Effect of dorsiflexion shoes on the energy cost of running

Science & Sports (2010) 25, 81—87

ORIGINAL ARTICLE

Effect of dorsiflexion shoes on the energy cost of running Effet de la dorsiflexion sur le coût énergétique de la course à pieds M. Buchheit a,∗, P.B. Laursen b, F. Leblond a, S. Ahmaidi a a

Research Laboratory, EA 3300 ‘‘Exercise physiology and rehabilitation’’, Faculty of Sport Sciences, University of Picardie, Jules-Verne, 80025 Amiens, France b School of Exercise, Biomedical and Health Sciences, Edith Cowan University, Joondalup, WA, Australia Received 14 April 2009; accepted 29 August 2009 Available online 19 January 2010

KEYWORDS Aerobic energy expenditure; Springboost shoes; Submaximal running; repeated sprinting

MOTS CLÉS Coût énergétique ;

∗

Summary Objectives. — To evaluate the effect of dorsiflexion shoes on the energy cost of running (Cr), with or without noticeable acute neuromuscular fatigue. Methods. — On four separate occasions, thirteen trained subjects (23.1 ± 4.2 y), wearing standard (twice) or dorsiflexion (twice) shoes, performed two successive 5-min submaximal running exercise bouts (Sub1 and Sub2 , 45% of the speed reached at the end of the 30—15 Intermittent Fitness Test), interspersed by a repeated sprint exercise aimed at inducing acute neuromuscular ˙ O2 ), heart rate (HR), blood lactate ([La]b ) and rating of perceived fatigue. Oxygen uptake (V ˙ O2 measured exertion (RPE) were computed for all tests. Cr was calculated using the mean V over the last three minutes of Sub1 and Sub2 , and expressed in ml kg−1 m−1 . Results. — HR, [La]b and RPE were significantly higher for Sub2 (P < 0.01). [La]b were lower for dorsiflexion compared with standard shoes (P = 0.03), without a ‘shoe x repetition’ interaction for any parameter (P > 0.46). Nevertheless, for Sub2 , qualitative analyses suggested that HR, ˙ O2 and Cr tended to be higher, and [La]b tended to be lower wearing dorsiflexion (i.e., ‘very V likely’ (99%) substantial effect on Cr). Conclusions. — Present results suggest that wearing dorsiflexion shoes may reduce blood lactate levels but increase Cr under conditions of acute neuromuscular fatigue. © 2009 Elsevier Masson SAS. All rights reserved. Résumé Objectifs. — Évaluer l’effet d’une dorsiflexion sur le coût énergique de la course à pied, avec et sans fatigue neuromusculaire apparente.

Corresponding author. E-mail address: [email protected] (M. Buchheit).

0765-1597/$ – see front matter © 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.scispo.2009.08.001

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M. Buchheit et al. Chaussures Springboost ; Course sous-maximales ; Sprints répétés

Méthodes. — Lors de quatre occasions distinctes, 13 sujets participèrent, en portant des chaussures avec dorsiflexion (2 fois) ou non (2 fois), à deux exercices sous-maximaux (Sub1 and Sub2 , courus à 45 % de la vitesse maximale atteinte à la fin du 30—15 Intermittent Fitness Test), entrecoupés par un exercice de répétition de sprints (destiné à induire une fatigue musculaire). La ˙ O2 ), la fréquence cardiaque (HR), le lactate sanguin ([La]b ) et la perconsommation d’O2 (V ˙ O2 sur les trois dernières ception de l’effort (RPE) ont été mesurés pour tous les tests. Le V minutes d’effort était utilisé pour calculer le coût énergétique. Résultats. — HR, [La]b et RPE étaient supérieurs pour Sub2 (p < 0.01). [La]b était inférieur pour les chaussures avec dorsiflexion (p = 0.03), mais aucune interaction « chaussure x répétition » n’était observée, quels que soient les paramètres (p > 0.46). Cependant, pour Sub2 , ˙ O2 et Cr avaient tendance à être supérieurs, et [La]b , l’analyse qualitative suggérait que HR, V inférieur avec dorsiflexion (soit un « très probable » (99 %) effet pour Cr). Conclusions. — Ces résultats suggèrent que les chaussures avec dorsiflexion peuvent diminuer le niveau de lactate sanguin mais augmenter le coût énergétique dans des conditions de fatigue neuromusculaire. © 2009 Elsevier Masson SAS. Tous droits réservés.

1. Introduction

cular) fatigue, which generally leads to a higher energy cost [3,27], might have a greater impact on energy expenditure when combined with dorsiflexion compared with standard shoes. In light of our limited understanding into the effect that dorsiflexion priming plays on the energy Cr and total energy expenditure, we compared the cardiorespiratory responses to submaximal running exercises in subjects wearing either dorsiflexion or standard fitness shoes. Moreover, to account for an eventual differential effect of fatigue on the energy Cr, metabolic analyses were performed while subjects were running either before or after a supramaximal fatiguing repeated sprint exercise [17,18].

The effect of shoes or shoe devices that lower the heel in relation to the forefoot, thereby increasing dorsiflexion, has received growing interest in recent years [2,15,16,25]. Studies examining dorsiflexion shoes have reported increases in jumping height [16], as well as improvements in sprinting speed or jumping ability compared with using standard shoes [15,25]. The effect of dorsiflexion shoes on the energy cost of locomotion however has not been examined. As running economy can markedly influence endurance performance [19], understanding how dorsiflexion shoes might affect the energy cost of locomotion is important for practitioners and athletes alike. Furthermore, if dorsiflexion shoes have the capacity to increase energy expenditure, their use might eventually be recommended for people on weight loss programs. Such propositions may in fact be possible. For example, it has been recently shown that dorsiflexion priming activates lower limb musculature differently during locomotion compared with standard shoes. For instance, Bourgit et al. [2] showed a significant increase in triceps surae and tibialis anterior electromyographic (EMG) activity and suggested that changes in muscle recruitment patterns related to the balancing role of these muscles may have caused this effect. Another explanation for these finding could be the specific foot position that mechanically induces a longer lever, thereby increasing neuromuscular activity of the engaged muscle due to the greater forces applied. Nevertheless, the specific impact that dorsiflexsion shoes have on energy cost of running (Cr) and thus total energy expenditure is difficult to predict. On one hand, the increase in lower limb muscular activity [2] could increase the energetic requirements due to the greater number of muscle fibers being recruited. Conversely, it is possible that the foot position might enhance the amount of stored elastic energy (eccentric phase), which could in turn lower the metabolic demand of running [1]. Prestretched calf muscles and tendons could also lead to further reductions in vertical center of gravity oscillations, inducing an increased overall leg stiffness [9], which in turn could lead to a reduced energy Cr [9]. Finally, as body balance may be more affected with dorsiflexion shoes, it is also possible that (neuromus-

2. Methods 2.1. Subjects Thirteen well-trained males (age 23 ± 4 years, stature 179.5 ± 5.6 cm, mass 76.9 ± 9 kg) participated in the study. Participants had no history or clinical signs of cardiovascular or pulmonary diseases, were not taking prescribed medications and presented with normal levels of blood pressure and electrocardiographic patterns. The procedures followed were in accordance with the recommendations of the declaration of Helsinki and participants gave voluntary written consent to participate in the experiment, which was approved by the hospital’s human research ethics committee.

2.2. Study overview Participants came to the laboratory on five separate occasions. To minimize possible circadian effects, all tests were initiated at the same time of day (±1 h). Subjects first performed a graded maximal aerobic test (30—15 Intermittent Fitness Test, 30—15IFT [4]) for the determination of the first ventilatory threshold (VTh1 ), peak oxygen uptake ˙ O2 peak) and a reference velocity (VIFT ) for the ongoing (V moderate-intensity exercise bouts. Following this test, subjects returned to the laboratory on four separate occasions to perform two submaximal-intensity exercise runs (Sub1

Dorsiflexion and energy expenditure

83 with 15-s passive recovery periods. For this test, velocity was set at 8 km h−1 for the first 30-s run, and speed was increased by 0.5 km h−1 every 30-s stage thereafter. The velocity attained during the last completed stage was determined as the subject’s VIFT .

Figure 1 Experimental schedule with continuous submaximal running exercises performed before (Sub1 ) and after (Sub2 ) a supramaximal intermittent exercise bout (SE). Sub1 , Sub2 and SE were each performed twice with standard, and twice with dorsiflexion shoes. Moreover, to maximize randomization, shoes were changed between Sub1 and Sub2 , before or after SE, so that there was a unique shoe combination for each of the four test sequences.

and Sub2 ) at the speed associated with VTh1 , interspersed with a 2-min supramaximal exercise run (SE) followed by 5 min of passive recovery (Fig. 1). Sub1 and Sub2 were 5 min in duration and were preceded by 2 min of rest. No warm-up was allowed before Sub1 . The time between the end of Sub1 and SE was 5 min (i.e., standardized warm-up consisting of a few athletic drills and short bursts of progressive accelerations on the track), so that the total session time was: 2’ (passive standing) + 5’ (Sub1 ) + 5’ (passive recovery) + 5’ (warm-up for SE) + 2’ (SE) + 5’ (passive recovery including 2’ of passive standing) + 5’ (Sub2 ) = 29 min. The time chosen between the end of SE and Sub2 was to ensure an optimal neuromuscular fatigue effect on Sub2 [17]. Participants wore, in random order, either standard fitness or dorsiflexion (B-Volley, 2◦ , Springboost SA, Switzerland) [2,25] shoes throughout the four 29-min test series. Thus, Sub1 , Sub2 and SE were each performed twice with standard, and twice with dorsiflexion shoes. Moreover, to maximize randomization, shoes were changed between Sub1 and Sub2 , before or after SE, so that there was a unique shoe combination for each of the four test sequences. Respiratory gas exchange and heart rate (HR) were recorded during the entire session. Blood lactate was also measured 3 min after each test. Participants indicated their rating of perceived exertion (RPE, 0—10 Borg’s scale) immediately at the end of each test. All tests were performed on an indoor synthetic track where ambient temperature ranged from between 18 to 22 ◦ C. Subjects were told not to perform exercise on the day prior to testing, and to consume their (usual) last meal at least 3 h before the scheduled test time.

2.3. Exercise testing 2.3.1. Maximal graded aerobic test Cardiorespiratory capacity of each subject was assessed with the 30—15 Intermittent Fitness Test, with subjects wearing their standard shoes [6]. This intermittent shuttle ˙ O2 peak compared to a field test elicits similar levels of V standard continuous incremental test (i.e., r = 0.71; 95% CI for mean difference, −1.0—3.5 ml min−1 kg−1 ) [6], and has been shown to be accurate to estimate VTh1 [6]. Achievement of the final running speed (VIFT ) during this test has also been shown to be reliable (intraclass correlation coefficient = 0.96; typical error = 0.33 (95 CI, 0.26 — 0.46) km h−1 ) [4]. The 30—15IFT consisted of 30-s shuttle runs interspersed

2.3.2. Submaximal running exercise bouts Exercise intensity for Sub1 and Sub2 was set at 45% of VIFT . ˙ O2 correThis was chosen to enable subjects to reach a V sponding to ≈95—100% of that observed at VTh1 [6]. Running pace was dictated by a prerecorded beep that sounded at appropriate intervals in order to allow participants to adjust their running speed as they passed through specific zones of the field (i.e., a cone placed every 20 m). Particular attention was focused by ensuring that subjects reached the required running speed within at least 5 s. 2.3.3. Supramaximal exercise The SE test chosen was adapted from previous repeated sprint running tests [5,26]. Prior to the study, all subjects were familiarized with the repeated-sprint protocol. Subjects performed six repetitions of maximal 25-m sprints, starting every 25 s (Wireless Timing-Radio Controlled, Brower Timing System, Colorado, USA). During the first ∼18 s of recovery between sprints, subjects performed an active running recovery (2.0 m s−1 ). Three seconds prior to the commencement of each sprint, subjects were asked to assume the ready position and await the start signal. During recovery, audio feedback (i.e., time countdown) was provided so that participants could maintain the required running speed. Participants were instructed to complete all sprints as fast as possible, and strong verbal encouragement was provided to each during all sprints. Percentage of speed decrease (%Dec) was calculated for each session [12] in order to confirm neuromuscular fatigue [17,18].

2.4. Measurements 2.4.1. Cardiorespiratory measures ˙E ], oxygen uptake Respiratory gas exchanges (ventilation [V ˙ O2 ], carbon dioxide production [V ˙ CO2 ] and respiratory [V exchange ratio [RER]) and HR were measured during the 30—15IFT , Sub1 and Sub2 using an automated breath-bybreath system (K4b2 , Cosmed, Rome, Italy) [11]. Before each test, the O2 and CO2 analysis systems were calibrated as recommended by the manufacturer. Data were automatically filtered for outliers (i.e., greater than 4 standard deviations (SD) from the local mean [22]). For the 30—15IFT , data were first averaged in 20-s increments. Estimated VTh1 was defined ˙ O2 at which V ˙E /V ˙ O2 and PETO2 began to increase as the V ˙ O2 peak was without a simultaneous increase in PETCO2 . V ˙ O2 values attained during arbitrarily defined as the highest V two consecutive 20-s periods [6]. A HR peak attained near predicted maximum (220-age), a [la]b higher than 8 mmol l−l and a RER > 1.1 were additionally required to confirm the maximal nature of the test [6]. 2.4.2. Blood lactate measurement Three minutes after the end of each exercise set, a fingertip blood sample (5 ␮L) was collected and blood lactate concentration was determined (Lactate Pro, Arkray Inc, Japan).

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The accuracy of the analyzer was checked before each test using standards. The suitability and reproducibility of this analyzer has been previously established throughout the physiological range of 1.0 to 18.0 mmol L−1 [24]. Delta blood lactate values ([La]b ) were then calculated for each exercise. Before Sub1 , preexercise levels were assumed to be 1 mmol l−1 [10]. Previous observations have shown no difference in postexercise blood lactate measurements taken 3 and 5 min after repeated sprint running in subjects with comparable cardiorespiratory fitness levels and training history [5], so that blood lactate concentration at the onset of the Sub2 exercise (i.e., 5 min after the end of SE) was considered equivalent to that measured 3 min after the sprint sequence. 2.4.3. Energy cost of running and aerobic energy expenditure during submaximal exercises ˙ O2 measured Energy Cr was calculated using the mean V over the last three minutes of Sub1 and Sub2 , and expressed in ml kg−1 m−1 [19]. Aerobic energy expenditure (AEE) was obtained while converting the total O2 cost of each exercise into Kcal, assuming that the consumption of 1 L O2 in the human body yields from 4.68 to 5.05 Kcal throughout the range of RER values from 0.71 to 1.00 (i.e., AEE = TO2 × [(1RER)/0.3 × 19.6]+[(RER-0.7)/0.3 × 21]/4.18 Kcal) [26].

no difference on any parameters (all P > 0.46), a ‘trial’ effect was not investigated. A two-way repeated measures ANOVA with Bonferroni’s post-hoc tests (if an interaction was found) were used to determine the respective effect of ‘shoes’ (between factor, dorsiflexion vs. standard shoes) and ‘repetition’ (within factor, Sub1 vs. Sub2 ) on cardiorespiratory parameters, Cr and AEE. In addition, trends in cardiorespiratory parameters and energy expenditure were interpreted using adjusted Cohen’s effect sizes and thresholds (aES > 0.2, small; > 0.5, moderate; > 0.8, large) [14]. Smallest worthwhile differences were also calculated for all variables to determine the likelihood that the true effect was substantially beneficial, trivial, or harmful (detrimental). The threshold value for the smallest worthwhile change in cardiorespiratory parameters and AEE was set at 0.2 Cohen’s units [14]. If both benefit and harm were calculated to be > 5%, the true effect was assessed as unclear. Where clear interpretation could be made, chances of benefit were assessed as follows: < 1%, almost certainly not; 1—5%, very unlikely; 5—25%, unlikely; 25—75%, possibly; 75—95%, likely; 95—99%, very likely; and > 99%, almost certainly [14].

3. Results

2.5. Statistical analyses

3.1. Cardiorespiratory responses to maximal and submaximal exercise bouts

Statistical analyses were carried out using Minitab 14.1 Software (Minitab Inc., Paris, France) and data are presented as means and SD. The distribution of each variable was examined with the Shapiro-Wilk normality test. Homogeneity of variance was verified by a Levene test. Paired T-tests were used to compare mean values for cardiorespiratory parameters and energy expenditure data between the repeated trials with similar shoes. Since there was

˙ O2 , HR, [La]b and RPE measured during Values for peak V the 30—15IFT were 51 ± 7 ml.min−1 kg−1 , 182 ± 7 beat min−1 , 11.1 ± 1.8 mmol l−1 and 8.7 ± 0.7, respectively. VIFT was 19.6 ± 0.7 km h−1 . As expected, running at 45% VIFT (9.0 ± 0.3 km h−1 ) during the submaximal exercise bouts equated to moderate-intensity exercise; mean steady-state ˙ O2 was just below VTh1 (60 ± 4% V ˙ O2 peak, corresponding V to 93 ± 5% of VTh1 ). Mean [La]b reached after SE was

Table 1 shoes.

Cardiorespiratory measures and rating of perceived exertion during submaximal running using standard and dorsiflexion Sub1

Sub2

Standard −1

˙E (l min ) V ˙ O2 (ml min−1 kg−1 ) V ˙ CO2 (ml min−1 kg−1 ) V RER HR (beat.min−1 ) [La]b (mmol l−1 ) RPE Cr (ml(O2 ) kg−1 m−1 )

55.6 30.0 30.1 1.01 135 1.4 2.3 0.204

± ± ± ± ± ± ± ±

7.0 3.5 5.1 0.1 14 0.8 1.0 0.02

Dorsiflexion 56.6 30.0 30.0 1.01 137 1.0 2.3 0.204

± ± ± ± ± ± ± ±

7.4 4.0 4.0 0.1 14a 0.5b 1.0 0.02

Standard 66.2 30.4 29.4 0.98 148 −5.3 2.8 0.207

± ± ± ± ± ± ± ±

7.6 4.1 3.9 0.1 13 1.0 1.2 0.03

Shoes effect (P)

Repetition effect (P)

0.66 0.59 0.52 0.81 0.52 0.03 0.51 0.55

< 0.001 0.59 0.92 0.19 < 0.001 < 0.001 < 0.01 0.29

Dorsiflexion 66.9 31.6 30.9 0.99 150 −5.9 3.0 0.214

± ± ± ± ± ± ± ±

6.6 3.7b 4.1 0.1 11b 1.1c 1.0c 0.02b

˙ O2 ), carbon dioxide production (V ˙ CO2 ), respiratory exchange ratio Values are mean ± SD for minute ventilation (VE), oxygen uptake (V (RER), heart rate (HR), blood lactate ([La]b ), rating of perceived exertion (RPE) and energy cost of running (Cr) measured during the first (Sub1 ) and second (Sub2 ) submaximal running exercises wearing standard or dorsiflexion shoes. Main effects from the two-way Anova are presented. No interaction effect was noted (all P > 0.46). a Qualitative within-exercise difference with small adjusted effect size (aES > 0.2). b Qualitative within-exercise difference with moderate adjusted effect size (aES > 0.5). c Qualitative within-exercise difference with large adjusted effect size (aES > 0.8).

Dorsiflexion and energy expenditure

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4.1. Effect of dorsiflexion on blood lactate accumulation Present results showed a significant main ‘shoe’ effect on lactate accumulation during submaximal exercise (Table 1). Blood lactate accumulation was lower during the first submaximal exercise bout, and showed higher delta values (−5.9 ± 1.1 vs. −5.3 ± 1.0 mmol l−1 ) following the second run (after the repeated fatiguing exercise). These findings suggest a greater lactate oxidation (and/or a lower production) using dorsiflexion compared with standard fitness shoes (Table 1). This beneficial effect of dorsiflexion shoes is novel, and might be related to the presumed increase in stored elastic energy [1] or leg stiffness [9] that could have lowered the energetic needs of leg muscles. Biomechanical analyses using force plate measurements are however warranted in order to confirm these hypotheses. Figure 2 Mean (±SD) values for aerobic energy expenditure (AEE) calculated for the first (Sub1 ) and second (Sub2 ) submaximal running exercise with subjects wearing either standard or dorsiflexion (i.e., Springboost) shoes. ‡: qualitative difference with large adjusted effect size (aES > 0.8).

10.1 ± 1.8 mmol l−1 , with no difference between the four trials (P = 0.81). Similarly, there was no difference between the four trials for %Dec (mean value = 3.3 ± 0.8%, P = 0.91).

3.2. Effect of dorsiflexion shoes on cariorespiratory parameters and energy expenditure during submaximal running ˙E , V ˙ O2 , V ˙ CO2 , RER, HR, [La]b , RPE and Cr Mean values for V are for each exercise are presented in Table 1. Fig. 2 shows ˙ O2 (i.e., data for AEE. There was no ‘shoe effect’ on basal V during the 2-min periods preceding each test) (P = 0.76). Values were significantly higher for Sub2 compared with Sub1 ˙E (all P < 0.001) and RPE (P < 0.01). There for HR, [La]b , V ˙ O2 (P = 0.59), V ˙ CO2 was however no ‘repetition’ effect for V (P = 0.92) or Cr (P = 0.29). A ‘shoe’ effect was only apparent for [La]b (P = 0.03) (P > 0.13 for all other parameters). No ‘repetition x shoe’ interaction was found for any parameter (all P > 0.46). Despite the non-significant interaction effects, qualitative analysis suggested that, during Sub1 , there was a tendency for HR (aES = 0.4) to be higher, and for [La]b to be lower (aES = −0.7) for dorsiflexion compared to standard shoes. For Sub2 , there was a tendency for RPE (aES = 0.3), HR ˙ O2 (aES = 1.0), Cr (aES = 1.0) and AEE (aES = 1.2) (aES = 1.1), V to be higher for dorsiflexion compared to standard shoes, with a ‘possibly’ substantial effect for RPE (32%) and HR ˙ O2 (86%), Cr (63%), and a ‘very likely’ substantial effect for V (99%) and AEE (90%). [La]b tended to be higher with dorsiflexion during Sub2 (aES = −0.7), but the true effect was rated ‘unclear’.

4. Discussion The main findings of the present study in well trained individuals were that dorsiflexion shoes significantly reduced blood lactate accumulation following repeated sprinting and were very likely to increase total energy expenditure under acute neuromuscular fatigue conditions.

4.2. Effect of dorsiflexion on energy cost of running and aerobic energy expenditure Since we found neither a significant ‘shoe’ effect, nor a ‘shoe x repetition’ interaction, it was not possible to analyze differences between shoes throughout the two experimental conditions (i.e., with or without fatigue) using a classical statistical approach (based on P values). In order to fully examine our hypotheses, we used a more contemporary qualitative approach [8,14] that allowed us to draw inferences based on observed effects within each condition. Although the O2 equivalent of lactate is sometimes included in the calculation of the total O2 cost of exercise [13], aerobic sources were exclusively used here. Nevertheless, below VTh1 , anaerobic participation to total energy expenditure is thought to be negligible (i.e., 1 mmol l−1 after Sub1 ). Moreover, sub2 was initiated with high blood lactate levels (i.e., > 10 mmol l−1 ), so that the exact anaerobic contribution during this exercise was impossible to evaluate. Cr values observed here were however similar to those measured previously under similar conditions using standard shoes [3,23,27]. Our results showed neither a significant main effect, nor any qualitative trend for dorsiflexion to affect Cr or AEE during the first running bout (Table 1 and Fig. 1, left bars). This was surprising given the significant differences in muscular recruitment patterns [2] that were expected to either increase (greater number of muscle fibers involved) or decrease (increased stored elastic energy [1] or leg stiffness [9]) Cr and AEE. It is possible that the running velocity tested here (9.5 km h−1 ) was too low to allow the previously noted EMG differences to play a role in the modification of muscle metabolism. Higher running velocities must therefore be performed in future experimental studies.

4.3. Effect of dorsiflexion on energy cost of running and total energy expenditure under acute fatigue conditions The short (150 s, including recovery periods) repeated running sprint sequence [5,26] was designed to induce a marked level of neuromuscular fatigue [17,18] that we expected

86 would increase the energy Cr [3,27]. Although we did not have EMG data [7,2], the percentage of speed decrement during the repeated sprints (i.e., > 3%) reflected neuromuscular impairment [17]. In this specific context, Cr and AEE were almost possibly (>90%) increased with the Springboost compared with standard shoes, suggesting that the effect of dorsiflextion priming is only apparent when lower limbs experience a certain level of fatigue. Since dorsiflexion priming induces motor pattern reorganization [2] and is likely to perturb body balance more than standard shoes, it is possible that fatigue may have exacerbated the lower limb muscular activity as a consequence of the longer groundcontact time [20] and/or the increased needs for center of gravity readjustment. An increase in upper-body muscular activity (e.g., back or arm muscles more involved in body balance), as well as an accentuated sympathetic activity (i.e., ‘stress’ effect related to balance perturbation, inferred from higher exercise HR values, Table 1) could also have accounted for this substantially higher energy expenditure. Even if previous investigations [15,25] did not report higher injury rates with dorsiflexion compared with standard shoes in athletes following eight-week athletic programs, whether dorsiflexion does not exaggerate muscular strain (inferred from ‘possibly’ higher RPE values observed here and increased EMG activity [2]) on calf muscles needs to be evaluated in a general (untrained) population.

4.4. Practical applications Present results suggest that, at least during the first minutes of low-intensity running, dorsiflexion priming does not affect the aerobic energy Cr in well-trained individuals. Conversely, in the situation of an acute fatigue, wearing Springboost shoes might be substantially beneficial for people willing to increase energy expenditure (i.e., people engaging in weight-loss training program).

In conclusion, the dorsiflexion shoes used in the present study by well-trained individuals were shown to significantly reduce blood lactate levels and were very likely to increase aerobic energy expenditure under acute neuromuscular fatigue conditions. Further studies investigating a larger range of running speeds are warranted to examine whether or not blood lactate accumulation is consistently lower with dorsiflexion shoes, which could be beneficial for competitive runners engaged in short duration events.

5. Conflicts of interest None.

Acknowledgments The authors would like to thank the subjects for their enthusiastic participation, Bachar Haydar and Paul Raoul Delors for their appreciable help in data analysis, as well as Irmant Cadjjiof for his assistance with the preparation of the manuscript. The authors have no affiliation with Springboost SA.

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