The purpose of this study was to compare lower extremity mechanics that occur during running with a RFS pattern compared to those that occur when running with a FFS pattern and those occurring in a BF condition. In general, FFS and BF conditions demonstrated similar mechanical changes when ‐compared to the shod RFS condition. Specifically, BF and FFS runners demonstrated increased plantarflexion at initial contact, increased peak ankle power absorption and decreased peak knee and hip power absorption. This is consistent with what has been previously shown in FFS runners and BF runners.17,18
In both FFS and BF running the forces at initial contact are transmitted through the comparably smaller midfoot bones and muscles rather than through the calcaneus, talus and tibia directly. While a structurally sound foot may be able to absorb these forces effectively, it is likely that different foot types may respond differently to these increased forces to the forefoot. Foot type was not assessed in the current study so it is unclear which specific foot types would be more vulnerable. The difference between FFS and RFS ankle angle at initial contact was much greater than the difference between BF and RFS. This results in a much greater shortening of the gastrocnemius and soleus, which may require the muscle to work harder due to the compromised length tension relationship.19
Additionally, because of the eccentric to concentric transition that occurs at midstance,20
the muscles of the calf may be further stressed during midstance.
There was no increase in knee flexion angle at initial contact in the BF or FFS conditions. While the values in the current study are lower than previously reported,18
they are consistent with the findings presented in recent reports that examined BF runners.17
If the ankle is in more plantarflexion at initial contact, the knee would be in more flexion in order to establish the strike position closer to the projection of the center of mass (COM). If runners are not habitual FFS or BF runners, they may not change the knee angle, resulting in a strike further anterior to the COM projection. Manipulation of strike position relative to the COM independent of strike pattern (FFS versus RFS) may help clarify if strike pattern or strike position is the more important factor in changes in lower extremity mechanics. For example, increasing stride frequency decreases anterior strike position and increases knee position at initial contact.21
These changes are present without a change in foot strike pattern. Finally, the decrease in the passive tension in the gastrocnemius may result in more extension of the knee at initial contact in inexperienced FFS or BF runners. Modification of the complex interactions of stride frequency, stride length, foot strike pattern, and lower extremity mechanics as they relate to running performance and injury is not yet fully understood. Simply instructing runners to “run on your toes” or “on the ball of your foot” may not result in the desired strike pattern and perhaps place some individuals at risk for injury if the change in strike pattern is incomplete or incompatible with the runner's lower extremity structure.
Interestingly, the FFS and BF conditions did not result in changes compared to the RFS condition when considering hip angle at initial contact. While runners during the BF condition demonstrated a trend toward decreased hip flexion, these differences were not significant. Decreased hip flexion is consistent with modification of the position of the COM projection more posteriorly under the body. Because trained BF runners strike in less plantarflexion than FFS, they would be less likely to strike anteriorly. With no concurrent increase in knee angle at initial contact, the hip would need to remain in less flexion, bringing the COM posteriorly and allowing the forefoot to contact the ground. This may contribute to the concurrent increase reported in peak vertical ground reaction force in FFS.18
This increase in vertical GRF combined with the more vertical orientation of the lower extremity segments in FFS and BF running, there is potentially increased compressive forces compared to torsional forces in the ankle, knee and hip joints. Additionally, this may partially explain the reported decrease in the deceleration component of the posterior GRF.
The FFS pattern and BF conditions both reduced the peak knee extensor power. FFS demonstrated the greatest reduction in magnitude. These findings are consistent with what has been previously reported in research performed on forefoot strikers.17,18
Comparatively, the FFS conditions demonstrated the greatest reduction in knee extensor power. It is important to note that these changes in knee power occurred independent of changes in knee position at initial contact. Since there were no changes in knee position and similar magnitude of the vertical GRF, the differences in knee power suggest that the line of the vertical GRF may be passing closer to the knee joint center throughout the stance phase. Therefore, it may be important to evaluate contact forces in the joints of the lower extremities during BF and FFS running. Further, changes in the moment arm of the vertical GRF have implications for extensor demand and metabolic cost.22
Plantarflexion power (absorption) was significantly greater in FFS and BF conditions when compared to the shod RFS condition. FFS had the greatest increase in ankle power absorption. This is likely present due to the fact that running in the BF condition resulted in a more midfoot strike pattern that reduces the load on the plantarflexors. In fact, running BF resulted in an average strike index of 45.7% but only 60% of the runners in this condition actually adopted a midfoot or FFS pattern. This suggests that while BF running, on average, results in a different strike pattern, a number of runners still maintain a RFS pattern. Therefore, a switch to running BF may not sufficient to make comprehensive changes in the lower extremity mechanics in all runners. Running BF without subsequent changes in strike pattern is unlikely to result in reduction of injury or other benefits. In contrast, 100% of the subjects ran with a midfoot or FFS pattern in the FFS condition with an average strike index of 65.8%. It is important to note that none of the subjects in the current study were experienced or trained BF runners. Further study of whether trained BF runners demonstrate similar strike patterns as those observed in the current study will help to clarify the potential changes associated with BF running.
A midfoot strike pattern potentially places the perpendicular position of the vertical GRF further from the ankle joint center compared to FFS. While this may reduce the vertical impact through the long axis of the metatarsals, it is likely to increase the torsional forces imparted on the midfoot and forefoot as a result of changing the “gear ratio” as described by Braunstein et al.23
These torsional forces are most commonly directed toward dorsiflexion of the metatarsals on the cuboid and cuneiforms. The morphology of these plane joints help to establish stability in the midfoot. However, it is not known how these joints respond to repetitive dorsiflexion stress associated with midfoot and forefoot strike patterns. This may partially explain some of the recent evidence associating strike pattern and metatarsal stress fractures in BF runners.24