In recent years, the use of treadmills that provide partial body weight support (BWS) have become increasingly commonplace among elite athletes as a supplemental training and rehabilitation tool. Several technologies for achieving BWS on a treadmill exist, including harness systems, underwater treadmills, and the most recent development, the application of Lower Body Positive Pressure (LBPP). These LBPP treadmill, also called the "Antigravity treadmill[R]", uses positive air pressure applied within a sealed chamber surrounding the subject's pelvis and legs to support the user's body weight. These LBPP treadmills have been used to reduce the ground reaction forces (GRFs) associated with running, while still maintaining a cardiovascular training stimulus via increased treadmill speed (Grabowski and Kram, 2008).
Previous research among non-elite runners has shown oxygen consumption to decrease as BWS is increased using a LBPP treadmill, (Figueroa et al., 2012; Grabowski, 2010; Grabowski and Kram, 2008; Hoffman and Donaghe, 2011; Kline et al., 2015; Raffalt et al., 2013; Ruckstuhl et al., 2010). Furthermore, the percentage reduction in oxygen consumption appears in close proportion to the amount of BWS provided at relatively less supportive conditions, but increasingly less than proportional to the percentage of BWS provided at the more supportive conditions (Grabowski and Kram, 2008; Kline et al., 2015). For example, Grabowski and Kram (2008) reported that with the application of approximately 25%, 50% and 75% BWS, the gross reduction in metabolic power was approximately 25%, 36% and 45%, respectively at a velocity of 3m/s, and 31%, 43%, and 53% at a velocity of 4m/s. Studies have also demonstrated deviation in the actual amount of BWS provided by a LBPP treadmill device when compared to the machinecalibrated levels of support. One paper demonstrated the device to be over-supportive (Hoffman and Donaghe, 2011), while others found the device to be undersupportive, except when the device was inflated and the level of BWS was set to 0% (Grabowski, 2010; Grabowski and Kram, 2008; McNeill et al., 2015). Such deviations may impact interpretation of the relationship between metabolic cost and BWS.
Despite reductions in metabolic cost, it has also been shown that equivalent maximal and sub-maximal oxygen consumption rates (V[O.sub.2]) can be achieved while running on LBPP treadmills by increasing treadmill velocity to offset the reduction in oxygen consumption associated with running with BWS (Gojanovic et al., 2012; Kline et al., 2015; Raffalt et al., 2013). Studies have also demonstrated linear increases in V[O.sub.2] with increases in velocity across a range of BWS conditions, with the slope of the velocity vs V[O.sub.2] relationship tending to decrease with increasing BWS (Grabowski and Kram, 2008; Hoffman and Donaghe, 2011; Raffalt et al., 2013). Hoffman and Donaghe (2011) contend that the smaller slope is a product of the effect of speeding up on metabolic demand with increasing BWS.
While these studies provide valuable insight into the metabolic demands of using the LBPP treadmill among recreational athletes, it is not well documented how these findings might apply to the effect of BWS among highly trained runners at the running speeds that they use, which are considerably faster than those of recreational runners. Professional athletes pioneered the machine and have recently dominated popular media exposure of the technology, with reports elite athletes use LBPP treadmills for both rehabilitation and training purposes. For example, the first group of professional athletes to use the LBPP treadmill were the long distance runners of the Nike Oregon Project, who used a prototype treadmill in 2005 (www..AlterG[R].com). Despite the focus on elite athletes in development and use, current research presents data on the effects of BWS across only a relatively slow and narrow range of velocities that are not applicable to the range of training paces of highly-trained, elite distance runners.
The purpose of the present study was to add data on elite runners to the growing body of literature on LBPP treadmills. Specifically, the goal was to determine the relationship between velocity and metabolic cost while running on an LBPP treadmill, and to examine how the application of BWS affected this relationship. Additionally, due to the highly trained and elite nature of the runners recruited and their ability to comfortably run at relatively fast velocities sub-maximally, we were better able to evaluate the relationship between unloading and metabolic cost at velocities previously unattainable by research subjects without generating significant proportions of energy from non-oxidative pathways. Consistent with the existing LBPP literature, it was hypothesized that 1) as BWS support increased, the metabolic cost associated with running would decrease; 2) this decrease in metabolic cost would be proportionately less than the percentage of BWS provided at greater levels of BWS (i.e. 40% support would lead to less than 40% reduction in V[O.sub.2]); and 3) the slope of the relationship between BWS and oxygen consumption across velocity would be less steep with greater BWS (indicating that increasing velocity is relatively easier when running with more BWS).
Six elite male long distance runners (mean age 26.4, SD=4.0 years, mean weight 64.2, SD=4.3 kg) were recruited from the local community of professional and collegiate runners in Flagstaff, Arizona to participate in the study. Inclusion criteria were to have a 5km personal record of less than 14 minutes, a 10km personal record of less than 29 minutes or a half marathon personal record of less than 64 minutes, achieved in the preceding 12 months. All subjects regularly ran on standard running treadmills, and were thus well accommodated to treadmill running (Morgan et al., 1991; 1994; Williams et al., 1991). All participants had also either regularly incorporated LBPP treadmill running into their weekly training using an LBPP treadmill, or had spent at least one hour running on the LBPP treadmill utilized in this study before commencing the study. Previous work by this lab (McNeill et al., 2014) had found that stable V[O.sub.2] measurements were achieved after approximately 60 minutes of accommodation to running on the LBPP treadmill; therefore these participants were considered accommodated to LBPP treadmill running. Approval for the protocol was given by the Institutional Review Board of Northern Arizona University, and prior to testing, each participant signed an informed consent.
Protocol and design
The protocol involved two testing days, separated by approximately one week, and not scheduled within 2 days after a hard workout. Testing was done in the morning, and participants consumed the same...