle activity exist between natural RFS runners and natural FFS runners because muscle activities in natural FFS runners have not yet been reported.
The goal of this study was to identify how muscle activities differ between runners with a natural RFS pattern and runners with a natural FFS pattern. Since FFS runners tend to run with a more plantarflexed ankle around the time of foot contact, we hypothesized that FFS runners would show significantly lower average muscle activity in the tibialis anterior during both the end of swing phase and early stance phase. The larger peak plantarflexion moments generated by FFS runners (Kulmala et al., 2013) led us to test the hypothesis that the soleus and gastrocnemius would have significantly higher average activity in FFS runners during late swing and early stance phases.
Section snippets
Subjects
Twelve natural RFS runners (age: 27.9±5.2 years; height: 171±11 cm; weight: 63.8±11.0 kg) and ten natural FFS runners (age: 29.0±6.3 years; height: 176±6 cm; weight: 64.9±7.6 kg) participated in this study. Foot strike type was confirmed after the data collection, as described below. All runners were healthy, experienced long distance runners, who reported running a minimum of 25 km/week. Each subject gave informed consent prior to participation according to a protocol approved by the Stanford
Results
We found significant differences between natural FFS and RFS runners in muscle activity of the ankle plantarflexors and dorsiflexors during late swing phase (Fig. 2). The tibialis anterior had lower RMS muscle activity during late swing phase in FFS runners compared to RFS runners (p=0.001; Table 1); no significant difference was detected during early stance phase (p=0.590; Table 2).
The medial and lateral gastrocnemius had higher muscle activity during late swing phase in FFS runners, compared
Discussion
Our goal was to identify differences in muscle activity between natural RFS and natural FFS runners. Because FFS runners tend to land with greater ankle plantarflexion, we hypothesized that FFS runners would show significantly lower tibialis anterior activity than RFS runners during the terminal swing and early stance phases. In partial support of this hypothesis, FFS runners ran with less tibialis anterior activity during the terminal swing phase compared to RFS runners; however, RMS activity
Conflict of interest statement
None of the authors had any financial or personal conflict of interest with regard to this study.
Acknowledgments
The authors thank Samuel Hamner, Edith Arnold, and the Stanford University Human Performance Lab. This work was supported by a Stanford Bio-X Graduate Student Fellowship and NIH Grants U54 GM072970, R24 HD065690.
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