**3. Results**

A final sample of 10 athletes completed the present study, with no drop-outs. These athletes were national to international level runners/triathletes, with two of them having participated in major sporting events (Olympic Games and World Championships). Table 1 presents the mean and individual descriptive characteristics of the sample, showing a fairly homogeneous fitness level across all runners (i.e., mean VO2max of 78.4 ± 3.8 mL kg−<sup>1</sup> min<sup>−</sup>1).


**Table 1.** Descriptive characteristics of the participants.

A Student's t-test for paired samples revealed no significant difference in RE values between TTS and GS conditions (51.1 ± 4.2 vs. 50.9 ± 5.1 mL kg−<sup>1</sup> min−1, respectively, *p* = 0.779, Cohen's *d* = 0.092). Figure 3 shows both mean and individual values for VO2. Additionally, blood lactate was not different between shoe conditions at min 1 (*p* = 0.793), min 3 (*p* = 0.250), and min 15 (*p* = 0.641) post-exercise (Figure 4). Both whole-body and legs-only RPE values were also not significantly different between TTS and GS at min 1 (*p* = 1.0 and *p* = 0.273, respectively), min 3 (*p* = 0.443 and *p* = 0.591, respectively), and min 15 (*p* = 0.168 and *p* = 0.591, respectively) post-exercise (Figure 4). Finally, HR values were not significantly different between TTS and GS during exercise (150.1 ± 15 vs. 151.0 ± 16 bpm, respectively, *p* = 0.461, Cohen's *d* = 0.244; Figure 4).

**Figure 3.** Mean and individual running economy values (mL kg−<sup>1</sup> min<sup>−</sup>1) of the 10 athletes running in traditional training shoes (grey column) and in grounding shoes (black column).

**Figure 4.** Blood lactate (**A**), whole-body rate of perceived exertion (RPE; **B**), and legs-only RPE (**C**) during the recovery period after running in the traditional training shoe (TTS, gray solid line) or grounding shoe (GS, black solid line). Heart rate during the running economy trial in both TTS and GS trials (**D**). Dashed lines represent overlapping mean values between shoes.

#### **4. Discussion**

The main findings of the present study show that grounding technology applied to shoe designs does not provide a physiological/perceptual response over traditional training shoes in well-trained athletes. The RE, blood lactate, heart rate, and perceptual response of these athletes, exercising at 80% of their IAT during 20 min on a 400 m dirt track, were not different between shoes conditions.

Despite previous promising findings suggesting that grounding technology has positive effects on the physiological responses (i.e., reduced acute inflammatory processes) of humans at rest [7,8], very limited research has focused on the implementation of grounding during exercise, with only two studies focusing on the effectiveness of grounding in reducing muscular damage after exercise-induced DOMS. This is the first study to examine the impact of grounding in shoes during running, which makes the comparison with previous studies challenging due to the unique nature of running for the implementation of this technology (i.e., intermittent contact time with the ground). Our findings, however, differ from those of Sokal et al. [8], who claimed that all recreational cyclists within their study experienced physiological attenuation at rest, during a 30 min exercise at 50% of their VO2max, and during recovery, indicated by decreases in blood urea; however, these authors failed to include any individual data. It is also worth noting that these biochemical parameters were not measured immediately prior to grounding/placebo conditions, and therefore group-by-time interactions could not be determined, which limits the interpretation of these results. Additionally, one would expect both blood urea and creatinine concentrations to remain unchanged following the exercise protocol used by these authors (a single bout of light exercise for 30 min). Blood urea and creatinine levels have been shown to increase after prolonged, strenuous exercise as a result of increased protein catabolism and/or impaired renal function [19], which is unlikely to have occurred during the exercise protocol proposed by Sokal et al. The difference between the groups observed by Sokal et al., interpreted in the context of our present findings, are more likely due to dayto-day inter-individual variability in blood urea, or some potential methodological issues during data collection, rather than due to physiological stress attenuation during exercise. In a subsequent study, Sokal et al. presented additional data from the same aforementioned experiment [20], focusing on the effects of grounding on VO2 uptake, blood glucose, lactate, and bilirubin concentrations. The 42 subjects included in this study were divided into two subgroups (*n* = 21) according to their VO2max, therefore, both groups had a comparable cardiorespiratory fitness (Group A = 50.8 vs. Group B = 50.7 mL kg−<sup>1</sup> min−1). The study design followed a double-blind, crossover protocol between Groups A and B. During the first testing day, Group A was under the placebo condition and Group B was under the grounding stimulus, with these conditions interchanged during the second day of testing. These authors reported a significantly reduced VO2 uptake (numeric data not shown by the authors) at the end of the exercise with the grounding stimulus only in Group B, when compared to the placebo. The study design employed by Sokal et al. [8,20] has limited reliability, given that their experimental tests were performed on different days, which may have biased the results. Day-to-day variability and the lack of a familiarization trial may have potentiated the learning effects only for Group B (i.e., the group with the grounding stimulus during the second day). These results should, therefore, be interpreted with caution.

To our knowledge, the two aforementioned studies are the only two experiments focusing on the effects of grounding on the biophysiological responses of humans during submaximal exercise. However, the important methodological issues described above, and the use of cycling being the only mode of exercise, limits the interpretation of the current literature and its comparison with the present study. In our experiment, we used a doubleblind, randomized, crossover design, with tests for all experimental conditions performed on the same day. We are aware that the conductor element within the shoe was not in permanent contact with the ground (i.e., intermittent contact time during running), and we did not measure muscle activity, nor foot/stride mechanics, during running, which may have provided more information and potentially revealed an effect. However, to ensure a sufficient

contact time, we designed a longer than usual RE protocol (i.e., 20 min bouts; Figure 2), so that we could identify a potential dose–response relationship over time. Despite these rigorous experimental procedures, our results show that grounding technology did not have any impact on the measured responses during running when compared to traditional training shoes. Previous research showed a decrease in muscle damage in response to high-intensity strength exercises in subjects under grounding conditions [13,14] when compared to a placebo. These findings would suggest that grounding technology may have a role to play as a muscle recovery method, which in turn could translate into a benefit for runners when performing higher intensity exercise (i.e., above the anaerobic threshold) in which muscle fatigue and acidosis occur to a greater extent. Nonetheless, future research using larger sample sizes and examining foot mechanics (especially contact times) would be required to confirm our findings. Other shoe designs currently available on the market that include a CFP and a high midsole stack height made of compliant, resilient, and lightweight foam, seem the most effective shoe modality to date. This technology has shown to improve RE by increasing the midsole longitudinal bending stiffness, favoring a decrease in the range of motion of the metatarsophalangeal joint [3,21,22].

#### **5. Conclusions**

In conclusion, our results suggest that grounding in shoe designs is not an effective alternative for well-trained athletes to improve their running efficiencies, and/or their physiological/perceptual responses during submaximal exercise. However, there are intrinsic limitations that should be considered. Potential grounding effects could have been missed during our study as running does not allow constant contact between the athlete and the ground, which could have potentially biased the results. In relation to this, lower caliber athletes may have benefited from this technology given their ground contact times are greater than faster, elite athletes; an issue that could not be addressed in the current study. Future research may therefore consider additional sports in which athletes remain in constant contact with the ground (e.g., race-walking, cross-country skiing, powerlifting). Despite these limitations, our study followed a high-quality methodological protocol (double-blind, randomized, crossover design) using a homogeneous sample of highly trained athletes (as represented in Table 1), which suggests that our conclusions are reliable for this specific population.

**Author Contributions:** Conceptualization and methodology: B.M.-P., I.Z., M.K., D.R., J.A.C. and Y.P.P.; formal analysis: A.G.-A., J.A.C., I.Z., B.M.-P.; writing—original draft preparation: B.M.-P., I.Z., A.G.-A.; review and editing: B.M.-P., I.Z., A.G.-A., M.K., T.B., M.G., D.R., J.A.C. and Y.P.P.; supervision: Y.P.P. and J.A.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was supported by a contract from adidas AG with the University of Zaragoza, Spain (Project: "Testing support for innovation project"; number 2021/0348).

**Institutional Review Board Statement:** The present study was approved by the Ethics Committee of Aragon, Spain (CEICA, num. 17/2021).

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** The datasets used and analyzed within the present manuscript will be available from the corresponding author/first author upon request.

**Acknowledgments:** We wish to thank the athletes involved in this study for participating.

**Conflicts of Interest:** M.K., T.B., M.G., D.R. are employees of adidas AG. B.M.P., I.Z., A.G.A., J.A.C., Y.P.P. have no conflicts of interest relevant to the content of this article.

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