August 2016

Foot strike while running and patellofemoral pain

8runner-iStock_49375282-copyRecent research supports the idea that increased patellofemoral stress is a contributing factor in runners with patello­femoral pain, and that gait retraining to promote a forefoot strike pattern is associated with reductions in patellofemoral stress and self-reported pain.

By Jenevieve L. Roper, PhD; Deborah L. Doerfler, PT, DPT, PhD; Christine M. Mermier, PhD; and Janet S. Dufek, PhD

Running is a popular mode of exercise, with more than 17 million people participating in running events each year1 and even more participating recreationally. This number is significantly higher compared with the 1990s (4.6 million) and is likely due at least in part to numerous studies reporting significant health benefits from cardiovascular exercise.2-4

Alongside the increasing popularity of running events, clinicians have observed an increase in the number of associated running injuries. The most commonly reported running-related injury is patellofemoral pain (PFP),5 commonly called anterior knee pain. The cause of PFP appears to be multifactorial;6-8 however, it appears patellofemoral stress may play a key role in the severity and occurrence of PFP.9

Increased patellofemoral stress is believed to lead to pain due to greater patellofemoral joint contact force between the patella and femur.10 It is affected by several factors, including quadriceps force,11,12 knee abduction angle,13,14 and hip internal or external rotation angle.15 Additionally, stride characteristics during gait are known to contribute to PFP;16 therefore, alterations in specific characteristics are likely to affect the severity of PFP. Reducing step length by at least 10% was associated with a significant decrease in patellofemoral joint stress per step in healthy female runners,17 supporting the idea that altering running mechanics can help reduce joint specific forces.

Recreational runners with patellofemoral pain who transition to a forefoot strike pattern should be cognizant of a potential increase in ankle or Achilles injury risk.

Much of the research regarding PFP and gait retraining has focused on hip weakness and altered hip mechanics. Studies suggest individuals with PFP demonstrated greater peak hip adduction and hip internal rotation range of motion than healthy individuals,18,19 and therefore researchers have focused on hip mechanics for gait retraining.18,20 Studies have found a reduction in knee pain, but researchers did not measure patellofemoral stress. Therefore, it’s possible that patellofemoral stress did not change, which could have left runners susceptible to recurring PFP.

Foot strike patterns

8Runner-iStock_19347954-copyThe influence of foot strike patterns on PFP and patellofemoral stress has only recently been studied.14,21 The majority of runners naturally use a rearfoot strike (RFS) pattern.22 Studies suggest these runners are up to 3.4 times more likely to sustain an injury compared with runners who use a forefoot strike (FFS) pattern.23,24 Therefore, identifying runners with PFP who are also rearfoot strikers may be a key component in PFP management.

Several measures can be used to determine foot strike, including location of the center of pressure at initial contact,25 foot strike angle,26 and measurement of heel and metatarsal acceleration peaks.27 Classically, foot strike pattern is classified as RFS, midfoot strike (MFS), or  FFS. However, because MFS and FFS patterns have similar gait characteristics, they are often grouped together and collectively termed FFS. We have adopted that convention in this article.

Kinematically, there are several differences between FFS and RFS running. The greatest difference is seen at the ankle, where there is plantar flexion at initial contact with a FFS pattern compared with dorsiflexion at initial contact with a RFS pattern.14,28-30 Research to date on foot strike pattern and knee range of motion (ROM) has been equivocal. Some investigators have found sagittal plane knee ROM differs significantly between foot strike patterns,29 while others have found no significant difference.30 Only recently has it been determined that there is a significant difference in knee abduction14,31 and knee flexion31 between RFS and FFS.

Researchers have demonstrated a significant difference between foot strike patterns for several key kinetic variables. Loading rate, or the rate at which forces are applied to the body, has been shown to be significantly lower in FFS runners than RFS runners,14,32,33 and studies have found associations between elevated loading rates and injury risk in runners.34 Additionally, RFS patterns are associated with greater shock attenuation than FFS patterns35—an indication that the magnitude of shock absorbed by the foot is higher for rearfoot strikers than forefoot strikers; the lower magnitudes of shock associated with FFS running may be due to this strike pattern’s shorter stride length.36,37 When measuring impact peaks in the different directions (anterior-posterior, mediolateral, and vertical), it was determined that FFS running has a lower vertical impact peak than RFS running, but greater anterior-posterior and mediolateral impact peaks,38 suggesting the loading rate in the vertical direction is reduced at the expense of increasing loading rate in the other two directions.

Patellofemoral contact force and patellofemoral stress also have been found to be significantly reduced with FFS running.14,21 Kulmala et al found that patellofemoral contact force and patello­femoral stress were significantly lower in 19 female forefoot strikers than in 19 female rearfoot strikers.14 Vannata et al reported that, in 16 habitual rearfoot strike runners, peak patellofemoral joint stress and stress-time integral over the stance phase of gait decreased significantly when those runners used a FFS pattern.21 However, both studies involved healthy participants, not runners with PFP.

We felt it would be clinically useful to retrain runners with PFP to transition from a RFS pattern to a FFS pattern, as this could potentially reduce patellofemoral stress and other biomechanical variables that could contribute to or exacerbate PFP. We found the most convenient way to retrain runners was to use a mirror for visual feedback during the retraining period, as most recreational runners would not have access to expensive real-time feedback equipment often used in gait labs.

Our research

To better understand if a FFS pattern was better than a RFS pattern for runners with PFP, we performed a study to examine the effects of changing foot strike pattern on knee abduction angles at initial contact, patellofemoral contact force, and patellofemoral stress, in addition to pain severity and occurrence.31 A physical therapist screened male and female recreational runners (habitual rearfoot strikers) for PFP. They assigned 16 to either an experimental group, which completed eight gait retraining sessions over two weeks, or a control group, which completed normal running sessions during the same time period. All the runners wore the same neutral conventional running shoe for an initial baseline running trial, a post-retraining running trial, and one-month follow-up trial. At the end of the training period all runners consistently used a FFS pattern. Biomechanical variables were recorded during the overground portion of the running trial.

We found that retraining from RFS running to FFS running was associated with a significant reduction in running-related knee pain reported by the participants from pre-retraining to the one-month follow-up (p = .02). Additionally, there was a significant increase in knee flexion at initial contact (p = .01), knee abduction at initial contact (p = .02), and ankle flexion at initial contact (p < .001), as well as a significant increase in ankle range of motion throughout ground contact (p < .001) from pre-retraining to one-month follow-up. We also found a significant reduction in patellofemoral contact force (p = .02) and patellofemoral stress (p = .02), which are previously reported to be factors in PFP.9

Based on our findings, we concluded that retraining from RFS to FFS significantly reduced PFP and therefore, could be an effective self-management strategy for recreational runners experiencing PFP.

Previous gait retraining studies in runners with PFP did not change foot strike pattern, but instead focused on gluteal and hip mechanics.18,21,39-41 However, a subsequent systematic review and meta-analysis found a discrepancy between prospective and cross-sectional studies regarding the role of hip strength and mechanics in PFP.42 Although all studies reported retraining was associated with a reduction in self-reported knee pain, it is possible the reported pain reduction was a result of changes in knee mechanics—not hip mechanics—driven by retraining.

At initial contact, there is typically an impact transient present with RFS running, which is absent during FFS running. The absence of the impact transient in FFS running likely reduces the amount of patellofemoral contact force and patellofemoral stress that is seen at initial contact, as well as the loading rate,34 which has been associated with injury risk. Additionally, forefoot striking is associated with greater stride frequency than rearfoot striking as well as a shorter stride length, which reduces shock attenuation.36,37

One observation we noted in our study was an increase in Achilles tendon force in the intervention group as a result of retraining, though it was not statistically significant. Previous studies have found greater Achilles tendon force with FFS.14 Despite the lack of significance in our study it should be mentioned, as this is an indicator of increased mechanical work at the ankle, which could make runners susceptible to ankle injuries. Therefore, a slow transition from RFS to FFS is recommended to reduce that risk. However, if a runner has a history of Achilles tendon injury or other ankle injuries, transitioning to a FFS pattern may exacerbate the symptoms, lead to reinjury, or both, and is therefore not recommended.

Additionally, we measured running economy during the study. We hypothesized that transitioning to a new gait pattern could potentially increase metabolic cost during running, as runners learn to make technical and physiological adaptations to the novel pattern. A reduction in running economy could potentially impact performance in running events. However, our results (unpublished data, 2015) determined that there was no significant change in running economy as a result of retraining from a RFS pattern to a FFS pattern.

Collectively, our results suggest recreational runners affected by PFP should transition to a FFS pattern, but should be cognizant of a potential increase in ankle injury risk. Also, during the transition, runners may experience a reduction in performance resulting from decreased training and running economy. However, this reduction is likely temporary and will resolve as runners adjust to the modified gait pattern and return to their normal training regimen.

We also note the effects of footwear on our results. Because runners were habitually shod, we opted to use a neutral running shoe, which may not have the same effects on patellofemoral joint contact force and patellofemoral joint stress as a FFS pattern while barefoot. However, previous research has found the only kinematic or kinetic variable affected by footwear in FFS runners was stride length, which was shorter for barefoot running than shod running.43 Although that study did not include runners with PFP and did not assess patellofemoral joint contact force or patellofemoral joint stress, its findings suggest the effect of footwear on FFS mechanics is minimal. We acknowledge there may be a small effect, and that using minimalist shoes may produce the greatest benefits similar to barefoot FFS running. Further research is needed to determine the true difference between shod FFS and barefoot FFS running, particularly with respect to patellofemoral joint contact force and patellofemoral joint stress in runners with PFP.

Unresolved questions

Although our study has provided evidence that a FFS pattern is beneficial for runners affected by PFP, some questions still need to be answered. Our study included only one month of follow-up; therefore, we are unaware if any injuries, especially at the ankle, may have occurred as runners continued to use a FFS pattern. Furthermore, because we did not measure performance, we cannot be certain whether performance will be impacted as a result of retraining.

Also, our study population consisted of recreational runners. Therefore, we are unsure whether the same results would be seen in more competitive runners with higher levels of training. Caution should be used when trying to apply our results to other running populations.

Conclusion

Although some investigations have suggested hip weakness is a likely contributor to PFP, more recent studies42,44 indicate the role of hip strength is still unclear. Recent research supports the idea that increased patellofemoral stress is a factor in PFP, and that gait retraining to promote a FFS pattern is associated with reductions in patellofemoral stress and self-reported pain.

Further studies are needed to determine whether injuries occur with use of a FFS pattern for more than one month after retraining. Furthermore, studies have yet to assess whether running performance is affected as a result of retraining, which could be a major determining factor in whether runners decide to change foot strike pattern.

Although we are still trying to understand the cause—or causes—of PFP, the data do suggest that patellofemoral stress plays a major role. More research is needed to develop clinical tests clinicians can use to screen for increased patellofemoral stress, which may help with early identification of this pathology and potentially reduce the occurrence and severity of the pain experienced by runners.

Jenevieve L. Roper, PhD, is an assistant professor of kinesiology at California State University, San Bernardino. Deborah L. Doerfler, PT, DPT, PhD, is an assistant professor of physical therapy at the University of New Mexico Health Sciences Center in Albuquerque. Christine M. Mermier, PhD, is an assistant professor of exercise science at the University of New Mexico in Albuquerque. Janet S. Dufek, PhD, is a professor in the Department of Kinesiology and Nutrition Sciences at the University of Nevada, Las Vegas.

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