PARTIAL BODY WEIGHT SUPPORT during weightbearing

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1 960 ORIGINAL ARTICLE Physiological Responses to Body Weight Supported Treadmill Exercise in Healthy Adults Martin D. Hoffman, MD, Haylee E. Donaghe, BS ABSTRACT. Hoffman MD, Donaghe HE. Physiological responses to body weight supported treadmill exercise in healthy adults. Arch Phys Med Rehabil 2011;92: Objective: To determine whether the relationships of heart rate, rating of perceived exertion (RPE), and ground reaction forces (GRFs) with oxygen consumption rate (V O2 ) during treadmill exercise are altered by partial body weight support (BWS) via lower-body positive pressure. Design: Repeated-measures design. Setting: Exercise physiology laboratory. Participants: Healthy, active adults (N 12); mean age SD, years. Interventions: Not applicable. Main Outcome Measures: V O2, heart rate, RPE, and GRFs were measured during walking and running at 3 levels (0%, 25%, 50%) of BWS. Before exercise, standing heart rate and blood pressure were measured under each BWS condition. Results: Standing heart rates were 7 beats/min lower (P.05) and systolic blood pressures were 10mmHg higher (P.001) at 50% BWS compared with 0% BWS, but mean blood pressure while standing and the relationship of heart rate with V O2 during walking and running were not altered by BWS. While walking, the RPE at a V O2 of 10 ml kg 1 min 1 was statistically lower (P.05) at 0% BWS compared with 25% and 50% BWS (mean values, 7 vs 8 points), but RPE was not different among conditions while running at av O2 of 25 ml kg 1 min 1. Peak normal GRFs at specified V O2 levels and RPE values were reduced (P.05) with increasing BWS for walking and running. Conclusions: Because partial BWS does not alter the relationship of heart rate with V O2 during exercise and has minimal effect on the relationship of RPE with V O2, training heart rate and RPE values do not appear to require adjustment with partial BWS. Reduced GRFs at specified V O2 levels from partial BWS suggest that there are important clinical applications of this technology. Key Words: Oxygen consumption; Physical exertion; Rehabilitation; Running; Walking; Weightlessness simulation by the American Congress of Rehabilitation Medicine From the Department of Physical Medicine and Rehabilitation, Department of Veterans Affairs, Northern California Health Care System, and University of California Davis Medical Center, Sacramento, CA (Hoffman); and the Exercise Science Graduate Group, University of California Davis, Davis, CA (Hoffman, Donaghe). Presented to the American College of Sports Medicine, June 4, 2010, Baltimore, MD. Supported by AlterG and the National Center for Research Resources, a component of the National Institutes of Health (NIH grant no. UL1 RR024146) and the National Institutes of Health Roadmap for Medical Research. This material is also the result of work supported with resources and the use of facilities at the VA Northern California Health Care System. A commercial party having a direct financial interest in the results of the research supporting this article has conferred or will confer a financial benefit on the author or 1 or more of the authors. Hoffman has received grant support from AlterG, Inc, the manufacturer of the partial body weight support system used in this report. Reprint requests to Martin D. Hoffman, MD, Department of Physical Medicine and Rehabilitation (117), Sacramento VA Medical Center, Hospital Way, Mather, CA , martin.hoffman@va.gov /11/ $36.00/0 doi: /j.apmr PARTIAL BODY WEIGHT SUPPORT during weightbearing exercise has long been used as a rehabilitative and training tool. Previous research on unloading systems has shown that body weight supported exercise could have many advantageous effects, such as reducing peak ground reaction forces (GRFs), 1-3 aiding in gait retraining, 2,4-7 and providing aerobic stimuli to people with various disabilities who might otherwise have difficulty exercising. 2,6,8,9 A unique approach to provision of body weight support (BWS) during treadmill exercise makes use of computer-regulated lower-body positive pressure to create a net lifting force on the user. Pressurization of a flexible enclosure surrounding the treadmill and user s lower body creates the lifting force, which is capable of reducing effective body weight by approximately 50% at pressure differentials of less than 30mmHg. 1 Because of this design, the system does not cause discomfort or respiratory restrictions, as have been reported with some harness systems, 6,10,11 and does not alter kinematics or muscle activation patterns to the extent that would be expected from water resistance when water immersion is used to provide partial BWS. Limited research has been performed with partial BWS through lower-body positive pressure, but preliminary work has found that peak GRFs are lower when running with partial BWS compared with unsupported running at the same energy expenditure. 3 In other words, the partial BWS through lowerbody positive pressure appears to provide an effective means of reducing impact forces while maintaining metabolic demand during running. Clinical application of this discovery has been demonstrated in a recent case report of a distance runner who was able to use partial BWS to continue running training acutely after a lumbar disk herniation. 9 Other people who have pain from loading forces or impaired strength and exercise tolerance limited to walking speeds could also benefit from a system that allows reduced GRFs while sustaining an adequate aerobic demand. However, to our knowledge, the effect of varying levels of BWS via lower-body positive pressure on the relationship of GRF with metabolic demand has not been examined for walking. The current study aimed to investigate further the potential utility of this new approach to partial BWS during treadmill walking and running. The effect of different levels of partial BWS on the relationships between various physiological and biomechanical variables was examined in healthy subjects. Specifically, the relationships of heart rate, rating of perceived exertion (RPE), and GRF with V O2 were of primary interest to ANOVA BWS GRF IPAQ RPE V O2 List of Abbreviations analysis of variance body weight support ground reaction force International Physical Activity Questionnaire rating of perceived exertion oxygen consumption rate

2 BODY WEIGHT SUPPORTED TREADMILL EXERCISE, Hoffman 961 Table 1: Selected Subject Characteristics Characterstics Age (y) Height (m) Mass (kg) Body mass index (kg/m 2 ) IPAQ score (MET min wk 1 ) NOTE. Values are presented as mean SD. Abbreviation: MET, metabolic equivalent of task. determine whether training heart rates or RPEs require adjustment with partial BWS to achieve the same aerobic demand as unsupported exercise, and whether GRFs for given aerobic demands are altered during walking and running with partial BWS. METHODS Value Participants Twelve healthy subjects (6 men, 6 women) from 21 to 59 years of age participated in the study (table 1). Based on International Physical Activity Questionnaire (IPAQ) scores, 1 subject was categorized as having a moderate physical activity level, while all others were categorized as having a high physical activity level. Most were regular runners. Exclusion criteria included known cardiac, pulmonary, or metabolic disorders, musculoskeletal disorders preventing participation in exercise, and pregnancy. In addition, persons using tobacco or nicotine-containing products, mood-altering medications or drugs (including anti-inflammatory drugs, antihistamines, pain medications, or antidepressants), or beta-blockers were excluded. All participating subjects were directed to refrain from strenuous exercise on the day of their tests, to keep pretest meals as consistent as possible, and to bring comfortable exercise clothing to the testing facility. Also, all subjects were instructed to wear the same running shoes for exercise tests throughout the study. All procedures were approved by the institutional review board, and each subject voluntarily gave written informed consent. Information pertaining to the general purpose of the study was provided to each subject in order to obtain informed consent, but specific hypotheses were not discussed with subjects prior to their completion of the study. Subjects were awarded a $10 gift card after the completion of each trial. Experimental Design Subjects came to the laboratory for a total of 3 visits at approximately the same time of day. A graded submaximal exercise test was completed on the AlterG P200 Anti-Gravity Treadmill a during each visit, 1 for each of the following AlterG settings: 0%, 25%, and 50% BWS. The order of BWS conditions was counterbalanced across participants by randomly assigning each subject within a block of 6 subjects to a different 1 of the 6 potential orders. Subjects were blind to the condition and were unable to visualize the AlterG display panel because it was covered with the RPE chart. On their first visit, subjects were advised of the protocol; were familiarized with the Borg 6 to 20 point RPE scale 12 ; completed the long-form, last 7 days, self-administered version of the IPAQ 13,14 ; and underwent their first submaximal exercise test, as described. During their second and third visits, subjects were reminded of the protocol before starting their exercise tests. Height was measured during the first visit, and weight was measured at the beginning of each visit. Each test was separated by 3 to 7 days. Exercise Testing The graded submaximal exercise tests consisted of two 3-minute standing stages, followed by four 4-minute walking stages, and then as many 4-minute running stages as required for the subject to reach an RPE of 13 (somewhat hard). The treadmill remained level for all conditions. The first standing stage was with the subject fully positioned in the AlterG and at 0% BWS. At the start of the second standing stage, the AlterG was pressurized for the 25% and 50% BWS conditions, and the pressure then remained constant for the remainder of the test. Previous work 3 has demonstrated minimal pressure fluctuation during running across the levels of BWS used in this study. Blood pressure was measured during the last minute of the standing stages with an automated device b and the same cuff for all 3 tests on a given subject. Heart rate c and V O2 (Parvo- Medics TrueOne 2400 d ) (system calibrated prior to each test) were measured continuously throughout the duration of the exercise test. GRFs were continuously recorded from integrated force transducers (calibrated prior to each test) at each corner of the treadmill, sampled at 250Hz. The force transducers were capable of measuring vertical GRFs, and because incline was not used, these measurements correspond to forces in the normal direction. The subject was reminded to think about their RPE near the beginning of the last minute of each stage and was requested to provide an RPE during the last 30 seconds of each stage. Preliminary testing was performed with the goal of defining treadmill speeds to allow matching of V O2 for each stage across the 3 BWS conditions. However, in order to match V O2 for walking at a moderate speed in the unsupported condition, an unrealistically fast walking speed for the highest level of BWS was required. Therefore, we chose to use identical walking speeds (0.49, 0.85, 1.25, and 1.56m/s) over the 3 BWS conditions, but attempted to use speeds that would provide approximately similar V O2 levels for each running stage. Accordingly, the running speeds were faster for a given stage with increasing levels of BWS beginning at 1.79, 2.59, and 3.49m/s for 0%, 25%, and 50% BWS, respectively, and increasing by 0.31m/s for each stage. Data Analysis Representative samples of force data while standing at 0%, 25%, and 50% BWS were used to determine the actual level of BWS for each condition. Mean values SD for each condition were determined. Standing heart rate values were averaged over a 1-minute interval, 1.5 minutes into each stage. Mean blood pressure was calculated as diastolic pressure plus one third of the difference between systolic and diastolic pressures. Comparisons of standing heart rates and systolic, diastolic, and mean blood pressures among the 3 conditions were made with 1-way repeated-measures analysis of variance (ANOVA) and Tukey posttests. For the walking and running stages, heart rate, V O2, and peak GRFs were averaged over a 1-minute interval, 2.5 minutes into each stage. GRF data were processed with a custom LabVIEW program e. The raw data were filtered with a Butterworth highpass filter with cutoff frequencies of 20Hz for walking data and 40Hz for running data. Using the filtered data, maximum normal GRFs for each foot strike were averaged over the 1-minute interval in order to determine a peak normal GRF for each stage. Within this same 1-minute interval of data collec-

3 962 BODY WEIGHT SUPPORTED TREADMILL EXERCISE, Hoffman Table 2: Heart Rate and Blood Pressure During Standing With Different Levels of Partial BWS Variable 0% BWS 25% BWS 50% BWS Heart rate (beats/min) * Systolic blood pressure (mm Hg) Diastolic blood pressure (mm Hg) Mean blood pressure (mm Hg) NOTE. Values are presented as mean SD. Different from 0% BWS condition at *P.05 and P.01. tion for each running stage, maximum vertical loading rates were averaged across 20 peaks, or 10 strides. Maximum vertical loading rate (in body weight per second) was defined as the greatest slope of the GRF versus time curve between heel strike and peak normal GRF across 4 data points (an interval of 12ms). Linear regression analyses were used to compare among conditions the relationships of walking and running V O2, walking and running peak normal GRF, and running maximum vertical loading rate with speed. In order to determine V O2 as a function of both speed and degree of BWS, separate multiple linear regression analyses were performed for walking and running. Equations were determined using the BWS defined by the AlterG setting as well as the actual measured BWS. The relationship of heart rate with V O2 was compared among BWS conditions with 1-way repeated-measures ANOVA and Tukey posttests on the slopes and intercepts of individual linear regressions for each subject and BWS condition. Comparisons among BWS conditions of walking and running RPE, walking and running peak normal GRF, and running maximum loading rate with V O2 were performed by first determining individual values for each dependent variable at V O2 levels of 10 ml kg 1 min 1 for walking and ml kg 1 min 1 for running from individual linear regressions. These V O2 levels were selected because they were generally within the range experienced by most subjects under each condition. In all cases but one in which at least 2 running stages were completed, the V O2 range was within 2mL kg 1 min 1 for walking and 3mL kg 1 min 1 for running of these specified levels. Because the individual relationships between peak normal GRF and V O2 were not always well fit with a regression equation, peak normal GRF was linearly interpolated from the 2 data points on either side of the specified V O2 levels except where extrapolation was required. Similarly, comparisons among BWS conditions of walking and running peak normal GRF and running maximum loading rate with RPE were performed at an RPE value of 9 for walking and 12 for running because they were generally within the range experienced by nearly all subjects under each condition. The calculated individual values were then compared among conditions with 1-way repeated-measures ANOVA and Tukey posttests. For all analyses, a probability value of less than.05 was accepted as significant. RESULTS The AlterG settings of 0%, 25%, and 50% BWS resulted in actual (mean SD) percentages of BWS of 5 2, 30 2, and 55 4, respectively. In other words, the actual amount of BWS was 5% greater than the AlterG settings. For ease of presenting the results, the conditions are described based on the AlterG settings except where specifically noted. Partial BWS significantly altered standing heart rate (P.02) and systolic blood pressure (P.01) values (table 2). Diastolic (P.7) and mean blood pressures (P.5) were not altered by partial BWS. The relationships between V O2 and speed across varying levels of BWS are shown in figure 1. For walking, slopes of the relationships between V O2 and speed were not different (P.14), but the intercepts were different (P.001). For running, slopes were different (P.001), and because of the extent of this difference, it was not possible to test whether the intercepts differed. Equations from multiple linear regression analyses defining V O2 (ml kg 1 min 1 ) as functions of speed (m/s) and fraction of BWS were determined for walking from complete data for all 12 subjects across all 4 speeds and for running from complete data on 6 subjects who finished at least 3 speeds under each condition. Based on the AlterG BWS setting, the equations were and V O2 5.8 v 6.2 BWS 7.1 (r 2.98) V O2 8.3 v 34.4 BWS 8.9 (r 2.97) for walking and running, respectively, where v is treadmill speed. When using the actual fractional BWS, the respective equations were and V O2 5.8 v 6.2 BWS 7.4 (r 2.98) V O2 8.4 v 34.7 BWS 10.5 (r 2.97) for walking and running. As evidenced by the nearly 6-fold difference in BWS multiplier in the equation for running compared with that for walking, increases in BWS decrease V O2 at a given speed much more for running than walking. The relationships for heart rate and RPE with V O2 are shown in figure 2. BWS did not alter the relationship between heart rate and V O2 (P.5 for slope; P.3 for intercept). The RPE for walking at a V O2 of 10 ml kg 1 min 1 was statistically affected (P.014) by BWS (table 3). For running, the RPE at Fig 1. Relationships of V O2 with treadmill speed for walking (left) and running (right) at 0% (circles), 25% (triangles), and 50% (squares) BWS. The respective linear regressions are shown as solid (0% BWS), dashed (25% BWS), and dotted (50% BWS) lines. Data are for all 12 subjects for the 4 walking stages and the 6 subjects who finished at least 3 running stages. Brackets represent 1 SD and for clarity are shown in only 1 direction for the 0% and 50% conditions.

4 BODY WEIGHT SUPPORTED TREADMILL EXERCISE, Hoffman 963 not statistically different (P.12) among BWS conditions (see table 3). Fig 2. Linear regressions for heart rate and RPE (walking on left, running on right) with V O2 for 0% (circles), 25% (triangles), and 50% (squares) BWS. The respective linear regressions are shown as solid (0% BWS), dashed (25% BWS), and dotted (50% BWS) lines, although those for 50% BWS overlie the 25% BWS lines and do not appear evident. Symbols are displayed at the mean V O2 for each stage. Brackets represent 1 SD and for clarity are shown in only 1 direction for the 0% and 50% conditions. Statistical results are described in the text and table 3. av O2 of 25mL kg 1 min 1 was not statistically different (P.064) among BWS conditions (see table 3). Linear regression analyses of peak normal GRF with speed revealed slopes to be similar (P.16 for walking; P.96 for running) across BWS conditions, but intercepts were different (P.001) for both walking and running (fig 3). Additionally, peak normal GRFs at specified V O2 levels were reduced (P.001) with increasing BWS for both walking and running (see table 3). Partial BWS did not significantly affect the relationship between maximum vertical loading rate and speed during running because the slopes (P.89) and intercepts (P.35) were similar among conditions (fig 4). However, the maximum loading rate for running at a V O2 of 25mL kg 1 min 1 was affected by BWS (P.0085) (see table 3). The relationships of peak normal GRF and maximum vertical loading rates with RPE are shown in figure 5. Peak GRFs for walking at an RPE of 9 and for running at an RPE of 12 were reduced (P.001) by increasing levels of BWS (see table 3). Maximum loading rates for running at an RPE of 12 were DISCUSSION This work found that the relationship between heart rate and V O2 was unaltered among the 3 BWS conditions during walking and running. From a practical standpoint, this finding suggests that training programs based on target heart rates do not require adjustment at BWS levels up to 50%. As such, moderate-to-high fit exercisers who establish intensity by heart rate can expect a similar metabolic demand from workouts with partial BWS as with unsupported exercise. Although others have presented data on the metabolic demand of ambulating across different speeds and levels of partial BWS with lower-body positive pressure, 3,15 a practical quantification of these relationships for walking and running has not been previously reported. The present work offers such equations for walking and running as determined through multiple linear regression analyses. Of note is that comparison of the equations for walking and running demonstrates that, in order to maintain a given V O2, increasing levels of BWS require much greater increases in speed with running than with walking. Furthermore, even though the multiple linear regression analysis for running resulted in a good fit (r 2.97) across the range of speeds that could be examined, the slopes for the linear regressions of V O2 with speed differed statistically among the 3 conditions for running. In other words, increases in running speed have less impact on metabolic demand at higher levels of BWS than at lower levels of BWS. This effect from increasing BWS has been previously observed 3,15 and is similar to what occurs with running on progressively more negative slopes In a practical sense, this effect means that, at higher levels of BWS, high-fit persons will be constrained in reaching adequate aerobic demands for training purposes and are likely to find it necessary to limit their levels of partial BWS. Table 3: RPE, Peak Normal GRF, and Maximum Loading Rate Values at Specified V O2 Levels, and Peak Normal GRF and Maximum Loading Rate Values at Specified RPE Levels for Different Proportions of Partial BWS Variable/Condition 0% BWS 25% BWS 50% BWS RPE (points) Walking at V O2 10 ml kg 1 min * * Running at V O2 25 ml kg 1 min Peak normal GRF (BW) Walking at V O2 10 ml kg 1 min Running at V O2 25 ml kg 1 min Walking at RPE Running at RPE Maximum loading rate (BW/s) Running at V O2 25 ml kg 1 min Running at RPE NOTE. Values were determined from individual regressions. Values are presented as mean SD. Abbreviation: BW, body weight. Different from 0% BWS condition at *P.05, P.01, and P.001. Different from 25% BWS condition at P.01 and P.001.

5 964 BODY WEIGHT SUPPORTED TREADMILL EXERCISE, Hoffman Fig 3. Relationships of peak normal GRF with speed and V O2 for walking (left) and running (right) at 0% (circles), 25% (triangles), and 50% (squares) BWS. The respective linear regressions are shown as solid (0% BWS), dashed (25% BWS), and dotted (50% BWS) lines. For the relationships of peak GRF with V O2, symbols are displayed at the mean V O2 for each stage. Brackets represent 1 SD and for clarity are shown in only 1 direction for the 0% and 50% conditions. Statistical results are described in the text and table 3. Abbreviation: BW, body weight. The present work also found that the RPE at a specified V O2 was not significantly affected by BWS during running. Also, although RPE at a specified V O2 was found to be statistically altered for walking, it is unlikely that a 1-point difference in RPE has important practical implications. Yet it should be noted that differences among levels of BWS in the relationship of RPE with V O2 could expand in a less fit population in which walking RPE values would be higher. Thus, it would appear that, at least for a fit population, minimal if any adjustment in RPE is required to achieve similar aerobic workloads when walking or running with partial BWS compared with unsupported walking or running. Because the speed necessary to achieve a given V O2 must be increased with partial BWS, the unaltered relationship between RPE and V O2 for running suggests that the stimulus for RPE is related more to aerobic demand than Fig 4. Relationships of maximum vertical loading rate with speed and V O2 for running at 0% (circles), 25% (triangles), and 50% (squares) BWS. The respective linear regressions are shown as solid (0% BWS), dashed (25% BWS), and dotted (50% BWS) lines. For the relationships of maximum loading rate with V O2, symbols are displayed at the mean V O2 for each stage. Brackets represent 1 SD and for clarity are shown in only 1 direction for the 0% and 50% conditions. Statistical results are described in the text and table 3. Abbreviations: BW, body weight; Max, maximum.

6 BODY WEIGHT SUPPORTED TREADMILL EXERCISE, Hoffman 965 upright resting situation, lower-body positive pressure during upright exercise has been found to have no effect on heart rate. 20 This is consistent with our findings and suggests that the lower-extremity skeletal muscle pump and other exerciseinduced cardiovascular responses contribute more to venous return than lower-body positive pressure of this magnitude. Increases in BWS resulted in reductions in peak normal GRFs at given speeds and V O2 levels for both walking and running. These findings are in agreement with previous work focused on running 3 and suggest that partial BWS may be a useful adjunct in rehabilitation of conditions causing pain with weight-bearing such as arthritis or fractures, and after lowerextremity operative procedures. The ability to reduce GRFs while still maintaining aerobic demand through partial BWS may also prove to be valuable for injury prevention among those at risk for certain overuse injuries, such as runners who develop stress fractures as a result of repetitive loading forces. The present work determined that the relationship between maximum vertical loading rate and speed was not statistically altered with BWS during running. In contrast, Grabowski and Kram 3 found that increased BWS reduced maximum loading rate for a particular speed. More importantly, though, is that both the present and previous work have demonstrated that loading rate increases for a specified V O2 level with partial BWS. This has potential clinical implications in that some research suggests that loading rate may be an important contributor to the development of certain types of overuse injuries. 21,22 The relationships of peak normal GRF and maximum vertical loading rate with RPE were also examined in the present study. These relationships have importance because training intensity is generally established by RPE. At specified RPE values, peak GRFs were reduced by partial BWS for both walking and running, while maximum loading rate did not appear to be affected by partial BWS. This suggests that under typical training conditions where intensity is established by RPE, peak GRFs would be lower with partial BWS. Fig 5. Relationships of peak normal GRF and maximum vertical loading rate with RPE for running at 0% (circles), 25% (triangles), and 50% (squares) BWS. The respective linear regressions are shown as solid (0% BWS), dashed (25% BWS), and dotted (50% BWS) lines. Symbols are displayed at the mean RPE for each stage. Brackets represent 1 SD and for clarity are shown in only 1 direction for the 0% and 50% conditions. Statistical results are described in the text and table 3. Abbreviations: BW, body weight; Max, maximum. movement rate. At the lower intensities of walking, however, RPE may be influenced by movement rate. Application of lower-body positive pressure to humans in the upright position can enhance venous return, resulting in an increase in stroke volume and a compensatory decrease in heart rate. 19,20 This effect is evident while at rest with pressures as low as 25mmHg, 20 which were of the same order of magnitude as were required in the present work to achieve 50% BWS. 1 As such, it is not surprising that we found heart rate to be decreased while standing with 50% BWS compared with 0% BWS. Also, as in previous work, 20 we found that mean blood pressure was not altered by the magnitude of lower-body positive pressures used in this study. In contrast with the Study Limitations Study limitations include the relatively small sample size and the use of multiple comparisons, which increase the risk for type II and I errors, respectively. In addition, the subject group was healthy and active, so the present findings would not necessarily be expected to transfer to a less fit population. In particular, the findings relative to RPE would be altered by the fitness level of the subjects. Future research should study less fit subjects. Research should also be conducted to determine whether decreases in GRFs through partial BWS result in decreased pain in subjects with lower-extremity pain conditions and allow for exercise at higher aerobic demands than could be achieved without BWS. Previous work among patients with osteoarthritis did not seem to support the premise that partial BWS, achieved through the use of a harness system, reduces pain while walking at a given V O2 level. 8 However, a case study of a distance runner with an acute lumbar disk herniation suggests that pain from impact forces can be effectively reduced while simultaneously allowing adequate aerobic demands through the use of lower-body positive pressure. 9 In addition, patients who had recently undergone anterior cruciate ligament reconstruction surgery who were unable to tolerate unsupported walking because of pain were found to tolerate walking with 40% and 80% BWS through lower-body positive pressure. 2

7 966 BODY WEIGHT SUPPORTED TREADMILL EXERCISE, Hoffman CONCLUSIONS The findings of this study suggest that the lower-body positive pressures required for achieving 50% BWS enhance venous return while standing but do not alter the relationship of heart rate with V O2 during walking or running among healthy, active adults. This combined with little, if any, effect on the relationship of RPE with V O2 indicates that training heart rate and RPE do not require adjustment when walking and running with partial BWS via lower-body positive pressure. The regression equations determined in this study relating V O2 to degree of BWS and speed for both walking and running provide future users with a framework on which to build training and rehabilitative programs. Important potential clinical applications of this technology, especially related to the observed reductions in GRFs for specified V O2 from partial BWS, should be further investigated in clinical populations, especially those limited by pain from joint loading. Acknowledgment: We thank Keith Williams, PhD, for his time and assistance in developing the program that was used to analyze force data. References 1. 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: 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;(431): Grabowski AM, Kram R. Effects of velocity and weight support on ground reaction forces and metabolic power during running. J Appl Biomech 2008;24: Finch L, Barbeau H, Arsenault B. Influence of body weight support on normal human gait: development of a gait retraining strategy. Phys Ther 1991;71: Franceschini M, Carda S, Agosti M, Antenucci R, Malgrati D, Cisari C. Walking after stroke: what does treadmill training with body weight support add to overground gait training in patients early after stroke? Stroke 2009;40: Norman KE, Pepin A, Ladouceur M, Barbeau H. A treadmill apparatus and harness support for evaluation and rehabilitation of gait. Arch Phys Med Rehabil 1995;76: Sullivan KJ, Knowlton BJ, Dobkin BH. Step training with body weight support: effect of treadmill speed and practice paradigms on poststroke locomotor recovery. Arch Phys Med Rehabil 2002; 83: Mangion KK, Axen K, Haas F. Mechanical unweighting effects on treadmill exercise and pain in elderly people with osteoarthritis of the knee. Phys Ther 1996;76: Moore MN, Vandenakker-Albanese C, Hoffman MD. Use of partial body-weight support for aggressive return to running after lumbar disk herniation: a case report. Arch Phys Med Rehabil 2010;91: MacKay-Lyons M, Makrides L, Speth S. Effect of 15% body weight support on exercise capacity of adults without impairments. Phys Ther 2001;81: Ruckstuhl H, Kho J, Weed M, Wilkinson MW, Hargens AR. Comparing two devices of suspended treadmill walking by varying body unloading and Froude number. Gait Posture 2009;30: Borg G. Perceived exertion as an indicator of somatic stress. Scand J Rehabil Med 1970;2: Booth ML. Assessment of physical activity: an international perspective. Res Q Exerc Sport 2000;71: Craig CL, Marshall AL, Sjöström M, et al. International physical activity questionnaire: 12-country reliability and validity. Med Sci Sports Exerc 2003;35: Ruckstuhl H, Schlabs T, Rosales-Velderrain A, Hargens AR. Oxygen consumption during walking and running under fractional weight bearing conditions. Aviat Space Environ Med 2010;81: Liefeldt G, Noakes TD, Dennis SC. Oxygen delivery does not limit peak running speed during incremental downhill running to exhaustion. Eur J Appl Physiol 1992;64: Margaria R, Cerretelli P, Aghemo P, Sassi G. Energy cost of running. J Appl Physiol 1963;18: Robergs RA, Wagner DR, Skemp KM. Oxygen consumption and energy expenditure of level versus downhill running. J Sports Med Phys Fitness 1997;37: Ng AV, Hanson P, Aaron EA, Demment RB, Conviser JM, Nagel FJ. Cardiovascular responses to military antishock trouser inflation during standing arm exercise. J Appl Physiol 1987;63: Nishiyasu T, Nagashima K, Nadel ER, Mack GW. Effects of posture on cardiovascular responses to lower body positive pressure at rest and during dynamic exercise. J Appl Physiol 1998; 85: Hreljac A. Impact and overuse injuries in runners. Med Sci Sports Exerc 2004;36: Hreljac A, Marshall RN, Hume PA. Evaluation of lower extremity overuse injury potential in runners. Med Sci Sports Exerc 2000; 32: Suppliers a. AlterG P200 Anti-Gravity Treadmill; AlterG, Inc, Milmont Dr, Fremont, CA b. GE Dinamap Carescape V100 Monitor; GE Healthcare, 3000 N Grandview Blvd, Waukesha, WI c. Polar Electro Inc, 370 Crossways Park Dr, Woodbury, NY d. ParvoMedics, Inc, 8152 S 1715 East, Sandy, UT e. National Instruments Corp, N Mopac Expwy, Austin, TX