Michiel Twiss BScPT, MScPT i.a. Contact:

Quest for the Holy Strike Contemporary considerations for the clinical management and prevention of running related injuries: kinematics and kinetics in forefoot and rearfoot strike patterns Michiel Twiss BScPT, MScPT i.a. Contact: mrmtwiss@gmail.com

Content INTRODUCTION 3 METHODS 4 RESULTS 5 Kinematics of RFS and FFS 5 Kinetics of RFS and FFS 5 DISCUSSION 7 CONCLUSION 9 REFERENCES 11 APPENDIX FIGURES 1-4 13 2

Introduction Running has become increasingly popular with over 17 million finishers in more than 30000 races in the United States in 2015 (Chan et al., 2017). However, running entails the risk of running-related injury. Reports on the incidence of lower extremity injuries sustained from long distance running range from 20-80% in runners (van Gent et al., 2007). The most frequent injury site is at the knee with patellofemoral pain syndrome (PFPS) being the most common knee injury followed by iliotibial band syndrome (ITBS or runners knee ), plantar fasciitis and meniscal injuries (Taunton, 2002). Some runners land on their heels (heel-strikers) and some runners land on the forefoot and differences in both kinematics and kinetics during different foot-strike patterns have an effect on injury susceptibility. The purpose of this essay is to determine the influence of different footstrike patterns on prevention and clinical management of running-related injuries by comparing kinematics and kinetics of forefoot and rearfoot strike patterns during running. 3

Methods Searching for relevant literature was conducted in Pubmed and Google Scholar using combinations and variants of different terms such as `running`, `runners`, `foot strike pattern`, `forces`, `forefoot`, `rearfoot`, `barefoot`, `shod`, `kinetics`, `kinematics`, `injury prevention`. The choice of relevant articles was narrowed down after reading titles and abstracts. Additional articles were handsearched ensuring comprehensiveness. The literature should help to lead the discussion of this essay. 4

Results Two distinct foot strike patterns can be identified among runners: forefoot strike pattern (FFS) and rearfoot strike pattern (RFS). About 85% of shoed runners typically adopt an RFS or heelstrike, i.e. the heel being the first part of the food contacting the ground whereas barefoot or minimalistic-shoewear runners typically adopt a mid- or forefoot strike (Larson et al., 2011). Midfoot striking (MFS) can be considered an intermediate in the continuum of these landing patterns and will not be discussed here as a distinct pattern. Kinematics of RFS and FFS The difference in kinematics between RFS and FFS can be described as follows: At the moment of impact during RFS and FFS the hip and knee are both flexed. At initial contact during RFS, the ankle is dorsiflexed with the runner typically landing in the middle or outside of the heel. In FFS the ankle is plantarflexed with the runner typically landing on the lateral side of the forefoot on the 4th and 5th metatarsal heads or ball of the foot. During the loading response in RFS, the ankle will continue to plantarflex with the forefoot coming down to flat foot. In FFS the ankle will dorsiflex - whilst stretching and loading the plantar fascia, the calf muscles, and Achilles tendon controlling the heel descent. Knee and hip continue to flex in both RFS and FFS. However, in FFS, the knee generally flexes more compared to RFS (Lieberman, 2012). Following flat foot to midstance pronation occurs eversion, dorsiflexion, and arch stretching from heel to toe in RFS with the ankle dorsiflexing. At the same time the lower leg moves forward relative to the foot causing eversion. The foot (flat on the ground) will start to stretch the arch. In FFS pronation occurs in opposite direction from toe to heel. In both RFS and FFS knee and hip extension occur during midstance to toe off with the ankle plantarflexing and lifting the heel of the ground. The arch recoils, flexing the toes causing upward and forward acceleration of the body preparing for the next rearfoot impact. ( Running Barefoot: Biomechanics of Foot Strike, n.d.) Kinetics of RFS and FFS The ground reaction force (GRF) with its corresponding torque and the controlling/counteracting muscle forces and their corresponding torques are the source of joint loading in the lower leg during distinct foot strike patterns. The difference in kinetics of both strike patterns is described below. 5

RFS and FFS kinetics at initial contact and loading response At the moment of impact during RFS, the vertical GRF vector lies behind the axis of the ankle joint causing a clockwise ankle moment. In contrast, at the moment of impact during FFS, the GRF vector is in front of the axis of the ankle joint causing an anti-clockwise moment, i.e. the top of the foot will come towards the shank. During the loading response in RFS, a counteracting muscle force is delivered through the eccentric activity of the M. tibialis anterior (TA) controlling any slapping down of the foot and pronation whereas in FFS the counteracting muscle force will be delivered by the eccentric activity of the M. gastrocnemius (GAS) and M. Soleus (SOL) which both control the downward motion of the heel pad (Fig.1). RFS and FFS kinetics at midstance and swing There are significant differences in joint torques when running either RFS or FFS across the stride (midstance and swing). Knee moments are significantly larger and ankle moments are significantly smaller during RFS in comparison to FFS. Furthermore, across the stride during RFS the knee shows greater peak instantaneous power absorption though much smaller peak instantaneous power absorption at the ankle compared to FFS (Stearne, Alderson, Green, Donnelly, & Rubenson, 2014) (Fig. 2). Additional kinetic data from Stearne et al. regarding energy absorption at the individual joints during stance indicate that energy is more or less equally distributed at the ankle and knee during RFS (45.2% at the ankle and 41.1% at the knee). In contrast, in habitual FFS runners, the ankle contributes much more to negative work (energy absorption) compared to the knee (62.4% ankle vs 25.7% knee) and also compared to RFS (62.4% ankle FFS and 45.2% ankle RFS) (Fig. 3). Hip joint moments and power absorption do not show significant differences when comparing both patterns. 6

Discussion Comparison of the biomechanics of habitual RFS- with habitual FFS runners shows different strains within the lower limb joints. Habitual RFS runners place more demand on the knee joint in both sagittal and frontal planes compared to FFS whereas habitual FFS runners place more demand on the ankle joint in the sagittal plane (Stearne et al., 2014). Stearne et al. measured vertical forces based on the GRF using inverse dynamics to calculate the net joint moments and energy absorption (power) in both patterns. Given the knee being the predominant site of injury (van Gent et al., 2007) and 85% of shoed runners adopt an RFS pattern (Knorz et al., 2017) these results suggest that habitual RFS runners with knee problems could benefit from switching to an FFS pattern. However, shear forces (AP and ML components) need to be considered. These were not calculated in the study by Stearne. Other research shows that shear forces can amount to relevant levels for previously injured joints with instability and/or pre-existing cartilage damage during running (Knorz et al., 2017): Knorz was able to demonstrate that for the ankle, the knee and the hip moderate but significantly higher shear forces arise during FFS with the exception of the knee in APdirection. Loading rates (LR) in FFS in all three joints remained lower in most directions compared to LR in RFS, with the exception of higher AP-LR at both the ankle and the hip as well as higher ML-LR at the hip during FFS (Fig. 4). Regarding the hip, Knorz et al. write that biomechanical analyses are difficult, `due to the anatomic position of the hip joint close to the center of the body` with data difficult to interpret. Albeit, these findings are consistent with other research revealing that FFS is associated with higher ankle joint stress. For physiotherapists and other healthcare practitioners, this could mean that specific recommendations can be made to runners with unstable ankle joints, tendon pathology or hip problems. These runners may benefit from or switching to RFS. Also, habitual FFS runners switching from FFS to RFS are able to replicate the joint mechanical characteristics demonstrated by habitual RFS runners. Interestingly, style switching did not increase frontal plane knee loads as in habitual RFS runners and it required less positive mechanical average power in the limb at the same running speed of habitual RFS runners (Stearne et al., 2014). Switching to RFS - while maintaining mechanical performance - in injury rehabilitation of abovementioned pathologies may be a useful strategy. As can be seen in Figure 4, because FFS was associated with significantly lower shear forces in the AP-direction in the knee, FFS may be favored for runners with previous specific knee injuries such as ACL-tears or patellofemoral pain syndrome (PFPS). Data from a case series showed that adopting an FFS was beneficial in patients with patellofemoral pain 7

(Cheung & Davis, 2011) and another study demonstrated the effectiveness of a two-week gait modification training in reducing vertical impact loads (Chan et al., 2017). According to the authors, the gait retraining group may have shifted to a more midfoot or forefoot strike. Also, at follow up at 12-months, the gait retraining group had a much lower injury rate compared to the controls, 16% to 38% respectively. Several limitations of this study will be discussed below, also with regards to potential future research. 8

Conclusion Running is an incredibly popular fitness activity. In particular, running is of low cost and an easy-to-implement intervention to maintain overall fitness and prevent cardiovascular, musculoskeletal and psychological disease. However, running is associated with increased risk of injury. This essay sought after the influence of two distinct strike patterns on running mechanics and tissue loading (at the joint, muscle, tendon and bone level) to determine if the application of one strike pattern over the other might provide beneficial effects with regards to prevention and management of running related injuries. Running mechanics, in general, have been studied for hundreds of years and researchers are still trying to better understand the relationship between running biomechanics and injury (McClay, 2000). Biomechenical research comparing both strike patterns is booming and generally indicates that FFS may lead to higher MPF (shear forces) and RFS seem to lead to higher LR. Higher LR in RFS probably results from the impact peak at initial contact (heel landing) whereas higher shear forces in FFS are a consequence of reduced ground contact time (Knorz et al., 2017). Higher LR is considered an important factor for running injuries (McClay, 2000) with RFS runners being more at more risk for running injuries. Converting to an FFS pattern may positively influence the course of running injuries (Davis, Rice, & Wearing, 2017). Higher shear forces may be detrimental in previously injured joints with instability and/or pre-existing cartilage and thus moderately higher AP and ML components during FFS in the ankle and hip joints (Knorz et al., 2017) should be taken into consideration when managing pre-existing injuries in these joints. Up until recently, kinetic data of running mechanics was only accessible through biomechanical research in the laboratory setting. The developments of wearable technology (GPS watches, tibial accelerometers) now allow for analysis outside the laboratory (Willy, 2017). As mentioned in the discussion, a gait retraining program was proven to be effective in reducing vertical loading rates. The study found a reduction in injury rates for the gait retraining group suggesting that by changing gait style, mechanical loads and injury rates could be influenced. Among several limitations in this study are that the gait retraining group requires a biomechanics laboratory. Furthermore, the study did not assess gait mechanics outside the laboratory thus it is unknown if the participants maintained gait changes after training. Also, the study did not measure volume or intensity (faster or slower running) between both groups thus leaving the interpretation of why injury rates were different open for discussion. An interesting future research project could be to determine if wearable technology could ontrol for these confounding factors and overcome the abovementioned study limitations? GPS watches may be more accurate in measuring training loads parameters such as running speed and total volume compared to self-reported measures. Additional controlling for these confounding factors in the field will be valuable in interpreting biomechanical data obtained in the laboratory. Tibial accelerometers and 9

pressure sensitive insoles could also provide additional biomechanical data and real-time feedback could assist in gait retraining. However, additional data provided by wearable technology must be accurate and reliable.thus, further research will be needed to test accuracy, reliability, and validity of wearable technology. 10

References C2ST TV. (n.d.). Biomechanics of Running: The Science of Movement - Steven McCaw. Retrieved from https://www.youtube.com/watch?v=7zstuct47wo Chan, Z. Y. S., Zhang, J. H., Au, I. P. H., An, W. W., Shum, G. L. K., Ng, G. Y. F., & Cheung, R. T. H. (2017). Gait Retraining for the Reduction of Injury Occurrence in Novice Distance Runners: 1-Year Follow-up of a Randomized Controlled Trial. The American Journal of Sports Medicine, 36354651773627. https://doi.org/10.1177/0363546517736277 Cheung, R. T. H., & Davis, I. S. (2011). Landing pattern modification to improve patellofemoral pain in runners: a case series. The Journal of Orthopaedic and Sports Physical Therapy, 41(12), 914 919. https://doi.org/10.2519/jospt.2011.3771 Davis, I. S., Rice, H. M., & Wearing, S. C. (2017). Why forefoot striking in minimal shoes might positively change the course of running injuries. Journal of Sport and Health Science, 6(2), 154 161. https://doi.org/10.1016/j.jshs.2017.03.013 Knorz, S., Kluge, F., Gelse, K., Schulz-Drost, S., Hotfiel, T., Lochmann, M., Krinner, S. (2017). Three-Dimensional Biomechanical Analysis of Rearfoot and Forefoot Running. Orthopaedic Journal of Sports Medicine, 5(7), 232596711771906. https://doi.org/10.1177/2325967117719065 Larson, P., Higgins, E., Kaminski, J., Decker, T., Preble, J., Lyons, D., Normile, A. (2011). Foot strike patterns of recreational and sub-elite runners in a long-distance road race. Journal of Sports Sciences, 29(15), 1665 1673. https://doi.org/10.1080/02640414.2011.610347 Lieberman, D. E. (2012). What we can learn about running from barefoot running: an evolutionary medical perspective. Exercise and Sport Sciences Reviews, 40(2), 63 72. https://doi.org/10.1097/jes.0b013e31824ab210 McClay, I. (2000). The evolution of the study of the mechanics of running. Relationship to injury. Journal of the American Podiatric Medical Association, 90(3), 133 148. https://doi.org/10.7547/87507315-90-3-133 11

Richards, J. (2008). Biomechanics in clinic and research: an interactive teaching and learning course. Edinburgh ; New York: Churchill Livingstone/Elsevier. Running Barefoot: Biomechanics of Foot Strike. (n.d.). Retrieved December 12, 2017, from http://barefootrunning.fas.harvard.edu/4biomechanicsoffootstrike.html Stearne, S. M., Alderson, J. A., Green, B. A., Donnelly, C. J., & Rubenson, J. (2014). Joint kinetics in rearfoot versus forefoot running: implications of switching technique. Medicine and Science in Sports and Exercise, 46(8), 1578 1587. https://doi.org/10.1249/mss.0000000000000254 Taunton, J. E. (2002). A retrospective case-control analysis of 2002 running injuries. British Journal of Sports Medicine, 36(2), 95 101. https://doi.org/10.1136/bjsm.36.2.95 van Gent, R. N., Siem, D., van Middelkoop, M., van Os, A. G., Bierma-Zeinstra, S. M. A., Koes, B. W., & Taunton, J. E. (2007). Incidence and determinants of lower extremity running injuries in long distance runners: a systematic review * COMMENTARY. British Journal of Sports Medicine, 41(8), 469 480. https://doi.org/10.1136/bjsm.2006.033548 Willy, R. W. (2017). Innovations and pitfalls in the use of wearable devices in the prevention and rehabilitation of running related injuries. Physical Therapy in Sport: Official Journal of the Association of Chartered Physiotherapists in Sports Medicine, 29, 26 33. https://doi.org/10.1016/j.ptsp.2017.10.003 12

Appendix Figures 1-4 Fig.1 Vertical ground reaction force causing clockwise moment in RFS pattern (left) and anti-clockwise moment in FFS pattern (right) (C2ST TV, n.d.) Fig. 2 Ankle, knee and hip joint moments (A) and instantaneous powers (B) Stearne et al., 2014) across the stride (stance and swing) (Stearne et al., 2014) 13

Fig. 3 Distribution of positive and negative work during the stance phase (Stearne et al., 2014) Fig. 4 Maximum peak force (MPF) and loading rates (LR) in RFS and FFS in mean pair differences. (Knorz et al., 2017) 14