Effects

Effects of Footwear and Strike Type on Running Economy DANIEL P. PERL, ADAM I. DAOUD, and DANIEL E. LIEBERMAN Department of Human Evolutionary Biology, Harvard University, Cambridge, MA ABSTRACT PERL, D. P., A. I. DAOUD, and D. E. LIEBERMAN. Effects of Footwear and Strike Type on Running Economy. Med. Sci. Sports Exerc., Vol. 44, No. 7, pp. 1335–1343, 2012. Purpose: This study tests if running economy differs in minimal shoes versus standard running shoes with cushioned elevated heels and arch supports and in forefoot versus rearfoot strike gaits. Methods: We measured the cost of transport (mL O2Ikg j1Imj1) in subjects who habitually run in minimal shoes or barefoot while they were running at 3.0 mIsj1 on a treadmill during forefoot and rearfoot striking while wearing minimal and standard shoes, controlling for shoe mass and stride frequency. Force and kinematic data were collected when subjects were shod and barefoot to quantify differences in knee flexion, arch strain, plantar flexor force production, and Achilles tendon–triceps surae strain. Results: After controlling for stride frequency and shoe mass, runners were 2.41% more economical in the minimal-shoe condition when forefoot striking and 3.32% more economical in the minimal-shoe condition when rearfoot striking (P G 0.05). In contrast, forefoot and rearfoot striking did not differ significantly in cost for either minimal- or standard-shoe running. Arch strain was not measured in the shod condition but was significantly greater during forefoot than rearfoot striking when barefoot. Plantar flexor force output was significantly higher in forefoot than in rearfoot striking and in barefoot than in shod running. Achilles tendon–triceps surae strain and knee flexion were also lower in barefoot than in standard- shoe running. Conclusions: Minimally shod runners are modestly but significantly more economical than traditionally shod runners regardless of strike type, after controlling for shoe mass and stride frequency. The likely cause of this difference is more elastic energy storage and release in the lower extremity during minimal-shoe running.KeyWords: RUNNING ECONOMY, BAREFOOTRUNNING, MINIMAL-SHOE RUNNING, FOREFOOT STRIKE, REARFOOT STRIKE Hominins evolved to run long distances more than2 million years ago (6), but the last few decadeshave seen two major related changes in human running biomechanics. The first is shoes. Footwear such as sandals or moccasins were invented less than 50,000 yr ago (35), but the modern running shoe with a cushioned elevated heel, arch supports, and a stiffened midsole (hereafter called a standard shoe) was created only in the 1970s. The second likely change has been running form, especially foot strike. More than 75% of today’s shod runners typically rearfoot strike (RFS), in which the heel first contacts the ground (18,22), but barefoot or minimally shod runners more often forefoot strike (FFS), with the ball of the foot landing before the heel, or they sometimes midfoot strike (MFS), with the heel and ball of the foot landing simultaneously (12,23). Barefoot and minimally shod runners especially tend to FFS on hard or rough surfaces because FFS landings, unlike RFS landings, generate no impact peak, which is painful without a cushioned heel that slows the rate of impact loading about sevenfold (9,21,23,31). Elevated heels also encourage a runner to RFS, even when the foot is slightly plantar flexed, facilitating a longer stride and eliminating controlled dorsi- flexion by the plantar flexors during landing. If humans evolved to run barefoot, most often with an FFS gait, it follows that natural selection did not adapt the human body to RFS in shoes. One question of interest is whether shoes and strike types affect running economy. To date, several studies have compared running economy in barefoot and shod conditions but with different experimental treatments that did not control for all relevant variables. The first study was conducted by Burkett et al. (8), who mea- sured running economy in 21 habitually shod runners (all orthotics users) at 3.35 mIsj1 without controlling for shoe APPLIED SCIENCES Address for correspondence: Daniel E. Lieberman, Ph.D., Department of Human Evolutionary Biology, Harvard University, 11 Divinity Avenue, Cambridge, MA 02138; E-mail: danlieb@fas.harvard.edu. Submitted for publication June 2011. Accepted for publication December 2011. Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal’s Web site (www.acsm-msse.org). 0195-9131/12/4407-1335/0 MEDICINE & SCIENCE IN SPORTS & EXERCISE! Copyright " 2012 by the American College of Sports Medicine DOI: 10.1249/MSS.0b013e318247989e 1335 Copyright © 2012 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited. type, shoe weight, or strike type. Runners were about 1%–2% less costly when barefoot than shod (with or without orthot- ics), approximately the difference expected from the extra shoe mass (15). A similar result was obtained by Divert et al. (14), who measured running economy in 12 habitually shod male runners at 3.61 mIsj1 barefoot, in socks (50, 150, and 350 g), and in shoes (150 and 350 g). Because runners were 3% more costly in the 350-g shoes and socks than when barefoot, the cost difference was interpreted to be a mass effect. Divert et al. (14), however, noted that 75% of the subjects had no impact peak when barefoot or in socks, suggesting a switch from an RFS gait in shoes to an FFS gait in socks or barefoot. Squadrone and Gallozzi (32) analyzed eight experienced barefoot runners at 3.32 mIsj1 barefoot, wearing 148-g minimal shoes (Vibram FiveFingers REI, Kent, WA), and in 341-g shoes. As with Divert et al. (14), shod runners were 1.3%–2.8% more costly when shod, but shoe mass was not controlled, and runners switched from an RFS gait in shoes to an MFS or FFS gait when barefoot or minimally shod. Recently, Hanson et al. (16) compared running economy in 10 habitually shod runners at 70% of V˙O2max in barefoot and shod conditions on a treadmill and overground. Although the barefoot condition was 3.8% more economical, shoe mass and strike type were uncontrolled. Several factors likely complicate the interpretation of these results. Shoe mass was controlled only by Divert et al. (14), but a typical shoe increases the lower extremity’s moment of inertia by adding 300 g to the foot, thus augmenting leg swing cost, which may comprise 20% of total running cost (24,26). At a given speed, the cost of transport (COT (mL O2Ikg j1Imj1)) during running increases approximately 1% for every 100 g of added shoe mass (15), poten- tially explaining the 1%–3% lower costs previously mea- sured in barefoot versus shod conditions. Another factor to consider is strike type because RFS and FFS gaits have slightly different mass–spring mechanics. Tendons, ligaments, and muscles of the lower extremity store elastic energy during the first half of stance and then recoil during the second half of stance, helping push the body’s center of mass upward and forward (4). These struc- tures, which are derived in humans relative to great apes (6), may be used more effectively in barefoot or FFS running through several mechanisms. The first is more elastic energy storage in the Achilles tendon, which recovers approximately 35% of the mechanical energy that the body generates with each step (2,21). Although the initial ground reaction force (GRF) is lower in an FFS than in an RFS, it creates a larger external dorsiflexion moment around the ankle that is coun- tered by an internal plantar flexor moment (13,37). Although higher external dorsiflexion moments in FFS gaits cause higher triceps surae contractile costs, more controlled dor- siflexion during an FFS could permit more elastic energy storage and return because the heel descends substantially under controlled dorsiflexion, stretching the Achilles tendon while the triceps surae contracts eccentrically or isometri- cally (19). Further, an elevated heel limits ankle dorsiflex- ion, which may lessen Achilles tendon strain in shod versus barefoot running. It is reasonable to assume that in an RFS gait, the Achilles tendon does not stretch at impact and stretches primarily from dorsiflexion after foot flat as the tibia passes over the foot. Therefore, we predict that the Achilles tendon is likely to store and return more elastic en- ergy in FFS versus RFS running and even more during FFS running in minimal shoes or when barefoot versus in stan- dard shoes. However, a related factor with opposite effects on economy is the force the triceps surae must produce to counter higher sagittal plane moments in FFS versus RFS gaits (Fig. 1). Consequently, the length of the tuber calca- neus, which creates the Achilles tendon’s moment arm, has a strong inverse effect on economy because shorter moment arms allow for greater storage and release of elastic strain energy (28,30). Another biomechanical difference between FFS and RFS running is knee flexion. RFS runners typically land with the foot in front of the knee, which is more extended and less compliant at strike but then flexes more during stance; in contrast, FFS runners land with an initially more flexed knee and have more knee flexion during impact (23,27) but flex the knee less thereafter (5). Because the gastrocnemius orig- inates on the distal femur, knee flexion slackens the Achilles tendon–triceps surae complex (ATTSC) during the first half of stance but differently in RFS and FFS gaits. Because knee flexion lessens ATTSC elongation during the first half of FIGURE 1—Model of different forces (top) acting on the longitudinal arch at the moment of impact and thus before foot flat in an FFS (A) and RFS (B). Major kinematic differences in a lateral view are illus- trated at the bottom, and circles indicate locations of landmarks used to measure arch strain. Fv is the vertical GRF, Fat is the tibialis an- terior force, Fa is the Achilles tendon force, and Fb is the body force. In the FFS, Fv is smaller in magnitude, and the Achilles tendon exerts a plantar flexing force to control dorsiflexion; in the RFS, Fv is greater in magnitude, there is no Fa, and the tibialis anterior must produce a dorsiflexing force, Fat, to counter plantarflexion. Because the FFS is loaded in three-point bending before foot flat, the longitu- dinal arch is predicted to stretch more during this period of stance (dashed lines). http://www.acsm-msse.org1336 Official Journal of the American College of Sports Medicine A PP LI ED SC IE N C ES Copyright © 2012 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited. stance and is controlled by the quadriceps, one predicts a pos- itive correlation between COT and total knee flexion dur- ing stance. Energy storage in the arch is another potential source of differences in running economy between the FFS and RFS gaits and between runners who are barefoot, in minimal shoes, or in standard shoes (Fig. 1). The longitudinal and transverse arches of the foot include many elastic structures that recover an estimated 17% of the mechanical energy gen- erated per step (21), making barefoot and minimally shod running likely to store more elastic energy because external arch supports in standard shoes lessen vertical arch com- pression during stance, limiting how much the arch can stretch and recoil. Another contrast is that FFS runners ini- tially load the arch in three-point bending (Fig. 1A) from the instant the ball of the foot contacts the ground, with a GRF applied upward anterior to the ankle at the metatarsal heads, an upward balancing force applied posterior to the ankle by the Achilles tendon, and a downward force applied by the body’s mass through the ankle. In contrast, an RFS runner experiences little or no arch compression at impact (Fig. 1B) because the arch is subject to a GRF below or slightly pos- terior to the ankle where it is opposed by the downward force of the body’s mass and the force from the tibialis an- terior applied near the arch’s apex at the medial cuneiform. These forces likely stiffen the arch until foot flat, prevent- ing elastic storage of any energy that impact generates. One therefore predicts the arch will store and recover more en- ergy in FFS than RFS running and more so in barefoot or minimally shod runners. A related factor is foot strength. Individuals who wear stiff-soled shoes with arch supports possibly have weaker intrinsic foot muscles than individuals who are habitually barefoot or minimally shod (7). Because foot muscles affect elastic energy storage in the arch, run- ning economy between barefoot, minimally shod, and stan- dardly shod conditions may differ in runners who habitually run in standard shoes versus barefoot or in minimal shoes. A final factor to consider when comparing cost among different conditions is stride frequency. Experimental stud- ies indicate that the optimal COT in shod runners occurs at stride frequencies of 170–185 steps per minute regardless of incline, leg length, and body mass (10). The explanation for this phenomenon is not well understood, but many jog- gers in standard shoes adopt a slower preferred cadence compared with barefoot or minimally shod runners who tend to have shorter strides and higher stride frequencies (8,14,32) more common among experienced shod runners. Why some runners prefer lower stride frequencies is unknown, but dif- ferences in stride frequency could be a confounder that ex- plains some of the variation in cost previously measured between barefoot/minimally shod and standardly shod con- ditions. Because there is no a priori reason to predict that optimal stride frequency should vary with footwear, this study controlled for stride frequency. In short, we predicted that footwear usage and strike type have independent effects on running economy after con- trolling for stride frequency, previous footwear history, and shoe mass. First, we hypothesized that habitual barefoot/ minimally shod runners will have a lower COT when min- imally shod than in standard shoes, independent of strike type and after controlling for shoe mass and stride frequency, because of more elastic energy storage in the lower ex- tremity. Second, we hypothesized that FFS runners are more economical than RFS runners independent of footwear be- cause of more elastic energy storage in the Achilles tendon and possibly the foot. However, these gains may be offset by higher contractile costs for the triceps surae and the in- trinsic foot muscles in an FFS than in an RFS. Finally, we predicted that within a given condition, COT correlates neg- atively with how much the arch of the foot and the ATTSC stretch and positively with knee flexion. METHODS Subjects. Running biomechanics and economy were measured in 15 subjects (13 men, 2 women), all experienced barefoot or minimally shod runners with no major injuries in the past 6 months and with no lower extremity abnormali- ties. Mean T SD subject height was 1.75 T 0.06 (SD); mean body mass was 73.3 T 10.6 (SD); mean BMI was 23.8 T 2.6 (SD); mean was 41.3 T 9.8 (SD); mean weekly mileage was 33.4 T 16.5 (SD). Subjects had been running barefoot or in minimal footwear for an average of 2.1 years T 1.1 (SD) (range, 0.6–4.0). These subjects preferred to FFS, but most of them used to run in standard shoes, and all of them were comfort- able running with an RFS gait. Subjects who were not com- fortable with an RFS were excluded from the study. The collection of data on all subjects was approved by the Harvard University Committee on the Use of Human Subjects, and prior written informed consent was obtained from all subjects. Treatment. Each subject ran in shoes defined as stan- dard (having a cushioned elevated heel, arch supports, and a stiff sole) and minimal (lacking these features) using both FFS and RFS gaits. Standard shoes used were Asics GEL-Cumulus 10i, a neutral shoe; Vibram FiveFingersi shoes were used for the minimally shod condition instead of barefoot running to prevent injury on the treadmill; these shoes have previously been shown to have no significant effect on barefoot running kinematics or economy (23,32). All footwear and socks were weighed before each trial to the nearest 0.1 g, and ankle weight belts filled with the appro- priate mass of metal washers were strapped around each an- kle during minimally shod running. All trials for each subject were completed on the same day, and the order of the running conditions was randomized across subjects. Different tre- admills, however, were used for measuring running cost and biomechanics because the instrumented treadmill used for measuring GRF (see below) is not as comfortable for long- term running. To measure running cost, subjects ran on a treadmill (Vision Fitness T9250; Cottage Grove, WI) at 3.0 mIsj1 for approximately 2 min to determine preferred stride frequency FOOTWEAR, STRIKE TYPE, AND RUNNING ECONOMY Medicine & Science in Sports & Exercised 1337 A PPLIED SC IEN C ES Copyright © 2012 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited. and to habituate them to the treadmill. Subjects were then con- nected by a two-valve mouthpiece to a gas analyzer (see be- low) connected to a flexible lightweight tube with a nasal clip to ensure solely oral breathing. After 2 min of habituation, subjects then performed four trials in a random order: FFS in minimal shoes and ankle weights, FFS in standard shoes, RFS in minimal shoes and ankle weights, and RFS in stan- dard shoes. Each trial lasted a minimum of 5 min, with at least 1 min of running after V˙O2 levels reached a steady state. A metronome was used to keep the runner at his/her preferred stride frequency. Subjects were given 5-min breaks between trials. Respirometry. Expired gas was collected using a Sable flow generator and controller (500H FlowKit; Sable Sys- tems International, Las Vegas, NV) with an airflow rate of 150 LIminj1. A subsample of expired air was then pushed at 300 mLIminj1 into an open-ended syringe where it was then pulled at 100 mLIminj1 by a subsampler (SS-4; Sable Systems International) through a Drierite column to scrub water vapor. Subsampled air was then pushed at 100 mLIminj1 through a paramagnetic oxygen analyzer (PA-10 Oxygen Analyzer; Sable Systems International), which measured the fractional amount of oxygen at 100 Hz. Room air oxygen levels were measured before and after each condition, and windows were kept closed. To correct for any drift in oxygen measurement, V˙O2 at steady state was computed as follows: ðFiO2i þ ððFiO2f # FiO2iÞTss=ðTf # TiÞÞÞ # FeO2ss ! " FR where FiO2i is the initial fractional amount of oxygen in the incurrent air stream measured before each trial at equilib- rium without the subject connected, FiO2f is the final frac- tional amount of oxygen present in the incurrent air stream measured after each trial without the subject connected, Tss is the time into each trial when the subject’s oxygen con- sumption reached steady state, Tf is the time when the final incurrent oxygen fraction was measured, Ti is the time when the initial incurrent oxygen fraction was measured, FeO2ss is the mean fractional amount of oxygen in the excurrent air stream measured for each subject at steady state for at least 1 min at the end of each trial, and FR is the mean gas flow rate in the mask when steady-state V˙O2 was measured. COT was then calculated as milliliters of oxygen per kilogram per meter. Kinematics. Kinematic data were collected with an eight-camera Oqus kinematics system (Qualysis, Gothenburg, Sweden) at 500 Hz for 30-s intervals with subjects running in the four conditions at 3.0 mIsj1 with