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Metabolic Cost of Running Barefoot versus Shod: Is Lighter Better?

FRANZ, JASON R.; WIERZBINSKI, CORBYN M.; KRAM, RODGER

Medicine & Science in Sports & Exercise: August 2012 - Volume 44 - Issue 8 - p 1519–1525
doi: 10.1249/MSS.0b013e3182514a88
APPLIED SCIENCES
Free

Purpose Based on mass alone, one might intuit that running barefoot would exact a lower metabolic cost than running in shoes. Numerous studies have shown that adding mass to shoes increases submaximal oxygen uptake (V˙O2) by approximately 1% per 100 g per shoe. However, only two of the seven studies on the topic have found a statistically significant difference in V˙O2 between barefoot and shod running. The lack of difference found in these studies suggests that factors other than shoe mass (e.g., barefoot running experience, foot strike pattern, shoe construction) may play important roles in determining the metabolic cost of barefoot versus shod running. Our goal was to quantify the metabolic effects of adding mass to the feet and compare oxygen uptake and metabolic power during barefoot versus shod running while controlling for barefoot running experience, foot strike pattern, and footwear.

Methods Twelve males with substantial barefoot running experience ran at 3.35 m·s−1 with a midfoot strike pattern on a motorized treadmill, both barefoot and in lightweight cushioned shoes (∼150 g per shoe). In additional trials, we attached small lead strips to each foot/shoe (∼150, ∼300, and ∼450 g). For each condition, we measured the subjects’ rates of oxygen consumption and carbon dioxide production and calculated metabolic power.

Results V˙O2 increased by approximately 1% for each 100 g added per foot, whether barefoot or shod (P < 0.001). However, barefoot and shod running did not significantly differ in V˙O2 or metabolic power. A consequence of these two findings was that for footwear conditions of equal mass, shod running had ∼3%–4% lower V˙O2 and metabolic power demand than barefoot running (P < 0.05).

Conclusions Running barefoot offers no metabolic advantage over running in lightweight, cushioned shoes.

Locomotion Lab, Department of Integrative Physiology, University of Colorado, Boulder, CO

Address for correspondence: Jason R. Franz, M.S., UCB 354, University of Colorado, Boulder, CO 80309-0354; E-mail: jason.franz@colorado.edu.

Submitted for publication October 2011.

Accepted for publication February 2012.

Advocates of barefoot running (26,27) claim that it is more “efficient” than running in shoes (shod). Obviously, barefoot running eliminates the mass of the shoes, and numerous studies have shown uniformly that adding mass to the feet or shoes increases the rate of oxygen uptake, V˙O2 (which reflects metabolic rate), during submaximal running. In the most comprehensive of these studies, Frederick et al. (17) quantified that, across a range of running speeds, V˙O2 increases by ∼1% for each 100 g of mass added to each shoe. Thus, if all other factors remain constant, running with shoes that each have a mass of ∼300 g should increase V˙O2 by 3% compared with barefoot running. However, of the seven studies that have compared barefoot and shod running (5,12,13,17,18,25,29), only two (12,13) have found a difference of that proportion. In fact, only those two studies have found any statistically significant difference in V˙O2 between barefoot and shod running. We depict the results of these studies in Figure 1. The lack of difference found in most of the studies suggests that factors other than shoe mass (e.g., foot strike pattern, barefoot running experience, shoe construction) play important roles in determining the metabolic costs of barefoot versus shod running.

FIGURE 1

FIGURE 1

Previous studies that compared barefoot and shod running have not controlled for those factors. When barefoot, runners tend to use a mid or forefoot strike pattern but most runners rearfoot strike when shod (8,10,11,19,21,23). None of the seven aforementioned studies of running economy controlled for foot strike pattern, and several explicitly note that they were comparing barefoot with a mid or forefoot strike to shod with a rearfoot strike (12,13). Because they mixed two factors, interpretation is difficult. Ardigo et al. (2) and Perl et al. (24) found no difference in V˙O2 between mid and rearfoot striking shod, and Cunningham et al. (9) found similar economy for both strike patterns barefoot. However, there may be interactive effects, and not controlling for foot strike pattern could confound barefoot versus shod comparisons. One might also expect that barefoot running economy would improve with practice. Only two of the barefoot V˙O2 studies (25,29) included exclusively subjects with extensive barefoot running experience. The others did not specify (12,17), studied runners without barefoot experience (5,13), or had subjects with varying amounts of experience (18).

Recognizing the pioneering efforts of our colleagues and with the benefit of hindsight, we designed the present experiment. We compared the metabolic effects of added mass during both barefoot and shod running and further compared V˙O2 and metabolic power during barefoot versus shod running. We controlled for foot strike pattern, barefoot running experience, and footwear. Based on the study by Frederick et al. (17), we first hypothesized that adding mass to the foot/shoe would increase metabolic cost by 1% per 100 g added per foot. Given the equivocal results of previous studies, our second hypothesis was that barefoot running would have the same metabolic cost as running in lightweight shoes. A logical extension of hypotheses 1 and 2 led to our third hypothesis: for conditions of equal foot/shoe mass, shod running would have a lower metabolic cost than barefoot running.

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METHODS

We based our sample size on the study of Frederick (15), who reported that, with an expected coefficient of variation of 1.5%–2% for repeated, within-day measurements of oxygen uptake (28), a 1%–2% mean difference between conditions could be resolved with a sample size of 10–15 subjects. Thus, we recruited 14 healthy male runners. Because of exclusions, our analysis comprised 12 subjects (mean ± SD; age = 29.8 ± 7.3 yr, body mass = 75.5 ± 7.1 kg, and height = 179.2 ± 5.3 cm). Inclusion criteria were mid-foot strike preference both barefoot and with shoes, run at least 25 km·wk−1 in total, including at least 8 km·wk−1 of barefoot or in minimal running footwear (e.g., Vibram Five Fingers) for at least 3 months of the last year, free from injury, self-reported ability to sustain 5 min·km−1 (3.3 m·s−1) running pace for at least 60 min, and meeting the criteria of the American College of Sports Medicine for minimal risk for exercise (1). From a self-report of their typical training, subjects averaged a speed of 3.5 ± 0.4 m·s−1 (range = 3.4–4.5 m·s−1) for a weekly average of 46.1 ± 21.3 km (range = 25–80 km). Based on these reports, our inclusion criteria, and subject feedback, completing this experimental protocol was of moderate intensity and duration for each subject. We did not exclude females but none who volunteered met all of the inclusion criteria. The University of Colorado Institutional Review Board approved the study protocol, and all subjects gave their written consent after being informed of the nature of the study.

To verify that the subjects preferred to run with a midfoot strike pattern (6), we asked them to run at their typical pace for a 10-km training run across a 30-m runway equipped with a force plate (Advanced Mechanical Technology, Inc., Watertown, MA) to which a sheet of paper was affixed. We taped small pieces of marker pen felt to each subject’s right foot at 90%, 70%, and 33% of foot length (measured along the line between the heel and the distal end of the second toe; Fig. 2). Force plate data were collected at 1000 Hz. We tracked the center of pressure relative to the foot outline provided by the pen marks left on the paper as per Cavanagh and LaFortune (6). We classified subjects as midfoot strikers if the center of pressure at initial contact was between 33% and 70% of foot length and rearfoot strikers if the center of pressure started posterior to the 33% mark. Only midfoot strikers were included in the study. We excluded one potential subject during this initial screening because of his rearfoot strike pattern preference while running barefoot.

FIGURE 2

FIGURE 2

Subjects completed a standing trial and a series of seven randomized 5-min running trials in the different footwear conditions during a single experimental session. A 4-min rest period separated each of the running trials. In all running trials, subjects ran at a speed of 3.35 m·s−1 for 5 min on a steel deck motorized treadmill (Quinton 18-60, Quinton Instruments, Seattle, WA) that we modified to have a calibrated digital readout of speed. For the duration of the experiment, subjects wore very thin, slip-resistant yoga socks for safety and hygienic purposes. For the shod trials, we purchased several pairs of the same model of very lightweight running shoes (Nike Mayfly). There is some evidence that shoe design characteristics other than mass (e.g., cushioning [16]) may influence metabolic cost. Thus, we selected this running shoe in particular because it has some cushioning but no other features such as medial posting/arch support or various other motion control elements. The Mayfly has a fused midsole–outsole made from Phylite®, a proprietary injection-molded ethylene vinyl acetate mixture with a high resilience. The Mayfly has a midfoot sole thickness of approximately 14 mm and a durometer of 58–62 shore C. We offered verbal instructions to each subject to maintain a mid- foot strike pattern whether barefoot or shod, which we then visually confirmed throughout each trial. The baseline mass for this experiment was defined as M, the mass of each lightweight shoe. Thus, M was specific to shoe size (M = 135.6 g for size 9, M = 142.3 g for size 10, and M = 150.2 g for size 11). Subjects completed four “barefoot” running conditions: barefoot (BF0M), barefoot with ∼150 g (BF1M), ∼300 g (BF2M), and ∼450 g (BF3M) of added mass per foot. Subjects also completed three shod running conditions: shoes alone (SH1M), shoes with ∼150 g (SH2M), and ∼300 g (SH3M) of added mass per shoe. We did not control for stride length so as to compare normal barefoot and shod running. Ten of the subjects also repeated the BF0M, BF1M, and SH1M conditions for 2 min while wearing a pressure-sensitive insole inside the sock of their right foot, sampled at 1000 Hz (B&L Engineering, Tustin, CA). We used these insoles to determine the timing of foot initial contact and calculated the average stride frequency for 10 consecutive strides. By knowing the treadmill speed, we then calculated the average stride length.

To add mass to each foot during barefoot trials, we modified the uppers of a different model of running shoes to allow for easy attachment of small lead strips while still simulating barefoot running (Fig. 3). We removed the outsole, midsole, and the entire front portion of the shoe upper, anterior to the midshaft of the fifth metatarsal, leaving only the heel counter, the thin fabric arch section, the tongue, and the laces. Because all of our subjects were midfoot strikers, this design exposed the plantar surface of the foot that would normally contact the ground during barefoot running. These modified uppers had a mass of 106.5 g each. We secured the lead strips by lacing them above the center of mass of the foot, along the tongue. The amount of added lead was adjusted so that the sum of the mass of the modified uppers plus lead mass was equal to 1M, 2M, and 3M.

FIGURE 3

FIGURE 3

During the standing and running trials, we collected the subject’s expired gases and calculated the STPD rates of oxygen consumption (V˙O2) and carbon dioxide production (V˙CO2) using an open-circuit respirometry system (TrueOne 2400; ParvoMedics, Sandy, UT). Before each experiment, we calibrated the gas analyzers using reference gases and a calibrated 3-L syringe. We averaged V˙O2, V˙CO2, and RER for the last 2 min of each trial. Subjects’ RER had to remain below 1.0 in order for their data to be included. One subject was excluded based on that criterion. We report gross V˙O2 values in milliliters of O2 per kilogram per minute but also the average standing value (mean ± SD; 4.74 ± 0.61 mL O2·kg−1·min−1) to allow calculation of net V˙O2. We normalized using the subject’s body mass while not wearing shoes. From V˙O2 and V˙CO2, we calculated gross metabolic power in watts per kilogram using Brockway’s (4) equation. Fletcher et al. (14) recommend that metabolic power is a more representative expression of running economy than V˙O2 alone.

We confirmed a steady-state (i.e., no time-dependent change) V˙O2 over minutes 3–5 for each trial using 15-s intervals and a repeated-measures ANOVA (all P values > 0.226). We also confirmed that the average V˙CO2 exhibited no time-dependent change during the experimental session (P = 0.436). A repeated-measures ANOVA tested for significant main effects of added mass and of footwear (barefoot vs shod). We then used a Student’s paired t-test to compare BF0M and SH1M. We used a criterion of P < 0.05 for statistical significance.

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RESULTS

Adding mass to the feet significantly increased oxygen uptake (V˙O2) whether running barefoot (P < 0.001) or shod (P < 0.001). Table 1 and Figure 4 summarize our metabolic results. Linear regressions of these data (Fig. 4A) indicated that for each 100 g of mass added per foot, V˙O2 increased by 0.92% and 1.19% during barefoot and shod running, respectively.

TABLE 1

TABLE 1

FIGURE 4

FIGURE 4

Without added mass, the mean V˙O2 and gross metabolic power were both 2.1% lower when running in shoes compared with barefoot (BF0M vs SH1M), but these differences were not statistically significant (P = 0.092 and P = 0.118, respectively). For 8 of the 12 participants, running in the lightweight, cushioned shoes was less metabolically demanding than running barefoot, despite the greater mass. Subjects selected 3.3% longer stride lengths during shod running (Table 1; P = 0.001). Importantly, the longer strides adopted when running in shoes reflected an effect of footwear and not of added mass; stride length did not significantly differ between BF0M and BF1M (P = 0.342).

For footwear conditions of equal mass, metabolic demand was significantly lower for running in shoes than running barefoot (Fig. 4). At 1M (BF1M vs SH1M), V˙O2 and gross metabolic power were both 3.4% lower when running in shoes compared with barefoot (P = 0.035 and P = 0.018, respectively). At 2M (BF2M vs SH2M), V˙O2 and gross metabolic power were 4.0% and 4.2% lower during shod running, respectively (P = 0.008 and P = 0.007, respectively). Finally, at 3M (BF3M vs SH3M), although V˙O2 was not significantly different (2.6% lower in shoes, P = 0.099), gross metabolic power was 3.3% lower when running in shoes compared with barefoot (P = 0.041).

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DISCUSSION

In the present study, we systematically quantified the metabolic effects of adding mass to the feet and compared V˙O2 and metabolic power during barefoot versus shod running while controlling for foot strike pattern, barefoot running experience, and footwear. Our findings support all three of our hypotheses. In a group of experienced midfoot striking barefoot runners: 1) V˙O2 increased by approximately 1% for each 100 g added per foot, whether barefoot or shod; 2) barefoot and shod running did not significantly differ in V˙O2 or metabolic power; and 3) for footwear conditions of equal mass, shod running had lower V˙O2 and metabolic power than barefoot running. Our results demonstrate no metabolic advantage of running barefoot over running in lightweight, cushioned shoes.

Adding mass to the feet predictably increases oxygen uptake during both barefoot and shod running. However, as hypothesized, V˙O2 and metabolic power when running in lightweight shoes are not significantly different compared with barefoot running, despite the greater mass. These two findings complement the accumulating body of evidence, indicating that factors other than shoe mass play important roles in determining the metabolic cost of barefoot versus shod running (5,16,17,24,25,29). Of the candidates for those factors, differences in foot strike pattern seem to have little effect on running economy. The three published studies on the topic have found midfoot striking and rearfoot striking to have equal V˙O2 (2,9,24). Arch support could affect V˙O2, but this has not been specifically or systematically investigated. Two possible factors to consider are differences in stride length and shoe cushioning characteristics. Their influences are difficult to discern until one examines barefoot and shod running conditions of equal mass.

For barefoot and shod running conditions of equal mass, V˙O2 and metabolic power are ∼3%–4% lower for running in shoes compared with barefoot. What factors produce these metabolic differences? First, our participants adopted a 3.3% greater stride length when running in shoes. Cavanagh and Williams (7) showed that shod runners naturally choose a stride length that minimizes metabolic cost. Thus, it is plausible that the shorter, faster strides preferred by barefoot runners increase V˙O2 and metabolic power. However, based on Cavanagh and Williams (7), we estimate that the 3.3% greater stride length during shod running would account for less than a 0.4% metabolic savings. Second, Frederick et al. (16) found that among different shoes matched for mass, more cushioned shoes reduce V˙O2. Similarly, they proposed that V˙O2 during barefoot running includes a “[metabolic] cost of cushioning the body.” Unlike during shod running, more of this “cushioning” is presumably accomplished through the actions of leg muscles during barefoot running, incurring a metabolic penalty (3). Here, we controlled for shoe cushioning properties and found uniformly lower values of metabolic power at 1M, 2M, and 3M for running in shoes compared with barefoot. Our findings lend support to the “cost-of-cushioning” hypothesis.

A minority of studies published on this topic have found a statistically significant difference in metabolic cost between barefoot and shod running (12,13,18). Flaherty (13) found that running in shoes of mass 356 g elicited a 4.6% greater V˙O2 than running barefoot. More recently, Divert et al. (12) compared V˙O2 during barefoot running to that when running in 150 and 350 g shoes and showed that only the heavier shoes elicited a greater V˙O2 (3.4%) than barefoot. Further, they found that, for conditions of equal mass, barefoot and shod running showed equivalent values of oxygen uptake. However, 9 of the 12 participants in the study of Divert et al. (12) preferred a midfoot strike pattern when barefoot, and all were rearfoot strikers when shod. Also, the 150 and 350 g shoes had substantially different shoe cushioning properties. Hanson et al. (18) recently quantified V˙O2 in participants with varying degrees of barefoot running experience. In that study, when running speed was maintained on a treadmill, V˙O2 was a statistically insignificant 2.0% greater even when running in shoes of considerable mass (353 g) compared with barefoot. Hansen et al. (18) also reported a 5.7% difference in V˙O2 during overground running. However, we suspect that a systematic error occurred in the way they controlled overground running speed such that subjects ran slower barefoot compared with shod (22). Our finding that barefoot and shod running did not significantly differ in V˙O2 or metabolic power agrees with the majority of earlier studies (5,17,25,29). Conversations with Divert et al. (12) confirmed that they used the same model ∼150 g shoe and found no increase in V˙O2 compared with running barefoot.

Our study provides new insight into the question: is lighter better? Although lighter, we find that running barefoot offers no metabolic advantage over running in lightweight, cushioned shoes. Certainly, because of the predictable increase in V˙O2 and metabolic power with added mass, lighter shoes are more economical than heavier shoes of equivalent construction. Further, despite the added mass, some features of shoes (e.g., cushioning) likely offer a metabolic savings compared with barefoot running. Figure 1 shows that, compared to barefoot, only one study reported an increase in V˙O2 for shod running above that expected by the greater mass. Thus, it seems that various aspects of shoe construction positively affect running economy. However, for typical running shoes, the positive effects are not enough to offset the penalty incurred by greater shoe mass. It remains to be systematically determined how individual shoe design features affect running economy.

Our results suggest that if the shoes presently tested had been marginally lighter, metabolic demand during shod running would have been statistically significantly lower than barefoot. To evaluate this, we modeled the relationship between oxygen uptake and added shoe mass. Specifically, we iteratively decreased each subject’s average V˙O2 during shod running by 0.01% (corresponding to ∼1 g per shoe), thereby increasing the mean difference in V˙O2 between barefoot and shod running (BF0M vs SH1M) until we found a statistically significant 2.5% difference between conditions (P = 0.049). We then used the linear regression for adding mass during shod running to predict the mass that would elicit this statistically predicted V˙O2. Based on this technique, we propose that running in a shoe of comparable design, but of average mass 129.0 g (only 16 g less than the shoe model we tested), would offer a metabolic advantage over running barefoot.

There are several limitations of this study. First, by design, our findings are limited to runners who use a midfoot strike pattern. Further, our findings are limited to the model of lightweight shoes that we investigated. Running shoes vary considerably in mass, cushioning properties, amount of arch support, motion control characteristics, heel lift, and flexibility. It is unclear the extent to which each of these factors affects running economy. We also studied a relatively slow treadmill running speed (equivalent to a 5-min·km−1 pace). We were unable to find a sufficient number of subjects with considerable barefoot running experience who could aerobically maintain faster running speeds. Because none of the females who volunteered met all of our inclusion criteria, we included only male participants. Finally, our sample size may have been statistically underpowered to find a significant difference between BF0M and SH1M (effect size = 0.57, 1 − β = 0.58). A post hoc power analysis revealed that the 2.1% greater metabolic energy consumption for BF0M compared to SH1M would require an additional 15 subjects to reach statistical significance.

Future research on this topic should seek to establish the independent effects of stride length and shoe cushioning on metabolic cost in barefoot versus shod running. To examine the effects of stride length, one could enforce the preferred shod stride frequency during barefoot running at the same running speed. To test the “cost-of-cushioning” hypothesis, one could have subjects run barefoot on a surface that has cushioning characteristics similar to those of cushioned running shoes (e.g., Kerdok et al. [20]). Such research could identify if there is an optimal amount of shoe cushioning that minimizes metabolic demand during running.

In summary, although adding mass to the feet predictably increases oxygen uptake, V˙O2 and metabolic power do not differ between running barefoot and in lightweight, cushioned shoes. Further, for conditions of equal mass, shod running provides a metabolic savings of ∼3%–4% compared to barefoot. Despite the added mass, we predict that running in ultralightweight but cushioned shoes should be more economical than running barefoot.

This study was supported by the University of Colorado Undergraduate Research Opportunity Program, but independent of any shoe company sponsorship.

This study did not receive financial support from the National Institutes of Health, the Welcome Trust, or the Howard Hughes Medical Institute.

The authors have no conflicts of interests to disclose.

The results of the present study do not constitute endorsement by the American College of Sports Medicine.

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Keywords:

ECONOMY; ENERGETIC COST; OXYGEN CONSUMPTION; SHOE; FOOTWEAR; EFFICIENCY

©2012The American College of Sports Medicine