Evidence of variable performance responses to the Nike 4% shoe: Definitely not a game-changer for all recreational runners

Purpose: We compared running economy (RE) and 3-km time-trial (TT) performances of male recreational runners wearing the Nike Vaporfly 4% (NIKE), lightweight racing flats (FLAT), and their habitual footwear (OWN). Methods: Eighteen male recreational runners [age: 33.5 (11.9) y, V̇O2peak: 55.8 (4.4) mL·kg-1·min-1] attended 4 sessions ~7 days apart. The first session consisted of a V̇O2peak test to inform subsequent RE speeds set at 60, 70, and 80% of the speed eliciting V̇O2peak. In subsequent sessions, treadmill RE and 3-km TT were assessed in the three footwear in a randomised, counterbalanced crossover design. Results: RE was improved in NIKE (3.6 to 4.5%, p ≤ 0.002) and FLAT (2.4 to 4.0%, p ≤ 0.042) versus OWN across intensities, with a trivial difference between NIKE and FLAT (1.0 to 1.6%, p ≥ 0.325). NIKE 3-km TT (11:07.6 ± 0:56.6 mm:ss) was superior to OWN by 16.6 s (2.4%, p = 0.005) and FLAT by 13.0 s (1.8%, , p = 0.032), with similar times between OWN and FLAT (0.5%, , p = 0.747). Only 29% of runners were more economical across intensities and faster in NIKE. Conclusions: Overall, our findings indicate that NIKE could benefit RE in male recreational runners at relative speeds when compared to OWN, but not when compared to FLAT. More runners exhibited better TT performances in NIKE (61%) versus FLAT (22%) and OWN (17%). The high variability in individual RE (-3.1 to 12.1%) and TT (-3.8 to 8.2%) shoe-responses suggests that individualisation of running footwear prescription is warranted.


INTRODUCTION
Running economy (RE) defines the rate of oxygen consumption when running at a given submaximal speed and is a key measure linked with distance running performance [1,2]. RE varies considerably between individuals [3] and is influenced by multiple variables, including metabolic, cardiorespiratory, biomechanical, and neuromuscular efficiency [1,2]. Although maximal aerobic power (VȮ2peak) and fractional utilisation of VȮ2peak are also considered key factors in distance running performance [3], runners with superior RE at a similar VȮ2peak and lactate threshold (i.e., estimate of fractional utilisation) generally outperform their peers [3]. Given the direct link between RE and performance [4,5], acute changes in RE via footwear interventions has become an active area of research [6,7].
Until recently, shoe mass was one of the few footwear characteristics consistently linked with meaningful improvements in RE and time-trial (TT) performance [8,9,4]. The energetic cost of running has been shown to increase ~0.7 to 1% for each 100 g of added mass per shoe [4,8], explaining why certain elite runners prefer racing in lightweight racing flats. In May 2017, the Breaking2 Nike Inc. sponsored event introduced the Nike Vaporfly Elite. These prototype shoes were lighter than equivalent marathon racing shoes; had a midsole composed of a novel compliant and resilient ZoomX foam from polyether block amide (PEBA) promoted to enhance energy return; and a carbon fibre panel embedded in the midsole stipulated to promote longitudinal bending stiffness [10,7,6]. ing performance to consider, these monumental achievements have sparked debate in the running community regarding the 'fairness' of the Vaporfly [11] and whether their use constitute 'technology doping'.
Nike-sponsored [7] and Nike-independent [6,10] experiments have confirmed RE improvements under laboratory conditions in high-calibre runners (sub 32:00 men and 35:30 women 10 km) wearing Nike Vaporfly 4% mechanistically driven by the elastic properties and energy return from midsole compression [12]. Using public race reports and big data from Strava, Quealy and Katz [13] have corroborated improvements in runners wearing the now commercially-available Nike Vaporfly in both faster and slower runners.
Their analyses indicate that runners wearing Nike Vaporfly 4% or Next% run 4 to 5% faster and have a 73% chance of setting a personal best compared to wearing other popular running shoes (i.e., their habitual footwear) [13]. However, it is debatable whether such a comparison is fair given that most recreational runners wear shoes that are ~100 g heavier, the analysis feeds into the 'fairness' debate of the Vaporfly shoes [11] both at the elite and recreational level. In addition, since previous studies have not blinded participants to footwear, it is unsure whether any 'placebo effect' is contributing to the reported benefits of the Vaporfly [13,4,6], or whether the improvements stem solely from their material and lightweight characteristics.
Evidence of variable performance responses to the Nike 4% shoe: Definitely not a game-changer for all recreational runners  For a given percent improvement in RE; Kipp et al. [5] predict slightly greater percent improvements at running velocities slower than ~3 m·s -1 , and slightly lower percent improvements at faster velocities given the non-linear relationship between oxygen uptake and running velocity. Accordingly, recreational runners might reap greater benefits than elite from wearing Nike Vaporfly, although laboratory-based data from recreational runners are not available. Hence, our aims were to compare running economy (RE) at relative running speeds and 3-km time-trial (TT) performances of male recreational runners wearing the commercially available Nike Vaporfly 4% (NIKE), lightweight racing flats (FLAT), and their habitual footwear (OWN).

Participants
Based on published data [10], a sample of 17 runners was required to detect 0.25 effect size improvements in RE between footwear conditions with an 80% power (β = 0.20) and 5% significance level (α = 0.05).
According to test-retest reliability data [14] and one-sample comparison of means, a sample of 18 runners was required to detect meaningful improvements in treadmill TT performance with an 80% power and 5% significance level.  [15] as rearfoot (n = 14) or non-rearfoot (mid/forefoot, n = 4) from 2D video recordings (50 Hz) at 70% of the speed found to elicit VȮ2peak. Runners were recruited through personal contacts, running clubs, social media, newsletter postings, and word-of-mouth. Inclusion criteria were male runners with a 5-km run time of ~20 to 25 minutes within the 3 months prior to testing, and with a men US shoe size of 8.5 to 12. Runners with current or recent (< 3 months) injuries or elite runners were excluded.
All participants provided written informed consent prior to participation, and were informed of the potential injury risk of running in novel footwear. The study protocol was approved by our institution's Human Research Ethics Committee [HREC(Health)2018#81] and abided to the ethical standards of the Declaration of Helsinki.

Experimental protocol
The effect of footwear on RE and TT treadmill performance was assessed using a randomised counterbalanced experimental design that required participants to attend four laboratory sessions (Fig. 1). The first session was designed to collect baseline measures, establish VȮ2peak, ensure proper shoe fit, and familiarise runners to running in the Nike Vaporfly 4% (NIKE) and the lightweight racing flat (FLAT) Saucony Endorphin Racer 2 road racing shoe. The two experimental shoes were spray-painted black to blind individuals to their make and model ( Fig. 1) and reduce potential for placebo, [4,6] adding ~2 g to each shoe. In the second, third, and fourth sessions, RE at 60, 70, and 80% of the speed eliciting VO2peak (ʋVO2peak) and 3-km TT performance were assessed in one of three footwear conditions allocated in a randomised counterbalanced Evidence of variable performance responses to the Nike 4% shoe: Definitely not a game-changer for all recreational runners

Visit 2
Baseline information, anthropometric characteristics, and the make and model of participants' OWN shoes were recorded in visit 1. Participants then trialled the two experimental shoes to ensure proper fit, walking and jogging around the laboratory. Immediate shoe comfort and experience in the three footwear conditions (NIKE, FLAT, and OWN) were recorded using a visual analogue scale (VAS) based on work by Lindorfer et al. [16] The corresponding anchor points for the two scales were '0 = Not comfortable at all -10 Evidence of variable performance responses to the Nike 4% shoe: Definitely not a game-changer for all recreational runners of minimalism of shoes were assessed using a minimalist index [17]. Shoe-related characteristics are presented in Table 1.
Participants subsequently completed a 4-minute warm-up at 10 km·h -1 running with their own shoes on a motorised treadmill (Steelflex PT10 Fitness, Steelflex Fitness, Taipei, Taiwan) prior to completing a VȮ2peak ramp test using an incremental speed protocol and 1% incline to assess maximal aerobic power [18].
The test started at 10 km·h -1 and increased 1 km·h -1 per minute until volitional exhaustion. The mean ʋVȮ2peak was 18.4 (1.0) km·h -1 . After a 10-minute rest, participants ran 2 x 3 minutes at a self-selected speed on the treadmill once in NIKE and once in FLAT for shoe familiarisation.
Throughout the 3-minute bouts, heart rate (HR, Polar RS800CX, Kempele, Finland) was recorded at 15-s intervals and expired gases were continuously measured using a calibrated metabolic cart (True One 2400, Parvo Medics, Salt Lake City, Utah, USA) to determine oxygen uptake (VȮ2) and respiratory exchange ratio (RER). The two highest consecutive 15-s VȮ2 readings registered in the last minute of each bout were averaged to determine RE (mL·kg -1 ·min -1 ). Following the last bout, participants rated their perceived shoe comfort on the comfort VAS and rested 5 minutes before the TT.
The starting speed for the blinded 3-km TT was set at 90% of ʋVȮ2peak [16.4 (0.9) km·h -1 ]. Prior to the start, participants were reminded to run the 3-km TT as fast as they could and provide a maximal effort.
Participants were familiarised with the starting speed, speed increases, and speed decreases used during the TT (± 0.5 km·h -1 ) for 1 minute in total. Runners were blinded to their elapsed time and speed during the TT [14]. Participants rested 1 minute following familiarisation and then started the TT.
During the TT, participants verbally communicated 'up' or 'down' when they wanted corresponding 0.5 km·h -1 increases or decreases in treadmill speed. During the TT, the researcher verbally communicated the covered distance to participants in 400 m increments to 2400 m, and 100 m increments during the last 400 m. The researcher provided no other verbal communication or encouragement. At TT completion, RPE was collected and a perceived shoe performance VAS was obtained to examine whether participants perceived the shoes had aided their performance. The corresponding anchor points were '0 = No help in performance -10 = Maximal help in performance'.

Statistics
Descriptive statistics are reported as mean (standard deviation) unless stated otherwise. Data were analysed using repeated measures analyses of variance (RM ANOVA) and covariance (RM ANCOVA). Identity was the between-subjects error term and footwear (NIKE, FLAT, OWN) was the repeated-measures term in all analyses. Shoe mass was added in RE and TT data analyses as a covariate to evaluate any potential effect of shoe mass on outcomes. When not significant, the covariate was removed. Tukey's honest significant difference was used in post-hoc pairwise comparisons to determine which shoe-by-shoe comparisons significantly differed. Statistical significance was set at p ≤ 0.05 in all analyses.
To interpret practical meaningfulness, magnitudes of the standardized effects were calculated using Cohen's d and standard deviations from the three footwear conditions. Cohen's d magnitudes were interpreted using thresholds of 0.2, 0.5, and 0.8 for small, moderate, and large [20]. An absolute effect size of 0.2 was considered the smallest worthwhile effect, with smaller effects considered trivial. The effect was deemed unclear if its 95% confidence interval [lower, upper] overlapped the thresholds for small positive and negative effects (i.e., d ± 0.2). To consider individual responses, percentage differences between conditions were computed for each individual. The data from one RE test in the NIKE condition did not save in the software; hence, RE data from NIKE derive from n = 17 individuals.

Running economy
Shoe mass had no significant effect on any of the RE variables (p ≥ 0.108) and was removed as a covariate. Footwear significantly impacted RE across intensities (p < 0.002, Fig. 2

Time trial
Shoe mass had no significant effect on any of the TT variables (p ≥ 0.190) and was removed as a covariate. Footwear significantly impacted TT performance (p = 0.005, Fig. 3

(22%) in FLAT, and 3 (17%) in OWN. Two of the four non-rearfoot strikers (50%) performed their best TT
in FLAT, with one in NIKE (25%) and one in OWN (25%). There was no significant difference in RPE measures between footwear, and no perceived difference regarding the effect of shoe on performance ( Table   5).
Of the 17 runners that presented with complete RE and TT data, 5 exhibited their best RE across intensities and TT performance in NIKE (29%). The remaining 12 (71%) performed their best RE across intensities and TT performances in different shoes, which included the four non-rearfoot strikers.

DISCUSSION
Our study adds to the body of knowledge on the Nike Vaporfly from an independent laboratory, and is the first to observe that NIKE can benefit RE laboratory-based measures in recreational runners compared to their habitual footwear and at relative (i.e., relative to ʋVȮ2peak) rather than absolute speeds. al. [7] and Barnes and Kilding [10] reported an ~3 to 4% improvement in NIKE at absolute speeds ranging from 14 to 18 km·h -1 after adding lead weights to equalise shoe mass. The variation in the reported percentage RE gains from NIKE between studies are likely of multi-factorial origin, and probably relate to running speed differences [5]; the type of runners and footwear examined [13]; individual responses to footwear; as well as differences in treadmill compliance given that more compliant surfaces can enhance RE [21]. The repeatedmeasure design of studies somewhat minimise the influence of treadmill compliance however, as individuals act as their own controls. The placebo effect has also been cited as a potential reason for changes in performance with footwear [4,6], and could contribute to between-study differences in results. Unlike previous Evidence of variable performance responses to the Nike 4% shoe: Definitely not a game-changer for all recreational runners studies, we attempted to minimise the placebo effect by spray-painting our shoes black, and only one runner was able to identify the NIKE shoe at the completion of all experimental trials. Our VAS score analyses relating to perceived shoe experience, immediate shoe comfort, shoe comfort subsequent the RE trials, and perceived impact of shoe on 3-km TT suggest no clear relationship between physiological or performance responses of our participants to footwear conditions, with our runners generally being the most comfortable wearing their OWN shoes, followed by NIKE.
Under mechanical testing with a peak force and contact time similar to running at 18 km·h -1 , NIKE has been shown to return 87% of the mechanical energy stored (7.46 J energy return per step) [7]. This greater energy return has been attributed to the greater compliance, rather than resilience, of the NIKE midsole [7].
Although the amount of energy returned from NIKE during running is relatively small compared to that from musculoskeletal structures [22,23], it is sufficient to diminish the energetic cost of running locomotion. Our data demonstrated that, at the two greatest intensities (70 and 80% of ʋVȮ2peak), lactate concentration levels were lower in NIKE than OWN. Accordingly, a shift in lactate accumulation to greater intensities would enable athletes to train or race at a similar speed for a lower metabolic cost, with the potential to improve running endurance performance [24]. Lactate concentrations were also lower in FLAT than OWN at the greatest intensity, indicating a lower metabolic cost in FLAT.
It has been well documented that for every 100 g of added shoe mass, the energetic cost of running increases ~0.7 to 1.0% [4,25,8,26]. In our study, the mean mass of NIKE was 209 g, FLAT was 156 g, and OWN was 313 g. In agreement with the shoe mass -energy cost relationship, there was a 2.0% increase in energy cost for each 100 g of added mass when contrasting FLAT to OWN. On the other hand, considering shoe mass only would mean a 4% increase in energy cost for each 100 g of added mass when comparing NIKE to OWN. Thus, the mass -energy relationship does not fully explain the overall RE advantage observed in the current study, and highlights the positive effects of the NIKE construction on the metabolic cost of running; underpinning the non-significant effect of shoe mass in our analyses.
Frederick et al. [27] were some of the first researchers to address the 'cost of cushioning' after noting no change in RE between barefoot and well-cushioned shoes. Subsequent research has confirmed that 10 mm cushioning compared to no cushioning reduced metabolic cost, but that the detrimental effects of shoe mass on energy expenditure counteracted the benefits of cushioning when comparing barefoot to shod treadmill running [26]. When matched for mass and controlling for other footwear features, shoes with a more compliant (i.e., more cushioned) and resilient (i.e., less energy loss) midsole can reduce oxygen cost by ~1% [25].
Similarly, inserting carbon fibre plates to midsoles to increase the longitudinal bending stiffness of running footwear has also been shown to improve RE ~1% [28], although the location of the plate [29], running speed [30], and induced changes in running biomechanics [29,31] can influence this relationship. The 'cost of cushioning' concept and 87% of stored mechanical energy return from the NIKE midsole potentially involve lesser muscular efforts to moderate impacts, which could underpin the 3.6 to 4.5% improvement in RE compared to OWN across running intensities alongside the lighter shoe mass and stiffer midsole, although more mechanistic investigations are required to confirm these assumptions.
Runners from this study also performed better during the 3-km TT in NIKE by 16 [7] differences in the footstrike patterns of cohorts, and the more substantial difference between our runners' OWN and FLAT footwear compared to NIKE (e.g., the observed variability in the minimalist ratings in Table 1).
NIKE significantly improved 3-km TT performance compared to FLAT, whereas RE measures were similar between these two footwear conditions. Typically, improvements in RE lead to improved TT performances in a predictable manner [4]. It is possible that the greater difference in shoe characteristics (i.e., shoe drop, stack height, and minimalist index, Table 1) between FLAT and OWN compared to NIKE and OWN was more challenging for runners to adapt to at a sustained maximal effort. The lower perceived comfort expressed at baseline in FLAT compared to OWN might have made runners more conservative during the 3km TT and refrain from requesting speed increments as readily. It is also possible that 3-km TT performance would have improved in FLAT versus OWN had a period of habituation or training in FLAT been provided.
Although our findings overall indicate that NIKE can benefit RE and long distance racing performance in male recreational runners, the non-rearfoot strikers in our study appeared to respond less favourably to NIKE than the rearfoot strikers, as also observed by Hoogkamer et al. [12]. Based on our 3-km TT results, 21 runners per group would be needed to detect a significant difference (α = 5%) in response to footwear between non-rearfoot and rearfoot strikers with 80% power (β = 20%). The potential for greater benefits of NIKE in rearfoot strikers might be due to the greater compression of the midsole at the heel region [32] where the amount of ZoomX foam is the greatest, as well as the evidenced -albeit limited -greater vertical loading rates and impact peaks of rearfoot strikers [33]. Research specifically examining responses to NIKE footwear based on footstrike pattern is warranted to elucidate the interaction between footstrike characteristics and the performance enhancing effects of footwear.
Although reliable [14], conducting a TT on a treadmill is one limitation of this study. The TT difference between NIKE and OWN equates to approximately one speed increment difference for the duration of the trial (i.e., 0.5 km·h -1 ), with non-treadmill TT performances potentially more ecologically valid being not tester-dependent for speed changes. Even though the spray-painting of the shoes minimised the potential for a placebo effect, this method did not completely blind the runner or examiner to footwear worn. Furthermore, running in NIKE compared to other footwear has been shown to influence biomechanics [10,12,6], with typical findings of increased stride length, longer flight times, and decreased peak plantar-flexion velocities in NIKE. The RE speeds in this study were slower than those examined previously and in different footwear; hence, investigating the biomechanical adaptation in recreational runners to NIKE could shed light on mechanisms that contribute to improved performance.

PRACTICAL APPLICATIONS
Evidence of variable performance responses to the Nike 4% shoe: Definitely not a game-changer for all recreational runners Our study provides evidence that NIKE can benefit RE and 3-km TT performance and could potentially represent a viable ergogenic aid in recreational running; but only 47% of runners were more economical across intensities in NIKE, only 61% faster in NIKE, and just 29% more economical and faster in NIKE.
Hence, given the considerable variability between runners and intensities, our data does not support a generalised benefit in recreational runners seeking to improve performance. Certain runners were more economical or performed better in FLAT, whereas no runner was the most economical in their OWN footwear across intensities. Furthermore, one must consider the risks associated with changing biomechanical patterns in uninjured runners [33] and transitioning to novel footwear too quickly [34]. We only recruited male runners because of shoe cost considerations; however, we speculate that recreational female runners would respond similarly given the findings of similar responses to NIKE footwear from elite [10] and Strava [13] cohorts.

CONCLUSIONS
We provide evidence that the commercially available Nike Vaporfly can improve RE and effectively enhance performance in some male recreational runners using laboratory-based data, particularly when compared to participants' habitual shoes. Lightweight racing flats were also effective in improving RE and enhancing TT performance in some -albeit fewer -runners. When compared with their own footwear, the Nike '4%' benefits may extend to recreational runners, relative submaximal running speeds, and maximal efforts (albeit to a lesser extent).
Funding: This study was internally funded by Te Huataki Waiora School of Health, University of Waikato, and The Running Clinic, Canada. This study was not endorsed by any footwear company.

Conflict of interest:
Blaise Dubois and Jean-François Esculier are employed by The Running Clinic, a continuing education organization which translates scientific evidence to healthcare professionals and the general public. Kim Hébert-Losier is a speaker for The Running Clinic.