Oxygen consumption during walking and running under fractional weight bearing conditions by Paul

RESEARCH ARTICLE

Oxygen Consumption During Walking and Running Under Fractional Weight Bearing Conditions Heidi Ruckstuhl, Thomas Schlabs, Armando Rosales-Velderrain, and Alan R. Hargens RUCKSTUHL H, SCHLABS T, ROSALES-VELDERRAIN A, HARGENS AR. ered during unloaded exercise. The extent of decrease in Oxygen consumption during walking and running under fractional oxygen consumption over a wide range of speeds (from weight bearing conditions. Aviat Space Environ Med. 2010; 81:550–4. slow walking to moderately fast running) is new and Introduction: Knowledge of changes in oxygen consumption during the emphasis of our present study. reduced body weight loading at different speed levels is important, in particular in astronauts who may lose aerobic fitness as well as in clinical patients (e.g., in obese individuals at walking speeds or in injured METHODS athletes at running speeds). Therefore, the aim of this study was to analyze oxygen consumption during Subjects . unloaded walking and running. Methods: Oxygen consumption (VO2), heart rate (HR), and Borg rating of We studied 10 physically active, nonsmoking female perceived exertion (Borg RPE) were quantified in 10 healthy young female and male volunteers at 3 body weight (BW) conditions (100%, and male volunteers (5 females and 5 males) aged 23 6 66%, and 33% BW) and 4 treadmill speeds (slow walking at 0.4 m z s21, 3 yr (mean 6 SD) with body mass index of 21.8 6 2.0 comfortable walking at 1.3 m z s21, slow running at 2.2 m z s21, and (mean 6 SD). They were recreationally involved in 21 moderately fast running at 3.1 m z s ). Unloading was achieved in a sports such as running, strength training, water aerowaist-high chamber with increased pressure. called Lower Body Positive bics, and team sports (soccer or basketball). Subjects did Pressure (LBPP). Results: All parameters (VO2, HR, and Borg RPE) decreased during unloaded walking and running. Interestingly, our findings not drink alcohol or caffeine for at least 12 h before testconfirm a specific linear relationship between each parameter and tread o ) ing as both may increase oxygen consumption (V 2 Delivered by Publishing Technology to: Guest User mill speed for each BW condition with smaller slope angles at higher . ( 1,11 ). Moreover, subjects were asked to abstain from 76.167.134.242 On: angle decreased from 11.9 at Sat, 15 Mar 2014 22:46:28 levels of unloading (e.g., for VO2, the slopeIP: Copyright: Aerospace Medical food 2 hAssociation before the testing as moderately fast running 100% BW to 4.4 at 33% BW). Discussion: Oxygen consumption, heart rate, and Borg rating of perceived exertion are reduced during unloaded was included within the protocol. The study protocol exercise with a relatively greater decline at higher treadmill speeds. That was approved in advance by the authors’ institutional is, the higher the treadmill’s speed in unloaded conditions, the relatively review board. Each female and male subject provided “easier” it is. to exercise. written informed consent before participating. Keywords: VO2, suspended exercise, body weight unloading, LBPP.

CCORDING TO RECENT studies, body unloading reduces oxygen consumption during unloaded walking (3,6,17) and unloaded running (7), referenced as walking energy cost per unit of time (17) and as gross metabolic power (7). To our knowledge, there is no such study investigating walking and running simultaneously. Such knowledge would improve the basic understanding of the influence of reduced gravity on aerobic fitness in astronauts. Furthermore, a broad spectrum of clinical patients could profit from this knowledge during unloaded treadmill exercises, e.g., to improve fitness of obese individuals at walking speeds, to improve muscle strength in the elderly at walking speeds, or to improve rehabilitation in sports medicine patients such as injured athletes at running speeds. Our main purpose was to investigate whether the relative amount of oxygen decline during unloaded exercise is similar or different within a wide spectrum of speeds ranging from slow walking to moderately fast running. The purpose of this study was to quantify oxygen consumption, heart rate, and Borg rating of perceived exertion during unloaded walking and running. We hypothesized that oxygen consumption would be low550

Equipment and Design Unloading was achieved using the Lower Body Positive Pressure (LBPP) chamber investigated in previous clinical (4) and experimental (2,13) studies. During LBPP treadmill walking, the subject’s lower body was enclosed by a waist-high pressure chamber that contains a treadmill. Pressure inside was increased such that the pressure difference across the waist seal produces an upward force, unloading the subject’s body weight (Fig. 1). Subjects walked and ran at three body weight (BW) conditions (100%, 66%, and 33% BW) and four treadmill speeds (slow walking at 0.4 m z s21, comfortable walking at 1.3 m z s21, slow running at 2.2 m z s21, and moderately From the Department of Orthopaedic Surgery, University of California, San Diego, CA. This manuscript was received for review in October 2009. It was accepted for publication in January 2010. Address correspondence and reprint requests to: Heidi Ruckstuhl, Ph.D., Dept of Orthopaedic Surgery, University of California, San Diego UCSD Medical Center, 350 Dickinson St., #121, MC 8894, San Diego, CA 92103; heidi.ruckstuhl@gmail.com. Reprint & Copyright © by Aerospace Medical Association, Alexandria, VA. DOI: 10.3357/ASEM.2693.2010 Aviation, Space, and Environmental Medicine x Vol. 81, No. 6 x June 2010

. Vo2 DURING UNLOADED EXERCISE—RUCKSTUHL ET AL. o , HR, Borg RPE) and treadmill speed. parameters (V 2 Paired-samples t-tests were used to evaluate statistically the percentage decline during unloading between all speed conditions. The significance level a was corrected according to Bonferroni: acorrected 5 0.05/k with k 5 12 as the number of tests. RESULTS During unloading, oxygen consumption, heart rate, and Borg RPE decreased significantly (P , 0.001 for all parameters, Fig. 2). Analysis of simple effects showed that oxygen consumption was significantly different at each BW condition except between 100% and 66% BW at 1.3 m z s21. HR differed significantly between 100% and 33% BW at 0.4 and 1.3 m z s21, for higher speeds between all BW conditions. Also, the Borg RPE was influenced by running speeds (2.2 and 3.1 m z s21) with an exception Fig. 1. Pressure inside (P1) the Lower Body Positive Pressure (LBPP) chamber was increased such that the pressure difference across the waist between 66% and 33% BW at 2.2 m z s21. seal produces an upward force, unloading the subject’s body weight. With higher speeds, oxygen consumption, heart rate, P.2 refers to the pressure outside of the chamber. Oxygen consumption and Borg RPE all increased significantly (P , 0.001 for (VO2), heart rate, and Borg rating of perceived exertion were quantified. all parameters). Analysis of simple effects showed that o , HR, and Borg RPE were significantly different beV 2 fast running at 3.1 m z s21). At a given BW condition, the tween all speed conditions. protocol started with a 5 min walking warm-up at 1.3 Oxygen consumption, heart rate, and Borg RPE were m z s21. After a rest of 3 min, subjects completed all four signifi cantly influenced by the interaction of body loadinvestigated speeds in ascending order each for 5 min. ing and treadmill speed (P , 0.001 for all parameters). There was a break of 30 min between BW conditions. The linear relationship between all three parameters and The protocol was randomized by changing the order of speed was more acute (smaller slope angle) at higher BW loading to avoid order and fatigue effects. levels of unloading (see slopes in Fig. 3). Thereby, high Delivered by Publishing Technology to: Guest User Overall, three parameters were analyzed: oxygen conPearson correlation coefficients (between 0.94 and 0.99) IP: 76.167.134.242 On: Sat, 15 Mar 2014 22:46:28 V sumption ( o2), heart rate (HR), and Borg rating of perCopyright: Aerospace Medical Association were found (Fig. 3). ceived exertion (Borg RPE). We used an open-circuit Oxygen consumption, heart rate, and Borg rating of perrespirometry system (Vmax Encore, VIASYS Healthcare, ceived exertion did not differ between genders. Further Conshohoken, PA) to monitor Vo2. Steady state Vo2 more, it was found that the percentage decline of oxygen was calculated as average over the last 2 min of each consumption during unloading was greater at running 5-min interval because Vo2 steady state for moderate inthan at walking speeds (P , 0.004, Table I). For heart rate, tensities is reached within 2–3 min (12,15). HR was meathe percentage decrease was greater for running than walksured with a Polar heart watch and chest strap transmitter ing ( P , 0.002) except when comparing 100–66% BW be(in beats per minute, bpm). Moreover, the subjects’ inditween slow walking and running. The percentage decline vidual exertions were assessed using Borg RPE ranging of Borg RPE during unloaded exercise, however, was only from 6 (no exertion at all) to 20 (maximal exertion). HR different when comparing 100–33% BW between slow and Borg RPE were determined at the end of each 5-min walking and slow running (P , 0.004). interval. Statistics

DISCUSSION

All parameters were analyzed statistically using SPSS. To investigate parameter differences between all BW and speed conditions, the General Linear Model for repeated measures with two factors was used: body loading at three levels (100%, 66%, and 33% BW) and treadmill speed at four levels (0.4 m z s21, 1.3 m z s21, 2.2 m z s21, and 3.1 m z s21). To identify gender differences, we used the same model with gender as a between-subjects factor. Analysis of simple effects was performed in the case of significant interaction by investigating the effect of one factor (either BW or speed) on dependent variables following Bonferroni’s correction. Significance was set at P less than 0.05. Linear regression analysis performed on the mean values defined the linear relationship (linear equations and Pearson correlation coefficients) between

Inﬂuence of Body Loading

Aviation, Space, and Environmental Medicine x Vol. 81, No. 6 x June 2010

As hypothesized, oxygen consumption significantly decreased during unloaded walking and running. Our results are in accordance with previous studies of unloaded treadmill walking (3,6,17) and running (7). To our knowledge, there is no previous study investigating both walking and running speeds at reduced simulated fractional gravity. Such knowledge will improve our understanding of the influence of reduced gravity on aerobic fitness in astronauts. These results also aid understanding of unloaded exercise over a wide range of treadmill speeds in a broad spectrum of subjects (e.g., obese or elderly individuals, injured athletes). In addition to a reduction of oxygen consumption, we also found a decrease of heart rate and Borg rating of perceived exertion 551

. Vo2 DURING UNLOADED EXERCISE—RUCKSTUHL ET AL.

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. Fig. 2. Oxygen consumption (VO2), heart rate (HR), and Borg rating of perceived exertion (Borg RPE) at three body weight (BW) conditions and four treadmill speeds (0.4 m z s21, 1.3 m z s21, 2.2 m z s21, and 3.1 m z s21). Markers are somewhat displaced in order not to overlap each other. Dotted lines are visual aids to distinguish BW conditions. Means 6 SDs are provided.

Fig. 3. Linear regression analysis was performed on the mean val. ues of oxygen consumption (VO2), heart rate (HR), and Borg rating of perceived exertion (Borg RPE) at three body weight (BW) conditions. Equations and Pearson correlation coefficients (R2) are listed within the graphs. Also illustrated are the parameters’ mean values at 100% BW (crosses), 66% BW (open circles), and 33% BW (black diamonds).

during unloaded walking and running. As a major finding, we confirm a specific linear relationship between each parameter and treadmill speed that is dependent on the amount of loading. Thereby, slopes of all specific linear relationships are more acute at higher levels of unloading. That is, the higher the treadmill speed in unloaded conditions, the “easier” it is to exercise because of a relatively greater decline of oxygen consumption, heart rate, and Borg rating of perceived exertion. In addition, each parameter’s value at any desired speed can be approximated based on the equations for our investigated BW conditions (100%, 66%, and 33% BW).

Mars gravity and 17% Moon gravity per se, we believe our data help understand oxygen consumptions over a linear well-defined range of unloading. Moreover, these results on control female and male subjects are needed prior to studies in clinical patients. In this regard, al o , HR, and Borg RPE during unloaded terations of V 2 walking and running may be different in various population groups. We only unloaded the subjects’ body weights to simulate partial gravity. However, under these conditions, the cardiovascular system is not well simulated to match the reduced gravity environment. There was a significant decrease of the parameters’ values during unloading at all investigated speeds. However, in some subjects, there was an individual increase. This might be due to the fact that the order of BW condition was randomized and the break of 30 min between BW conditions was too short to allow full recovery in these subjects. We chose this type of protocol

Limitations Our findings are based on 10 healthy young female and male subjects over three levels of BW loading: 100%, 66%, and 33% BW. Although we didn’t test 38% 552

Aviation, Space, and Environmental Medicine x Vol. 81, No. 6 x June 2010

. Vo2 DURING UNLOADED EXERCISE—RUCKSTUHL ET AL. TABLE I. CHANGE OF OXYGEN CONSUMPTION, HEART RATE, AND BORG RATING OF PERCEIVED EXERTION DURING UNLOADING. 0.4 m z s21 From 100 to 66% BW From 100 to 33% BW From 100 to 66% BW From 100 to 33% BW From 100 to 66% BW From 100 to 33% BW

[%] [ml z kg21 z min21] [%] [ml z kg21 z min21]

210 6 8% 21.0 6 0.7 226 6 16% 22.4 6 1.7

[%] [bpm] [%] [bpm]

28 6 11% 27 6 11 214 6 7% 213 6 6

[%] [Borg units] [%] [Borg units]

20 6 8% 20.0 6 0.7 22 6 8% 20.2 6 0.6

1.3 m z s21

2.2 m z s21

Change in Oxygen Consumption 28 6 10% 226 6 11% 21.2 6 1.3 27.9 6 3.9 228 6 15% 254 6 13% 23.8 6 2.4 216.4 6 4.4 Change in Heart Rate 26 6 9% 216 6 7% 26 6 9 224 6 11 213 6 7% 233 6 9% 214 6 8 249 6 13 Change in Borg Rating of Perceived Exertion 20 6 17% 211 6 10% 20.1 6 1.4 21.3 6 1.1 26 6 14% 218 6 11% 20.7 6 1.3 22.2 6 1.3

3.1 m z s21 228 6 9% 210.8 6 4.1 253 6 10% 220.5 6 4.9 217 6 5% 229 6 8 234 6 8% 259 6 14 26 6 6% 20.9 6 0.9 216 6 8% 22.4 6 1.3

Values are means 6 SDs (N 5 10). Listed are changes in the parameters’ values (percentage and numerical values) from 100 to 66% body weight (BW) and from 100 to 33% BW, respectively. For statistical analysis, the reader is referred to the text.

(4). Furthermore, there is no adverse impact of LBPP on to limit the experimental time (, 3 h) to avoid subject cardiovascular parameters (2). Therefore, we propose fatigue and impatience. Furthermore, we didn’t want to unloaded LBPP walking as a safe and suitable method test on different days, which might influence measurefor instituting physical activity in obese individuals bements as well. Thus, we consider this limitation as micause many obese individuals have difficulties walking nor. Also, there are probably different linear regression at full body weight due to relatively higher oxygen conequations for walking and running speeds as expressed o with running as sumption than normal-weight persons (5,10). We further by a greater percentage decline of V 2 speculate that exercise time in obese individuals can be compared to walking (Table I). The observed relatively increased when unloaded, balancing the effect of regreater decline during running may be due to less en o . In rehaDelivered by Publishing to: Guest User duced energy expenditure due to reduced V ergy efficiency of running compared to walking as evi- Technology 2 IP: 76.167.134.242 On: Sat, 15 Mar 2014 22:46:28 denced by a lower percentage recovery (14). In other bilitation of injured athletes, unloaded LBPP running Copyright: Aerospace words, running is metabolically more demanding than Medical may beAssociation effective to hasten rehabilitation and return to walking. When unloaded at running speeds, the mussport (7). However, during unloaded running, aerobic cles’ work may be relatively more reduced than during capacity may not be maintained as oxygen consumption unloaded walking, resulting in relatively low oxygen is clearly reduced during unloading as further hypothconsumption. However, as we just measured two speeds esized by Grabowski and Kram (7). for each walking and running, we are not able to quanFuture studies of unloaded walking and running should focus on astronauts and on patient groups (e.g., tify this observation. obese individuals or injured athletes) to study feasibility Potential Applications and Future Directions and success of unloaded walking and running for exercise and rehabilitation. Furthermore, it would be interDue to reduced gravity, oxygen consumption is reesting to quantify the linear relationship of oxygen duced in astronauts during long-duration stays in space consumption and treadmill speed for walking and run(9). In order to maintain aerobic capacity, aerobic trainning separately. ing is necessary while exercise intensity is an important Oxygen consumption decreases with body weight unparameter (8,16). Based on our findings of reduced oxyloading over a range of walking and running speeds. Ingen consumption during unloaded walking and runterestingly, our findings confirm a linear relation between ning, astronauts should exercise at higher intensity oxygen consumption and treadmill speed for each BW levels than what the gravitational levels of the Moon condition with more acute slopes at higher levels of unand Mars permit. Our results suggest that increased loading. Therefore, there is a relatively greater decline of intensity and, thus, increased oxygen consumption is oxygen consumption at higher treadmill speeds, facilitateither reached by higher speeds or higher loading coning unloaded exercise at faster speeds. In addition, heart ditions (i.e., higher gravity levels). Whereas treadmill rate and Borg rating of perceived exertion during unloadspeed is easily adapted in space, exercise within lower ing decreases in a similar way to oxygen consumption. body negative pressure (8,16) or another form of artificial gravity is necessary to increase gravity levels. Furthermore, we suggest that LBPP ambulation may ACKNOWLEDGMENTS We thank the Swiss National Science Foundation for financial supbe a valuable tool for exercise and rehabilitation in a port. These studies were carried out in part in the General Clinical broad spectrum of patients. As shown in patients after Research Center, University of California, San Diego, with funding knee surgery, unloaded LBPP walking reduces pain and provided by the National Center for Research Resources, M01RR ground reaction forces while safely facilitating exercise 000827 United States Public Health Service. In particular, we thank Aviation, Space, and Environmental Medicine x Vol. 81, No. 6 x June 2010

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. Vo2 DURING UNLOADED EXERCISE—RUCKSTUHL ET AL. Paul Shragg for statistical assistance. We also thank Peter Wagner, M.D., for his advice on the experimental protocol. Authors and afﬁliations: Heidi Ruckstuhl, Ph.D., M.S., Thomas Schlabs, Armando Rosales-Velderrain, M.D., and Alan R Hargens, Ph.D., B.A., Department of Orthopaedic Surgery, University of California, San Diego, CA. REFERENCES 1. Ahrens JN, Crixell SH, Lloyd LK, Walker JL. The physiological effects of caffeine in women during treadmill walking. J Strength Cond Res 2007; 21:164–8. 2. Cutuk A, Groppo ER, Quigley EJ, White KW, Pedowitz RA, Hargens AR. Ambulation in simulated fractional gravity using lower body positive pressure: cardiovascular safety and gait analyses. J Appl Physiol 2006; 101:771–7. 3. Danielsson A, Sunnerhagen KS. Oxygen consumption during treadmill walking with and without body weight support in patients with hemiparesis after stroke and in healthy subjects. Arch Phys Med Rehabil 2000; 81:953–7. 4. Eastlack RK, Hargens AR, Groppo ER, Steinbach GC, White KK, Pedowitz RA. Lower body positive-pressure exercise after knee surgery. Clin Orthop Relat Res 2005 Feb; (431):213–9. 5. Farrell PA, Gustafson AB, Kalkhoff RK. Assessment of methods for assigning treadmill exercise workloads for lean and obese women. Int J Obes 1985; 9:49–58. 6. Fox EL, Bartels RL, Chaloupka EC, Klinzing JE, Hoche J. Oxygen cost during exercise in simulated subgravity environments. Aviat Space Environ Med 1975; 46:300–3. 7. Grabowski AM, Kram R. Effects of velocity and weight support on ground reaction forces and metabolic power during running. J Appl Biomech 2008; 24:288–97.

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