Physiological and Metabolic Reaction to Lower Body Positive Pressure Treadmill Running

Aim: The aim of the present study was to investigate the reactions of oxygen consumption (V˙O2), heart rate (HR) and lactate accumulation (La) when running on lower body positive pressure treadmills (LBPPT).

Methods: 15 well-trained male athletes (V˙O2peak: 60.2±3.8ml kg-1 min-1) completed in randomized order three analogous maximal incremental treadmill tests,recording spiroergometrical data using breath-by-breath analysis. Two tests were held on a LBPPT, with 80% and 60% body weight (80% BWSet and 60% BWSet), respectively. The third test was completed on a conventional treadmill (100% BWSet).

Results: Average of all running speed stages from 10 to 18 kmh-1, V˙O2 decreased significantly from 48.1±8.4 via 39.7±6.8 to 33.5±7.3ml kg-1 min-1 at 100%, 80% and 60% BWSet (p<0.001). HR was on average 15 bpm and 27 bpm lower at 80% and 60% BWSetcompared to 100% BWSet (p<0.001), while La decreased from 2.5±2.3 via 1.5±1.1 to 1.1±0.5 mmol l-1 (p<0.001).

Conclusion: V˙O2, HR and La are clearly changed by LBPPT running. Furthermore, regression analyses showed that training at a fixed V˙O2stimulus leads to higher lactate values on the LBPPT compared to the conventional treadmill, which may indicate a change in energy contributions.

KEY WORDS: AlterG, Hypogravity, LBPPT, Anti-Gravity, Oxygen Consumption

Zielstellung: Das Ziel der Studie bestand darin, die physiologischen und metabolischen Anpassungen in Form von Sauerstoffaufnahme (V ˙O2), Herzfrequenz (HF) und kapillarer Laktatkonzentration (La) während des Laufens auf einem Anti-Schwerkraft-Laufband (ASL) zu erfassen.

Methodik: 15 trainierte männliche Ausdauerathleten (V ˙O2peak: 60,2±3,8ml kg-1 min-1) absolvierten drei identische Laufband-Stufentests in randomisierter Reihenfolge, bei denen spiroergometrische Daten sowie kapillare Laktatkonzentrationen erfasst wurden. Zwei Tests fanden auf dem ASL statt, bei 80% bzw. 60% des Körpergewichts (80% KG und 60% KG). Der dritte Test wurde auf einem herkömmlichen Laufband bei gesamtem Körpergewicht absolviert (100% KG).

Ergebnisse: Über die Geschwindigkeiten von 10-18km h-1 reduzierte sich die V ˙O2 im Mittel signifikant von 48,1±8,4 über 39,7±6,8 auf 33,5±7,3 ml kg-1 min-1 bei 100%, 80% und 60% KG (p<0.001). Die HF sank um 15 (80% KG) bzw. 27 (60% KG) Schläge pro Minute im Vergleich zum Wert bei 100% KG. Die La reduzierte sich von 2,5±2,3 via 1,5±1,1 auf 1,1±0,5mmol l-1 (p<0.001).

Fazit: V˙O2, HF und La sind während des Laufens auf einem ASL signifikant reduziert. Zudem zeigen Regressionsanalysen, dass bei einer festgelegten Belastung (% V˙O2peak) auf dem ASL höhere La ermittelt werden. Diese Anpassung gilt es bei Trainingsinterventionen zu beachten.

SCHLÜSSELWÖRTER: AlterG, Teilschwerelosigkeit, Teilgewichtsentlastung, Sauerstoffaufnahme

Middle-distance and long-distance running success is influenced by the physiological factors of maximal oxygen uptake (V˙O2max) (16, 29), running economy (RE) (5, 27, 29, 33), utilization of V ˙O2max (% V ˙O2max) (2, 26) and the running speed at the anaerobic threshold (vAT) (7, 40, 41). To improve those parameters, high training volumes are commonly realized (3, 39).

Inevitably, high training volumes imply increased mechanical stress to the musculoskeletal system. Altogether, 37-56% of all runners incur at least one running-related injury per year, while 1000 hours of training statistically result in 2.5-12.1 injuries (42). To avoid overuse injuries and their repercussions of training reduction or cessation, elite coaches and runners have been seeking and adopting new training tools, such as underwater treadmills and lower body positive pressure treadmills (LBPPT). Both technological concepts allow runners to maintain high training volumes, whereas their musculoskeletal load is reduced because of partial body-weight support by static buoyance (water) or lift (air).

With regard to oxygen consumption (V˙O2), previous studies revealed that running on a LBPPT implies a decrease in V˙O2 as compared to running under standard conditions at the same speed (1, 18, 23, 24, 30). Concerning cardiac reactions, maximal heart rate (HRmax) seems to be unaffected from LBPPT running, whereas submaximal heart rate (HR) is reduced.

As for metabolic reactions, the understanding of how LBPPT affects lactate accumulation (La) has been incomplete. To date, only one study has explicitly addressed that question, reporting that peak lactate (Lapeak) after an incremental treadmill test was lower for running on a LBPPT at 85% BWSet(6.4 ± 1.8 mmol l-1) than on a standard treadmill (7.7±2.2mmol l-1) (9). At 95% BWSetand 90% BWSet, however, no significant differences between LBPPT and standard treadmill running were found.

Because V˙O2, HR and La belong to the most important physiological parameters for exercise monitoring and training control (8, 25, 34, 39), we investigated the simultaneous reactions of those parameters in well-trained runners during an incremental treadmill test on the LBPPT. The explicit aim of the present study was to elucidate V ˙O2, HR and La reactions in a combined multifactorial parameter analysis in order to advance and refine practical training implications and recommendations of LBPPT running for sports scientists, physicians and practitioners. To the best of our knowledge, the present study is the first multifactorial approach to combine metabolic and respiratory parameters in a holistic view.

Fifteen well-trained male runners and triathletes (Table 1) performed three identical incremental treadmill tests until volitional exhaustion in randomized order. The inclusion criterion was a personal best better than 40:00 min at an official 10km road race. All participants were familiar with treadmill running, performing several sessions every year. The study was approved by the ethics committee (HU Berlin) and is in accordance with high ethical standards (11) and the Declaration of Helsinki. Written consent to participate in the study was given by all participants.

One test was completed on a conventional treadmill h/p/cosmos saturn®250/100 (100% BWSet; h/p/cosmos sports & medical GmbH, Nußdorf-Traunstein, Germany). The other two tests were performed on a LBPPT AlterG®Anti-Gravity Treadmill®Pro 200 Plus (AlterG®, Fremont, California, USA), with 80% and 60% BWSet, respectively. For each participant, the tests were performed on the same weekday and at the same time of day (±30min) in three consecutive weeks. Prior to each test, body weight and height were measured (Seca Vogel & Halke Hamburg 910, seca GmbH & Co. KG, Hamburg, Germany). Spiroergometrical data were acquired throughout each test using a stationary system with breath-by-breath analysis (Quark CPET, COSMED, Pavona di Albano, Italy). Data processing was completed via the software OMNIA 1.6 (COSMED, Pavona di Albano, Italy). HR measurements were performed using the HRM RunTM system (Garmin Ltd., Canton Schaffhausen, Switzerland). At rest, between the stages and after exhaustion, a sample of 20 μl of arterialized capillary blood was taken from the earlobe, solubilized in a 1000μl hemolysate solution and analyzed using the SUPER GL ambulance system (Dr. Müller Gerätebau GmbH, Freital, Germany). In addition, between the stages and after exhaustion, rating of perceived exertion (RPE) was inquired on the basis of the Borg Scale (4).

For each treadmill condition, the initial running speed was set to 6km h-1and increased by 2km h-1 after each completed stage. The individually achieved maximal running speed was that at volitional exhaustion and differed between participants (see below). The maximal running speeds were observed on the LBPPT and reached 28km h-1 for some participants. Each stage lasted 3 min, except for the last one at individual volitional exhaustion, which could be stopped earlier. Only complete stages were considered in later data analysis. Between the stages, a rest break of 30s was made for taking blood samples and ask for RPE. All participants performed their tests with the same individual, familiar pair of running shoes, thereby avoiding footwear-related intra-individual distorting effects (13, 36). Furthermore, participants were instructed to replicate a similar pattern of training and nutrition during the last 48h before their tests.

The first two stages (6km h-1 and 8km h-1) of each trial were considered as a standardized warm-up and excluded from data analysis. All participants successfully completed the stages from 10km h-1 to 18km h-1, irrespective of BWSet condition. Running at 20km h-1, some participants reached volitional exhaustion before the end of the stage. To maintain data homogeneity for each BWSetcondition and participant, those additional stages were disregarded in later data analysis. Hence, for each BWSet condition and running speed, the same number of participants (n=15) was included.

To obtain a precise approximation for the individual maximal running speed (vmax) of each participant in each trial, vmaxwas extrapolated linearly from the actually completed stage duration (i.e., ≤3:00min) and the nominal running speed of the last stage, e.g. vmax=25km h-1 for running 1:30min at 26km h-1. Based on those vmaxvalues, an additional normalization of individual absolute running speeds (vabs, in terms of km h-1) with respect to vmaxwas done, yielding relative running speeds vrelin terms of 50-100% vmax, i.e. vrel:=vabs/vmax x 100%. For retrieving V ˙O2, HR and La figures at those specific relative running speeds, a linear interpolation of mean measurement values was conducted.

Following recommended procedure, acquired V˙O2 data were averaged over 30s (31), while the last 30s value of each stage was used in later statistical analysis. HR data were manually saved 15s before the end of each stage and directly after exhaustion. For enabling an easy transfer of our study result into practice, regression equations were computed for V ˙O2, HR (linear model) and La (exponential model) as functions of absolute running speed by means of Origin 2017 (OriginLab Corp., Massachusetts, USA), solely using the data of stages from 10-18km h-1. Moreover, a standard low-intensity training stimulus of ≈75% V˙O2peak (14, 15, 39) was selected for comparing responses in running speed, HR and La for running on a LBPPT vs. a standard treadmill, and deducing practical implications therefrom.

Data were processed with Microsoft Excel 2016 (Microsoft Corporation, Redmond, USA) and IBM SPSS Statistics 23 (IBM, Armonk, USA). Results are presented as mean±standard deviation. To identify a bias between the three BWSet conditions, three individual cases of a one-way analysis of variance (ANOVA) for the factor BWSet were performed on the dependent variables vmax, V ˙O2peak and Lapeak. For RPE, a Friedman Test was conducted, instead. Furthermore, a two-way ANOVA was computed based on the two factors BWSet and vabs(for 10-18km h-1) for the dependent variables V ˙O2, HR and La. In consideration of the two mathematically equivalent representations of running speed in terms vabs(km h-1) and vrel(percentage of vmax), a second ANOVA was computed for the dependent variables V˙O2, HR and La on the basis of the factors BWSet and vrel(for 50%-100%). Notably, in spite of the mathematical equivalence of running speed representations in terms of vabsand vrel, corresponding statistics for V ˙O2, HR and La as functions of vabsand vrelare not equivalent. The reason for this is given by the definition of vrel, by which it not only depends on nominal running speed but also on the individual’s maximal running speed vmax, which differs from participant to participant. Hence, the vrelrepresentation implicitly covers differences in performance level of the participants, which the vabsrepresentation does not include.

For all ANOVA computations, variance homogeneity was assessed first using Levene’s test (p>0.05). Standard level of significance was set to p=0.05. 95% confidence intervals (CI) were calculated. Effect sizes (ES) with partial eta-squared (η²) values were calculated as following (20): 0.01≥η2≤0.09, small ES; 0.09≥η²≤0.25, medium ES; and η²≥0.25, large ES. Bonferroni correction was used for post hoc multiple comparison of means for main effects and significant interactions.

Maximal Running Speed vmax, Peak Oxygen Consumption V˙O2peak, Peak Lactate Accumulation Lapeak and Perceived Exertion RPE at Exhaustion
With increasing level of unloading and thus decreasing BWSet, the participants achieved significantly higher maximal running speeds vmax(p<0.001; Figure 1). In relative terms, vmaxgrew by 18±5% for 80% vs. 100% BWSet and by 30±7% for 60% vs. 100% BWSet. In contrast, V ˙O2peak, Lapeak and RPE at exhaustion were unaffected by BWSet.

Oxygen Consumption (V˙O2)
In terms of absolute running speed vabs, an increase in speed resulted in a higher V ˙O2 (p<0.001, η² = 0.743) across all BWSetconditions, the changes proving significant for each stage from 10-18km h-1 (p<0.05, Figure 2). Conversely, mean V ˙O2 significantly decreased with lower BWSet, where mean decreases amounted to 8.4 (CI: 6.9-9.9) and 14.7 (CI: 13.1-16.2) ml kg-1 min-1 for 80% vs. 100% and 60% vs. 100% BWSet, respectively (p<0.001, η²=0.720; Figure 2). Expressed in terms of relative running speed vrel, V ˙O2significantly increased with vrel(p<0.001, η²=0.909) and BWSet (p<0.001, η²=0.100; Figure 2, right), the latter, however with only medium effect size. On average, the decrease in V˙O2 from 100% to 80% and 60% amounted to 2.0 (CI: 0.8-3.2) and 2.4 (CI: 1.2-3.5) ml kg-1 min-1, respectively. No interaction effect of relative running speed and BWSet was observed for V˙O2 (p=0.999, η²=0.006).

Heart Rate (HR)
In terms of vabs, higher running speeds resulted in significantly increased HR across all unloading conditions (p<0.001, η²=0.766), with each difference proving significant (p<0.05) from 10-18 km h-1. Moreover, HR significantly decreased with decreasing BWSet (p<0.001, η²=0.612; Figure 3, left), the mean reduction amounting to 15 (CI: 11-18) and 27 (CI: 23-30) bpm for 80% vs. 100% and 60% vs. 100% BWSet, respectively. No interaction effect between absolute running speed and BWSet was found for HR (p=0.661, η²=0.027). Also in terms of vrel, HR significantly increased with BWSet (p<0.05, η²=0.053). Regarding the factor BWSet, a significant decrease of 4 bpm (CI: 1-6) was observed only for 100% vs. 60% BWSet (Figure 3, right). No interaction of vreland BWSet was observed for HR (p=1.000, η²=0.003).

Lactate Accumulation (La)
Expressed as a function of vabs, higher La values were observed for increasing running speeds (p<0.001, η²=0.472; Figure 4, left). As for unloading condition, significant reductions from 100% to 80% and from 100% to 60% BWSet were found, amounting to 1.04 (CI: 0.6-1.4) and 1.44 (CI: 1.0-1.8) mmol l-1, respectively. For example, running with 16 km h-1at 100% BWSet led to a La of 3.05±1.48 mmol l-1, whereas 80% and 60% BWSet resulted in 1.71±1.01 and 1.21±0.37 mmol l-1, respectively (Figure 4, left). In summary of all running speeds, there was a La decrease of 1.04mmol l-1at 80% BWSet and 1.44mmol l-1 at 60% BWSet, corresponding to a relative reduction of 29% and 42%, respectively.

In terms of vrel, La significantly increased with BWSet, yet with only a small effect size (p<0.05, η²=0.024; Figure 4, right). No interaction between vrel and BWSet was found in La (p=0.932, η²=0.017).

Regression Analyses
Regression analyses of V˙O2, HR and La with respect to BWSet are presented in Table 2, yielding high values for the coefficient of determination R2>0.98 through all BWSetconditions. For achieving a practically relevant training stimulus of V ˙O2=45ml kg-1min-1 (≈75% V ˙O2peak), running speed should be increased by 3.8km h-1 at 80% BWSet and by 7.4km h-1 at 60% BWSet as compared to 100% BWSet, as our data show. With that increase in running speed, HR elevates by 6 bpm at 80% BWSet and by 12bpm at 60% BWSet. Furthermore, La values raise disproportionally from 1.31 to 1.90 and 2.45mmol l-1 at 100%, 80% and 60% BWSet, respectively (Table 3).

There is evidence, for instance, that LBPPT running implies lower vertical ground reaction forces and thus reduces biomechanical loading (6, 10, 12, 17, 30). Regardless of that biomechanical unloading, it is essential for elite athletes to account for the changes that BWSetinduces in their physiological training load. First findings showed a nearly proportional decrease of V˙O2 with body-weight support for 70%, 80% and 90% BWSet (18). However, beyond that range of body-weight support, i.e. <70% BWSet or >90% BWSet, the relative change in V ˙O2 differed from that of BWSet(18). Generally, a V ˙O2 decrease on LBPPT has been confirmed in the present study. We obtained decreases of -17% and -30% in V˙O2 from 100% to 80% and 60% BWSet, respectively. Interestingly, data of elite runners showed significantly higher V˙O2 decreases of 34% and 38% at 80% and 60% BWSet (24). As a possible explanation, the authors suspected that 80% BWSet may be an optimal level of horizontal support for the elite runners (24).

Furthermore, HR data from the present study are in accordance with previous studies, reporting a reduction in HR from 123±6bpm to 105±8 and 101±12bpm at 100%, 80% and 60% BWSet, respectively (24). Barnes and Janecke (1) showed a HR decrease from 143±5 bpm at 100% BWSet to 126±3 bpm at 80% BWSet (12% reduction). The pertinent result of the present study supports this outcome. In analogy to the V ˙O2 reactions, with respect to elite runners, however, the relative reduction proved slightly lower at 80% BWSet. Also for higher running speeds, the results are qualitatively comparable with the findings of the present study: We found changes of -9% for 80% vs. 100% BWSet and -17% for 60% vs. 100% BWSet, while previous studies on elite runners showed a HR decrease of -16% (80% BWSet) and -22% (60% BWSet) (24). In view of the quantitative deviations, it may be hypothesized that highly trained runners benefit “better” from BWSet than well-trained runners do. The variability of the running economy may be one explanation (24). McNeill et al. (24) demonstrated that even elite runners, who had developed an economical stride for several years, showed an increased variability from 100% BWSet to 80% BWSet and even more through 60% BWSet. Furthermore, running economy on LBPPT seems to be higher (“poorer”) (38). It seems likely that this variability is more pronounced in recreational or less-trained runners (28). This high variability may lead to a less economical stride and therefore to higher physiological reactions, e.g. heart rate. On the other hand, Stucky et al. (37) showed that stroke volume is increased during steady-state treadmill running on LBPPT. It may be hypothesized that elite runners – provided with an already high stroke volume – benefit in such a way that heart rate can be decreased in a disproportional way. Such presumptions should be substantiated or refuted by future investigations. In addition, it was shown that the LBPPT had no influence on venous return during submaximal running (37). The authors explained that the produced forces of the muscle pump (~90mmHg) during physical stress override the generated forces of the LBPPT (~30mmHg at 60% BWSet and ~15mmHg at 80% BWSet, respectively).

To date, only little has been known about how LBPPT affects submaximal La values. Trying to bridge that gap, the present study revealed that La significantly decreased on LBPPT (p<0.001). La at lower running speeds was unaffected by LBPPT, while clear differences were observed for faster running.

In essence, V ˙O2, HR and La are clearly changed by LBPPT running. Since those three quantities are often used for monitoring and controlling training (8, 25, 35, 39), it is essential that coaches carefully instruct their athletes on how they should alter their session targets on LBPPT. While some studies show that submaximal intensities of 70-85% V ˙O2max seem to be most beneficial to increase V˙O2max, other authors suggest training at higher intensities (90-100% V ˙O2max) (25). Based on a mean V˙O2peak of 60.2±3.8ml kg-1 min-1 in this study, a submaximal training speed of, e.g., 12.7km h-1(≈75% V ˙O2peak) at 100% BWSet should be increased to 16.5km h-1at 80% BWSetin order to maintain the same V ˙O2 stimulus. In that case, La increases from 1.31mmol l-1 at 100% BWSet to 1.90mmol l-1 at 80% BWSet, suggesting that anaerobic energy contributions increase. It may be hypothesized that biomechanical changes on the LBPPT (32) as well as altered muscular innervation patterns (21, 22) lead to a less economical stride that goes along with increased La values. The relationship between V ˙O2 and La (Figure 5) for the three BWSet conditions supports this assumption. Bearing in mind that elite athletes are used to complete at least 70% of their training at intensities below La levels of 2mmol l-1 (19, 25), an increase in running speed on LBPPT should thus be carried out carefully – as long as aerobic adaptations are desired. Despite that, LBPPT running allows to train closer to the race pace at reduced physiological effort.

Farina et al. (6) described that the biomechanical and physiological adaptations appear “to become more exaggerated at body weight settings <70 %”. Considering the results of the present study, it seems advisable from a practical training perspective not to fall below this threshold. As described in the results, the running speed to reach the same V ˙O2 stimulus has to be increased so much that the lactate values rise significantly. While at 80% BWSetonly minor changes in lactate and thus in the anaerobic energy contribution are observed (still being classified as a low intensity training), lactate level at 60% BWSet clearly exceeds the 2mmol l-1threshold. Hence, for low intensity training, we recommend keeping the body weight reductions and associated speed increases low (around 80% BWSet) to prevent the risk of significant adjustments in the aerobic-anaerobic energy contribution.

Regarding previous research, a current review by Farina et al. (6) described that every 10% decrease in BWSet leads to a V˙O2 decrease of 3.4ml kg-1 min-1 (ranging from 1.79 to 5.36m s-1and LBPPT settings of 50-100% BWSet), which would amount to 6.8ml kg-1min-1for 20% BWSetand 13.6ml kg-1 min-1 for 40% BWSet. However, the present study with well-trained athletes yielded slightly different adjustments of 8.4ml kg-1 min-1(80% BWSet) and 14.7ml kg-1 min-1(60% BWSet). The reductions in oxygen uptake found by McNeill et al. (24) for elite runners (at 4.47m s-1) were also clearly different: 14.3ml kg-1 min-1 (at 80% BWSet) and 17.0ml kg-1 min-1 (at 60% BWSet). Those results suggest that absolute changes in V ˙O2 can be very different, depending on the tested participants. As already discussed above, the reasons might be the higher performance capacity and the associated different running economy. In future, it will be necessary to address the questions of physiological reactions on the LBPPT for different performance levels to be able to deduce more specific training instructions.

We thank Mr. Peter Rüdrich for practical help and inspiring discussions during the working process.

Conflict of Interest
The authors have no conflict of interest.