Recent 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.
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
The 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.
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.
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.
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.
- 2015 state of the sport – US road race trends. Running USA website. http://www.runningusa.org/state-of-sport-us-trends-2015-?returnTo=running-usa-blog. Published July 13, 2015. Accessed August 1, 2016.
- Dangardt FJ, McKenna WJ, Lusher T, Deanfield JE. Exercise: friend or foe? Nat Rev Cardiol 2013;10(9):495-507.
- Dhaliwal SS, Welborn TA, Howat PA. Recreational physical activity as in independent predictor of multivariate cardiovascular disease risk. PLoS One 2013;8(12):e83435.
- Kravitz L. The 25 most significant health benefits of physical activity and exercise. Idea Fitness J 2007;4(9):55-63.
- Taunton JE, Ryan MB, Clement DB, et al. A retrospective case-control analysis of 2002 running injuries. Br J Sports Med 2002;36(2):95-101.
- Davis IS, Powers C. Patellofemoral pain syndrome: proximal, distal, and local factors – an international research retreat. J Orthop Sports Phys Ther 2010;40(3):A1-A48.
- Lankhorst NE, Bierma-Zeinstra SM, Van Middlekoop M. Risk factors for patellofemoral pain syndrome: a systematic review. J Sports Orthop Sports Phys Ther 2012;42(2):81-94.
- Petersen W, Ellermann A, Gosele-Koppenburg A, et al. Patellofemoral pain syndrome. Knee Surg Sports Traumatol Arthrosc 2014;22(10):2264-2274.
- Farrokhi S, Keyak JH, Powers CM. Individuals with patellofemoral pain exhibit greater patellofemoral joint stress: a fine element analysis study. Osteoarthritis Cartilage 2011;19(3):287-294.
- Ho KY, Keyak JH, Powers CM. Comparison of patella bone strain between females with and without patellofemoral pain: a finite element analysis study. J Biomech 2014;47(1):230-236.
- Lenhart RL, Thelen DG, Wille CM, et al. Increasing running step rate reduces patellofemoral joint forces. Med Sci Sports Exerc 2014;46(3):557-564.
- Roos PE, Barton N, van Deursen RWM. Patellofemoral joint compression forces in backward and forward running. J Biomech 2012;45(9):1656-1660.
- Connolly KD, Ronsky Jl, Westover LM, et al. Differences in patellofemoral contact mechanics associated with patellofemoral pain syndrome. J Biomech 2009;42(16):2802-2807.
- Kulmala JP, Avela J, Pasanen K, Parkkari J. Forefoot strikers exhibit lower running-induced knee loading than rearfoot strikers. Med Sci Sports Exerc 2013;45(12):2306-2313.
- Souza RB, Draper CE, Fredericson M, Powers CM. Femur rotation and patellofemoral joint kinematics: a weight-bearing magnetic resonance imaging analysis. J Orthop Sports Phys Ther 2010;40(5):277-285.
- Heiderscheit BC, Chumanov ES, Michalski MP, et al. Effects of step rate manipulation on joint mechanics during running. Med Sci Sports Exerc 2011;43(2):296-302.
- Willson JD, Sharpee R, Meardon SA, Kernozek TW. Effects of step length on patellofemoral joint stress in female runners with and without patellofemoral pain. Clin Biomech 2014;29(3):243-247.
- Noehren B, Scholz J, Davis I. The effect of real-time gait retraining on hip kinematics, pain and function in subjects with patellofemoral pain syndrome. British J Sports Med 2010;45(9):691-696.
- Souza RB, Powers CM. Differences in hip kinematics, muscle strength, and muscle activation between subjects with and without patellofemoral pain. J Orthop Sports Phys Ther 2009;39(1):12-19.
- Willy RW, Scholz JP, Davis IS. Mirror gait retraining for the treatment of patellofemoral pain in female runners. Clin Biomech 2012;27(10):1045-1051.
- Vannatta CN, Kernozek TW. Patellofemoral joint stress during running with alterations in foot strike pattern. Med Sci Sports Exerc 2015;47(5):1001-1008.
- Hasegawa H, Yamauchi T, Kraemer WJ. Foot strike patterns of runners at the 15-km point during an elite-level half marathon. J Strength Cond Res 2007;21(3):888-893.
- Daoud AI, Geissler GJ, Wang F, et al. Foot strike and injury rates in endurance runners: a retrospective study. Med Sci Sports Exerc 2012;44(7):1325-1334.
- Goss DL, Goss MT. A review of mechanics and injury trends among various running styles. US Army Med Dep J 2012:62-71.
- Cavanagh PR, LaFortune MA. Ground reaction forces in distance running, J Biomech 1980;13(5):397-406.
- Altman AR, Davis IS. A kinematic method for foot strike pattern detection in barefoot and shod runners. Gait Posture 2012;35(2):298-300.
- Giandolini M, Poupard T, Gimenez P, et al. A simple field method to identify foot strike pattern during running. J Biomech 2014;47(7):1588-1593.
- Nunns M, House C, Fallowfield J, et al. Biomechanical characteristics of barefoot footstrike modalities. J Biomech 2013;46(15):2603-2610.
- Shih Y, Lin K-L, Shiang T-Y. Is the foot striking pattern more important than barefoot or shod conditions in running? Gait Posture 2013;38(3):490-494.
- Williams DS 3rd, Green DH, Wurzinger B. Changes in lower extremity movement and power absorption during forefoot striking and barefoot running. Int J Sports Phys Ther 2012;7(5):525-532.
- Roper JL, Harding EM, Doerfler D, et al. The effects of gait retraining in runners with patellofemoral pain: A randomized trial. Clin Biomech 2016;35:14-22.
- Lieberman DE, Venkadesan M, Werbel WA, et al. Foot strike patterns and collision forces in habitually barefoot versus shod runners. Nature 2010;463(7280):531-535.
- Goss DL, Goss MT. A comparison of negative joint work and vertical ground reaction force loading rates in chi runners and rearfoot-striking runners. J Orthop Sports Phys Ther 2013;43(10):685-692.
- Davis IS, Bowser BJ, Mullineaux DR. Reduced vertical impact loading in female runners with medically diagnosed injuries: a prospective investigation. British J Sports Med 2015; bjsports-2015-094579v2.
- Delgado TL, Kubera-Shelton E, Robb RR, et al. Effects of foot strike on low back posture, shock attenuation, and comfort in running. Med Sci Sports Exerc 2013;45(3):490-496.
- Mercer JA, Bates BT, Dufek JS, Hreljac A. Characteristics of shock attenuation during fatigued running. J Sport Sci 2003;21(11):911-919.
- Squadrone R, Gallozzi C. Biomechanical and physiological comparison of barefoot and two shod conditions in experienced barefoot runners. J Sports Med Phys Fitness 2009;49(1):6-13.
- Nordin AD, Dufek JS, Mercer JA. Three-dimensional impact kinetics with foot-strike manipulations during running. J Sport Health Sci 2015;(In press).
- Barrios JA, Crossley KM, Davis IS. Gait retraining to reduce the knee adduction moment through real-time visual feedback of dynamic knee alignment. J Biomech 2010;43(11):2208-2213.
- Noehren B, Davis I. The effect of gait retraining on hip mechanics, pain, and function in runners with patellofemoral pain syndrome. J Orthop Sports Phys Ther 2009;40(3):A40-A41.
- Crowell HP, Davis IS. Gait retraining to reduce lower extremity loading in runners. Clin Biomech 2011;26(1):78-83.
- Rathleff MS, Rathleff CR, Crossley KM, et al. Is hip strength a risk factor for patellofemoral pain? A systematic review and meta-analysis. Br J Sports Med 2014;48(14):1088.
- Thompson MA, Lee SS, Seegmiller J, McGowan CP. Kinematic and kinetic comparison of barefoot and shod running in mid/forefoot and rearfoot strike runners. Gait Posture 2015;41(4):957-959.
- Herbst KA, Barber Foss KD, Fader L, et al. Hip strength is greater in athletes who subsequently develop patellofemoral pain. Am J Sports Med 2015;43(11):2747-2752.