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Greater vertical loading rate in obese compared to normal weight young adults
Clinical Biomechanics 33 (2016) 61–65
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Clinical Biomechanics journal homepage: www.elsevier.com/locate/clinbiomech
Greater vertical loading rate in obese compared to normal weight young adults Derek N. Pamukoff a,b,c,⁎, Michael D. Lewek b,c,e, J. Troy Blackburn b,c,d a
Department of Kinesiology, California State University, Fullerton, 800 N State College, Fullerton, CA 92831, USA Department of Exercise and Sport Science, The University of North Carolina at Chapel Hill, 209 Fetzer Hall, Chapel Hill, NC 27599-8700, USA Curriculum in Human Movement Science, The University of North Carolina at Chapel Hill, Bondurant Hall, Suite 3000, Chapel Hill, NC 27599-7135, USA d Department of Orthopedics, The University of North Carolina at Chapel Hill, 209 Fetzer Hall, Chapel Hill, NC 27599-8700, USA e Division of Physical Therapy, The University of North Carolina at Chapel Hill, Bondurant Hall, Suite 3000, Chapel Hill, NC 27599-7135, USA b c
a r t i c l e
i n f o
Article history: Received 26 September 2015 Accepted 15 February 2016 Keywords: Gait Obesity Osteoarthritis Biomechanics Walking
a b s t r a c t Background: Obesity is a risk factor for knee osteoarthritis. Altered gait biomechanics are common in obese individuals, and may contribute to the development of knee osteoarthritis. Research has focused on older obese adults with knee osteoarthritis, and it is unclear if young obese individuals display similar aberrant biomechanics. The purpose of this study was to compare gait biomechanics between normal-weight and obese young adults. Methods: 15 normal-weight (body mass index = 21.5 (1.1)) and 15 obese (body mass index = 33.5 (3.7)) young adults were recruited and categorized by body mass index. Lower extremity kinematics and kinetics were collected while participants walked at standardized (1 m/s) and self-selected speeds. Analysis of variance (group by condition) was used to compare peak vertical ground reaction force, vertical loading rate, peak internal knee extension moment, peak internal knee abduction moment, peak knee flexion angle, and knee flexion excursion between groups. Findings: Gait biomechanics did not differ between groups during walking at a self-selected speed. When walking at a standardized speed, obese subjects displayed greater instantaneous vertical loading rates (46.2 vs. 35.0 N/s, P b 0.001), and lesser knee flexion excursion (5.5° vs. 7.7°, P = 0.04). Instantaneous vertical loading rate was greater during walking at a self-selected speed compared to a standardized speed in the obese (P = 0.007) and normal weight groups (P = 0.001). Interpretation: As greater loading rates are related to cartilage degeneration, these results suggest that obesity may contribute to knee osteoarthritis. Prospective studies are needed to identify the influence of higher loading rates on knee osteoarthritis. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Approximately one third of adults in the United States are considered obese (body mass index N 30.0) (Ogden et al., 2006). Moreover, the percentage of overweight children and young adults is nearly 40% (Ogden et al., 2006), and being overweight or obese at a young age elevates the risk for early onset of co-morbidities such as diabetes (Bloomgarden, 2004), cancer (Pi-Sunyer, 1993), and heart disease (Pi-Sunyer, 1993). Additionally, obesity in those aged 50 and older increases the risk of developing knee osteoarthritis (KOA) (Silverwood et al., 2014). OA is common among older adults, with approximately 12% of older adults suffering from KOA (Lawrence et al., 2008). Greater body weight places additional stress on the articular cartilage within the ⁎ Corresponding author at: Department of Kinesiology, California State University, Fullerton, Fullerton, CA 92831, USA. E-mail addresses: [email protected] (D.N. Pamukoff), [email protected] (M.D. Lewek), [email protected] (J.T. Blackburn).
http://dx.doi.org/10.1016/j.clinbiomech.2016.02.007 0268-0033/© 2016 Elsevier Ltd. All rights reserved.
knee joint and increases the rate of cartilage breakdown (Lementowski and Zelicof, 2008). Importantly, being obese from a young age is likely to reduce the age of onset for co-morbidities associated with obesity, such as OA. Therefore, it is necessary to understand the influence of obesity on risk factors for OA among young adults. The pathogenesis of OA is multifactorial including mechanical and metabolic factors (Allen and Golightly, 2015; Kluzek et al., 2015) resulting in inflammation and gradual breakdown of articular cartilage. Mechanically, body mass index is associated with greater tibiofemoral compression and shear force during gait, regardless if OA is present or not (Harding et al., 2016). Greater forces observed at the knee joint during gait could be due to alterations in knee joint kinematics and kinetics. For instance, older adults with KOA commonly have lesser knee flexion excursion (Kaufman et al., 2001; Zeni and Higginson, 2009), lesser internal knee extensor moments (Harding et al., 2012; Kaufman et al., 2001), greater internal knee abduction moments (Astephen and Deluzio, 2005; Harding et al., 2012; Segal et al., 2009), greater vertical ground reaction forces (Messier et al., 1996), and greater
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vertical loading rates compared to control individuals (Messier et al., 1996). Obese individuals without KOA may display similar aberrant gait biomechanics (Allen and Golightly, 2015; Harding et al., 2012). However, the majority of research in this area has focused on older adults or individuals who are already experiencing symptoms of KOA, and some evidence (Harding et al., 2012) indicates that body mass index has differential effects on knee biomechanics depending on the presence of KOA. Additionally, obesity is associated with a loss of the positive relationship between cartilage thickness and ambulatory joint loads in individuals without KOA (Blazek et al., 2014). Unfortunately, there is limited evidence available on gait biomechanics in obese young adults who are otherwise healthy. Therefore, it is unclear if obese young adults have deviations in gait biomechanics that may elevate their risk for future knee joint disease. Overall, evidence is conflicting regarding gait biomechanics of obese young adults without KOA. Previous research (Browning and Kram, 2007) has found greater absolute sagittal plane knee moments and ground reaction forces in obese young adults compared to normalweight adults, but did not normalize their data to subject anthropometrics. As such, the relative influence of body weight and obesity on gait biomechanics is unclear. Alternatively, other studies have reported no differences in knee biomechanics (normalized joint moments and knee kinematics) during gait between obese, overweight, and normalweight young adults (DeVita and Hortobagyi, 2003; Freedman Silvernail et al., 2013). The discrepancy in findings could be due to differing protocols in gait analyses such as the use of an instrumented treadmill (Browning and Kram, 2007) versus overground gait (DeVita and Hortobagyi, 2003; Freedman Silvernail et al., 2013). Furthermore, the aforementioned studies did not report additional characteristics of the ground reaction force such as the vertical loading rate, which is increased among obese patients with KOA (Messier et al., 1996). Articular cartilage is viscoelastic, thus it is sensitive to the rate at which it is loaded. When articular cartilage is loaded at a higher rate, it stiffens, thus elevating its risk for failure and breakdown (Ewers et al., 2002). Animal models demonstrate that repetitive high rate loading of the limbs results in rapid degeneration of the articular cartilage, regardless of the magnitude of the load (Radin et al., 1984). Therefore, it is reasonable that the rate at which the ground reaction force is applied may be a more sensitive measure to differentiate lower extremity loading characteristics in obese and normal-weight young adults, but to our knowledge, studies to date have only reported vertical loading rate among obese patients who also have KOA (Wearing et al., 2006). The purpose of this study was to compare gait biomechanics between normal-weight and obese young adults. We hypothesized that obese individuals would display gait biomechanics that may exacerbate their risk for joint disease including a greater vertical ground reaction force, greater vertical loading rate, lesser knee flexion excursion, lesser internal knee extension moment, and greater internal knee abduction moment. 2. Methods 2.1. Subjects Thirty (15 obese and 15 normal-weight) young adults participated in this study (Table 1) and were recruited from the University and Table 1 Participant demographics (mean and 95% confidence interval). Group
Normal
Obese
Age (years) Height (cm) Mass (kg)⁎ Body mass index (kg/m2)⁎ Preferred walking speed (m s−1)⁎
20.4 (19.7, 21.1) 171.7 (166.9, 175.3) 63.3 (59.9, 66.7) 21.6 (21.0, 22.1) 1.34 (1.27, 1.39)
21.2 (20.6, 21.8) 170.8 (165.1, 176.5) 97.7 (87.0, 107.9) 33.5 (30.2, 36.7) 1.09 (1.07, 1.17)
⁎ Indicates P b 0.05.
surrounding community. Subjects were between the ages of 18– 35 years, and subjects in the normal-weight group had a body mass index between 18.5 and 24.9, and subjects in the obese group had a body mass index greater than 30.0 (Calle et al., 1999). Subjects were excluded for any lower extremity injury within the 6 months prior to participation, any lower extremity surgery, a body mass index between 25.0 and 29.9, participation in resistance training or intercollegiate sport, or diagnosis of lower extremity osteoarthritis or knee pain. Prior to the start of the study, all subjects provided written informed consent, and all procedures were approved by the University's Institutional Review Board. 2.2. Gait biomechanics assessment Subjects completed 10 walking trials at a self-selected pace and 10 trials at a standardized pace of 1 m/s in a randomized order on a 10 m walkway. At least 5 practice trials were performed to ensure subjects could strike the force plate without “aiming”, and they were instructed to look straight ahead at a point on the wall during each trial. Gait speed was monitored using infrared timing gates centered over the force plate, and trials were repeated if gait speed fell outside 1 ± 0.05 m/s for the standardized speed and ±5% for the self-selected speed. Trials were considered acceptable if the subject made contact with the entire foot on the force plate without any noticeable gait deviations. Electromagnetic tracking sensors (Motion Star, Ascension Corp., Burlington, VT, USA) were positioned on the pelvis, thigh, shank, and foot of the dominant limb (Coren and Duncan, 1979) using double sided tape. The hip joint center was estimated using the Bell method (Bell et al., 1990) following digitization of the left and right anterior superior iliac spines, and the ankle and knee joint centers were estimated as the midpoints between the digitized medial and lateral malleoli and femoral epicondyles, respectively. The Motion Monitor motion capture system (Innovative Sports Training, Chicago, IL, USA) was used for model generation and data acquisition, and all kinematic data were sampled at 100 Hz. Kinetic data were sampled from a nonconductive force plate (model 4060-NC, Bertec Corp., Columbus, OH, USA) at 1000 Hz and time-synchronized with the kinematic data. 2.3. Data reduction All data were analyzed using a custom LabVIEW program (National Instruments, Austin, TX, USA). Kinematic data were lowpass filtered at 10 Hz and extracted from the gait cycle for which kinetic data was obtained. Knee joint angles were calculated as motion of the shank segment relative to the thigh segment using Grood and Suntay angles (Grood and Suntay, 1983). All kinematic and kinetic variables were calculated during the first 50% of the stance phase (i.e. weight acceptance and mid stance), defined as the interval from ground contact (vertical ground reaction force N 20 N) to toe off (vertical ground reaction force b 20 N). Peak knee flexion angle was identified, and knee flexion excursion was calculated as the peak angle minus the knee flexion angle at ground contact. Ground reaction force data were lowpass filtered at 75 Hz. The peak vertical ground reaction force was identified, and the average vertical loading rate was calculated as the peak vertical ground reaction force divided by the time to peak vertical ground reaction force (Hunt et al., 2010; Liikavainio et al., 2007; Pietrosimone et al., 2015). The instantaneous vertical loading rate was calculated as the first time derivative of the vertical ground reaction force, and the peak value was used for analysis (Liikavainio et al., 2010). Vertical ground reaction force and loading rates were normalized to body weight. Ground reaction forces were combined with kinematics via standard inverse dynamic procedures to yield net internal moments (Gagnon and Gagnon, 1992), and peak internal knee extension and abduction moments (i.e. the internal responses to external flexion and adduction joint loading, respectively) were identified. Joint moments were normalized to the product of height (meters) and weight (Newtons). The mean of 10 trials
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was used for analysis for both the standardized and self-selected speeds, respectively. 2.4. Statistical analysis Minimum sample size was estimated for an alpha level of 0.05 and a beta of 0.20 based on differences in gait biomechanics between normalweight and obese adults reported elsewhere (Segal et al., 2009). A minimum of 14 subjects (7 per group) was indicated, thus 30 subjects (15 per group) were recruited to provide a priori power to detect meaningful differences between groups. Additional subjects were recruited because this sample size calculation was based on expected differences in the frontal plane knee moment (Segal et al., 2009), and we also studied additional biomechanical variables. Data were screened for outliers using boxplots, inspected for normality using the Shapiro– Wilk test, and homogeneity of variance using Levene's test to confirm assumptions for inferential statistics. 2 (group: obese, normal weight) by 2 (condition: self selected speed, standardized speed) analysis of variance was used to evaluate all dependent variables (peak knee flexion angle, knee flexion excursion, peak internal knee extension moment, peak internal knee abduction moment, peak vertical ground reaction force, and the linear and instantaneous vertical ground reaction force loading rates). Post hoc comparisons were assessed using a Bonferroni adjustment (α = 0.013). Pearson correlations were calculated between kinematic (knee flexion angle and excursion) and kinetic (ground reaction force and vertical loading rate) variables. All data were analyzed using SPSS version 19.0. 3. Results Descriptive statistics of demographic information are presented in Table 1. All data were found to be normal and were treated as such. As expected, the obese group had a higher body mass index (Table 1, P b 0.001) and body mass (Table 1, P b 0.001). Obese subjects also utilized a slower self-selected walking speed (Table 1, P = 0.004). The group by condition interaction was significant for the instantaneous vertical loading rate (Table 2, F1,28 = 4.43, P = 0.03) and knee flexion excursion (Table 2, F1,28 = 3.57, P = 0.05). Post hoc analyses suggest that when walking at a standardized speed, obese subjects displayed greater instantaneous (Table 2, P b 0.001) vertical ground reaction force loading rates and lesser knee flexion excursion (Table 2, P = 0.04). The instantaneous vertical loading rate was also greater during the self-selected speed compared to the standardized speed in both the obese (P = 0.007) and normal weight groups (P = 0.001). Furthermore, weak to moderate correlations were found between knee flexion excursion and the average loading rate (r = −0.32, P = 0.04), and knee flexion excursion and the instantaneous loading rate (r = −0.28, P = 0.05) when walking at standardized speeds. No differences were observed at either speed in peak internal knee extension or abduction moments or vertical ground reaction force.
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4. Discussion The purpose of this study was to evaluate differences in gait biomechanics between normal-weight and obese young adults. We hypothesized that obese individuals would exhibit characteristics indicative of greater risk for KOA (e.g. lesser internal knee extension moment and knee flexion angle, and greater vertical ground reaction force, loading rate, and internal knee abduction moment) compared to normalweight individuals. Obese subjects displayed a greater instantaneous vertical loading rate and lesser knee flexion excursion compared to normal-weight subjects when walking at a standardized speed. Contrary to our hypotheses, we observed no differences in internal knee extension or abduction moments, or in the magnitude of the vertical ground reaction force. Greater instantaneous loading rates among obese individuals compared to normal weight individuals agreed with our hypotheses. Obese individuals have greater mass attributed to adipose tissue compared to skeletal muscle in the lower extremity. Additional fat mass likely contributes to a reduction of muscle strength relative to body weight, and induces fatigue of the lower extremity muscles such as the quadriceps (Syed and Davis, 2000). Insufficient lower extremity strength relative to body weight increases the loading rate and impairs shock attenuation (Syed and Davis, 2000). Habitual loading during walking is necessary for the maintenance of articular cartilage health (Seedhom, 2006; Yao and Seedhom, 1993). However, a higher loading rate during gait may place one at greater risk for the development of KOA (Ewers et al., 2002; Radin et al., 1984). As such, higher instantaneous loading rates among our obese group may explain the discrepancy in osteoarthritis prevalence between obese and normal-weight adults (Allen and Golightly, 2015; Lementowski and Zelicof, 2008). Articular cartilage is viscoelastic and is sensitive to the rate at which it is loaded. Higher loading rates contribute to stiffening of the articular cartilage, and increase the risk for failure and breakdown (Ewers et al., 2002). Furthermore, previous research in animals indicates that repetitive loading of the limbs at higher rates results in quick degeneration of articular cartilage, regardless of the overall magnitude (Ewers et al., 2002; Radin et al., 1984). As such, greater instantaneous loading rates rather than peak ground reaction force in obese individuals may be particularly relevant to articular cartilage health in obese individuals. We did not observe a difference in average loading rate as observed in another study involving obese individuals with KOA (Messier et al., 1996). However, our calculation of average loading rate used the peak ground reaction force in the numerator, which did not differ between groups and may explain this finding. Interestingly, we only observed a difference in the instantaneous vertical loading rates between obese and normal weight subjects at a standardized speed. Previous research (Browning and Kram, 2007) suggests that obese adults walking at 1.1 m/s experience similar lower extremity loading as normal weight adults walking at 1.4 m/s (a biomechanical equivalent), and self-selected speeds in this investigation were
Table 2 Gait biomechanics (mean and 95% confidence interval). Self-selected speed
Knee flexion excursion (°)⁎⁎ Peak knee flexion angle (°) Internal knee extension moment (bw × ht) Internal knee abduction moment (bw × ht) Vertical ground reaction force (bw) Average vGRF LR (bw/s) Instantaneous vGRF LR (bw/s)⁎⁎
Standardized speed
Normal
Obese
Normal
Obese
8.8 (8.9, 11.5) 14.4 (10.7, 15.2) 0.028 (0.013, 0.048) 0.036 (0.013, 0.021) 1.14 (1.09, 1.18) 6.33 (6.01, 6.84) 52.9 (46.7, 61.5)
7.6 (5.4, 9.8) 13.0 (11.1, 17.8) 0.018 (0.012, 0.024) 0.037 (0.011, 0.025) 1.09 (1.06, 1.13) 6.33 (6.03, 6.94) 58.3 (49.9, 60.7)
7.7 (6.2, 9.1)⁎ 9.8 (6.9, 12.7) 0.017 (0.010, 0.021) 0.016 (0.011, 0.019) 1.12 (1.06, 1.18) 6.30 (6.09, 6.45) 35.0 (31.3, 38.7)⁎,Ŧ
5.5 (4.1, 6.9) 9.4 (7.5, 11.4) 0.014 (0.010, 0.028) 0.016 (0.008, 0.023) 1.05 (1.01, 1.12) 6.32 (6.01, 7.34) 46.2 (41.89, 50.45)Ŧ
vGRF: vertical ground reaction force, LR: loading rate. ⁎⁎ Indicates significant group by condition interaction (P b 0.05). ⁎ Indicates different from obese group (P b 0.013). Ŧ Indicates different from self-selected speed (P b 0.013).
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similar (obese = 1.09 m/s; normal weight = 1.34 m/s). As such, since our obese individuals walked at a slower self-selected pace, it is likely that differences in loading characteristics were masked at self-selected speeds. Additionally, walking at slower speeds may be a compensatory mechanism to limit lower extremity loading in obese adults. We did consider that group differences at standardized speeds could have been from alterations in the normal weight group rather than in the obese group. However, the instantaneous loading rate was different between conditions in both groups, yet still greater in the obese group compared to the normal weight group during the standardized speed condition. We also observed lesser knee flexion excursion in the obese subjects compared to normal-weight subjects, which was in agreement with our hypotheses and with previous studies (Kaufman et al., 2000; Zeni and Higginson, 2009). Lesser knee flexion excursion may also explain why obese subjects had greater loading rates. This was evidenced by weak to moderate correlations between knee flexion excursion and the average loading rate, and knee flexion excursion and the instantaneous loading rate at standardized speeds. Knee flexion during the weight acceptance phase of stance is requisite for proper absorption of ground reaction forces, and is indicative of quadriceps function during gait. Lesser knee flexion reduces the time interval over which ground reaction forces are absorbed, and therefore increases the rate at which these forces are applied throughout the lower extremity. However, loading rate is a global rather than joint-specific indicator of loading, thus greater loading rates could also be explained by biomechanical alterations at other joints such as the hip or ankle. Interestingly, the difference in loading rate was only evident at an enforced speed. It could be that at self-selected speeds, obese individuals walk slower to increase the time interval over which force is applied and lower the rate of loading. The slower self-selected speed may be a compensatory action to protect articular cartilage from greater loading rates. Moreover, previous research indicates that BMI is correlated with loading rate only after adjusting for differences in walking velocity (Messier et al., 1996). Importantly, knee flexion can be improved through verbal cues during dynamic tasks such as landing (Mizner et al., 2008), and future studies are needed to examine the influence of modifying knee flexion on vertical loading rate. Interestingly, we did not observe a difference in the internal knee extension moment between groups, which was contrary to our hypotheses. However, these findings are in agreement with previous research (Freedman Silvernail et al., 2013), and subjects in this study were considered healthy aside from their body mass index. Furthermore, body mass index is often a poor discriminator of health, as it fails to account for differences in body composition. Secondly, hamstring cocontraction may have contributed to similar knee joint moments in the obese group in this study. The use of joint moments to make assumptions about forces acting on the knee joint ignores the contribution of muscle co-contraction, and the observed moment is the net effect of all muscle activity at a joint. Greater co-contraction results in greater intersegmental forces, even if the net moments calculated from inverse dynamic procedures are the same. As such, we suspect that greater co-contraction contributed to similar knee joint moments between obese and normal weight individuals in this study. Patients with knee OA utilize greater quadriceps–hamstring co-contraction compared to unimpaired individuals during walking (Bouchouras et al., 2015; Childs et al., 2004; Heiden et al., 2009; Zeni et al., 2010), and quadriceps–hamstring co-contraction contributes to a stiffened knee joint and greater loading rates (Schmitt and Rudolph, 2007). Importantly, weight loss reduces hamstring co-contraction in patients with KOA (Messier et al., 2011), and future research is needed to examine the influence of hamstring co-contraction on gait in healthy obese individuals. Secondly, the lack of significant differences between obese and normal weight adults could be related to the normalization technique adopted in this study. For instance, Browning and Kram (2007) reported absolute moments without normalization criteria,
and found greater sagittal plane knee moments in obese compared to normal weight adults. While we normalized joint moments to the product of height and weight, this procedure does not account for differences in fat free mass between obese and normal weight adults. Normalizing joint moments to the amount of fat free mass may be more indicative of joint moments due to muscle activity (Freedman Silvernail et al., 2013), and future studies should consider normalization technique when interpreting joint moments in obese individuals. Finally, the reduction in internal knee extension moments in patients with knee pathologies may also be attributed to arthrogenic muscle inhibition (AMI) of the quadriceps (Pietrosimone et al., 2014). AMI refers to a form of neural inhibition that limits the ability to fully activate the quadriceps muscles following knee injury (ACL reconstruction, patellofemoral pain syndrome etc.) or disease (KOA) (Palmieri-Smith and Thomas, 2009; Pietrosimone et al., 2011). AMI contributes to quadriceps dysfunction and may contribute to the development of KOA (Palmieri-Smith and Thomas, 2009). However, obese individuals without knee pathology may not have AMI, which may explain why we did not observe a difference in the internal knee extension moment between the normal-weight and obese groups. Finally, we did not observe a difference in the internal knee abduction moment between obese and normal-weight adults. This characteristic is an indicator of KOA risk and progression (Sharma et al., 1998), particularly in obese individuals (Sharma et al., 2000). However, our findings are similar to the results of another study in obese young adults (Freedman Silvernail et al., 2013). Furthermore, the frontal plane knee moment is largely influenced by knee alignment, and we did not specifically recruit individuals with misalignment. A previous study has suggested that obesity is only a risk factor for KOA progression in those with knee varus by increasing the external knee adduction moment (Sharma et al., 2000). As we did not assess knee varus alignment in this study, it is unclear if our findings are due to the absence of varus misalignment in this sample. There are limitations of this study that must be addressed when interpreting the results. Firstly, the cross sectional nature of the study precludes our ability to draw conclusions regarding KOA development in young obese subjects. While we observed differences in instantaneous loading rate and knee flexion excursion, these individuals were otherwise healthy. Prospective studies are needed to ascertain the effect of higher vertical loading rates and aberrant gait biomechanics on future osteoarthritis risk. Secondly, we classified our groups based on body mass index, which does not account for differences in body composition. Individuals classified as obese by body mass index due to greater muscle mass may be able to compensate for additional joint loading and attenuate ground reaction forces more effectively. Future studies should consider studying obese individuals classified using another method, such as body fat percentage. Thirdly, there may have been some movement of the electromagnetic sensors relative to bone segments, and motion artifact may have been greater in the obese group compared to the control group. However, all sensors were affixed with double-sided tape, and secured using pre-wrap and athletic tape. Future studies should consider the use of an optimization method for sensor placement to reduce the influence of any soft tissue artifact. Lastly, it is important to note that while our findings regarding loading rates were statistically significant, the absolute differences observed were fairly small and thus we interpret these results with caution. Additionally, this study focused on biomechanical contributors to knee joint health, and there are systemic and inflammatory associated with obesity that may contribute to KOA (Kluzek et al., 2015). 5. Conclusion In conclusion, our findings suggest that obese subjects had greater instantaneous loading rates when walking at standardized speeds, and lesser knee flexion excursion compared to normal-weight subjects at standardized and self-selected speeds. Future prospective studies are
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Clinical Biomechanics journal homepage: www.elsevier.com/locate/clinbiomech
Greater vertical loading rate in obese compared to normal weight young adults Derek N. Pamukoff a,b,c,⁎, Michael D. Lewek b,c,e, J. Troy Blackburn b,c,d a
Department of Kinesiology, California State University, Fullerton, 800 N State College, Fullerton, CA 92831, USA Department of Exercise and Sport Science, The University of North Carolina at Chapel Hill, 209 Fetzer Hall, Chapel Hill, NC 27599-8700, USA Curriculum in Human Movement Science, The University of North Carolina at Chapel Hill, Bondurant Hall, Suite 3000, Chapel Hill, NC 27599-7135, USA d Department of Orthopedics, The University of North Carolina at Chapel Hill, 209 Fetzer Hall, Chapel Hill, NC 27599-8700, USA e Division of Physical Therapy, The University of North Carolina at Chapel Hill, Bondurant Hall, Suite 3000, Chapel Hill, NC 27599-7135, USA b c
a r t i c l e
i n f o
Article history: Received 26 September 2015 Accepted 15 February 2016 Keywords: Gait Obesity Osteoarthritis Biomechanics Walking
a b s t r a c t Background: Obesity is a risk factor for knee osteoarthritis. Altered gait biomechanics are common in obese individuals, and may contribute to the development of knee osteoarthritis. Research has focused on older obese adults with knee osteoarthritis, and it is unclear if young obese individuals display similar aberrant biomechanics. The purpose of this study was to compare gait biomechanics between normal-weight and obese young adults. Methods: 15 normal-weight (body mass index = 21.5 (1.1)) and 15 obese (body mass index = 33.5 (3.7)) young adults were recruited and categorized by body mass index. Lower extremity kinematics and kinetics were collected while participants walked at standardized (1 m/s) and self-selected speeds. Analysis of variance (group by condition) was used to compare peak vertical ground reaction force, vertical loading rate, peak internal knee extension moment, peak internal knee abduction moment, peak knee flexion angle, and knee flexion excursion between groups. Findings: Gait biomechanics did not differ between groups during walking at a self-selected speed. When walking at a standardized speed, obese subjects displayed greater instantaneous vertical loading rates (46.2 vs. 35.0 N/s, P b 0.001), and lesser knee flexion excursion (5.5° vs. 7.7°, P = 0.04). Instantaneous vertical loading rate was greater during walking at a self-selected speed compared to a standardized speed in the obese (P = 0.007) and normal weight groups (P = 0.001). Interpretation: As greater loading rates are related to cartilage degeneration, these results suggest that obesity may contribute to knee osteoarthritis. Prospective studies are needed to identify the influence of higher loading rates on knee osteoarthritis. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Approximately one third of adults in the United States are considered obese (body mass index N 30.0) (Ogden et al., 2006). Moreover, the percentage of overweight children and young adults is nearly 40% (Ogden et al., 2006), and being overweight or obese at a young age elevates the risk for early onset of co-morbidities such as diabetes (Bloomgarden, 2004), cancer (Pi-Sunyer, 1993), and heart disease (Pi-Sunyer, 1993). Additionally, obesity in those aged 50 and older increases the risk of developing knee osteoarthritis (KOA) (Silverwood et al., 2014). OA is common among older adults, with approximately 12% of older adults suffering from KOA (Lawrence et al., 2008). Greater body weight places additional stress on the articular cartilage within the ⁎ Corresponding author at: Department of Kinesiology, California State University, Fullerton, Fullerton, CA 92831, USA. E-mail addresses: [email protected] (D.N. Pamukoff), [email protected] (M.D. Lewek), [email protected] (J.T. Blackburn).
http://dx.doi.org/10.1016/j.clinbiomech.2016.02.007 0268-0033/© 2016 Elsevier Ltd. All rights reserved.
knee joint and increases the rate of cartilage breakdown (Lementowski and Zelicof, 2008). Importantly, being obese from a young age is likely to reduce the age of onset for co-morbidities associated with obesity, such as OA. Therefore, it is necessary to understand the influence of obesity on risk factors for OA among young adults. The pathogenesis of OA is multifactorial including mechanical and metabolic factors (Allen and Golightly, 2015; Kluzek et al., 2015) resulting in inflammation and gradual breakdown of articular cartilage. Mechanically, body mass index is associated with greater tibiofemoral compression and shear force during gait, regardless if OA is present or not (Harding et al., 2016). Greater forces observed at the knee joint during gait could be due to alterations in knee joint kinematics and kinetics. For instance, older adults with KOA commonly have lesser knee flexion excursion (Kaufman et al., 2001; Zeni and Higginson, 2009), lesser internal knee extensor moments (Harding et al., 2012; Kaufman et al., 2001), greater internal knee abduction moments (Astephen and Deluzio, 2005; Harding et al., 2012; Segal et al., 2009), greater vertical ground reaction forces (Messier et al., 1996), and greater
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vertical loading rates compared to control individuals (Messier et al., 1996). Obese individuals without KOA may display similar aberrant gait biomechanics (Allen and Golightly, 2015; Harding et al., 2012). However, the majority of research in this area has focused on older adults or individuals who are already experiencing symptoms of KOA, and some evidence (Harding et al., 2012) indicates that body mass index has differential effects on knee biomechanics depending on the presence of KOA. Additionally, obesity is associated with a loss of the positive relationship between cartilage thickness and ambulatory joint loads in individuals without KOA (Blazek et al., 2014). Unfortunately, there is limited evidence available on gait biomechanics in obese young adults who are otherwise healthy. Therefore, it is unclear if obese young adults have deviations in gait biomechanics that may elevate their risk for future knee joint disease. Overall, evidence is conflicting regarding gait biomechanics of obese young adults without KOA. Previous research (Browning and Kram, 2007) has found greater absolute sagittal plane knee moments and ground reaction forces in obese young adults compared to normalweight adults, but did not normalize their data to subject anthropometrics. As such, the relative influence of body weight and obesity on gait biomechanics is unclear. Alternatively, other studies have reported no differences in knee biomechanics (normalized joint moments and knee kinematics) during gait between obese, overweight, and normalweight young adults (DeVita and Hortobagyi, 2003; Freedman Silvernail et al., 2013). The discrepancy in findings could be due to differing protocols in gait analyses such as the use of an instrumented treadmill (Browning and Kram, 2007) versus overground gait (DeVita and Hortobagyi, 2003; Freedman Silvernail et al., 2013). Furthermore, the aforementioned studies did not report additional characteristics of the ground reaction force such as the vertical loading rate, which is increased among obese patients with KOA (Messier et al., 1996). Articular cartilage is viscoelastic, thus it is sensitive to the rate at which it is loaded. When articular cartilage is loaded at a higher rate, it stiffens, thus elevating its risk for failure and breakdown (Ewers et al., 2002). Animal models demonstrate that repetitive high rate loading of the limbs results in rapid degeneration of the articular cartilage, regardless of the magnitude of the load (Radin et al., 1984). Therefore, it is reasonable that the rate at which the ground reaction force is applied may be a more sensitive measure to differentiate lower extremity loading characteristics in obese and normal-weight young adults, but to our knowledge, studies to date have only reported vertical loading rate among obese patients who also have KOA (Wearing et al., 2006). The purpose of this study was to compare gait biomechanics between normal-weight and obese young adults. We hypothesized that obese individuals would display gait biomechanics that may exacerbate their risk for joint disease including a greater vertical ground reaction force, greater vertical loading rate, lesser knee flexion excursion, lesser internal knee extension moment, and greater internal knee abduction moment. 2. Methods 2.1. Subjects Thirty (15 obese and 15 normal-weight) young adults participated in this study (Table 1) and were recruited from the University and Table 1 Participant demographics (mean and 95% confidence interval). Group
Normal
Obese
Age (years) Height (cm) Mass (kg)⁎ Body mass index (kg/m2)⁎ Preferred walking speed (m s−1)⁎
20.4 (19.7, 21.1) 171.7 (166.9, 175.3) 63.3 (59.9, 66.7) 21.6 (21.0, 22.1) 1.34 (1.27, 1.39)
21.2 (20.6, 21.8) 170.8 (165.1, 176.5) 97.7 (87.0, 107.9) 33.5 (30.2, 36.7) 1.09 (1.07, 1.17)
⁎ Indicates P b 0.05.
surrounding community. Subjects were between the ages of 18– 35 years, and subjects in the normal-weight group had a body mass index between 18.5 and 24.9, and subjects in the obese group had a body mass index greater than 30.0 (Calle et al., 1999). Subjects were excluded for any lower extremity injury within the 6 months prior to participation, any lower extremity surgery, a body mass index between 25.0 and 29.9, participation in resistance training or intercollegiate sport, or diagnosis of lower extremity osteoarthritis or knee pain. Prior to the start of the study, all subjects provided written informed consent, and all procedures were approved by the University's Institutional Review Board. 2.2. Gait biomechanics assessment Subjects completed 10 walking trials at a self-selected pace and 10 trials at a standardized pace of 1 m/s in a randomized order on a 10 m walkway. At least 5 practice trials were performed to ensure subjects could strike the force plate without “aiming”, and they were instructed to look straight ahead at a point on the wall during each trial. Gait speed was monitored using infrared timing gates centered over the force plate, and trials were repeated if gait speed fell outside 1 ± 0.05 m/s for the standardized speed and ±5% for the self-selected speed. Trials were considered acceptable if the subject made contact with the entire foot on the force plate without any noticeable gait deviations. Electromagnetic tracking sensors (Motion Star, Ascension Corp., Burlington, VT, USA) were positioned on the pelvis, thigh, shank, and foot of the dominant limb (Coren and Duncan, 1979) using double sided tape. The hip joint center was estimated using the Bell method (Bell et al., 1990) following digitization of the left and right anterior superior iliac spines, and the ankle and knee joint centers were estimated as the midpoints between the digitized medial and lateral malleoli and femoral epicondyles, respectively. The Motion Monitor motion capture system (Innovative Sports Training, Chicago, IL, USA) was used for model generation and data acquisition, and all kinematic data were sampled at 100 Hz. Kinetic data were sampled from a nonconductive force plate (model 4060-NC, Bertec Corp., Columbus, OH, USA) at 1000 Hz and time-synchronized with the kinematic data. 2.3. Data reduction All data were analyzed using a custom LabVIEW program (National Instruments, Austin, TX, USA). Kinematic data were lowpass filtered at 10 Hz and extracted from the gait cycle for which kinetic data was obtained. Knee joint angles were calculated as motion of the shank segment relative to the thigh segment using Grood and Suntay angles (Grood and Suntay, 1983). All kinematic and kinetic variables were calculated during the first 50% of the stance phase (i.e. weight acceptance and mid stance), defined as the interval from ground contact (vertical ground reaction force N 20 N) to toe off (vertical ground reaction force b 20 N). Peak knee flexion angle was identified, and knee flexion excursion was calculated as the peak angle minus the knee flexion angle at ground contact. Ground reaction force data were lowpass filtered at 75 Hz. The peak vertical ground reaction force was identified, and the average vertical loading rate was calculated as the peak vertical ground reaction force divided by the time to peak vertical ground reaction force (Hunt et al., 2010; Liikavainio et al., 2007; Pietrosimone et al., 2015). The instantaneous vertical loading rate was calculated as the first time derivative of the vertical ground reaction force, and the peak value was used for analysis (Liikavainio et al., 2010). Vertical ground reaction force and loading rates were normalized to body weight. Ground reaction forces were combined with kinematics via standard inverse dynamic procedures to yield net internal moments (Gagnon and Gagnon, 1992), and peak internal knee extension and abduction moments (i.e. the internal responses to external flexion and adduction joint loading, respectively) were identified. Joint moments were normalized to the product of height (meters) and weight (Newtons). The mean of 10 trials
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was used for analysis for both the standardized and self-selected speeds, respectively. 2.4. Statistical analysis Minimum sample size was estimated for an alpha level of 0.05 and a beta of 0.20 based on differences in gait biomechanics between normalweight and obese adults reported elsewhere (Segal et al., 2009). A minimum of 14 subjects (7 per group) was indicated, thus 30 subjects (15 per group) were recruited to provide a priori power to detect meaningful differences between groups. Additional subjects were recruited because this sample size calculation was based on expected differences in the frontal plane knee moment (Segal et al., 2009), and we also studied additional biomechanical variables. Data were screened for outliers using boxplots, inspected for normality using the Shapiro– Wilk test, and homogeneity of variance using Levene's test to confirm assumptions for inferential statistics. 2 (group: obese, normal weight) by 2 (condition: self selected speed, standardized speed) analysis of variance was used to evaluate all dependent variables (peak knee flexion angle, knee flexion excursion, peak internal knee extension moment, peak internal knee abduction moment, peak vertical ground reaction force, and the linear and instantaneous vertical ground reaction force loading rates). Post hoc comparisons were assessed using a Bonferroni adjustment (α = 0.013). Pearson correlations were calculated between kinematic (knee flexion angle and excursion) and kinetic (ground reaction force and vertical loading rate) variables. All data were analyzed using SPSS version 19.0. 3. Results Descriptive statistics of demographic information are presented in Table 1. All data were found to be normal and were treated as such. As expected, the obese group had a higher body mass index (Table 1, P b 0.001) and body mass (Table 1, P b 0.001). Obese subjects also utilized a slower self-selected walking speed (Table 1, P = 0.004). The group by condition interaction was significant for the instantaneous vertical loading rate (Table 2, F1,28 = 4.43, P = 0.03) and knee flexion excursion (Table 2, F1,28 = 3.57, P = 0.05). Post hoc analyses suggest that when walking at a standardized speed, obese subjects displayed greater instantaneous (Table 2, P b 0.001) vertical ground reaction force loading rates and lesser knee flexion excursion (Table 2, P = 0.04). The instantaneous vertical loading rate was also greater during the self-selected speed compared to the standardized speed in both the obese (P = 0.007) and normal weight groups (P = 0.001). Furthermore, weak to moderate correlations were found between knee flexion excursion and the average loading rate (r = −0.32, P = 0.04), and knee flexion excursion and the instantaneous loading rate (r = −0.28, P = 0.05) when walking at standardized speeds. No differences were observed at either speed in peak internal knee extension or abduction moments or vertical ground reaction force.
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4. Discussion The purpose of this study was to evaluate differences in gait biomechanics between normal-weight and obese young adults. We hypothesized that obese individuals would exhibit characteristics indicative of greater risk for KOA (e.g. lesser internal knee extension moment and knee flexion angle, and greater vertical ground reaction force, loading rate, and internal knee abduction moment) compared to normalweight individuals. Obese subjects displayed a greater instantaneous vertical loading rate and lesser knee flexion excursion compared to normal-weight subjects when walking at a standardized speed. Contrary to our hypotheses, we observed no differences in internal knee extension or abduction moments, or in the magnitude of the vertical ground reaction force. Greater instantaneous loading rates among obese individuals compared to normal weight individuals agreed with our hypotheses. Obese individuals have greater mass attributed to adipose tissue compared to skeletal muscle in the lower extremity. Additional fat mass likely contributes to a reduction of muscle strength relative to body weight, and induces fatigue of the lower extremity muscles such as the quadriceps (Syed and Davis, 2000). Insufficient lower extremity strength relative to body weight increases the loading rate and impairs shock attenuation (Syed and Davis, 2000). Habitual loading during walking is necessary for the maintenance of articular cartilage health (Seedhom, 2006; Yao and Seedhom, 1993). However, a higher loading rate during gait may place one at greater risk for the development of KOA (Ewers et al., 2002; Radin et al., 1984). As such, higher instantaneous loading rates among our obese group may explain the discrepancy in osteoarthritis prevalence between obese and normal-weight adults (Allen and Golightly, 2015; Lementowski and Zelicof, 2008). Articular cartilage is viscoelastic and is sensitive to the rate at which it is loaded. Higher loading rates contribute to stiffening of the articular cartilage, and increase the risk for failure and breakdown (Ewers et al., 2002). Furthermore, previous research in animals indicates that repetitive loading of the limbs at higher rates results in quick degeneration of articular cartilage, regardless of the overall magnitude (Ewers et al., 2002; Radin et al., 1984). As such, greater instantaneous loading rates rather than peak ground reaction force in obese individuals may be particularly relevant to articular cartilage health in obese individuals. We did not observe a difference in average loading rate as observed in another study involving obese individuals with KOA (Messier et al., 1996). However, our calculation of average loading rate used the peak ground reaction force in the numerator, which did not differ between groups and may explain this finding. Interestingly, we only observed a difference in the instantaneous vertical loading rates between obese and normal weight subjects at a standardized speed. Previous research (Browning and Kram, 2007) suggests that obese adults walking at 1.1 m/s experience similar lower extremity loading as normal weight adults walking at 1.4 m/s (a biomechanical equivalent), and self-selected speeds in this investigation were
Table 2 Gait biomechanics (mean and 95% confidence interval). Self-selected speed
Knee flexion excursion (°)⁎⁎ Peak knee flexion angle (°) Internal knee extension moment (bw × ht) Internal knee abduction moment (bw × ht) Vertical ground reaction force (bw) Average vGRF LR (bw/s) Instantaneous vGRF LR (bw/s)⁎⁎
Standardized speed
Normal
Obese
Normal
Obese
8.8 (8.9, 11.5) 14.4 (10.7, 15.2) 0.028 (0.013, 0.048) 0.036 (0.013, 0.021) 1.14 (1.09, 1.18) 6.33 (6.01, 6.84) 52.9 (46.7, 61.5)
7.6 (5.4, 9.8) 13.0 (11.1, 17.8) 0.018 (0.012, 0.024) 0.037 (0.011, 0.025) 1.09 (1.06, 1.13) 6.33 (6.03, 6.94) 58.3 (49.9, 60.7)
7.7 (6.2, 9.1)⁎ 9.8 (6.9, 12.7) 0.017 (0.010, 0.021) 0.016 (0.011, 0.019) 1.12 (1.06, 1.18) 6.30 (6.09, 6.45) 35.0 (31.3, 38.7)⁎,Ŧ
5.5 (4.1, 6.9) 9.4 (7.5, 11.4) 0.014 (0.010, 0.028) 0.016 (0.008, 0.023) 1.05 (1.01, 1.12) 6.32 (6.01, 7.34) 46.2 (41.89, 50.45)Ŧ
vGRF: vertical ground reaction force, LR: loading rate. ⁎⁎ Indicates significant group by condition interaction (P b 0.05). ⁎ Indicates different from obese group (P b 0.013). Ŧ Indicates different from self-selected speed (P b 0.013).
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similar (obese = 1.09 m/s; normal weight = 1.34 m/s). As such, since our obese individuals walked at a slower self-selected pace, it is likely that differences in loading characteristics were masked at self-selected speeds. Additionally, walking at slower speeds may be a compensatory mechanism to limit lower extremity loading in obese adults. We did consider that group differences at standardized speeds could have been from alterations in the normal weight group rather than in the obese group. However, the instantaneous loading rate was different between conditions in both groups, yet still greater in the obese group compared to the normal weight group during the standardized speed condition. We also observed lesser knee flexion excursion in the obese subjects compared to normal-weight subjects, which was in agreement with our hypotheses and with previous studies (Kaufman et al., 2000; Zeni and Higginson, 2009). Lesser knee flexion excursion may also explain why obese subjects had greater loading rates. This was evidenced by weak to moderate correlations between knee flexion excursion and the average loading rate, and knee flexion excursion and the instantaneous loading rate at standardized speeds. Knee flexion during the weight acceptance phase of stance is requisite for proper absorption of ground reaction forces, and is indicative of quadriceps function during gait. Lesser knee flexion reduces the time interval over which ground reaction forces are absorbed, and therefore increases the rate at which these forces are applied throughout the lower extremity. However, loading rate is a global rather than joint-specific indicator of loading, thus greater loading rates could also be explained by biomechanical alterations at other joints such as the hip or ankle. Interestingly, the difference in loading rate was only evident at an enforced speed. It could be that at self-selected speeds, obese individuals walk slower to increase the time interval over which force is applied and lower the rate of loading. The slower self-selected speed may be a compensatory action to protect articular cartilage from greater loading rates. Moreover, previous research indicates that BMI is correlated with loading rate only after adjusting for differences in walking velocity (Messier et al., 1996). Importantly, knee flexion can be improved through verbal cues during dynamic tasks such as landing (Mizner et al., 2008), and future studies are needed to examine the influence of modifying knee flexion on vertical loading rate. Interestingly, we did not observe a difference in the internal knee extension moment between groups, which was contrary to our hypotheses. However, these findings are in agreement with previous research (Freedman Silvernail et al., 2013), and subjects in this study were considered healthy aside from their body mass index. Furthermore, body mass index is often a poor discriminator of health, as it fails to account for differences in body composition. Secondly, hamstring cocontraction may have contributed to similar knee joint moments in the obese group in this study. The use of joint moments to make assumptions about forces acting on the knee joint ignores the contribution of muscle co-contraction, and the observed moment is the net effect of all muscle activity at a joint. Greater co-contraction results in greater intersegmental forces, even if the net moments calculated from inverse dynamic procedures are the same. As such, we suspect that greater co-contraction contributed to similar knee joint moments between obese and normal weight individuals in this study. Patients with knee OA utilize greater quadriceps–hamstring co-contraction compared to unimpaired individuals during walking (Bouchouras et al., 2015; Childs et al., 2004; Heiden et al., 2009; Zeni et al., 2010), and quadriceps–hamstring co-contraction contributes to a stiffened knee joint and greater loading rates (Schmitt and Rudolph, 2007). Importantly, weight loss reduces hamstring co-contraction in patients with KOA (Messier et al., 2011), and future research is needed to examine the influence of hamstring co-contraction on gait in healthy obese individuals. Secondly, the lack of significant differences between obese and normal weight adults could be related to the normalization technique adopted in this study. For instance, Browning and Kram (2007) reported absolute moments without normalization criteria,
and found greater sagittal plane knee moments in obese compared to normal weight adults. While we normalized joint moments to the product of height and weight, this procedure does not account for differences in fat free mass between obese and normal weight adults. Normalizing joint moments to the amount of fat free mass may be more indicative of joint moments due to muscle activity (Freedman Silvernail et al., 2013), and future studies should consider normalization technique when interpreting joint moments in obese individuals. Finally, the reduction in internal knee extension moments in patients with knee pathologies may also be attributed to arthrogenic muscle inhibition (AMI) of the quadriceps (Pietrosimone et al., 2014). AMI refers to a form of neural inhibition that limits the ability to fully activate the quadriceps muscles following knee injury (ACL reconstruction, patellofemoral pain syndrome etc.) or disease (KOA) (Palmieri-Smith and Thomas, 2009; Pietrosimone et al., 2011). AMI contributes to quadriceps dysfunction and may contribute to the development of KOA (Palmieri-Smith and Thomas, 2009). However, obese individuals without knee pathology may not have AMI, which may explain why we did not observe a difference in the internal knee extension moment between the normal-weight and obese groups. Finally, we did not observe a difference in the internal knee abduction moment between obese and normal-weight adults. This characteristic is an indicator of KOA risk and progression (Sharma et al., 1998), particularly in obese individuals (Sharma et al., 2000). However, our findings are similar to the results of another study in obese young adults (Freedman Silvernail et al., 2013). Furthermore, the frontal plane knee moment is largely influenced by knee alignment, and we did not specifically recruit individuals with misalignment. A previous study has suggested that obesity is only a risk factor for KOA progression in those with knee varus by increasing the external knee adduction moment (Sharma et al., 2000). As we did not assess knee varus alignment in this study, it is unclear if our findings are due to the absence of varus misalignment in this sample. There are limitations of this study that must be addressed when interpreting the results. Firstly, the cross sectional nature of the study precludes our ability to draw conclusions regarding KOA development in young obese subjects. While we observed differences in instantaneous loading rate and knee flexion excursion, these individuals were otherwise healthy. Prospective studies are needed to ascertain the effect of higher vertical loading rates and aberrant gait biomechanics on future osteoarthritis risk. Secondly, we classified our groups based on body mass index, which does not account for differences in body composition. Individuals classified as obese by body mass index due to greater muscle mass may be able to compensate for additional joint loading and attenuate ground reaction forces more effectively. Future studies should consider studying obese individuals classified using another method, such as body fat percentage. Thirdly, there may have been some movement of the electromagnetic sensors relative to bone segments, and motion artifact may have been greater in the obese group compared to the control group. However, all sensors were affixed with double-sided tape, and secured using pre-wrap and athletic tape. Future studies should consider the use of an optimization method for sensor placement to reduce the influence of any soft tissue artifact. Lastly, it is important to note that while our findings regarding loading rates were statistically significant, the absolute differences observed were fairly small and thus we interpret these results with caution. Additionally, this study focused on biomechanical contributors to knee joint health, and there are systemic and inflammatory associated with obesity that may contribute to KOA (Kluzek et al., 2015). 5. Conclusion In conclusion, our findings suggest that obese subjects had greater instantaneous loading rates when walking at standardized speeds, and lesser knee flexion excursion compared to normal-weight subjects at standardized and self-selected speeds. Future prospective studies are
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