Body weight independently affects articular cartilage catabolism.

Although obesity is associated with osteoarthritis, it is unclear whether body weight (BW) independently affects articular cartilage catabolism (i.e., independent from physiological factors that also accompany obesity). The primary purpose of this study was to evaluate the independent effect of BW on articular cartilage catabolism associated with walking. A secondary purpose was to determine how decreased BW influenced cardiovascular response due to walking. Twelve able-bodied subjects walked for 30 minutes on a lower-body positive pressure treadmill during three sessions: control (unadjusted BW), +40%BW, and -40%BW. Serum cartilage oligomeric matrix protein (COMP) was measured immediately before (baseline) and after, and 15 and 30 minutes after the walk. Heart rate (HR) and rate of perceived exertion (RPE) were measured every three minutes during the walk. Relative to baseline, average serum COMP concentration was 13% and 5% greater immediately after and 15 minutes after the walk. Immediately after the walk, serum COMP concentration was 14% greater for the +40%BW session than for the -40%BW session. HR and RPE were greater for the +40%BW session than for the other two sessions, but did not differ between the control and -40%BW sessions. BW independently influences acute articular cartilage catabolism and cardiovascular response due to walking: as BW increases, so does acute articular cartilage catabolism and cardiovascular response. These results indicate that lower-body positive pressure walking may benefit certain individuals by reducing acute articular cartilage catabolism, due to walking, while maintaining cardiovascular response. Key points Walking for 30 minutes with adjustments in body weight (normal body weight, +40% and -40% body weight) significantly influences articular cartilage catabolism, measured via serum COMP concentration. Compared to baseline levels, walking with +40% body weight and normal body weight both elicited significant increases in articular cartilage catabolism, while walking with -40% body weight did not. Cardiovascular response (HR and RPE) was not significantly different during walking with normal body weight and when compared to walking with -40% body weight. Key words: Positive pressure treadmill, obesity, osteoarthritis, body mass, cartilage oligomeric matrix protein Introduction 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.