Orthopedics & Biomechanics
789
Authors
S. Paulson, W. A. Braun
Affiliations
Exercise Science, Shippensburg University, Shippensburg, United States
Key words ▶ running economy ● ▶ minimalist footwear ● ▶ gait mechanics ●
Abstract
▼
Barefoot running and running using minimalist footwear have become increasingly popular in recent years. Footwear choice may affect running mechanics and the metabolic cost of running. To investigate these factors, 8 well-trained, female distance runners (mean age = 20.1 ± 1.4 years) were recruited to participate in the study. Following orientation to testing procedures, subjects completed 3 running economy tests on separate days. Treatment order (barefoot, minimalist footwear and running shoe) was counter-balanced. Each testing session consisted of a 5-min warmup at 2.24 m · s − 1, followed by the 7-min RE test at 3.13 m · s − 1. Biomechanical data were collected at the 3-min mark for 10 s, and expired gases were
Introduction
▼ accepted after revision January 01, 2014 Bibliography DOI http://dx.doi.org/ 10.1055/s-0034-1367064 Published online: February 27, 2014 Int J Sports Med 2014; 35: 789–793 © Georg Thieme Verlag KG Stuttgart· New York ISSN 0172-4622 Correspondence Dr. Sally Paulson Exercise Science Shippensburg University 1871 Old Main Dr. 17257 Shippensburg United States Tel.: + 1/717/477 1274 Fax: + 1/717/477 4083 [email protected]
Barefoot running has been used as a strategy for improving lower extremity muscular strength, particularly of the muscles surrounding the ankle. Research has shown that there are physiological and biomechanical differences between shod and barefoot running which could be beneficial to distance athletes [3, 6, 9, 10, 16, 27]. Running barefoot is thought to enhance performance and decrease the risk of overuse injuries [13, 18, 23, 26]. It has been suggested that the more natural condition of running barefoot is beneficial and that the wearing of shoes decreases running economy by altering the natural mechanics of the foot and/or adding mass to the foot [10, 16]. Robbins and Hanna [26] reported that the medial longitudinal arch rises and shortens due to the activation of the intrinsic muscles which causes the arch to absorb more of the vertical load upon impact. Furthermore, barefoot runners employ an ankle coordination strategy that reduces the ground reaction forces by allowing a greater surface area to absorb forces upon
collected from minutes 5–7. One-way repeated measures ANOVA revealed a statistically significant difference for running economy (p = 0.04), expressed as relative oxygen uptake per km in the barefoot condition (running shoe: 204.51 ± 2.84; minimalist footwear: 198.21 ± 3.04; barefoot: 193 .26 ± 3.62 ml · kg − 1· km − 1) vs. running shoe. The other physiological and biomechanical variables were not statistically significant (p > 0.05). However, moderate to large effect sizes suggested there were biomechanical changes that ensured between conditions. It should be further evaluated whether these mechanical adjustments and the running economy trend would translate into improved distance race performance while running barefoot or with minimalist footwear.
contact [15, 16]. Footwear is designed to reduce impact during the heel strike, to control rearfoot motion during loading, and to stabilize the forefoot during stance [10, 24]. However, footwear may cause a decrease in sensory feedback without decreasing the impact load [26]. Previous research has shown improvements in economy of transport (4–5 % lower oxygen requirement) while running barefoot as compared to shod running [16, 18, 23]. DiPrampero et al. [8] suggested that a 5 % increase in running economy resulted in a 3.8 % improvement in distance running performance. Additionally, Hanson et al. [16] reported similar increases for both overground and treadmill running. This physiological effect may be due to the biomechanical differences that have been shown in other studies [9, 10, 18]. Stride length, as well as contact and flight times, have been shown to be shorter and knee velocity during swing to be greater in barefoot running when compared to shod running [7, 9, 13, 29]. In addition, barefoot runners landed with greater plantar flexion to adopt a flatter foot placement upon contact and demonstrated
Paulson S, Braun WA. Mechanical and Physiological Examination … Int J Sports Med 2014; 35: 789–793
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Mechanical and Physiological Examination of Barefoot and Shod Conditions in Female Runners
790 Orthopedics & Biomechanics
Materials and Methods
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Subjects 8 female collegiate cross-country runners volunteered to participate in the study (age = 20.1 ± 1.4 years; mass = 54.1 ± 2.9 kg; height = 1.64 ± 0.06 m; body fat = 18.3 ± 2.9 %). Subjects were screened for risk factors using a health history questionnaire and were required to be free of lower extremity injury in the previous 6 months. All subjects were shod runners, but had regular training experience with BF running. Study participants were directed to abstain from running and other forms of exercise for the 24 h prior to testing and to arrive at the laboratory having fasted for a minimum of 3 h. Time of day for testing was consistent across all participants. The university’s institutional review board approved the study, and all subjects provided written consent prior to participation in the study. The study was conducted according to ethical standards [17].
Protocol The subjects were asked to report to the laboratory for 4 treadmill-running (Woodway Desmo-Pro treadmill, Waukesha, WI) sessions. During the first session, anthropometric variables were collected and a 30-min treadmill and equipment orientation run was performed [19]. The remaining testing sessions followed the same procedures with the exception that the condition [barefoot (BF), running shoe (RS), minimalist footwear (MF)] was randomized and counter-balanced. The Vibram FiveFingersTM KSO (Vibram USA, Concord, MA) multisport shoe was used for the MF condition, and subjects were asked to wear their regular running shoe for the RS testing session. The subjects completed a 5-min warm-up at 2.24 m · s − 1 (5 mph). Following the warm up, the
subjects ran on the treadmill for 7 min at 3.13 m · s − 1 (7 mph) and then performed a 5 min cool-down. Biomechanical data were collected at the 3-min mark for 10 s, and expired gases were collected from minutes 5–7. The testing sessions were separated by 5–7 days.
Biomechanical measurements Prior to the treadmill run, 6 reflective markers were applied along the right side of the body using double-sided adhesive tabs on the acromioclavicular joint, greater trochanter, lateral aspect of the knee joint, lateral malleolus, base of the fifth metatarsal (outside of the shoe) and lateral aspect of the heel (outside of the shoe). Lights were used to illuminate the reflective markers during data collection. Lower extremity (LE) kinematic data were collected (60 Hz) during the treadmill runs using a 2D motion-capture system (Vicon Motion Systems, v. 9.2, Englewood, CO) from the sagittal plane. A calibration frame was captured prior to each data collection session. All trials were digitized and analyzed using the Vicon Motus (v. 9.2) software. The camera (Panasonic GV-35, Secaucus, NJ) was attached to a stationary tripod (height = 0.85 m) and positioned 2.75 m from the treadmill. The camera field of view was perpendicular to the subject, and a shutter speed of 1/750 s was used during data collection. One trial was collected for 10 s for each condition. 30 s prior to filming, the subjects were instructed to drop back on the treadmill belt to prevent interference of the treadmill arm rails during digitizing. Subjects wore dark form-fitting attire for all sessions, and all reflective material was covered prior to data collection. The first 5 strides of the 10-s video were cropped in an attempt to minimize the effect of positioning the subjects toward the back of the treadmill. The next 3 consecutive strides were digitized and smoothed using a Butterworth 6 Hz cut-off frequency filter. The time to complete the 3 strides was obtained from the analysis of the video data. Stride rate (SR) was averaged from 3 strides and the time to complete the 3 strides. Stride length (SL) was calculated from the speed of the treadmill and the time to complete one stride. The speed of the treadmill was assumed to be equivalent to the runner’s speed [14]. The ankle and knee angles were obtained for each condition at initial contact (IC) and end contact (EC) with the treadmill belt. The first frame and last frame the subject contacted the treadmill belt were used to determine IC and EC, respectively [7]. Ankle and knee range of motion (ROM) [14] and vertical oscillation of the center of mass (COM) were calculated [30].
Running economy measurements Running economy was assessed by measuring oxygen uptake via indirect calorimetry using a calibrated metabolic measurement system (ParvoMedics TrueMax, Sandy UT). Expired gases were collected for a 2-min period starting at min 5. Standard mouthpiece and breathing tube apparatus were used. At approximately 4:30, the nose clip and mouthpiece apparatus were handed to the runner. While expired gases were collected, the respiratory tubing was supported by an investigator to minimize vibration and subsequent impact on the runner’s gait pattern. Expired gases were averaged over 1-min periods, and the last minute of each collection sample was used to determine oxygen uptake, respiratory exchange ratio and minute ventilation. Oxygen uptake was normalized for determination of running economy over a fixed distance and was expressed as ml · kg − 1· km − 1.
Paulson S, Braun WA. Mechanical and Physiological Examination … Int J Sports Med 2014; 35: 789–793
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greater leg stiffness throughout the stance phase [7, 10, 18, 29]. An increase in leg stiffness has been positively correlated with running economy [28]. The majority of the running literature has primarily examined male runners. Additionally, the research surrounding running economy and sagittal plane kinematics have used male subjects that may not have been accustomed to barefoot running [2, 7, 10]. While Squadrone et al. [29] used experienced barefoot runners, it was also an all-male sample. To our knowledge, differences between barefoot and shod running has not been previously investigated in an all-female cohort accustomed to barefoot running. In addition, previous research has suggested males and females adopt different running strategies at the hip and the knee [11, 21]. More specifically, females tend to have less knee flexion, greater valgus knee positioning, [1, 21] and displayed less lower extremity joint variability than their male counterpart [1]. The Vibram FiveFingersTM is a specialized shoe that is designed to mimic barefoot running by providing minimal support but also some level of protection to the foot [29]. However, it is unclear whether or not these shoes do indeed mimic barefoot running. Additionally, due to kinematic differences between the sexes female runners may respond differently to barefoot or minimalist footwear running. Therefore, the aim of this study was to examine running economy and sagittal plane kinematics in female distance runners accustomed to barefoot running during a continuous treadmill run under 3 conditions: barefoot (BF), minimalist (Vibram FiveFingersTM) footwear (MF) and regular running shoes (RS).
Orthopedics & Biomechanics
Variable ankle angle IC ( °) ankle angle EC ( °) ankle ROM ( °) knee angle IC ( °) knee angle EC ( °) knee angle swing ( °) knee ROM ( °) stride length (m) stride rate (strides/s) ground contact time (s) vertical COM (cm)
Regular running shoe 24.16 ± 2.13 53.98 ± 2.50 49.77 ± 4.03 151.06 ± 2.09 155.71 ± 1.52 81.62 ± 3.06 82.44 ± 3.05 1.15 ± 0.01 2.73 ± 0.03 0.12 ± 0.002 12.08 ± 0.43
Barefoot
Minimalist footwear
24.01 ± 2.87 43.51 ± 3.85 43.51 ± 1.93 152.85 ± 2.34 149.93 ± 5.94 84.23 ± 3.30 82.73 ± 3.82 1.12 ± 0.01 2.81 ± 0.02 0.12 ± 0.003 12.00 ± 0.47
20.07 ± 2.23 51.51 ± 2.94 47.16 ± 2.43 146.92 ± 2.20 154.62 ± 3.31 81.24 ± 3.71 85.23 ± 5.20 1.16 ± 0.04 2.71 ± 0.08 0.11 ± 0.007 11.78 ± 0.75
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Table 1 Kinematic data (M ± SE) of female distance runners wearing different footwear during treadmill running.
Expired gases and respiratory exchange ratio were used to derive rate of energy expenditure (kcal · min − 1).
A one-way repeated measures ANOVA and descriptive statistics (SPSS, v. 20.0, Chicago, IL) were used to analyze the variables. Differences between treatments were quantified using dependent t tests. The significance level was set at p < 0.05. For ANOVA analyses, the independent variable was treatment (BF, RS, MF). Dependent variables included running economy, respiratory exchange ratio (RER), minute ventilation (Ve), heart rate (HR), SL, SR and kinematic measures. The biomechanical variables measures were ankle and knee angles at IC and EC, ankle and knee ROM, knee angle at mid-swing, ground contact time (GCT) and vertical oscillation of COM (vCOM). All biomechanical dependent variables were averaged from 3 complete gait cycles for each condition. In addition, effect sizes (ES) were calculated. An ES less than 0.2 was a small effect, 0.5 a moderate effect and 0.8 a large effect [4].
VO2 (ml.kg–1.km–1)
Statistical analysis
210 205 200
**
195 190 185
BF
MF Condition
RS
Fig. 1 Relative oxygen uptake per kilometer under 3 testing conditions: barefoot (BF), minimalist footwear (MF), running shoe (RS); (M ± SE, N = 8). Running speed = 3.13 m · s − 1. *p = 0.04.
0.908 0.906 0.904
RER
0.902
Results
▼
The mean ± SE for the kinematic variables are presented ▶ Table 1. There was not a statistically significant difference in ● between the footwear conditions for ankle angle at IC [F(2,14) = 1.18, p = 0.34], knee angle at IC [F(2,14) = 2.49, p = 0.12], ankle angle at EC [F(2,14) = 2.64, p = 0.12], or the knee angle at EC [F(2,14) = 0.70, p = 0.51]. The knee flexion angle at mid-swing was not significantly different between the conditions [F(2,14) = 1.21, p = 0.33]. The ROM at the ankle [F(2,14) = 1.54, p = 0.25] and the knee [F(2,14) = 0.64, p = 0.54] were also not significantly different. Furthermore, there were no significant differences between GCT [F(2,14) = 1.64, p = 0.23], vCOM [F(2,14) = 0.23, p = 0.80], SL [F(2,14) = 1.04, p = 0.38] and SR [F(2,14) = 1.04, p = 0.38]. Although, the kinematic variables were not statistically significant, there was evidence suggesting changes were ensured. Several dependent variables had moderate to large ES. For instance, at IC the ankle angle was 16 % smaller while wearing the MF compared to the RS (ES = 0.54) or BF (ES = 0.66). Additionally, during the BF condition at EC the ankle angle was 19 % and 16 % smaller compared to the RS (ES = 1.14) and MF (ES = 0.83), respectively. The ankle had the greatest ROM during the RS and was 13 % and 5 % higher than the BF (ES = 0.59) and MF (ES = 0.28) conditions, respectively. Furthermore, the knee angle during the MF condition was flexed more at IC than RS (2.47 %, ES = 0.68) and BF (3.88 %, ES = 0.92) running. However, during EC the knee
0.9 0.898 0.896 0.894 0.892 0.89
BF
MF Condition
RS
Fig. 2 Respiratory exchange ratio (RER) under 3 testing conditions: barefoot (BF), minimalist footwear (MF), running shoe (RS); (M ± SE, N = 8). p > 0.05.
had greater flexion during the BF condition by 3.71 % and 3.13 % compared to the RS (ES = 0.47) and the MF (ES = 0.34) respectively. During the BF condition SR was greater than during RS (ES = 1.22) and MF (ES = 0.59) by 2.86 % and 3.56 %, respectively. Thus, SL was smaller in the BF condition by 3.6 % and 4.50 % compared to the RS (ES = 0.99) and MF (ES = 0.51), respectively. ▶ Fig. 1–3, The mean running economy, RER, and Ve are shown in ● respectively. Running economy, expressed as ml · kg − 1· km − 1, was significantly different across conditions [F(2,14) = 4.32, p = 0.04]. The BF condition required a 5.8 % lower oxygen uptake compared to the shod condition (t = − 3.06, p = 0.02; ES = 1.22). Though non-significant, the MF condition was associated with a 3.2 % lower oxygen uptake than the shod condition (t = − 1.60, p = 0.15; ES = 0.76). No other cardiorespiratory measures were found to be different across conditions, including RER [F(2,14) = 0.23, p = 0.80], Ve [F(2,14) = 1.02, p = 0.39] and VCO2
Paulson S, Braun WA. Mechanical and Physiological Examination … Int J Sports Med 2014; 35: 789–793
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IC is initial contact and EC is end contact
40
Ve (L·min–1)
39 38 37 36 35 34 33
BF
MF Condition
RS
Fig. 3 Minute ventilation (L · min − 1) during 3 testing conditions: barefoot (BF), minimalist footwear (MF), running shoe (RS); (M ± SE, N = 8). p > 0.05.
[F(2,14) = 2.68, p = 0.10]. Rate of caloric expenditure did not differ across conditions (BF = 9.62 ± 0.33; MF = 9.85 ± 0.31; RS = 10.16 ± 0.21 kcal · min − 1). Likewise, HR and RPE measures did not differ across conditions (p < 0.05).
Discussion
▼
In the present study differences in kinematics, running economy and other cardiorespiratory variables were studied to gain more insight as to how well-trained, female runners respond acutely to different footwear during treadmill running. Running economy was significantly lower in the BF compared to the RS condition. While none of the kinematic variables measured were significantly different, certain findings were consistent with previous research and provide valuable clinical significance. For instance, several studies have reported that BF runners demonstrate decreases in SL and increases in SR when compared to shod running [7, 9, 13, 20]. In the current study, the female runners displayed a similar non-significant pattern of increases in SR and decreases in SL while running barefoot. A majority of the studies examining foot placement during IC have reported an increase in plantarflexion at the ankle when running BF, leading to the adoption of a flatter foot upon contact [7, 13, 23]. De Clercq and colleagues [5] reported that heel-loading pressure was reduced when foot placement upon contact was more horizontal. The female distance runners in this investigation landed more flat-footed during the MF condition and demonstrated little difference between RS and BF. Thus, the subjects may have adopted a flatter foot positioning to decrease heel-loading pressure as this type of footwear has little cushioning. In addition, the knee flexion angle upon contact was more flexed while wearing the MF. De Wit et al. [7] reported a similar adaption at the knee with a flat-foot placement. Even though the runners were given the MF several weeks prior to data collection, this may have not been enough time to become accustomed to running in this type of footwear. McCarthy et al. [22] found that 12 weeks of MF training lead to the adoption of motor pattern changes similar to BF running, even during a shod condition. Interestingly, the female runners in the present study showed no significant kinematic changes between running BF and RS as previously reported with male runners. However, ankle ROM was 12.6 % (6 °) greater with the RS than BF. The lower ankle ROM of running BF may be attributed to the smaller ankle angle found during EC. The female runners displayed less plantarflexion and more knee flexion at EC with the treadmill belt when BF
(19.4 %, 10.5 ° and 3.7 %, 5.8 °, respectively as compared to RS). Furthermore, RS and MF displayed similar ankle angles at EC and ROM. Structural and running kinematics differences between males and females may also further explain why experienced BF male runner exhibited significant changes and the females in this study did not [1, 11, 21]. Despite non-significant adjustments among numerous gait variables, running economy was significantly higher with the BF condition vs. the RS condition. When comparing these 2 conditions, the oxygen uptake was 5.8 % lower during BF running, thus conferring the trend for a higher economy while running barefoot under controlled laboratory conditions. The MF condition elicited a 3.2 % lower oxygen uptake compared to the RS condition. Hanson et al. [16] reported similar findings when comparing shod running to barefoot running on a track and during treadmill running. In both cases, BF running elicited a significantly lower oxygen uptake requirement (5.7 % lower on ground and 2.0 % lower on the treadmill), conferring a greater economy of movement. Mass of the footwear could be an important factor in explaining differences between shod and BF running as it has been reported by Frederick [12] that the oxygen requirement for transport is increased by 1 % for each 100 g of mass added to a foot. Divert et al. [10] also found that when weight was added to the barefoot economy differences in comparison to shod running tended to become minimized. In the present study, the average mass of the RS was 268.7 g and the MF was 142 g. Thus, the increased mass of the RS may have contributed to the greater difference in running economy between the shod conditions. Furthermore, it was suggested that shock attenuation associated with a shod state can contribute to a loss of economy [10]. In contrast, Perl et al. [25] reported a small but significantly lower oxygen requirement (2.4–3.2 % depending on foot strike pattern) during running with MF (Vibram FiveFingersTM) vs. a RS while controlling for shoe mass and stride frequency. It seems that the deformation of the shoe upon foot strike may contribute to a loss of elastic energy that would not occur while running barefoot or when running with a minimally shod foot. Thus, shod running may lead to a loss of elastic energy, which would contribute to a lower economy when performing shod running. The higher energy cost of RS running, may also be associated with the behavioral property changes in the stretch-shortening cycle of the plantarflexor muscles. It has been suggested that BF running casuses greater leg stiffness allowing for improved elastic energy storage [9, 23]. In addition, greater activation of the gastrocnemius-soleus complex has been reported throughout the gait cycle while running BF [9, 10, 23]. This enhancement of energy storage may contribute to the lower metabolic cost of BF running. While these are important factors from a mechanistic perspective, the practical application of the statistical improvement in economy when barefoot may have greater relevance to the general running population in that footwear selection may affect total caloric expenditure. Energetic cost of running BF was approximately 5.6 % lower than while running shod. Over a long run, this could translate into a meaningful energy savings. Practically speaking, runners would be highly unlikely to add mass to a barefoot in an attempt to match limb mass when comparing footwear choices. Consequently, the general running population may be more inclined to experiment with footwear choices without considering the influence of choice on overall cost of the activity. Whether these differences in metabolic cost would
Paulson S, Braun WA. Mechanical and Physiological Examination … Int J Sports Med 2014; 35: 789–793
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792 Orthopedics & Biomechanics
translate into a performance difference is worthy of exploration. Notably, not a single participant indicated that she would opt to race in MF or to compete BF despite the findings of the study. Thus, it would be of applied value to establish whether performance differences may arise based on the economy trends shown in the present study. In conclusion, this study found a significant improvement in running economy despite no significant changes in cardiorespiratory measures or LE kinematics in a sample of well-trained female distance runners while performing a sub-maximal, fixed-velocity treadmill run under different footwear conditions. The increase of oxygen uptake in the RS condition was 5.8 % higher compared to BF. Such a difference could confer considerable energy conservation over a long-distance run in a BF runner. Whether this would translate into performance enhancement should be investigated. In addition, MF was characterized by landing with a flatter foot placement and more knee flexion upon contact in an attempt to decrease impact forces to the heel, whereas BF running exhibited a tendency for a shorter SL, greater SR, and decreased plantarflexion at EC. These minor kinematic changes, as well as no added footwear mass, may have led to greater economy during straight-ahead treadmill running. Future research should include a 3-dimensional analysis examining the chronic adaptions of BF and MF footwear to improve the understanding of LE kinematics. Additionally, analysis of potential energy conservation during long distance running warrants further investigation as to the performance enhancements that may be associated with BF running. Finally, there are physiological and anatomical differences between the sexes. Therefore, a similar study should be conducted examining differences in running economy and kinematics between the sexes under various footwear conditions.
Acknowledgements
▼
The Shippensburg University Undergraduate Research Grant Program assisted with funding for this project.
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789
Authors
S. Paulson, W. A. Braun
Affiliations
Exercise Science, Shippensburg University, Shippensburg, United States
Key words ▶ running economy ● ▶ minimalist footwear ● ▶ gait mechanics ●
Abstract
▼
Barefoot running and running using minimalist footwear have become increasingly popular in recent years. Footwear choice may affect running mechanics and the metabolic cost of running. To investigate these factors, 8 well-trained, female distance runners (mean age = 20.1 ± 1.4 years) were recruited to participate in the study. Following orientation to testing procedures, subjects completed 3 running economy tests on separate days. Treatment order (barefoot, minimalist footwear and running shoe) was counter-balanced. Each testing session consisted of a 5-min warmup at 2.24 m · s − 1, followed by the 7-min RE test at 3.13 m · s − 1. Biomechanical data were collected at the 3-min mark for 10 s, and expired gases were
Introduction
▼ accepted after revision January 01, 2014 Bibliography DOI http://dx.doi.org/ 10.1055/s-0034-1367064 Published online: February 27, 2014 Int J Sports Med 2014; 35: 789–793 © Georg Thieme Verlag KG Stuttgart· New York ISSN 0172-4622 Correspondence Dr. Sally Paulson Exercise Science Shippensburg University 1871 Old Main Dr. 17257 Shippensburg United States Tel.: + 1/717/477 1274 Fax: + 1/717/477 4083 [email protected]
Barefoot running has been used as a strategy for improving lower extremity muscular strength, particularly of the muscles surrounding the ankle. Research has shown that there are physiological and biomechanical differences between shod and barefoot running which could be beneficial to distance athletes [3, 6, 9, 10, 16, 27]. Running barefoot is thought to enhance performance and decrease the risk of overuse injuries [13, 18, 23, 26]. It has been suggested that the more natural condition of running barefoot is beneficial and that the wearing of shoes decreases running economy by altering the natural mechanics of the foot and/or adding mass to the foot [10, 16]. Robbins and Hanna [26] reported that the medial longitudinal arch rises and shortens due to the activation of the intrinsic muscles which causes the arch to absorb more of the vertical load upon impact. Furthermore, barefoot runners employ an ankle coordination strategy that reduces the ground reaction forces by allowing a greater surface area to absorb forces upon
collected from minutes 5–7. One-way repeated measures ANOVA revealed a statistically significant difference for running economy (p = 0.04), expressed as relative oxygen uptake per km in the barefoot condition (running shoe: 204.51 ± 2.84; minimalist footwear: 198.21 ± 3.04; barefoot: 193 .26 ± 3.62 ml · kg − 1· km − 1) vs. running shoe. The other physiological and biomechanical variables were not statistically significant (p > 0.05). However, moderate to large effect sizes suggested there were biomechanical changes that ensured between conditions. It should be further evaluated whether these mechanical adjustments and the running economy trend would translate into improved distance race performance while running barefoot or with minimalist footwear.
contact [15, 16]. Footwear is designed to reduce impact during the heel strike, to control rearfoot motion during loading, and to stabilize the forefoot during stance [10, 24]. However, footwear may cause a decrease in sensory feedback without decreasing the impact load [26]. Previous research has shown improvements in economy of transport (4–5 % lower oxygen requirement) while running barefoot as compared to shod running [16, 18, 23]. DiPrampero et al. [8] suggested that a 5 % increase in running economy resulted in a 3.8 % improvement in distance running performance. Additionally, Hanson et al. [16] reported similar increases for both overground and treadmill running. This physiological effect may be due to the biomechanical differences that have been shown in other studies [9, 10, 18]. Stride length, as well as contact and flight times, have been shown to be shorter and knee velocity during swing to be greater in barefoot running when compared to shod running [7, 9, 13, 29]. In addition, barefoot runners landed with greater plantar flexion to adopt a flatter foot placement upon contact and demonstrated
Paulson S, Braun WA. Mechanical and Physiological Examination … Int J Sports Med 2014; 35: 789–793
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Mechanical and Physiological Examination of Barefoot and Shod Conditions in Female Runners
790 Orthopedics & Biomechanics
Materials and Methods
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Subjects 8 female collegiate cross-country runners volunteered to participate in the study (age = 20.1 ± 1.4 years; mass = 54.1 ± 2.9 kg; height = 1.64 ± 0.06 m; body fat = 18.3 ± 2.9 %). Subjects were screened for risk factors using a health history questionnaire and were required to be free of lower extremity injury in the previous 6 months. All subjects were shod runners, but had regular training experience with BF running. Study participants were directed to abstain from running and other forms of exercise for the 24 h prior to testing and to arrive at the laboratory having fasted for a minimum of 3 h. Time of day for testing was consistent across all participants. The university’s institutional review board approved the study, and all subjects provided written consent prior to participation in the study. The study was conducted according to ethical standards [17].
Protocol The subjects were asked to report to the laboratory for 4 treadmill-running (Woodway Desmo-Pro treadmill, Waukesha, WI) sessions. During the first session, anthropometric variables were collected and a 30-min treadmill and equipment orientation run was performed [19]. The remaining testing sessions followed the same procedures with the exception that the condition [barefoot (BF), running shoe (RS), minimalist footwear (MF)] was randomized and counter-balanced. The Vibram FiveFingersTM KSO (Vibram USA, Concord, MA) multisport shoe was used for the MF condition, and subjects were asked to wear their regular running shoe for the RS testing session. The subjects completed a 5-min warm-up at 2.24 m · s − 1 (5 mph). Following the warm up, the
subjects ran on the treadmill for 7 min at 3.13 m · s − 1 (7 mph) and then performed a 5 min cool-down. Biomechanical data were collected at the 3-min mark for 10 s, and expired gases were collected from minutes 5–7. The testing sessions were separated by 5–7 days.
Biomechanical measurements Prior to the treadmill run, 6 reflective markers were applied along the right side of the body using double-sided adhesive tabs on the acromioclavicular joint, greater trochanter, lateral aspect of the knee joint, lateral malleolus, base of the fifth metatarsal (outside of the shoe) and lateral aspect of the heel (outside of the shoe). Lights were used to illuminate the reflective markers during data collection. Lower extremity (LE) kinematic data were collected (60 Hz) during the treadmill runs using a 2D motion-capture system (Vicon Motion Systems, v. 9.2, Englewood, CO) from the sagittal plane. A calibration frame was captured prior to each data collection session. All trials were digitized and analyzed using the Vicon Motus (v. 9.2) software. The camera (Panasonic GV-35, Secaucus, NJ) was attached to a stationary tripod (height = 0.85 m) and positioned 2.75 m from the treadmill. The camera field of view was perpendicular to the subject, and a shutter speed of 1/750 s was used during data collection. One trial was collected for 10 s for each condition. 30 s prior to filming, the subjects were instructed to drop back on the treadmill belt to prevent interference of the treadmill arm rails during digitizing. Subjects wore dark form-fitting attire for all sessions, and all reflective material was covered prior to data collection. The first 5 strides of the 10-s video were cropped in an attempt to minimize the effect of positioning the subjects toward the back of the treadmill. The next 3 consecutive strides were digitized and smoothed using a Butterworth 6 Hz cut-off frequency filter. The time to complete the 3 strides was obtained from the analysis of the video data. Stride rate (SR) was averaged from 3 strides and the time to complete the 3 strides. Stride length (SL) was calculated from the speed of the treadmill and the time to complete one stride. The speed of the treadmill was assumed to be equivalent to the runner’s speed [14]. The ankle and knee angles were obtained for each condition at initial contact (IC) and end contact (EC) with the treadmill belt. The first frame and last frame the subject contacted the treadmill belt were used to determine IC and EC, respectively [7]. Ankle and knee range of motion (ROM) [14] and vertical oscillation of the center of mass (COM) were calculated [30].
Running economy measurements Running economy was assessed by measuring oxygen uptake via indirect calorimetry using a calibrated metabolic measurement system (ParvoMedics TrueMax, Sandy UT). Expired gases were collected for a 2-min period starting at min 5. Standard mouthpiece and breathing tube apparatus were used. At approximately 4:30, the nose clip and mouthpiece apparatus were handed to the runner. While expired gases were collected, the respiratory tubing was supported by an investigator to minimize vibration and subsequent impact on the runner’s gait pattern. Expired gases were averaged over 1-min periods, and the last minute of each collection sample was used to determine oxygen uptake, respiratory exchange ratio and minute ventilation. Oxygen uptake was normalized for determination of running economy over a fixed distance and was expressed as ml · kg − 1· km − 1.
Paulson S, Braun WA. Mechanical and Physiological Examination … Int J Sports Med 2014; 35: 789–793
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greater leg stiffness throughout the stance phase [7, 10, 18, 29]. An increase in leg stiffness has been positively correlated with running economy [28]. The majority of the running literature has primarily examined male runners. Additionally, the research surrounding running economy and sagittal plane kinematics have used male subjects that may not have been accustomed to barefoot running [2, 7, 10]. While Squadrone et al. [29] used experienced barefoot runners, it was also an all-male sample. To our knowledge, differences between barefoot and shod running has not been previously investigated in an all-female cohort accustomed to barefoot running. In addition, previous research has suggested males and females adopt different running strategies at the hip and the knee [11, 21]. More specifically, females tend to have less knee flexion, greater valgus knee positioning, [1, 21] and displayed less lower extremity joint variability than their male counterpart [1]. The Vibram FiveFingersTM is a specialized shoe that is designed to mimic barefoot running by providing minimal support but also some level of protection to the foot [29]. However, it is unclear whether or not these shoes do indeed mimic barefoot running. Additionally, due to kinematic differences between the sexes female runners may respond differently to barefoot or minimalist footwear running. Therefore, the aim of this study was to examine running economy and sagittal plane kinematics in female distance runners accustomed to barefoot running during a continuous treadmill run under 3 conditions: barefoot (BF), minimalist (Vibram FiveFingersTM) footwear (MF) and regular running shoes (RS).
Orthopedics & Biomechanics
Variable ankle angle IC ( °) ankle angle EC ( °) ankle ROM ( °) knee angle IC ( °) knee angle EC ( °) knee angle swing ( °) knee ROM ( °) stride length (m) stride rate (strides/s) ground contact time (s) vertical COM (cm)
Regular running shoe 24.16 ± 2.13 53.98 ± 2.50 49.77 ± 4.03 151.06 ± 2.09 155.71 ± 1.52 81.62 ± 3.06 82.44 ± 3.05 1.15 ± 0.01 2.73 ± 0.03 0.12 ± 0.002 12.08 ± 0.43
Barefoot
Minimalist footwear
24.01 ± 2.87 43.51 ± 3.85 43.51 ± 1.93 152.85 ± 2.34 149.93 ± 5.94 84.23 ± 3.30 82.73 ± 3.82 1.12 ± 0.01 2.81 ± 0.02 0.12 ± 0.003 12.00 ± 0.47
20.07 ± 2.23 51.51 ± 2.94 47.16 ± 2.43 146.92 ± 2.20 154.62 ± 3.31 81.24 ± 3.71 85.23 ± 5.20 1.16 ± 0.04 2.71 ± 0.08 0.11 ± 0.007 11.78 ± 0.75
791
Table 1 Kinematic data (M ± SE) of female distance runners wearing different footwear during treadmill running.
Expired gases and respiratory exchange ratio were used to derive rate of energy expenditure (kcal · min − 1).
A one-way repeated measures ANOVA and descriptive statistics (SPSS, v. 20.0, Chicago, IL) were used to analyze the variables. Differences between treatments were quantified using dependent t tests. The significance level was set at p < 0.05. For ANOVA analyses, the independent variable was treatment (BF, RS, MF). Dependent variables included running economy, respiratory exchange ratio (RER), minute ventilation (Ve), heart rate (HR), SL, SR and kinematic measures. The biomechanical variables measures were ankle and knee angles at IC and EC, ankle and knee ROM, knee angle at mid-swing, ground contact time (GCT) and vertical oscillation of COM (vCOM). All biomechanical dependent variables were averaged from 3 complete gait cycles for each condition. In addition, effect sizes (ES) were calculated. An ES less than 0.2 was a small effect, 0.5 a moderate effect and 0.8 a large effect [4].
VO2 (ml.kg–1.km–1)
Statistical analysis
210 205 200
**
195 190 185
BF
MF Condition
RS
Fig. 1 Relative oxygen uptake per kilometer under 3 testing conditions: barefoot (BF), minimalist footwear (MF), running shoe (RS); (M ± SE, N = 8). Running speed = 3.13 m · s − 1. *p = 0.04.
0.908 0.906 0.904
RER
0.902
Results
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The mean ± SE for the kinematic variables are presented ▶ Table 1. There was not a statistically significant difference in ● between the footwear conditions for ankle angle at IC [F(2,14) = 1.18, p = 0.34], knee angle at IC [F(2,14) = 2.49, p = 0.12], ankle angle at EC [F(2,14) = 2.64, p = 0.12], or the knee angle at EC [F(2,14) = 0.70, p = 0.51]. The knee flexion angle at mid-swing was not significantly different between the conditions [F(2,14) = 1.21, p = 0.33]. The ROM at the ankle [F(2,14) = 1.54, p = 0.25] and the knee [F(2,14) = 0.64, p = 0.54] were also not significantly different. Furthermore, there were no significant differences between GCT [F(2,14) = 1.64, p = 0.23], vCOM [F(2,14) = 0.23, p = 0.80], SL [F(2,14) = 1.04, p = 0.38] and SR [F(2,14) = 1.04, p = 0.38]. Although, the kinematic variables were not statistically significant, there was evidence suggesting changes were ensured. Several dependent variables had moderate to large ES. For instance, at IC the ankle angle was 16 % smaller while wearing the MF compared to the RS (ES = 0.54) or BF (ES = 0.66). Additionally, during the BF condition at EC the ankle angle was 19 % and 16 % smaller compared to the RS (ES = 1.14) and MF (ES = 0.83), respectively. The ankle had the greatest ROM during the RS and was 13 % and 5 % higher than the BF (ES = 0.59) and MF (ES = 0.28) conditions, respectively. Furthermore, the knee angle during the MF condition was flexed more at IC than RS (2.47 %, ES = 0.68) and BF (3.88 %, ES = 0.92) running. However, during EC the knee
0.9 0.898 0.896 0.894 0.892 0.89
BF
MF Condition
RS
Fig. 2 Respiratory exchange ratio (RER) under 3 testing conditions: barefoot (BF), minimalist footwear (MF), running shoe (RS); (M ± SE, N = 8). p > 0.05.
had greater flexion during the BF condition by 3.71 % and 3.13 % compared to the RS (ES = 0.47) and the MF (ES = 0.34) respectively. During the BF condition SR was greater than during RS (ES = 1.22) and MF (ES = 0.59) by 2.86 % and 3.56 %, respectively. Thus, SL was smaller in the BF condition by 3.6 % and 4.50 % compared to the RS (ES = 0.99) and MF (ES = 0.51), respectively. ▶ Fig. 1–3, The mean running economy, RER, and Ve are shown in ● respectively. Running economy, expressed as ml · kg − 1· km − 1, was significantly different across conditions [F(2,14) = 4.32, p = 0.04]. The BF condition required a 5.8 % lower oxygen uptake compared to the shod condition (t = − 3.06, p = 0.02; ES = 1.22). Though non-significant, the MF condition was associated with a 3.2 % lower oxygen uptake than the shod condition (t = − 1.60, p = 0.15; ES = 0.76). No other cardiorespiratory measures were found to be different across conditions, including RER [F(2,14) = 0.23, p = 0.80], Ve [F(2,14) = 1.02, p = 0.39] and VCO2
Paulson S, Braun WA. Mechanical and Physiological Examination … Int J Sports Med 2014; 35: 789–793
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IC is initial contact and EC is end contact
40
Ve (L·min–1)
39 38 37 36 35 34 33
BF
MF Condition
RS
Fig. 3 Minute ventilation (L · min − 1) during 3 testing conditions: barefoot (BF), minimalist footwear (MF), running shoe (RS); (M ± SE, N = 8). p > 0.05.
[F(2,14) = 2.68, p = 0.10]. Rate of caloric expenditure did not differ across conditions (BF = 9.62 ± 0.33; MF = 9.85 ± 0.31; RS = 10.16 ± 0.21 kcal · min − 1). Likewise, HR and RPE measures did not differ across conditions (p < 0.05).
Discussion
▼
In the present study differences in kinematics, running economy and other cardiorespiratory variables were studied to gain more insight as to how well-trained, female runners respond acutely to different footwear during treadmill running. Running economy was significantly lower in the BF compared to the RS condition. While none of the kinematic variables measured were significantly different, certain findings were consistent with previous research and provide valuable clinical significance. For instance, several studies have reported that BF runners demonstrate decreases in SL and increases in SR when compared to shod running [7, 9, 13, 20]. In the current study, the female runners displayed a similar non-significant pattern of increases in SR and decreases in SL while running barefoot. A majority of the studies examining foot placement during IC have reported an increase in plantarflexion at the ankle when running BF, leading to the adoption of a flatter foot upon contact [7, 13, 23]. De Clercq and colleagues [5] reported that heel-loading pressure was reduced when foot placement upon contact was more horizontal. The female distance runners in this investigation landed more flat-footed during the MF condition and demonstrated little difference between RS and BF. Thus, the subjects may have adopted a flatter foot positioning to decrease heel-loading pressure as this type of footwear has little cushioning. In addition, the knee flexion angle upon contact was more flexed while wearing the MF. De Wit et al. [7] reported a similar adaption at the knee with a flat-foot placement. Even though the runners were given the MF several weeks prior to data collection, this may have not been enough time to become accustomed to running in this type of footwear. McCarthy et al. [22] found that 12 weeks of MF training lead to the adoption of motor pattern changes similar to BF running, even during a shod condition. Interestingly, the female runners in the present study showed no significant kinematic changes between running BF and RS as previously reported with male runners. However, ankle ROM was 12.6 % (6 °) greater with the RS than BF. The lower ankle ROM of running BF may be attributed to the smaller ankle angle found during EC. The female runners displayed less plantarflexion and more knee flexion at EC with the treadmill belt when BF
(19.4 %, 10.5 ° and 3.7 %, 5.8 °, respectively as compared to RS). Furthermore, RS and MF displayed similar ankle angles at EC and ROM. Structural and running kinematics differences between males and females may also further explain why experienced BF male runner exhibited significant changes and the females in this study did not [1, 11, 21]. Despite non-significant adjustments among numerous gait variables, running economy was significantly higher with the BF condition vs. the RS condition. When comparing these 2 conditions, the oxygen uptake was 5.8 % lower during BF running, thus conferring the trend for a higher economy while running barefoot under controlled laboratory conditions. The MF condition elicited a 3.2 % lower oxygen uptake compared to the RS condition. Hanson et al. [16] reported similar findings when comparing shod running to barefoot running on a track and during treadmill running. In both cases, BF running elicited a significantly lower oxygen uptake requirement (5.7 % lower on ground and 2.0 % lower on the treadmill), conferring a greater economy of movement. Mass of the footwear could be an important factor in explaining differences between shod and BF running as it has been reported by Frederick [12] that the oxygen requirement for transport is increased by 1 % for each 100 g of mass added to a foot. Divert et al. [10] also found that when weight was added to the barefoot economy differences in comparison to shod running tended to become minimized. In the present study, the average mass of the RS was 268.7 g and the MF was 142 g. Thus, the increased mass of the RS may have contributed to the greater difference in running economy between the shod conditions. Furthermore, it was suggested that shock attenuation associated with a shod state can contribute to a loss of economy [10]. In contrast, Perl et al. [25] reported a small but significantly lower oxygen requirement (2.4–3.2 % depending on foot strike pattern) during running with MF (Vibram FiveFingersTM) vs. a RS while controlling for shoe mass and stride frequency. It seems that the deformation of the shoe upon foot strike may contribute to a loss of elastic energy that would not occur while running barefoot or when running with a minimally shod foot. Thus, shod running may lead to a loss of elastic energy, which would contribute to a lower economy when performing shod running. The higher energy cost of RS running, may also be associated with the behavioral property changes in the stretch-shortening cycle of the plantarflexor muscles. It has been suggested that BF running casuses greater leg stiffness allowing for improved elastic energy storage [9, 23]. In addition, greater activation of the gastrocnemius-soleus complex has been reported throughout the gait cycle while running BF [9, 10, 23]. This enhancement of energy storage may contribute to the lower metabolic cost of BF running. While these are important factors from a mechanistic perspective, the practical application of the statistical improvement in economy when barefoot may have greater relevance to the general running population in that footwear selection may affect total caloric expenditure. Energetic cost of running BF was approximately 5.6 % lower than while running shod. Over a long run, this could translate into a meaningful energy savings. Practically speaking, runners would be highly unlikely to add mass to a barefoot in an attempt to match limb mass when comparing footwear choices. Consequently, the general running population may be more inclined to experiment with footwear choices without considering the influence of choice on overall cost of the activity. Whether these differences in metabolic cost would
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792 Orthopedics & Biomechanics
translate into a performance difference is worthy of exploration. Notably, not a single participant indicated that she would opt to race in MF or to compete BF despite the findings of the study. Thus, it would be of applied value to establish whether performance differences may arise based on the economy trends shown in the present study. In conclusion, this study found a significant improvement in running economy despite no significant changes in cardiorespiratory measures or LE kinematics in a sample of well-trained female distance runners while performing a sub-maximal, fixed-velocity treadmill run under different footwear conditions. The increase of oxygen uptake in the RS condition was 5.8 % higher compared to BF. Such a difference could confer considerable energy conservation over a long-distance run in a BF runner. Whether this would translate into performance enhancement should be investigated. In addition, MF was characterized by landing with a flatter foot placement and more knee flexion upon contact in an attempt to decrease impact forces to the heel, whereas BF running exhibited a tendency for a shorter SL, greater SR, and decreased plantarflexion at EC. These minor kinematic changes, as well as no added footwear mass, may have led to greater economy during straight-ahead treadmill running. Future research should include a 3-dimensional analysis examining the chronic adaptions of BF and MF footwear to improve the understanding of LE kinematics. Additionally, analysis of potential energy conservation during long distance running warrants further investigation as to the performance enhancements that may be associated with BF running. Finally, there are physiological and anatomical differences between the sexes. Therefore, a similar study should be conducted examining differences in running economy and kinematics between the sexes under various footwear conditions.
Acknowledgements
▼
The Shippensburg University Undergraduate Research Grant Program assisted with funding for this project.
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