A high V[Combining Dot Above]O2max is a prerequisite for competition in endurance events, such as distance running, although running economy (RE) is a stronger predictor of performance among athletes of comparable aerobic ability (4,7). Running economy represents the energy demands of running and is defined as submaximal oxygen consumption for a given velocity (8). Considering improvements in maximal oxygen consumption is dependent on training history (19), a more short-term immediate enhancement to endurance performance would be to improve RE. Hence, among trained athletes with similar V[Combining Dot Above]O2max profiles, RE is an important metric that explains a large and significant amount of the variation in distance running performance (7).
To improve RE, strategies targeting cardiorespiratory, metabolic, biomechanical, and/or neuromuscular components of performance have been most successful (2). Systematic reviews and meta analyses indicate that unshod (barefoot) running improves RE compared with shod (wearing shoes) running (6,14). A long-standing hypothesis accounting for this reduction in oxygen consumption while unshod is mass related; in other words, the oxygen cost of running increases and is directly proportional to the amount of mass added to the foot (5).
Given the attractiveness and potential utility of unshod running as a vehicle to improve RE (and race performance), several original investigations have appeared in the literature testing the hypothesis that running without shoes reduces oxygen consumption. Among these investigations, most include only male subjects (9,12,29,34,35), 1 includes both males and females (15), whereas only 2 studies have used female subjects (22,28). In regard to RE, conclusions from studies that have used both male and female subjects together may be equivocal because of the influence of gender as a confounding variable (3,16).
One major shortcoming in the RE literature among trained female distance runners in particular is the relatively low fractional utilization of V[Combining Dot Above]O2max elicited. For example, both Moore et al. (22) and Paulson and Braun (28) measured RE between unshod and shod conditions by administering velocities that required approximately 60–70% of relative V[Combining Dot Above]O2max. However, such intensities are moderate in nature and would not reflect the spectrum of exercise intensities used during competitive distance running. Hence, selecting an array of velocities that requires relatively high fractional utilization of V[Combining Dot Above]O2max (e.g., up to 85%) would correspond to the actual energy demands associated with long-distance (marathon) running (4). Therefore assessing RE at a velocity more closely matched to race pacing and intensity (i.e., 85% of relative V[Combining Dot Above]O2max) would be a more telling indicator of RE during competition.
The purpose of this study was to determine whether unshod compared with shod running reduced the physiologic and perceptual demands during submaximal treadmill running in recreationally trained long-distance female runners. Our hypothesis was that unshod running would reduce submaximal oxygen consumption, respiratory exchange ratio (RER), lactate, heart rate (HR), and ratings of perceived exertion (RPE), at velocities corresponding to 65, 75, and 85% of V[Combining Dot Above]O2max, respectively.
Experimental Approach to the Problem
To test the hypotheses that unshod running improves RE (submaximal oxygen consumption), RER, lactic acid, HR, and RPE, a within-subject randomized crossover study design was chosen. Recreationally trained long-distance female runners who were unaccustomed to unshod running were recruited to participate in the study. Twenty-four hours before data collection, each subject was instructed to refrain from exercise, caffeine, and alcohol consumption. The independent variable condition (unshod vs. shod) was chosen to test the main dependent variable, oxygen consumption, which was measured at velocities that required 65, 75, and 85% of V[Combining Dot Above]O2max. Respiratory exchange ratio, lactate, and HR responses were chosen as dependent variables because of the relevancy of these metabolic components during long-distance running performance (10,24). A perceptual variable, RPE, was chosen as a dependent measure of perceived intensity (11).
Fourteen recreationally trained long-distance female runners and who had been running 38.14 ± 7.59 km (mean ± SEM) per week for the previous 4 months were recruited. Nine of the 14 runners had completed at least a half or full marathon. Inclusion criteria included at least 1 competition race equal to or greater than 5 kilometers at a race pace of 4:20 to 5:35 min·km−1. Exclusion criteria included any previous experience with unshod running and current or previous lower extremity injury or other orthopedic limitations. The Institutional Review Board at Florida Atlantic University approved this study and written informed consent was obtained from all subjects before enrollment (Table 1).
As a brief overview of the study procedures, all subjects met for the initial day (day 1) of testing and 7.29 ± 0.66 days (mean ± SEM) later were randomized and completed either unshod or shod conditions, on days 2 and 3 to determine RE. Each subject completed the unshod and shod submaximal running trials (days 2 and 3) within 1.53 ± 0.14 days (mean ± SEM).
An informed consent, health history questionnaire, and running history questionnaire were completed. Height was measured on a wall-mounted stadiometer to the nearest 0.1 cm. Weight was assessed on a calibrated digital scale (Toledo Scale, Model # 8140; Toledo Electronic Weighing Scale, Worthington, OH, USA) to the nearest 0.1 kg. After body mass determination, each subject removed her running shoes, placed them on the same calibrated digital scale, and shoe mass was assessed to the nearest 0.001 kg. Circumference measurements were obtained from the largest portion of the thigh and calf to determine girth (20). Body fatness was determined with the 3-site skinfold method using calibrated Lange calipers, which included the triceps, suprailiac crest, and the front portion of the midthigh. A single skilled, and reliable, investigator completed body fat measurements for all subjects (1). A maximal graded exercise test (GXT) designed around race pace (4:20 to 5:35 min·km−1) on a motor-driven treadmill (Q65, Series 90; Quinton Instruments, Bothell, WA, USA) was administered. In brief, the GXT began at 8.85 km·h−1, 0% grade, and increased by 1.61 km·h−1 and 2% grade every 3 minutes until volitional failure. The highest level of oxygen consumption during a 1-minute period was considered as V[Combining Dot Above]O2max if the following 3 of the 4 criteria were met: (a) subject failed to increase oxygen consumption by 150 ml with an increase in workload, (b) subject had a RER greater than or equal to 1.10, (c) subject met or exceeded age-predicted maximum HR, and (d) subject chose greater than or equal to 17 for perceived exertion (Borg scale). Termination criteria were only accepted for completed stages. Fifteen seconds before the end of each stage, HR and RPE were recorded.
Days 2 and 3
The protocol was identical for both unshod and shod conditions. Running economy was assessed on a treadmill during 3 × 5-minute stages, with 2 minutes of standing rest separating each stage. Stage 1 required 5 minutes of running at 8.85 km·h−1 (∼65% of V[Combining Dot Above]O2max), stage 2 at 10.46 km·h−1 (∼70% of V[Combining Dot Above]O2max), and stage 3 at 12.07 km·h−1 (∼85% V[Combining Dot Above]O2max), all at 1% grade to reflect the energetic cost of outdoor running (17). The energy demand (oxygen cost = V[Combining Dot Above]O2 (ml·kg−1·min−1) was assessed by a calibrated metabolic system (TrueOne 2400 Metabolic Measurement System; ParvoMedics, Sandy, UT, USA). Fifteen seconds before the end of each stage, HR and RPE were recorded. Immediately after the completion of each stage a finger prick yielded a small capillary blood sample for lactate analysis (YSI 2300 STAT plus; YSI Instruments, Yellow Springs, OH, USA).
Shapiro-Wilk tests were run on all dependent variables to determine normality. A 2 (condition: shod, unshod) by 3 (intensity: 65, 75, and 85% of V[Combining Dot Above]O2max) analysis of variance (ANOVA) with repeated measures was conducted for submaximal oxygen consumption, RER, lactate, HR, and RPE. Holm-Sidak method was used for pairwise multiple comparison procedures post hoc. Univariate linear regression was run on select variables. An alpha level for statistical significance was chosen as p ≤ 0.05. For the sensitivity power analysis regarding the ANOVA, a large effect size of f = 0.41 was determined with G*Power version 3.1 given the sample size of n = 14. Statistical analyses were performed using SigmaPlot version 13.0 (Systat Software, Inc., San Jose, CA, USA).
All data passed Shapiro-Wilk normality tests. For submaximal oxygen consumption (RE), the 2-way ANOVA with repeated measures showed a nonsignificant trend for condition (p = 0.09), a significant main effect for intensity (p < 0.001), and a significant interaction between condition and intensity (p = 0.049). Holm-Sidak post hoc analyses showed significance for condition at 85% intensity (p = 0.018). Oxygen consumption between conditions was not significant for intensities at 65 and 75% of V[Combining Dot Above]O2max (p > 0.05) (Figure 1).
The 2-way ANOVA with repeated measures for RER, lactate, HR, and RPE showed a significant main effect for intensity (p < 0.001, all) only. There were no main effects for condition or (intensity × condition) interactions for these dependent variables (p > 0.05, all). For lactate, Holm-Sidak post hoc analyses showed a trend for significance for condition at 85% intensity (p = 0.11) (Figure 2 and Table 2).
Univariate linear regression showed statistical significance for thigh to calf ratio as a predictor variable and V[Combining Dot Above]O2max and body fat as response variables (V[Combining Dot Above]O2max: R = −0.66, Rsqr = 0.44, SEE = 3.72, y = 86.1 − (25.35 × thigh to calf ratio), p = 0.01; body fat: R = 0.68, Rsqr = 0.46, SEE = 1.96, y = −3.55 + (13.99 × thigh to calf ratio), p = 0.008) (Figure 3).
The principal finding in this study is that unshod running improves RE at 85% V[Combining Dot Above]O2max in recreationally trained long-distance female runners. It is speculative whether running unshod on a treadmill, as a means to reduce oxygen consumption, would translate to improved race performance during actual competition outdoors. However, competitive long-distance (marathon) running does require a similarly high fractional utilization of V[Combining Dot Above]O2max (4) used in this study, so it is plausible that unshod running may benefit trained endurance athletes who are able to use a large percentage of their V[Combining Dot Above]O2max. The subjects in this study were completely unaccustomed to unshod running, and thus the demonstrated improvement in RE was immediate. This discussion will highlight biomechanical aspects for the improvement in RE at 85% of V[Combining Dot Above]O2max and how this improvement may be applicable to the recreational runner.
According to load-carrying experiments, the improvement in RE is most likely due to the removal of the shoe mass from the foot. In fact, Frederick (13) indicated that a 1% increase in oxygen uptake is expected per 100 grams of mass added to each foot. In this study, the average pair of shoes weighed 590 grams (range = 460–800 g); therefore, the 4.27% improvement in RE (i.e., reduction in oxygen cost) while unshod, in part, is due to altered inertial properties and reduced mechanical work of the unloaded extremities (5,21,23). Reeves demonstrated a similar enhancement of RE in trained runners at high exercise intensities that was dependent on the mass of the shoe (29). Our data indicate the improvements in RE while unshod occur at an exercise intensity that closely matches race pace.
Athletes inexperienced with unshod running may perceive exposure of the barefoot to environmental conditions as high risk. This perception does not have scientific support. Several unshod running studies have reported that the barefoot allows for greater proprioception that is associated with a reduction in running-related injuries (31–33). For example, increased barefoot weight-bearing activities, such as indoor and outdoor walking and running, leads to medial longitudinal arch shortening, which is a beneficial adaptation that increases the natural shock absorbing capacity of the foot (33). When in shoes, however, a false “sense of security” is created because of deceptive advertising, which results in unfavorable landing strategies that increase ground reaction forces (30).
Furthermore, balance is also negatively impacted during shod conditions (30), which negatively affects the innate impact-moderating behavior of the unshod foot (32). For example, the arch of the foot acts as a natural spring (18), but the total forefoot stiffness of the shod foot is dominated by that of the unshod foot (27). Hence, bending stiffness of running shoes does not have a significant effect on racing performance but rather may reduce performance because of added work performed on the shoe (and not the ground where it can be returned for forward propulsion) during running (25,26). In summary, incorporating daily unshod activities in a variety of terrain may reduce running-related injuries because of positive structural and functional adaptations of the bare feet.
Other biomechanical factors favor RE such as low body fatness and leg mass that is distributed closer to the hip joint (20). In this study population, the thigh to calf ratio was inversely associated with V[Combining Dot Above]O2max and positively associated with body fatness. During initial statistical analyses, the thigh to calf ratio was used as a covariate in an analysis of covariance and was determined to not be related to or influence oxygen consumption between the shod and unshod conditions. In summary, our study provides continued support that unshod running reduces submaximal oxygen consumption at high exercise intensities in recreationally trained female distance runners.
In this study, recreationally trained female distance runners demonstrated significant improvements in RE on a treadmill at 85% of V[Combining Dot Above]O2max while unshod, but not at 65 or 75%. Considering RE is strongly correlated with endurance performance, and in our study, a reduction in submaximal oxygen consumption was seen immediately and unshod running may improve race performance by minimizing the energy demands during competitive or recreational running. In other words, greater RE enables endurance athletes to use a lower percentage of their V[Combining Dot Above]O2max for a given velocity. To that end, this study demonstrated in a controlled laboratory environment that unshod running is a performance aid. Coaches should consider using barefoot treadmill running as a safe training strategy, or tool, to improve RE at race pace, even among those lacking previous experience with barefoot running. Nevertheless, it is speculative whether these unshod running benefits would translate to improved distance running performance during actual competition outdoors.
The authors thank Eric Golinko of FAU for his statistical expertise. There was no financial support whatsoever from any source during the entirety of this project. The authors have no conflicts of interest to disclose. There are no professional relationships with companies or manufacturers who would benefit from the results of this study. The results of this study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association.
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Keywords:Copyright © 2016 by the National Strength & Conditioning Association.
endurance; performance; V[Combining Dot Above]o2max; economy; treadmill