TitleAuthorsAbstractFigure 1 Vertical ground reaction forces and foot kinematics for three foot strikes at 3.5 m s-1 in the same runner.Figure 2 Variation in impact transients.Figure 3 Differences during impact between shod RFS runners (group 1) and barefoot FFS runners (group 3) at approximately 4 m sMethods SummaryReferencesTable 1 Foot strike type and joint angles of habitual barefoot and shod runners from Kenya and the USAMethodsSubjectsTreatmentsKinematicsKineticsEstimation of effective massLETTERSFoot strike patterns and collision forces in habituallybarefoot versus shod runnersDaniel E. Lieberman1, Madhusudhan Venkadesan1,2*, William A. Werbel3*, Adam I. Daoud1*, Susan D’Andrea4,Irene S. Davis5, Robert Ojiambo Mang’Eni6,7& Yannis Pitsiladis6,7Humans have engaged in endurance running for millions of years1,but the modern running shoe was not invented until the 1970s. Formost of human evolutionary history, runners were either barefootor wore minimal footwear such as sandals or moccasins with smal-ler heels and little cushioning relative to modern running shoes.We wondered how runners coped with the impact caused by thefoot colliding with the ground before the invention of the modernshoe. Here we show that habitually barefoot endurance runnersoften land on the fore-foot (fore-foot strike) before bringing downthe heel, but they sometimes land with a flat foot (mid-foot strike)or, less often, on the heel (rear-foot strike). In contrast, habituallyshod runners mostly rear-foot strike, facilitated by the elevatedand cushioned heel of the modern running shoe. Kinematic andkinetic analyses show that even on hard surfaces, barefoot runnerswho fore-foot strike generate smaller collision forces than shodrear-foot strikers. This difference results primarily from a moreplantarflexed foot at landing and more ankle compliance duringimpact, decreasing the effective mass of the body that collides withthe ground. Fore-foot- and mid-foot-strike gaits were probablymore common when humans ran barefoot or in minimal shoes,and may protect the feet and lower limbs from some of the impact-related injuries now experienced by a high percentage of runners.Running can be most injurious at the moment the foot collideswith the ground. This collision can occur in three ways: a rear-footstrike (RFS), in which the heel lands first; a mid-foot strike (MFS), inwhich the heel and ball of the foot land simultaneously; and a fore-foot strike (FFS), in which the ball of the foot lands before the heelcomes down. Sprinters often FFS, but 75–80% of contemporary shodendurance runners RFS2,3. RFS runners must repeatedly cope withthe impact transient of the vertical ground reaction force, an abruptcollision force of approximately 1.5–3 times body weight, within thefirst 50 ms of stance (Fig. 1a). The time integral of this force, theimpulse, is equal to the change in the body’s momentum during thisperiod as parts of the body’s mass decelerate suddenly while othersdecelerate gradually4. This pattern of deceleration is equivalent tosome proportion of the body’s mass (Meff, the effective mass) stop-ping abruptly along with the point of impact on the foot5. The rela-tion between the impulse, the body’s momentum and Meffisexpressed asðT0{Fz(t)~Mbody(DvcomzgT)~Meff({vfootzgT) ð1Þwhere Fz(t) is the time-varying vertical ground reaction force, 02isthe instant of time before impact, T is the duration of the impacttransient, Mbodyis the body mass, vcomis the vertical speed of thecentre of mass, vfootis the vertical speed of the foot just before impactand g is the acceleration due to gravity at the Earth’s surface.Impact transients associated with RFS running are sudden forceswith high rates and magnitudes of loading that travel rapidly up thebody and thus may contribute to the high incidence of running-related injuries, especially tibial stress fractures and plantarfasciitis6–8. Modern running shoes are designed to make RFS runningcomfortable and less injurious by using elastic materials in a largeheel to absorb some of the transient force and spread the impulse overmore time9(Fig. 1b). The human heel pad also cushions impacttransients, but to a lesser extent5,10,11, raising the question of howrunners struck the ground before the invention of modern runningshoes. Previous studies have found that habitually shod runners tendto adopt a flatter foot placement when barefoot than when shod, thusreducing stresses on the foot12–15, but there have been no detailedstudies of foot kinematics and impact transients in long-term habitu-ally barefoot runners.We compared foot strike kinematics on tracks at preferred endurancerunning speeds (4–6 m s21) among five groups controlled for age andhabitual footwear usage (Methods and Supplementary Data 2). Adultswere sampled from three groups of individuals who run a minimum of20 km per week: (1) habitually shod athletes from the USA; (2) athle tesfrom the Rift Valley Province of Kenya (famed for endurance running16),most of whom grew up barefoot but now wear cushioned shoes whenrunning; and (3) US runners who grew up shod but now habitually runbarefoot or in minimal footwear. We also compared adolescents fromtwo schools in the Rift Valley Province: one group (4) who hav e neve rworn shoes; and another group (5) who have been habitually shod mostof their lives. Speed, age and distance run per week were not correlatedsignificantly with strike type or foot and ankle angles within or amonggroups. However, because the preferred speed was approximately1ms21slower in indoor trials than in outdoor trials, we made statisticalcomparisons of kinematic and kinetic data only between groups 1 and 3(Table 1).Strike patterns vary within subjects and groups, but these trials(Table 1 and Supplementary Data 6) confirm reports2,3,9that habitu-ally shod runners who grew up wearing shoes (groups 1 and 5) mostlyRFS when shod; these runners also predominantly RFS when barefooton the same hard surfaces, but adopt flatter foot placements by dorsi-flexing approximately 7–10u less (analysis of variance, P , 0.05). Incontrast, runners who grew up barefoot or switched to barefoot run-ning (groups 2 and 4) most often used FFS landings followed by heelcontact (toe–heel–toe running) in both barefoot and shod conditions.MFS landings were sometimes used in barefoot conditions (group 4)*These authors contributed equally to this work.1Department of Human Evolutionary Biology, 11 Divinity Avenue,2School