Thirty six percent of adult Americans are obese, and 69% are overweight (Flegal et al., 2012). Obesity is associated with knee osteoarthritis (OA) onset and progression (Felson et al., 2000; Griffin and Guilak, 2005; Hart and Spector, 1993). For each kilogram increase in body mass, knee OA genesis risk increases by 14% (Cicuttini et al., 1996), and a 5-kg body mass gain increases knee OA genesis likelihood by 35% (Hart and Spector, 1993). Relative to healthy-weighted peers, obese individuals experience greater vertical ground reaction forces (GRF) and altered GRF characteristics (Nimbarte and Li, 2011), which likely influences knee load and accelerates articular cartilage degradation (Creaby et al., 2013; Griffin and Guilak, 2005; Herzog et al., 2003; Whittle, 1999). Although obesity promotes knee OA due to mechanical and physiological factors (Felson et al., 2000), the role of each factor, independent from the other, is unclear. It is unclear how increased body mass, independent from physiological factors associated with obesity, influences articular cartilage catabolism. This knowledge is important to those who hope to solve the problem of knee OA, especially related to obesity.
Ambulation on a lower-body positive pressure treadmill is a relatively novel exercise mode that involves pressure differences between the lower- and upper-body (Hunter et al., 2014). The upward-directed pressure that is applied to the user reduces ambulatory GRF (Raffalt et al., 2013) and corresponding knee load (Patil et al., 2013). For this reason, these treadmills are being used in rehabilitative settings (Eastlack et al., 2005, Webber et al., 2014) by individuals with muscle and/or joint dysfunction (Kurz et al., 2011; Rose et al., 2013; Takacs et al., 2013). Although it has been established that low-impact aerobic exercise can help individuals decrease body mass and corresponding knee load (Hunter and Eckstein, 2009; Messier et al., 2005), low-impact exercise may elicit decreased cardiovascular response (Denning et al., 2010, Grabowski, 2010, Webber et al., 2014). Other researchers, however, have indicted no difference in cardiovascular measures during unloaded ambulation (Ruckstuhl et al., 2010; Thomas et al., 2007). The simultaneous effect of lower-body positive pressure on articular cartilage catabolism and cardiovascular response associated with walking has not yet been studied. It is important to determine whether walking with lower-body positive pressure can decrease articular cartilage catabolism, while simultaneously maintaining the cardiovascular benefit that is traditionally associated with walking.
Cartilage oligomeric matrix protein (COMP) is a noncollagenous extracellular matrix protein (Hedbom et al., 1992) that interacts with collagen and other matrix components to increase the structural integrity (Halasz et al., 2007) and load bearing ability of articular cartilage (Tseng et al., 2009). In able-bodied individuals, serum COMP concentration acutely increases after walking and running (Kersting et al., 2005; Kim et al., 2009; Mundermann et al., 2005; Niehoff et al., 2010). This increase is thought to represent articular cartilage catabolism due to exercise-induced load (Erhart-Hledik et al., 2012, Niehoff et al., 2011). Also, increased resting serum COMP concentration indicates chronic articular cartilage degradation (Neidhart et al., 1997; Verma and Dalal, 2013), and is associated with OA onset and progression (Chaganti et al., 2008, Sharif et al., 1995). Resting serum COMP concentration decreases after massive weight loss (Richette et al., 2011).
The primary purpose of this study was to evaluate the acute and independent effect of body weight (BW) on articular cartilage catabolism associated with walking. A secondary purpose was to evaluate how walking on a lower-body positive pressure treadmill simultaneously affects articular cartilage catabolism and cardiovascular response. We hypothesized that walking with increased BW would acutely and independently increase articular cartilage catabolism. We also hypothesized that walking with lower-body positive pressure, in order to potentially reduce articular cartilage catabolism, would reduce cardiovascular response due to walking.
In this cross-over designed study, a convenience sample of 12 able-bodied volunteers was used (Table 1). This sample size was chosen because previous researchers have found statistically significant differences using methods that were quite similar to the present methods, including the same dependent variables and similar sample sizes (Denning et al., 2014; Mundermann et al., 2005; Neidhart et al., 2000; Niehoff et al., 2011). Subjects were required to have no history of (1) lower-extremity injury six months prior to data collection, or (2) knee-related surgery in their lifetime. Subjects refrained from moderate to intense physical activity throughout participation in this study. Before participating, subjects read and signed an informed consent form that was approved by the Brigham Young University Institutional Review Board (IRB #X120398; Approved 1-2-2-13). All study procedures complied with the Helsinki Declaration.
Subjects completed three data collection sessions in a counterbalanced order: control (unadjusted BW), +40%BW (40% greater than unadjusted BW), and -40% BW (40% less than unadjusted BW). The sessions were completed 48 hours apart at the same time of day. For each session, subjects walked at the same self-selected speed for 30 minutes on a lower-body positive pressure treadmill (AlterG, Fremont, CA, USA). The self-selected walking speed was determined on the first day of data collection, 45 minutes before the start of the session: treadmill speed was increased, or decreased, until the subject described the speed to be the same speed that they would use to walk across a parking lot. For each session, subjects wore neoprene shorts that zipped into a lower-body positive pressure chamber that enclosed the subject from the waist down (Figure 1). For the control session, subjects walked with no BW manipulation. For the +40%BW session, subjects wore a weighted exercise vest (ZFO Sports, San Jose, CA, USA) that weighed 40% of their BW. For the -40%BW session, subjects walked while being unloaded 40% of BW, via the lower-body positive pressure chamber. For all sessions, subjects wore a HR monitor directly below the xiphoid process. HR and rate of perceived exertion (RPE; Borg 6-20 scale) were measured every three minutes, throughout each 30-minute walk, for each session, to quantify...