CONVERSION TABLE FOR RUNNING POSITIVE PRESSURE TREADMILLS
ON
LOWER BODY
JOHN R. KLINE,1 SCOT RAAB,1 J. RICHARD COAST,2 ROGER G. BOUNDS,3 DAVID K.P. MCNEILL,2 1 AND HENDRIK D. DE HEER Departments of 1Physical Therapy and Athletic Training; 2Biological Sciences; and 3Health Sciences, Northern Arizona University, Flagstaff, Arizona ABSTRACT
INTRODUCTION
Kline, JR, Raab, S, Coast, JR, Bounds, RG, McNeill, DKP, and de Heer, HD. A conversion table for running on lower body positive pressure treadmills. J Strength Cond Res 29(3): 854– 862, 2015—Lower body positive pressure (LBPP) or antigravity treadmills are becoming increasingly popular in sports and rehabilitation settings. Running at a decreased body weight (BW) reduces metabolic cost, which can be offset by running at faster speeds. To date, however, little is known about how much faster someone must run to offset the reduced metabolic cost. This study aimed to develop a user-friendly conversion table showing the speeds required on an LBPP treadmill to match the equivalent metabolic output on a regular, nonLBPP, treadmill across a range of body weight supports. A total of 20 recreational runners (11 males, 9 females) ran multiple 3-minute intervals on a regular treadmill and then on an LBPP treadmill at 6 different BWs (50–100%, 10% increments). Metabolic outputs were recorded and matched between the regular and LBPP treadmill sessions. Using regression analyses, a conversion table was successfully created for the speeds from 6.4 to 16.1 km$h21 (4 to 10 mph) in 0.8 km$h21 (0.5 mph) increments on the regular treadmill and BW proportions of 50, 60, 70, 80, 90, and 100% on an LBPP treadmill. The table showed that a greater increase in speed on the LBPP treadmill was needed with more support (p , 0.001) but that the proportion increase was smaller at higher speeds (p , 0.001). This research has implications for coaches or practitioners using or prescribing training on an LBPP treadmill.
KEY WORDS rehabilitation, oxygen consumption, anti-gravity, Alter-GÒ, Unloading, running speed
Address correspondence to Hendrik D. de Heer, [email protected]. 29(3)/854–862 Journal of Strength and Conditioning Research Ó 2015 National Strength and Conditioning Association
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ower body positive pressure (LBPP) treadmills that support the user’s body weight (BW) to facilitate walking or running have become increasingly popular for use in rehabilitation and injury prevention (2,9,12,15). A major aim of running on LBPP treadmills is to maintain or improve aerobic capacity while limiting the stress induced by ground reaction forces (GRFs) on the lower extremities, as compared with running outside or on a “regular” non-BW-supported treadmill (5). Previous research has shown that ambulating and running on an LBPP treadmill lowers metabolic expenditure, with more body weight support (BWS) reducing oxygen consumption (V_ O2) by a greater extent (3,5–7,14,16). The reduction in V_ O2 has been found to be slightly less than proportional to the amount of BWS provided, particularly at the higher levels of support. For example, Grabowski (5) found that while walking at 25% of full BW (75% BWS), metabolic expenditure was reduced by 45% compared with walking with no BWS. Although it is known that increasing BWS reduces oxygen consumption, several recent studies have shown that while running on LBPP treadmills, individuals can match both maximal (V_ O2max) and submaximal oxygen uptake achieved on non-BW supported treadmills (4,5,13). To achieve this, individuals must run at faster speeds to make up for the decreased metabolic demand associated with running at less than full BW (4,5,13). For example, Raffalt et al. (13) found that running at 10 km$h21 at full BW required about the same metabolic demand as running at 14 km$h21 at 75% BW and 18 km$h21 at 25% BW. Among trained runners, Gojanovic et al. (4) further found that running at 90% BW (10% BWS) required a roughly proportional increase in speed of 11.4% on average to achieve the same V_ O2max as on a regular treadmill. Despite these insights, it remains unclear how much faster one must run on an LBPP treadmill across a range of different running speeds and percentages of BW to match the metabolic demand of running without BWS. In the context of an increase in popularity of LBPP treadmill running, this is an important question to address from both a research and a practical perspective. For example, if a runner’s regular speed is 4 min$km21, but, due to a musculoskeletal injury, he or she
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above, the following hypotheses were also tested: (a) the metabolic demand of running on a LBPP treadmill will decrease as BWS increases, (b) the decrease in metabolic demand will be less than proportional to the BWS, and (c) the increase in speed needed to make up for the decrease in metabolic demand will be proportional to the amount of BWS.
METHODS Experimental Approach to the Problem
To develop a conversion table between methods of running, metabolic rate was measured during running on a regular treadmill and on the LBPP treadmill. The speeds at which the metabolic output was equivalent were then matched using regression analyses (see below). The regression equations for each BW predicting speed on an LBPP treadmill (dependent variable) from speed on a regular treadmill (independent variable) were used to create the conversion tables. Subjects
The study was approved by the institutional review board of the university. A total of 20 recreational runners were recruited (11 males, 9 females) through online announcements and snowball sampling (where existing study subject recruit subsequent participants). Participants’ mean age was 28.5 years old (SD = 8.8 years; range, 22–56 years old), their height was 171.1 cm (SD = 7.8 cm), and weight was 65.8 kg (SD = 8.7 kg). Participants completed an informed consent and a Physical Activity Readiness Questionnaire (PAR-Q) (17) before the study. Inclusion criteria were that each participant was at least 21 years old, had experience with treadmill running and was able to run for at least 3 minutes at 11.3 km$h21 (7 mph), and did not have any risk factors for engaging in physical activity as indicated by the PAR-Q (17).
Figure 1. Overview of study protocol assessing metabolic expenditure of running on a lower body positive pressure treadmill compared to a regular treadmill.
starts training on an LBPP treadmill, with part of his or her BW supported, running at the same speed of 4 min$km21 will cost substantially less effort. As a result, he or she will not receive the same training stimulus and risk deconditioning if maintained over a longer period of time. To date, however, it is unknown exactly how much easier it is to run with different levels BWS and how much speed increase would be needed to account for this reduced metabolic output. Understanding of precise conversion of speeds at defined levels of BWS will help in devising a training/rehabilitation program while not sacrificing cardiorespiratory fitness. To this end, the goal of this study was to develop a userfriendly conversion table for commonly used BW percentages (50% through 100%) to quickly tell how much faster one must run at different proportions of BWS to achieve a metabolic output equivalent to running at a certain speed on a regular, non-LBPP treadmill. Based on the previous research described
Procedures
The protocol included a total of 5 sessions (Figure 1). The first 2 sessions consisted of running on the LBPP treadmill (Alter-GÒ Anti-GravityÒ Treadmill; Alter-GÒ P200; Fremont, CA, USA) for 30 minutes nonstop, at a self-selected VOLUME 29 | NUMBER 3 | MARCH 2015 |
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LBPP Conversion Table
856 TABLE 1. Average observed V_ O2 from sessions on LBPP treadmill and proportion of regular treadmill V_ O2 across speeds and body weights.* LBPP treadmill speed (km$h21)
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Mean observed V_ O2 (ml$kg21$min21) Regular treadmill 50% BW 60% BW 70% BW 80% BW 90% BW 100% BW p† for slope Proportion of V_ O2 at same speed on regular treadmill (%) 50% BW 60% BW 70% BW 80% BW 90% BW 100% BW Overall
8
9.7
11.3
12.9
14.5
16.1
17.7
28.14 18.23 19.44 21.60 23.59 25.64 27.30 ,0.001§
32.11 20.15 21.36 23.67 25.87 28.86 30.38 ,0.001§
36.17 21.87 23.86 26.51 29.12 32.55 34.46 ,0.001§
39.94 24.15 25.88 28.70 31.57 34.89 37.43 ,0.001§
44.10 26.09 27.75 30.76 33.54 37.99 40.82 ,0.001§
48.24 30.14 31.46 34.24 37.38 41.82 44.04 ,0.001§
33.29 35.15 37.51 41.24 45.80 47.67 ,0.001§
64.64 68.89 76.64 83.69 91.07 97.04
62.66 66.54 73.74 80.81 90.17 94.69
60.43 66.01 73.29 80.55 90.08 95.33
57.05 60.60 68.94 76.36 85.35 92.77
59.39 63.06 69.97 76.19 86.30 92.73
63.66 65.04 70.54 76.28 85.84 90.91
NA NA NA NA NA NA
Mean % (SD)
61.11 64.84 72.02 78.84 87.91 93.85
(7.66) (6.91) (7.18) (6.48) (6.59) (6.95)
p value† for slope
p valuez for comparison
0.870 0.983 0.085 0.251 0.847 0.012k 0.166
,0.001§ 0.002§ 0.171 0.458 0.173 ,0.001§ 0.302
*LBPP = lower body positive pressure; BW = body weight. †p value using linear regression tested whether slope was significantly different from zero. _ O2 observed; for example, testing whether 50% BW was significantly different zComparison testing whether proportion of body weight is significantly different to proportion of V
from 61.11%, whether 60% BW was significantly different from 64.84% etc. §Significant on p , 0.01. kSignificant on p # 0.05.
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70%, 80%, 90%, and 100% of their original BW, split across 2 sessions. Participants were randomized between running at either 50, 70, and 100% or 60, 80, and 90% of BW for the fourth session while running the fifth session at the remaining BWs. Participants ran a total of 12 stages (3 body weights at each of 4 different speeds). The speeds were based on the last successful stage completed during their third (non-LBPP) session and were either below (3.2 km$h21 [2 mph] and 1.6 km$h21 [1 mph]) at the same speed or just above (1.6 km$h21 (1 mph]) their final non-LBPP speed. Participants rested between stages as they did in session 3. Participants started each of the last 2 sessions at the lowest speed and most BWS assigned. Figure 2. Metabolic demand of running on a lower body positive pressure treadmill expressed as a proportion of metabolic demand on a regular treadmill across different body weights.
speed above 6.4 km$h21 (4 mph). The first 5 minutes were run at 100% of the subject’s BW. The BW was subsequently dropped by 10% every 5 minutes until 50% BW was run at for the final 5 minutes. This allowed participants to accommodate to the Alter-GÒ treadmill, as previous studies indicated that there is an approximately 1-hour metabolic accommodation effect of treadmill running (10,11,18). For the 3 remaining sessions, participants were connected to a metabolic cart (TrueOne 2400; Parvo Medics, UT, USA) while wearing a heart rate monitor (FT60; Polar, Kempele, Finland). Participants’ height and weight without shoes and in light clothes were taken at the beginning of each session. Participants were asked to wear the same shoes for the last 3 sessions. The third session was completed on a “regular” (non-LBPP) treadmill (Woodway, Waukesha, WI, USA). The protocol for this session consisted of the participants running multiple 3-minute stages, starting at 6.4 km$h21 (4 mph) and increasing in 1.6 km$h21 (1 mph) increments to at least 11.3 km$h21 (7 mph). Participants were allowed to continue running until their self-reported rate of perceived exertion exceeded 16 in the final 30 seconds of each stage (on the original Borg scale) (1), their respiratory exchange ratio (RER) consistently reached above 1.0, or their HR got to within 10% of their theoretical maximum heart rate (HR) (220 2 age). Between stages, participants stood still until their HR fell below 110 b$min21 or 2 minutes elapsed, whichever came first. The minimum rest between stages was 30 seconds. For all metabolic tests, expired gas data were averaged for the final minute of each stage, consistent with previously published methods (13). Participants’ final stage reached during this session formed the basis for the final 2 sessions run on the LBPP treadmill. The fourth and fifth sessions were completed on an Alter-GÒ P200 LBPP treadmill. For each session on the LBPP treadmill, participants wore the Alter-G provided neoprene shorts that zip into the Alter-G treadmill enclosure and allow for running in a positive pressure environment. Participants ran at 50%, 60%,
Statistical Analyses
Data analysis was performed using SPSS version 20.0 (SPSS, Inc, Chicago, IL, USA). First, average metabolic expenditures from session 3 (the regular treadmill session) were calculated for each speed (6.4 km$h21 through 16.1 km$h21) (4–10 mph). Second, average metabolic expenditures were calculated for each speed (8.0 km$h21 through 17.7 km$h21) (5–11 mph) and each BW (50% BW through 100% BW) on the LBPP treadmill. Linear regression analyses using generalized estimating equations were conducted, taking into account the “nested” data structure (with multiple V_ O2 values on both the regular and LBPP treadmill for each person). Hypotheses were tested for p # 0.05. Hypothesis 1 (V_ O2 will significantly decrease with increased BWS) was tested by evaluating whether the slope of observed V_ O2 differed significantly from zero (downwards) across BWs. Hypothesis 2 addressed whether the V_ O2 decrease on the LBPP was equivalent to the proportion of BWS. For example, it was tested while running at 60% BW and the observed V_ O2 was significantly different from 60% of the V_ O2 used on a regular treadmill. Before testing hypothesis 3, full conversion tables were _ O2 for created. This was achieved by entering the average V each speed on the regular treadmill into regression equations _ O2 and speed on the that quantified the relationship between V LBPP treadmill. These regression equations were calculated separately for each level of BW. Hypothesis 3 stated that speed increase required to offset reduced metabolic demand on the LBPP would be proportional to the amount of BWS provided. Therefore, we tested whether the observed proportion of the increase in speed required was significantly different from the hypothesized proportion. For example, it was tested whether running at 80% BW would require significantly different than a 20% increase in speed to match the metabolic demand of running on a regular treadmill.
RESULTS Physiologic Data
The mean V_ O2 collected for each speed on the regular treadmill ranged from 23.9 ml$kg21$min21 (SD = 1.9) at 6.4 km$h21 (4 mph) to 48.2 ml$kg21$min21 (SD = 4.4) at VOLUME 29 | NUMBER 3 | MARCH 2015 |
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LBPP Conversion Table
858 TABLE 2. Proportion increase in speed required to match V_ O2 on a regular treadmill (%).* Regular treadmill speed
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Km$h21 Miles per hour
6.4 4
8.0 5
9.7 6
11.3 7
12.9 8
14.5 9
16.1 10
Body weight % 50% 60% 70% 80% 90% 100% Overall
87.44 76.78 55.50 44.00 30.18 9.98
73.17 63.42 48.38 35.96 22.78 8.12
65.58 55.55 44.88 31.52 18.58 7.91
60.19 49.93 42.38 28.36 15.75 7.76
56.04 45.71 40.43 25.91 13.48 7.57
52.31 41.88 38.36 23.58 11.29 7.01
49.53 39.06 36.95 21.91 9.73 6.75
Mean %
p value† for slope
% BWS
p valuez for comparison
62.93 52.71 43.61 29.89 17.18 7.84
0.010k 0.003§ 0.003§ 0.010k 0.006§ 0.009§ 0.017k
50% 40% 30% 20% 10% 0%
0.012k 0.020k 0.022k ,0.001§ 0.005§ 0.018k ,0.001§
*BWS = body weight support. †p-value for significant slope across speeds. zp-value for comparison of proportion speed increase to proportion body weight support, for example at 80% body weight (20% body weight support), we tested whether the
speed increase was greater than 20%, at 70% body weight, was the speed increase greater than 30% etc. §Significant on p , 0.01. kSignificant on p # 0.05.
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TABLE 3. Conversion table for speeds from 6.4 to 16.1 kilometers per hour (km$h21) and minutes per kilometer (min$km21) on a regular treadmill to a body-weight supported treadmill.* Regular treadmill speed km$h21 Min$km21
8.0 7:27
8.9 6:47
9.7 6:13
10.5 5:44
11.3 5:20
12.1 4:58
12.9 4:40
13.7 4:23
14.5 4:09
15.3 3:55
16.1 3:44
12.07
12.98
13.93
15.0
15.99
17.02
18.05
19.04
20.09
21.06
22.06
23.08
24.06
4:58 11.38 5:16 10.01 6:00 9.27 6:28 8.38 7:09 7.08 8:29
4:37 12.27 4:53 10.99 5:28 10.11 5:56 9.14 6:34 7.90 7:36
4:18 13.15 4:34 11.94 5:01 10.94 5:29 9.88 6:05 8.70 6:54
4:00 14.11 4:15 13.0 4:37 11.84 5:04 10.69 5:37 9.59 6:16
3:45 15.02 4:00 13.99 4:17 12.7 4:44 11.45 5:14 10.42 5:46
3:32 15.96 3:46 15.01 4:00 13.58 4:25 12.24 4:54 11.28 5:19
3:19 16.89 3:33 16.04 3:44 14.46 4:09 13.04 4:36 12.14 4:57
3:09 17.8 3:22 17.03 3:31 15.31 3:55 13.8 4:21 12.97 4:38
2:59 18.76 3:12 18.08 3:19 16.21 3:42 14.61 4:06 13.85 4:20
2:51 19.64 3:03 19.04 3:09 17.04 3:31 15.35 3:55 14.66 4:06
2:43 20.55 2:55 20.04 3:00 17.9 3:21 16.12 3:43 15.50 3:52
2:36 21.48 2:48 21.05 2:51 18.77 3:12 16.9 3:33 16.35 3:40
2:30 22.38 2:41 22.04 2:43 19.62 3:04 17.66 3:24 17.18 3:30
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*Regular treadmill speeds are the values in the top row; cells in the table are the equivalent speeds on a body-weight supported treadmill (up to 50% body weight support). Regression equations are as follows: for 50% BW speed in km$h21 on LBPP treadmill = 1.25 3 regular TM speed + 3.92; at 60% BW LBPP treadmill speed = 1.14 3 regular TM speed + 3.99; at 70% BW LBPP treadmill speed = 1.25 3 regular TM speed-1.93; at 80% BW LBPP treadmill speed = 1.08 3 regular TM speed + 2.32; at 90% BW speed = 0.97 3 regular TM speed + 2.15; and at 100% BW speed = 1.05 3 regular TM speed + 0.29.
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LBPP speed 50% (km$h21) Min$km21 60% (km$h21) Min$km21 70% (km$h21) Min$km21 80% (km$h21) Min$km21 90% (km$h21) Min$km21 100% (km$h21) Min$km21
6.4 9:19
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LBPP Conversion Table
TABLE 4. Conversion table for speeds from 4 to 10 miles per hour and minutes per mile.* Regular treadmill speed Miles per hour Minutes per mile
4 4.5 5 5.5 6 6.5 15:00 13:20 12:00 10:55 10:00 9:14
7 8:34
7.5 8:00
8 7:30
8.5 7:04
9 6:40
9.5 6:19
10 6:00
50% (mph) Minutes per mile 60% (mph) Minutes per mile 70% (mph) Minutes per mile 80% (mph) Minutes per mile 90% (mph) Minutes per mile 100% (mph) Minutes per mile
7.45 8:00 7.07 8:29 6.22 9:39 5.76 10:25 5.21 11:31 4.40 13:39
11.21 5:21 10.5 5:43 9.97 6:01 8.98 6:41 8.10 7:24 7.54 7:57
11.83 5:04 11.06 5:26 10.58 5:40 9.51 6:18 8.57 7:00 8.06 7:27
12.48 4:48 11.66 5:09 11.23 5:21 10.07 5:57 9.08 6:37 8.61 6:58
13.08 4:35 12.2 4:55 11.83 5:04 10.59 5:40 9.54 6:17 9.11 6:35
13.71 4:23 12.77 4:42 12.45 4:49 11.12 5:24 10.02 5:59 9.63 6:14
14.34 4:11 13.35 4:30 13.08 4:35 11.66 5:09 10.5 5:43 10.16 5:54
14.95 4:01 13.91 4:19 13.69 4:23 12.19 4:55 10.98 5:28 10.67 5:37
8.06 7:27 7.63 7:52 6.83 8:47 6.28 9:33 5.68 10:34 4.91 12:14
8.66 6:56 8.17 7:21 7.42 8:05 6.79 8:50 6.14 9:47 5.41 11:06
9.32 6:26 8.77 6:50 8.08 7:26 7.36 8:09 6.64 9:02 5.96 10:04
9.93 6:02 9.33 6:26 8.69 6:54 7.89 7:36 7.12 8:26 6.47 9:16
10.57 5:40 9.91 6:03 9.33 6:26 8.44 7:07 7.61 7:53 7.01 8:34
*Regular treadmill speeds are the values in the top row; cells in the table are the equivalent speeds on a body weight-supported treadmill (up to 50% body weight support). Regression equations are as follows: for 50% BW speed in miles per hour on LBPP treadmill = 1.26 3 regular TM speed + 2.41; at 60% BW LBPP treadmill speed = 1.14 3 regular TM speed + 2.48; at 70% BW LBPP treadmill speed = 1.25 3 regular TM speed + 1.20; at 80% BW LBPP treadmill speed = 1.08 3 regular TM speed + 1.44; at 90% BW speed = 0.96 3 regular TM speed + 1.34; and at 100% BW speed = 1.05 3 regular TM speed + 0.18.
16.1 km$h21 (10 mph). Variability on the regular treadmill was fairly limited, as SDs in observed V_ O2 were approximately 2 ml$kg21$min21 (range from 1.91 to 2.69 ml$kg21$min21) across all speeds excluding 10 mph (4.35 ml$kg21$min21). During the regular treadmill session, the average maximum HR reached 177.0 b$min21 (SD = 10.6), maximum RER was 1.05 (SD = 0.06), and maximum respiratory rate (RR) was 41.8 breaths per minute (SD = 8.7). Comparatively, the average maximum HR, RER, and RR reached on the LBPP treadmill were 179.2 b$min21 (SD = 12.1), 1.01 (SD = 0.07), and 44.9 breaths per minute (SD = _ O2 used 11.6), respectively. The variability in proportion of V on the LBPP treadmill was between 6 and 8% and was very similar across BWs (Table 1). Hypothesis 1: The Metabolic Demand of Running on a Lower Body Positive Pressure Treadmill Will Decrease as Support Increases. On average for all tests, as BWS increased, metabolic cost decreased (Table 1). For each speed separately, there was a significant linear downward trend, indicating that absolute V_ O2 decreased with more BWS (p , 0.001 for all trends). As an illustration, at 11.3 km$h21, average V_ O2 was 34.46 ml$kg21$min21 running at 100% BW, 29.12 ml$kg21$min21 at 80% BW, and 21.87 ml$kg21$min21 at 50% BW. Hypothesis 2: The Decrease in Metabolic Demand Will Not Be Proportional to the Decrease in BW. The outcome used in these analyses was the V_ O2 observed while running on the LBPP treadmill expressed as a proportion of the V_ O2 observed at the
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same speed on the regular treadmill (Table 1). We tested whether the proportion of observed V_ O2 was different from the proportion BW. In other words, if participants ran at 90% of their BW, did they use 90% of the V_ O2 on a regular treadmill at the same speed? The proportion was found to be roughly equivalent (within about 2% difference, not significantly different) to the BW for 90%, 80%, and 70% BW. At the higher and lower end of the BW scale, the proportion metabolic demand was significantly different from the proportion BW. At 100% BW on the LBPP treadmill, the proportion V_ O2 used was 93.85% on average, significantly lower than 100% (p , 0.001). At 50 and 60% BW, the average proportion V_ O2 used was significantly higher (61.11%, p , 0.001 and 64.84%, p = 0.002, respectively; see Figure 2 and Table 1). Development of Conversion Tables
Before testing hypothesis 3, conversion tables were developed for speeds between 6.4 and 16.1 km$h21 (4 and 10 mph) in 0.8 km$h21 (0.5 mph) increments and between the BWs of 50 and 100%, in 10% increments using linear regression equations. To assess the stability of the regression equations, a table was developed after the first 10 participants (see Tables 2, 3, and 4). We then assessed how much the regression equations and estimates in the table changed after adding in the second 10 participants. The absolute average change in each cell after adding in the data from the last 10 participants was 0.12 mph. Only at the lower left corner of the table, at 100% BW, and between the speeds 4 and 7 mph, did the table change more than 0.3 mph and have a change
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Journal of Strength and Conditioning Research larger than 3%. No value changed more than 0.5 mph after the first 10 participants. The slopes of the regression equations showed a similar pattern. For example, from 10 to 15 participants, the slopes did not change more than 4.5%, with the largest change at 90% BW, where the slope changed from b = 0.22 to b = 0.226. In each table, column heads represent speeds run on a normal treadmill and the numbers making up the table are the speeds one must run at on a LBPP treadmill at each different BW percentage (row). For example, if someone normally runs at 12.9 km$h21 (4:40 min$km21, 8 mph, 7:30 minutes per mile) on a regular treadmill and runs on a LBPP treadmill at 80% BW, the estimated required speed for the same metabolic effort would be 16.21 km$h21 (3:42 min$km21, 10.07 mph, 5:57 minutes per mile). If they progress to 90% BW, the estimated metabolic equivalent would be 14.61 km$h21 (4:06 min$km21, 9.08 mph, 6:37 minutes per mile). Hypothesis 3: The Increase in Speed Needed to Make up for the Decrease in Metabolic Demand Will Be Proportional to the Amount of BWS. Analyses for hypothesis 3 are summarized in Table 2 and based on Table 3. We tested whether the speed increase needed to offset the decrease in metabolic demand was significantly different from hypothesized. Averaged across speeds, we found that at each proportion of BW, the increase in proportion speed was greater than the amount of BWS (p values ranging from p , 0.001 to p = 0.22). For example, at 80% BW (20% support), an average 29.89% increase in speed was needed to make up for the decrease in metabolic cost. Speed, however, seemed to be very impactful. At higher speeds, lower proportional speed increases were required to match the metabolic demand of the regular treadmill (significant downward slopes for each proportion BW). For example, where the increase at 6.4 km$h21 and 80% BW was 44%, at 16.1 km$h21 and 80% BW, only a 21.91% speed increase was required. At higher speeds (16.1 km$h21), the proportion speed increase approached the proportion support: at 50% BW, a 49.5% increase in speed was required; at 90% BW, a 9.73% increase in speed was required. A notable exception was that at 100% BW, an increase of approximately 7% in speed was still required (Table 4).
DISCUSSION This study aimed to quantify how much faster individuals must run on LBPP treadmills to make up for the decreased metabolic cost associated with running at a percentage of full BW. Although previous research has looked at the effects BW supported treadmills can have on different physiological parameters, this study is the first to look specifically at the change in metabolic expenditure across a range of common speeds and proportions of BWS. The findings are presented in 2 conversion tables, with the results in kilometers per hour, minutes per kilometer, miles per hour, and minutes per mile.
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Some of the findings of this study are consistent with previous research. For example, it was found that at every speed and BW%, the metabolic demand was lower on the LBPP compared with the same speed on the regular treadmill. An interesting finding was that even at full BW on the LBPP, there was still a reduction in metabolic demand. Based on previous research, it has been shown that even while running at 100% BW, metabolic demand is decreased, explained either because 100% BW (0% BWS), according to the LBPP treadmill, is actually still lending approximately 3–5% of BW support (5,6,8) or lateral stabilization is still being provided when zipped into the enclosure (4). As BWS increased, metabolic demand significantly decreased with speed. At the lowest proportion BW (50% BW), the metabolic expenditure was approximately 61% of the metabolic expenditure at the same speed on the regular treadmill. These findings are consistent with previous research that showed an attenuation in the decrease in metabolic demand as BWS increased, which is why the decrease in metabolic demand was proportional at the higher proportions of BW (70, 80, and 90% BW) but not proportional at the lower proportion BWs. For example, Grabowski (5) found that at 25% BW (75% support), metabolic demand was between 55 and 60% of demand at full BW. This decreased metabolic demand can be offset by running at higher speeds. Gojanovic et al. (4) found that the increase in speed required at 90% BW to match subjects’ V_ O2max was roughly proportional to the amount of support provided during maximal exercise testing (10% BWS). Our data indicated that indeed about a 10% increase in speed was needed at 90% BW at speeds more than 14.5 km$h21. However, at lower speeds, the required increase in speed was significantly greater. For example, at 6.4 km$h21 and 90% BW on the LBPP treadmill, a 30.18% increase in speed was required to match the metabolic equivalent on the regular treadmill. These findings have implications for research and practice. It was confirmed that the LBPP treadmill allows for faster running at decreased metabolic demand. An important practical limitation is that if an LBPP treadmill user wants to complete a metabolically equivalent workout, it may not be feasible or increase the risk for other injuries due to biomechanical changes (4,13) to run at substantially faster speeds. Also, increasing speed will further increase GRFs. However, Raffalt et al. (13) found that GRFs were roughly equivalent at 10 km$h21 and full BW, 14 km$h21 and 75% BW, and 22 km$h21 and 50% BW. Comparing these increases to our table indicates that LBPP treadmill users can possibly achieve a metabolically equivalent workout while still experiencing reduced GRFs. For example, in our study, at 80% BW, an increase from 9.7 to 14.5 km$h21 requires an increase in speed of about 23%, whereas GRFs are estimated to be reduced by about 40% based on Raffalt et al (13). Future research is needed to define speeds in terms of metabolic expenditure and GRFs for a variety of runners in performance and rehabilitation settings. VOLUME 29 | NUMBER 3 | MARCH 2015 |
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LBPP Conversion Table An important limitation of this study is that we did not collect information on speeds more than 17.7 km$h21. Several speeds in the table are higher than this value and are extrapolated based on regression analyses. Second, the conversion tables represent average values, and individual variability in metabolic expenditure on the LBPP treadmill was greater than on the regular treadmill. In addition, the use of snowball sampling in recruiting subjects also presents limitations, including community bias and a nonrandom sample. Finally, it has to be noted that this research was conducted at 7,000-ft altitude.
PRACTICAL APPLICATIONS This is the first study to comprehensively evaluate how much faster the user of an LBPP treadmill must run to offset the decreased metabolic cost of running with BWS. It was confirmed that, consistent with previous literature, metabolic demand decreases with increasing BWS. In other words, running with BWS requires less effort than running without support, resulting in a lower cardiovascular training stimulus. The increase in speed needed to make up for the decrease in metabolic demand was highly dependent on the speed being run at. The findings are expressed in conversion tables, which can serve as a guideline for practitioners and coaches to develop training programs on LBPP treadmills without substantially reducing training of the cardiovascular system. The conversion tables summarize the required speed increases needed to match the metabolic output of running on the LBPP treadmill for speeds ranging from 6.4 to 16.1 km$h21 and BWs from 50 to 100% in 10% increments. These tables can further be used as a starting point for future research on finding the optimal rehabilitation protocols that maximize metabolic workload but limit GRFs.
ACKNOWLEDGMENTS This study was made possible by funding from the Technology Research Initiative Fund (TRIF) for purchase of the Alter-GÒ Anti-GravityÒ treadmill. The results of this study do not constitute endorsement of the product by the authors or the NSCA. The equipment used for this study was funded by the TRIF Research Equipment Acquisition Program (REAP) at Northern Arizona University.
REFERENCES 1. Borg, G. Perceived exertion as an indicator of somatic stress. Scand J Rehabil Med 2: 92–98, 1970. 2. Byl, N. Guidelines for Using the Alterg: Patients With Neurological Problems [Internet]. Fremont, CA: Alter-G. Available at: http://www.alterg.com/
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wp-content/uploads/2010/06/Neuro-Guidelines-by-Nancy-Byl.pdf. Accessed February 14, 2014. 3. Figueroa, MA, Wicke, J, Manning, J, Escamilla, P, Santillo, N, Wolkstein, J, and Weis, M. Validation of ACSM metabolic equations in an anti-gravity environment: A pilot study. Int J Appl Sci Technol 2: 204–210, 2012. 4. Gojanovic, B, Cutti, P, Shultz, R, and Matheson, GO. Maximal physiological parameters during partial body-weight support treadmill testing. Med Sci Sports Exerc 44: 1935–1941, 2012. 5. Grabowski, AM. Metabolic and biomechanical effects of velocity and weight support using a lower-body positive pressure device during walking. Arch Phys Med Rehabil 91: 951–957, 2010. 6. Grabowski, AM and Kram, R. Effects of velocity and weight support on ground reaction forces and metabolic power during running. J Appl Biomech 24: 288–297, 2008. 7. Hall, C, Figueroa, A, Fernhall, B, and Kanaley, JA. Energy expenditure of walking and running: Comparison with prediction equations. Med Sci Sports Exerc 36: 2128–2134, 2004. 8. Hoffman, MD and Donaghe, HE. Exercise responses during partial body-weight supported treadmill walking and running in healthy individuals. Arch Phys Med Rehabil 92: 960–966, 2011. 9. Kurz, MJ, Corr, B, Stuberg, W, Volkman, KG, and Smith, N. Evaluation of lower body positive pressure supported treadmill training for children with cerebral palsy. Pediatr Phys Ther 23: 232–239, 2011. 10. Morgan, D, Craib, M, Krahenbuhl, G, Woodall, K, Jordan, S, Filarski, K, Burleson, C, and Williams, T. Daily variability in running economy among well-trained male and female distance runners. Res Q Exerc Sci 65: 72–77, 1994. 11. Morgan, D, Martin, P, Krahenbuhl, G, and Baldini, F. Variability in running economy and mechanics among trained male runners. Med Sci Sports Exerc 23: 378–383, 1991. 12. Patil, S, Steklov, N, Bugbee, WD, Goldberg, T, Colwell, CW, and D’lima, DD. Anti-gravity treadmills are effective in reducing knee forces. J Orthop Res 31: 672–679, 2013. 13. Raffalt, PC, Hovgaard-hansen, L, and Jensen, BR. Running on a lower-body positive pressure treadmill: VO2max, respiratory response, and vertical ground reaction force. Res Q Exerc Sport 84: 213–222, 2013. 14. Ruckstuhl, H, Schlabs, T, Rosales-velderrain, A, and Hargens, AR. Oxygen consumption during walking and running under fractional weight bearing conditions. Aviat Space Environ Med 81: 550–554, 2010. 15. Tenforde, AS, Watanabe, LM, Moreno, TJ, and Fredericson, M. Use of an antigravity treadmill for rehabilitation of a pelvic stress injury. PM R 4: 629–631, 2012. 16. Teunissen, LP, Grabowski, A, and Kram, R. Effects of independently altering body weight and body mass on the metabolic cost of running. J Exp Biol 210: 4418–4427, 2007. 17. Thomas, S, Reading, J, and Shephard, RJ. Revision of the physical activity readiness questionnaire (PAR-Q). Can J Sport Sci 17: 338–345, 1992. 18. Williams, T, Krahenbuhl, G, and Morgan, D. Daily variation in running economy of moderately trained male runners. Med Sci Sports Exerc 23: 944–948, 1991.
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Journal of Strength and Conditioning Research
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ON
LOWER BODY
JOHN R. KLINE,1 SCOT RAAB,1 J. RICHARD COAST,2 ROGER G. BOUNDS,3 DAVID K.P. MCNEILL,2 1 AND HENDRIK D. DE HEER Departments of 1Physical Therapy and Athletic Training; 2Biological Sciences; and 3Health Sciences, Northern Arizona University, Flagstaff, Arizona ABSTRACT
INTRODUCTION
Kline, JR, Raab, S, Coast, JR, Bounds, RG, McNeill, DKP, and de Heer, HD. A conversion table for running on lower body positive pressure treadmills. J Strength Cond Res 29(3): 854– 862, 2015—Lower body positive pressure (LBPP) or antigravity treadmills are becoming increasingly popular in sports and rehabilitation settings. Running at a decreased body weight (BW) reduces metabolic cost, which can be offset by running at faster speeds. To date, however, little is known about how much faster someone must run to offset the reduced metabolic cost. This study aimed to develop a user-friendly conversion table showing the speeds required on an LBPP treadmill to match the equivalent metabolic output on a regular, nonLBPP, treadmill across a range of body weight supports. A total of 20 recreational runners (11 males, 9 females) ran multiple 3-minute intervals on a regular treadmill and then on an LBPP treadmill at 6 different BWs (50–100%, 10% increments). Metabolic outputs were recorded and matched between the regular and LBPP treadmill sessions. Using regression analyses, a conversion table was successfully created for the speeds from 6.4 to 16.1 km$h21 (4 to 10 mph) in 0.8 km$h21 (0.5 mph) increments on the regular treadmill and BW proportions of 50, 60, 70, 80, 90, and 100% on an LBPP treadmill. The table showed that a greater increase in speed on the LBPP treadmill was needed with more support (p , 0.001) but that the proportion increase was smaller at higher speeds (p , 0.001). This research has implications for coaches or practitioners using or prescribing training on an LBPP treadmill.
KEY WORDS rehabilitation, oxygen consumption, anti-gravity, Alter-GÒ, Unloading, running speed
Address correspondence to Hendrik D. de Heer, [email protected]. 29(3)/854–862 Journal of Strength and Conditioning Research Ó 2015 National Strength and Conditioning Association
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ower body positive pressure (LBPP) treadmills that support the user’s body weight (BW) to facilitate walking or running have become increasingly popular for use in rehabilitation and injury prevention (2,9,12,15). A major aim of running on LBPP treadmills is to maintain or improve aerobic capacity while limiting the stress induced by ground reaction forces (GRFs) on the lower extremities, as compared with running outside or on a “regular” non-BW-supported treadmill (5). Previous research has shown that ambulating and running on an LBPP treadmill lowers metabolic expenditure, with more body weight support (BWS) reducing oxygen consumption (V_ O2) by a greater extent (3,5–7,14,16). The reduction in V_ O2 has been found to be slightly less than proportional to the amount of BWS provided, particularly at the higher levels of support. For example, Grabowski (5) found that while walking at 25% of full BW (75% BWS), metabolic expenditure was reduced by 45% compared with walking with no BWS. Although it is known that increasing BWS reduces oxygen consumption, several recent studies have shown that while running on LBPP treadmills, individuals can match both maximal (V_ O2max) and submaximal oxygen uptake achieved on non-BW supported treadmills (4,5,13). To achieve this, individuals must run at faster speeds to make up for the decreased metabolic demand associated with running at less than full BW (4,5,13). For example, Raffalt et al. (13) found that running at 10 km$h21 at full BW required about the same metabolic demand as running at 14 km$h21 at 75% BW and 18 km$h21 at 25% BW. Among trained runners, Gojanovic et al. (4) further found that running at 90% BW (10% BWS) required a roughly proportional increase in speed of 11.4% on average to achieve the same V_ O2max as on a regular treadmill. Despite these insights, it remains unclear how much faster one must run on an LBPP treadmill across a range of different running speeds and percentages of BW to match the metabolic demand of running without BWS. In the context of an increase in popularity of LBPP treadmill running, this is an important question to address from both a research and a practical perspective. For example, if a runner’s regular speed is 4 min$km21, but, due to a musculoskeletal injury, he or she
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above, the following hypotheses were also tested: (a) the metabolic demand of running on a LBPP treadmill will decrease as BWS increases, (b) the decrease in metabolic demand will be less than proportional to the BWS, and (c) the increase in speed needed to make up for the decrease in metabolic demand will be proportional to the amount of BWS.
METHODS Experimental Approach to the Problem
To develop a conversion table between methods of running, metabolic rate was measured during running on a regular treadmill and on the LBPP treadmill. The speeds at which the metabolic output was equivalent were then matched using regression analyses (see below). The regression equations for each BW predicting speed on an LBPP treadmill (dependent variable) from speed on a regular treadmill (independent variable) were used to create the conversion tables. Subjects
The study was approved by the institutional review board of the university. A total of 20 recreational runners were recruited (11 males, 9 females) through online announcements and snowball sampling (where existing study subject recruit subsequent participants). Participants’ mean age was 28.5 years old (SD = 8.8 years; range, 22–56 years old), their height was 171.1 cm (SD = 7.8 cm), and weight was 65.8 kg (SD = 8.7 kg). Participants completed an informed consent and a Physical Activity Readiness Questionnaire (PAR-Q) (17) before the study. Inclusion criteria were that each participant was at least 21 years old, had experience with treadmill running and was able to run for at least 3 minutes at 11.3 km$h21 (7 mph), and did not have any risk factors for engaging in physical activity as indicated by the PAR-Q (17).
Figure 1. Overview of study protocol assessing metabolic expenditure of running on a lower body positive pressure treadmill compared to a regular treadmill.
starts training on an LBPP treadmill, with part of his or her BW supported, running at the same speed of 4 min$km21 will cost substantially less effort. As a result, he or she will not receive the same training stimulus and risk deconditioning if maintained over a longer period of time. To date, however, it is unknown exactly how much easier it is to run with different levels BWS and how much speed increase would be needed to account for this reduced metabolic output. Understanding of precise conversion of speeds at defined levels of BWS will help in devising a training/rehabilitation program while not sacrificing cardiorespiratory fitness. To this end, the goal of this study was to develop a userfriendly conversion table for commonly used BW percentages (50% through 100%) to quickly tell how much faster one must run at different proportions of BWS to achieve a metabolic output equivalent to running at a certain speed on a regular, non-LBPP treadmill. Based on the previous research described
Procedures
The protocol included a total of 5 sessions (Figure 1). The first 2 sessions consisted of running on the LBPP treadmill (Alter-GÒ Anti-GravityÒ Treadmill; Alter-GÒ P200; Fremont, CA, USA) for 30 minutes nonstop, at a self-selected VOLUME 29 | NUMBER 3 | MARCH 2015 |
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LBPP Conversion Table
856 TABLE 1. Average observed V_ O2 from sessions on LBPP treadmill and proportion of regular treadmill V_ O2 across speeds and body weights.* LBPP treadmill speed (km$h21)
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Mean observed V_ O2 (ml$kg21$min21) Regular treadmill 50% BW 60% BW 70% BW 80% BW 90% BW 100% BW p† for slope Proportion of V_ O2 at same speed on regular treadmill (%) 50% BW 60% BW 70% BW 80% BW 90% BW 100% BW Overall
8
9.7
11.3
12.9
14.5
16.1
17.7
28.14 18.23 19.44 21.60 23.59 25.64 27.30 ,0.001§
32.11 20.15 21.36 23.67 25.87 28.86 30.38 ,0.001§
36.17 21.87 23.86 26.51 29.12 32.55 34.46 ,0.001§
39.94 24.15 25.88 28.70 31.57 34.89 37.43 ,0.001§
44.10 26.09 27.75 30.76 33.54 37.99 40.82 ,0.001§
48.24 30.14 31.46 34.24 37.38 41.82 44.04 ,0.001§
33.29 35.15 37.51 41.24 45.80 47.67 ,0.001§
64.64 68.89 76.64 83.69 91.07 97.04
62.66 66.54 73.74 80.81 90.17 94.69
60.43 66.01 73.29 80.55 90.08 95.33
57.05 60.60 68.94 76.36 85.35 92.77
59.39 63.06 69.97 76.19 86.30 92.73
63.66 65.04 70.54 76.28 85.84 90.91
NA NA NA NA NA NA
Mean % (SD)
61.11 64.84 72.02 78.84 87.91 93.85
(7.66) (6.91) (7.18) (6.48) (6.59) (6.95)
p value† for slope
p valuez for comparison
0.870 0.983 0.085 0.251 0.847 0.012k 0.166
,0.001§ 0.002§ 0.171 0.458 0.173 ,0.001§ 0.302
*LBPP = lower body positive pressure; BW = body weight. †p value using linear regression tested whether slope was significantly different from zero. _ O2 observed; for example, testing whether 50% BW was significantly different zComparison testing whether proportion of body weight is significantly different to proportion of V
from 61.11%, whether 60% BW was significantly different from 64.84% etc. §Significant on p , 0.01. kSignificant on p # 0.05.
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70%, 80%, 90%, and 100% of their original BW, split across 2 sessions. Participants were randomized between running at either 50, 70, and 100% or 60, 80, and 90% of BW for the fourth session while running the fifth session at the remaining BWs. Participants ran a total of 12 stages (3 body weights at each of 4 different speeds). The speeds were based on the last successful stage completed during their third (non-LBPP) session and were either below (3.2 km$h21 [2 mph] and 1.6 km$h21 [1 mph]) at the same speed or just above (1.6 km$h21 (1 mph]) their final non-LBPP speed. Participants rested between stages as they did in session 3. Participants started each of the last 2 sessions at the lowest speed and most BWS assigned. Figure 2. Metabolic demand of running on a lower body positive pressure treadmill expressed as a proportion of metabolic demand on a regular treadmill across different body weights.
speed above 6.4 km$h21 (4 mph). The first 5 minutes were run at 100% of the subject’s BW. The BW was subsequently dropped by 10% every 5 minutes until 50% BW was run at for the final 5 minutes. This allowed participants to accommodate to the Alter-GÒ treadmill, as previous studies indicated that there is an approximately 1-hour metabolic accommodation effect of treadmill running (10,11,18). For the 3 remaining sessions, participants were connected to a metabolic cart (TrueOne 2400; Parvo Medics, UT, USA) while wearing a heart rate monitor (FT60; Polar, Kempele, Finland). Participants’ height and weight without shoes and in light clothes were taken at the beginning of each session. Participants were asked to wear the same shoes for the last 3 sessions. The third session was completed on a “regular” (non-LBPP) treadmill (Woodway, Waukesha, WI, USA). The protocol for this session consisted of the participants running multiple 3-minute stages, starting at 6.4 km$h21 (4 mph) and increasing in 1.6 km$h21 (1 mph) increments to at least 11.3 km$h21 (7 mph). Participants were allowed to continue running until their self-reported rate of perceived exertion exceeded 16 in the final 30 seconds of each stage (on the original Borg scale) (1), their respiratory exchange ratio (RER) consistently reached above 1.0, or their HR got to within 10% of their theoretical maximum heart rate (HR) (220 2 age). Between stages, participants stood still until their HR fell below 110 b$min21 or 2 minutes elapsed, whichever came first. The minimum rest between stages was 30 seconds. For all metabolic tests, expired gas data were averaged for the final minute of each stage, consistent with previously published methods (13). Participants’ final stage reached during this session formed the basis for the final 2 sessions run on the LBPP treadmill. The fourth and fifth sessions were completed on an Alter-GÒ P200 LBPP treadmill. For each session on the LBPP treadmill, participants wore the Alter-G provided neoprene shorts that zip into the Alter-G treadmill enclosure and allow for running in a positive pressure environment. Participants ran at 50%, 60%,
Statistical Analyses
Data analysis was performed using SPSS version 20.0 (SPSS, Inc, Chicago, IL, USA). First, average metabolic expenditures from session 3 (the regular treadmill session) were calculated for each speed (6.4 km$h21 through 16.1 km$h21) (4–10 mph). Second, average metabolic expenditures were calculated for each speed (8.0 km$h21 through 17.7 km$h21) (5–11 mph) and each BW (50% BW through 100% BW) on the LBPP treadmill. Linear regression analyses using generalized estimating equations were conducted, taking into account the “nested” data structure (with multiple V_ O2 values on both the regular and LBPP treadmill for each person). Hypotheses were tested for p # 0.05. Hypothesis 1 (V_ O2 will significantly decrease with increased BWS) was tested by evaluating whether the slope of observed V_ O2 differed significantly from zero (downwards) across BWs. Hypothesis 2 addressed whether the V_ O2 decrease on the LBPP was equivalent to the proportion of BWS. For example, it was tested while running at 60% BW and the observed V_ O2 was significantly different from 60% of the V_ O2 used on a regular treadmill. Before testing hypothesis 3, full conversion tables were _ O2 for created. This was achieved by entering the average V each speed on the regular treadmill into regression equations _ O2 and speed on the that quantified the relationship between V LBPP treadmill. These regression equations were calculated separately for each level of BW. Hypothesis 3 stated that speed increase required to offset reduced metabolic demand on the LBPP would be proportional to the amount of BWS provided. Therefore, we tested whether the observed proportion of the increase in speed required was significantly different from the hypothesized proportion. For example, it was tested whether running at 80% BW would require significantly different than a 20% increase in speed to match the metabolic demand of running on a regular treadmill.
RESULTS Physiologic Data
The mean V_ O2 collected for each speed on the regular treadmill ranged from 23.9 ml$kg21$min21 (SD = 1.9) at 6.4 km$h21 (4 mph) to 48.2 ml$kg21$min21 (SD = 4.4) at VOLUME 29 | NUMBER 3 | MARCH 2015 |
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LBPP Conversion Table
858 TABLE 2. Proportion increase in speed required to match V_ O2 on a regular treadmill (%).* Regular treadmill speed
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Km$h21 Miles per hour
6.4 4
8.0 5
9.7 6
11.3 7
12.9 8
14.5 9
16.1 10
Body weight % 50% 60% 70% 80% 90% 100% Overall
87.44 76.78 55.50 44.00 30.18 9.98
73.17 63.42 48.38 35.96 22.78 8.12
65.58 55.55 44.88 31.52 18.58 7.91
60.19 49.93 42.38 28.36 15.75 7.76
56.04 45.71 40.43 25.91 13.48 7.57
52.31 41.88 38.36 23.58 11.29 7.01
49.53 39.06 36.95 21.91 9.73 6.75
Mean %
p value† for slope
% BWS
p valuez for comparison
62.93 52.71 43.61 29.89 17.18 7.84
0.010k 0.003§ 0.003§ 0.010k 0.006§ 0.009§ 0.017k
50% 40% 30% 20% 10% 0%
0.012k 0.020k 0.022k ,0.001§ 0.005§ 0.018k ,0.001§
*BWS = body weight support. †p-value for significant slope across speeds. zp-value for comparison of proportion speed increase to proportion body weight support, for example at 80% body weight (20% body weight support), we tested whether the
speed increase was greater than 20%, at 70% body weight, was the speed increase greater than 30% etc. §Significant on p , 0.01. kSignificant on p # 0.05.
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TABLE 3. Conversion table for speeds from 6.4 to 16.1 kilometers per hour (km$h21) and minutes per kilometer (min$km21) on a regular treadmill to a body-weight supported treadmill.* Regular treadmill speed km$h21 Min$km21
8.0 7:27
8.9 6:47
9.7 6:13
10.5 5:44
11.3 5:20
12.1 4:58
12.9 4:40
13.7 4:23
14.5 4:09
15.3 3:55
16.1 3:44
12.07
12.98
13.93
15.0
15.99
17.02
18.05
19.04
20.09
21.06
22.06
23.08
24.06
4:58 11.38 5:16 10.01 6:00 9.27 6:28 8.38 7:09 7.08 8:29
4:37 12.27 4:53 10.99 5:28 10.11 5:56 9.14 6:34 7.90 7:36
4:18 13.15 4:34 11.94 5:01 10.94 5:29 9.88 6:05 8.70 6:54
4:00 14.11 4:15 13.0 4:37 11.84 5:04 10.69 5:37 9.59 6:16
3:45 15.02 4:00 13.99 4:17 12.7 4:44 11.45 5:14 10.42 5:46
3:32 15.96 3:46 15.01 4:00 13.58 4:25 12.24 4:54 11.28 5:19
3:19 16.89 3:33 16.04 3:44 14.46 4:09 13.04 4:36 12.14 4:57
3:09 17.8 3:22 17.03 3:31 15.31 3:55 13.8 4:21 12.97 4:38
2:59 18.76 3:12 18.08 3:19 16.21 3:42 14.61 4:06 13.85 4:20
2:51 19.64 3:03 19.04 3:09 17.04 3:31 15.35 3:55 14.66 4:06
2:43 20.55 2:55 20.04 3:00 17.9 3:21 16.12 3:43 15.50 3:52
2:36 21.48 2:48 21.05 2:51 18.77 3:12 16.9 3:33 16.35 3:40
2:30 22.38 2:41 22.04 2:43 19.62 3:04 17.66 3:24 17.18 3:30
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*Regular treadmill speeds are the values in the top row; cells in the table are the equivalent speeds on a body-weight supported treadmill (up to 50% body weight support). Regression equations are as follows: for 50% BW speed in km$h21 on LBPP treadmill = 1.25 3 regular TM speed + 3.92; at 60% BW LBPP treadmill speed = 1.14 3 regular TM speed + 3.99; at 70% BW LBPP treadmill speed = 1.25 3 regular TM speed-1.93; at 80% BW LBPP treadmill speed = 1.08 3 regular TM speed + 2.32; at 90% BW speed = 0.97 3 regular TM speed + 2.15; and at 100% BW speed = 1.05 3 regular TM speed + 0.29.
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LBPP speed 50% (km$h21) Min$km21 60% (km$h21) Min$km21 70% (km$h21) Min$km21 80% (km$h21) Min$km21 90% (km$h21) Min$km21 100% (km$h21) Min$km21
6.4 9:19
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LBPP Conversion Table
TABLE 4. Conversion table for speeds from 4 to 10 miles per hour and minutes per mile.* Regular treadmill speed Miles per hour Minutes per mile
4 4.5 5 5.5 6 6.5 15:00 13:20 12:00 10:55 10:00 9:14
7 8:34
7.5 8:00
8 7:30
8.5 7:04
9 6:40
9.5 6:19
10 6:00
50% (mph) Minutes per mile 60% (mph) Minutes per mile 70% (mph) Minutes per mile 80% (mph) Minutes per mile 90% (mph) Minutes per mile 100% (mph) Minutes per mile
7.45 8:00 7.07 8:29 6.22 9:39 5.76 10:25 5.21 11:31 4.40 13:39
11.21 5:21 10.5 5:43 9.97 6:01 8.98 6:41 8.10 7:24 7.54 7:57
11.83 5:04 11.06 5:26 10.58 5:40 9.51 6:18 8.57 7:00 8.06 7:27
12.48 4:48 11.66 5:09 11.23 5:21 10.07 5:57 9.08 6:37 8.61 6:58
13.08 4:35 12.2 4:55 11.83 5:04 10.59 5:40 9.54 6:17 9.11 6:35
13.71 4:23 12.77 4:42 12.45 4:49 11.12 5:24 10.02 5:59 9.63 6:14
14.34 4:11 13.35 4:30 13.08 4:35 11.66 5:09 10.5 5:43 10.16 5:54
14.95 4:01 13.91 4:19 13.69 4:23 12.19 4:55 10.98 5:28 10.67 5:37
8.06 7:27 7.63 7:52 6.83 8:47 6.28 9:33 5.68 10:34 4.91 12:14
8.66 6:56 8.17 7:21 7.42 8:05 6.79 8:50 6.14 9:47 5.41 11:06
9.32 6:26 8.77 6:50 8.08 7:26 7.36 8:09 6.64 9:02 5.96 10:04
9.93 6:02 9.33 6:26 8.69 6:54 7.89 7:36 7.12 8:26 6.47 9:16
10.57 5:40 9.91 6:03 9.33 6:26 8.44 7:07 7.61 7:53 7.01 8:34
*Regular treadmill speeds are the values in the top row; cells in the table are the equivalent speeds on a body weight-supported treadmill (up to 50% body weight support). Regression equations are as follows: for 50% BW speed in miles per hour on LBPP treadmill = 1.26 3 regular TM speed + 2.41; at 60% BW LBPP treadmill speed = 1.14 3 regular TM speed + 2.48; at 70% BW LBPP treadmill speed = 1.25 3 regular TM speed + 1.20; at 80% BW LBPP treadmill speed = 1.08 3 regular TM speed + 1.44; at 90% BW speed = 0.96 3 regular TM speed + 1.34; and at 100% BW speed = 1.05 3 regular TM speed + 0.18.
16.1 km$h21 (10 mph). Variability on the regular treadmill was fairly limited, as SDs in observed V_ O2 were approximately 2 ml$kg21$min21 (range from 1.91 to 2.69 ml$kg21$min21) across all speeds excluding 10 mph (4.35 ml$kg21$min21). During the regular treadmill session, the average maximum HR reached 177.0 b$min21 (SD = 10.6), maximum RER was 1.05 (SD = 0.06), and maximum respiratory rate (RR) was 41.8 breaths per minute (SD = 8.7). Comparatively, the average maximum HR, RER, and RR reached on the LBPP treadmill were 179.2 b$min21 (SD = 12.1), 1.01 (SD = 0.07), and 44.9 breaths per minute (SD = _ O2 used 11.6), respectively. The variability in proportion of V on the LBPP treadmill was between 6 and 8% and was very similar across BWs (Table 1). Hypothesis 1: The Metabolic Demand of Running on a Lower Body Positive Pressure Treadmill Will Decrease as Support Increases. On average for all tests, as BWS increased, metabolic cost decreased (Table 1). For each speed separately, there was a significant linear downward trend, indicating that absolute V_ O2 decreased with more BWS (p , 0.001 for all trends). As an illustration, at 11.3 km$h21, average V_ O2 was 34.46 ml$kg21$min21 running at 100% BW, 29.12 ml$kg21$min21 at 80% BW, and 21.87 ml$kg21$min21 at 50% BW. Hypothesis 2: The Decrease in Metabolic Demand Will Not Be Proportional to the Decrease in BW. The outcome used in these analyses was the V_ O2 observed while running on the LBPP treadmill expressed as a proportion of the V_ O2 observed at the
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same speed on the regular treadmill (Table 1). We tested whether the proportion of observed V_ O2 was different from the proportion BW. In other words, if participants ran at 90% of their BW, did they use 90% of the V_ O2 on a regular treadmill at the same speed? The proportion was found to be roughly equivalent (within about 2% difference, not significantly different) to the BW for 90%, 80%, and 70% BW. At the higher and lower end of the BW scale, the proportion metabolic demand was significantly different from the proportion BW. At 100% BW on the LBPP treadmill, the proportion V_ O2 used was 93.85% on average, significantly lower than 100% (p , 0.001). At 50 and 60% BW, the average proportion V_ O2 used was significantly higher (61.11%, p , 0.001 and 64.84%, p = 0.002, respectively; see Figure 2 and Table 1). Development of Conversion Tables
Before testing hypothesis 3, conversion tables were developed for speeds between 6.4 and 16.1 km$h21 (4 and 10 mph) in 0.8 km$h21 (0.5 mph) increments and between the BWs of 50 and 100%, in 10% increments using linear regression equations. To assess the stability of the regression equations, a table was developed after the first 10 participants (see Tables 2, 3, and 4). We then assessed how much the regression equations and estimates in the table changed after adding in the second 10 participants. The absolute average change in each cell after adding in the data from the last 10 participants was 0.12 mph. Only at the lower left corner of the table, at 100% BW, and between the speeds 4 and 7 mph, did the table change more than 0.3 mph and have a change
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Journal of Strength and Conditioning Research larger than 3%. No value changed more than 0.5 mph after the first 10 participants. The slopes of the regression equations showed a similar pattern. For example, from 10 to 15 participants, the slopes did not change more than 4.5%, with the largest change at 90% BW, where the slope changed from b = 0.22 to b = 0.226. In each table, column heads represent speeds run on a normal treadmill and the numbers making up the table are the speeds one must run at on a LBPP treadmill at each different BW percentage (row). For example, if someone normally runs at 12.9 km$h21 (4:40 min$km21, 8 mph, 7:30 minutes per mile) on a regular treadmill and runs on a LBPP treadmill at 80% BW, the estimated required speed for the same metabolic effort would be 16.21 km$h21 (3:42 min$km21, 10.07 mph, 5:57 minutes per mile). If they progress to 90% BW, the estimated metabolic equivalent would be 14.61 km$h21 (4:06 min$km21, 9.08 mph, 6:37 minutes per mile). Hypothesis 3: The Increase in Speed Needed to Make up for the Decrease in Metabolic Demand Will Be Proportional to the Amount of BWS. Analyses for hypothesis 3 are summarized in Table 2 and based on Table 3. We tested whether the speed increase needed to offset the decrease in metabolic demand was significantly different from hypothesized. Averaged across speeds, we found that at each proportion of BW, the increase in proportion speed was greater than the amount of BWS (p values ranging from p , 0.001 to p = 0.22). For example, at 80% BW (20% support), an average 29.89% increase in speed was needed to make up for the decrease in metabolic cost. Speed, however, seemed to be very impactful. At higher speeds, lower proportional speed increases were required to match the metabolic demand of the regular treadmill (significant downward slopes for each proportion BW). For example, where the increase at 6.4 km$h21 and 80% BW was 44%, at 16.1 km$h21 and 80% BW, only a 21.91% speed increase was required. At higher speeds (16.1 km$h21), the proportion speed increase approached the proportion support: at 50% BW, a 49.5% increase in speed was required; at 90% BW, a 9.73% increase in speed was required. A notable exception was that at 100% BW, an increase of approximately 7% in speed was still required (Table 4).
DISCUSSION This study aimed to quantify how much faster individuals must run on LBPP treadmills to make up for the decreased metabolic cost associated with running at a percentage of full BW. Although previous research has looked at the effects BW supported treadmills can have on different physiological parameters, this study is the first to look specifically at the change in metabolic expenditure across a range of common speeds and proportions of BWS. The findings are presented in 2 conversion tables, with the results in kilometers per hour, minutes per kilometer, miles per hour, and minutes per mile.
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Some of the findings of this study are consistent with previous research. For example, it was found that at every speed and BW%, the metabolic demand was lower on the LBPP compared with the same speed on the regular treadmill. An interesting finding was that even at full BW on the LBPP, there was still a reduction in metabolic demand. Based on previous research, it has been shown that even while running at 100% BW, metabolic demand is decreased, explained either because 100% BW (0% BWS), according to the LBPP treadmill, is actually still lending approximately 3–5% of BW support (5,6,8) or lateral stabilization is still being provided when zipped into the enclosure (4). As BWS increased, metabolic demand significantly decreased with speed. At the lowest proportion BW (50% BW), the metabolic expenditure was approximately 61% of the metabolic expenditure at the same speed on the regular treadmill. These findings are consistent with previous research that showed an attenuation in the decrease in metabolic demand as BWS increased, which is why the decrease in metabolic demand was proportional at the higher proportions of BW (70, 80, and 90% BW) but not proportional at the lower proportion BWs. For example, Grabowski (5) found that at 25% BW (75% support), metabolic demand was between 55 and 60% of demand at full BW. This decreased metabolic demand can be offset by running at higher speeds. Gojanovic et al. (4) found that the increase in speed required at 90% BW to match subjects’ V_ O2max was roughly proportional to the amount of support provided during maximal exercise testing (10% BWS). Our data indicated that indeed about a 10% increase in speed was needed at 90% BW at speeds more than 14.5 km$h21. However, at lower speeds, the required increase in speed was significantly greater. For example, at 6.4 km$h21 and 90% BW on the LBPP treadmill, a 30.18% increase in speed was required to match the metabolic equivalent on the regular treadmill. These findings have implications for research and practice. It was confirmed that the LBPP treadmill allows for faster running at decreased metabolic demand. An important practical limitation is that if an LBPP treadmill user wants to complete a metabolically equivalent workout, it may not be feasible or increase the risk for other injuries due to biomechanical changes (4,13) to run at substantially faster speeds. Also, increasing speed will further increase GRFs. However, Raffalt et al. (13) found that GRFs were roughly equivalent at 10 km$h21 and full BW, 14 km$h21 and 75% BW, and 22 km$h21 and 50% BW. Comparing these increases to our table indicates that LBPP treadmill users can possibly achieve a metabolically equivalent workout while still experiencing reduced GRFs. For example, in our study, at 80% BW, an increase from 9.7 to 14.5 km$h21 requires an increase in speed of about 23%, whereas GRFs are estimated to be reduced by about 40% based on Raffalt et al (13). Future research is needed to define speeds in terms of metabolic expenditure and GRFs for a variety of runners in performance and rehabilitation settings. VOLUME 29 | NUMBER 3 | MARCH 2015 |
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LBPP Conversion Table An important limitation of this study is that we did not collect information on speeds more than 17.7 km$h21. Several speeds in the table are higher than this value and are extrapolated based on regression analyses. Second, the conversion tables represent average values, and individual variability in metabolic expenditure on the LBPP treadmill was greater than on the regular treadmill. In addition, the use of snowball sampling in recruiting subjects also presents limitations, including community bias and a nonrandom sample. Finally, it has to be noted that this research was conducted at 7,000-ft altitude.
PRACTICAL APPLICATIONS This is the first study to comprehensively evaluate how much faster the user of an LBPP treadmill must run to offset the decreased metabolic cost of running with BWS. It was confirmed that, consistent with previous literature, metabolic demand decreases with increasing BWS. In other words, running with BWS requires less effort than running without support, resulting in a lower cardiovascular training stimulus. The increase in speed needed to make up for the decrease in metabolic demand was highly dependent on the speed being run at. The findings are expressed in conversion tables, which can serve as a guideline for practitioners and coaches to develop training programs on LBPP treadmills without substantially reducing training of the cardiovascular system. The conversion tables summarize the required speed increases needed to match the metabolic output of running on the LBPP treadmill for speeds ranging from 6.4 to 16.1 km$h21 and BWs from 50 to 100% in 10% increments. These tables can further be used as a starting point for future research on finding the optimal rehabilitation protocols that maximize metabolic workload but limit GRFs.
ACKNOWLEDGMENTS This study was made possible by funding from the Technology Research Initiative Fund (TRIF) for purchase of the Alter-GÒ Anti-GravityÒ treadmill. The results of this study do not constitute endorsement of the product by the authors or the NSCA. The equipment used for this study was funded by the TRIF Research Equipment Acquisition Program (REAP) at Northern Arizona University.
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