Obesity is associated with reduced joint range of motion (Park, 2010), which has been partially

INTRODUCTION Obesity is associated with reduced joint range of motion (Park, 2010), which has been partially attributed to adipose tissues around joints limiting inter-segmental rotations (Gilleard, 2007). These range of motion reductions help explain why obese individuals are likely to have limited functional reach capabilities in both the standing and seated positions when compared with normal weight individuals (Park, 2010). Restrictions in range of motion may cause modification in the movement strategy, with potential implications for increases in associated biomechanical stresses, in the form of joint moments and joint forces (Sibella, 2003). Obesity related changes in strategy have been demonstrated in performance of routine physical tasks (Larsson, 2001) and may also be evident during common rehabilitation and weight reduction exercises. Squat and forward lunge are the two commonly used rehabilitation exercises (Flanagan, 2004). The squat exercise is a classic multiple-joint exercise that has become an integral part of most lower-extremity strengthening and postoperative rehabilitation programs. Similarly, Lunge is commonly used in rehabilitation protocols especially for strengthening of the knee extensor muscles like quadriceps. Previous studies on squat and lunge mainly focused on electromyographic (EMG) data to study cocontraction and strengthening issues, and very few studies have looked at the biomechanics of squat and lunge activities (Reimann, 2012). These studies were conducted in healthy college age populations, but no studies have looked at biomechanical issues in obese individuals (Flanagan, 2003; Escamilla, 2001). Although there have been no studies in obese individuals, studies have been done on activities similar to squat and lunging. An increase in biomechanical stresses, as quantified by joint

moments has been reported during standing forward reaching tasks in obese subjects (Gilleard 2007). This study suggested that increased moments were likely due to biomechanically disadvantageous postures used by obese individuals, rather than their increased body mass. In addition, lower hip and higher knee extensor moments, attributed to limited trunk flexion were seen in obese, as compared to normal weight subjects during sit to stand activity (Sibella, 2003). Obese subjects may use similar postural modifications and strategies during performance of squat and lunge rehabilitation exercises, changing the biomechanical joint stresses, leading to unintended consequences. Despite the potential for biomechanical differences with normal weight subjects, there is no published data showing that physical therapists and clinicians make different recommendations while prescribing exercises to the obese individuals. Taking into consideration the biomechanical stresses and strategies during common exercises may help to inform rehabilitation approaches used for obese adults. The purpose of this study was to analyze the biomechanics of obese and normal weight individuals, as measured by hip and knee moments, while performing common physical therapy rehabilitation exercises. It was hypothesized that restricted joint mobility in obese subjects will be associated with decreased hip and increased knee joint moments as compared to normal weight subjects and that these differences will be more evident as the level of difficulty of squat and lunge increases. METHODS Ten obese female subjects mean age 37.4 ± 3.7 years; with BMI 39.2 ± 3.7 kg/m 2 and ten normal weight female age matched control subjects 38.1 ±4.5 years; BMI 22.6 ±2.3 kg/m 2 volunteered the study. The study was approved by the local institutional review board Height,

weight, waist circumference; hip circumference and tibial length were recorded. Waist circumference was measured at the level of the right iliac crest, with a Gulick II plus (Gulick II measuring tape; Country Technology Inc., Gays Mills, WI) tape measure. Cardio-respiratory fitness was assessed using an 8-minute submaximal Nemeth treadmill protocol. Triads of infrared emitting diodes (IREDs) were placed on the pelvis and trunk, and bilaterally on the thighs, legs, and feet. Markers were affixed to the lateral aspect of the foot, to the shaft of the tibia, and to the lateral aspect of the thigh. Femoral epicondyle motion was tracked by two markers mounted on a custom femoral tracking device (Houck & Yack, 2000). Pelvic markers were affixed on the sacrum using a 5 cm extension. A similar extension was placed on the lower cervical vertebrae, track the trunk segment. A link-based model was generated for tracking each segment. Anatomical landmarks were digitized, relative to segment local coordinate systems, with the subject standing in a neutral position, to create an anatomical model. Segment principal axes were defined by digitizing the following bony landmarks: Pelvis anterior and posterior superior iliac spines; Trunk or Head Arm Trunk (HAT): C-7 and L-1 vertebrae and glenohumeral joints; Thigh: hip joint center, lateral and medial condyles; Shank: lateral and medial condyles and malleoli; Foot: posterior heel, metatarsal head, and second toe (Figure 1). Kinematic data were collected using an Optotrak motion analysis system (Model 3020, Northern Digital Inc.,Waterloo, Ontario, Canada) operating at 60 Hz. Kinematic data were filtered at 6Hz, using a zero phase lag fourth-order Butterworth low pass filter. Kinetic data were obtained using a Kistler force plate (Kistler Instruments, Inc., Amherst, NY). The force plate data were sampled at 300 Hz, and were filtered at 6 Hz, thus providing ground reaction forces. Visual 3D software (C-Motion Inc. Kingston, Ontario) was used to perform link-segment calculations.

Testing session included two trials each: Squatting down, feet shoulder width apart with right foot on force plate and holding for 3 seconds at 3 different knee angles: 60, 70, and 80 degree (full knee extension being 0 degree). Forward lunging with the right lead foot on force plate at 3 different distances and holding for 3 seconds: 1, 1.1, 1.2 times subject s tibial length. Data Analysis: Visual 3D software (C-Motion) was used for processing and the moments were normalized to body mass. Mean values were calculated for extensor moments and range of motion over the 3 second hold. Lower extremity joint moment could be analyzed with the concept of 'support moment (Lower extremity joint kinetics and lumbar curvature during squat and stoop lifting, Hwang, 2009). As the subjects performed two trials at each level of squat and lunge, the mean of two trials was taken for further analysis. Statistical Analysis:. A Group (Obese vs Normal weight) by Level of difficulty ANOVA was used to find differences in hip, knee and ankle moments across three levels of difficulty for squat and lunge in obese and normal weight subjects. Pearson correlation coefficient was used to find correlation between moments and range of motion. Regression analysis was performed to find relationships between BMI/waist-to-hip ratio and extensor moments. RESULTS For the squat, hip, knee and ankle extensor moments in obese subjects were not different than normal weight subjects for all three levels of squat (table 1). Support moment (sum of hip, knee and ankle extensor moments) was higher in obese subjects, as compared to the normal weight subjects, for squat 70 (1.33 vs 1.03 Nm/kg) (p-value 0.03) and squat 80 (1.53 vs 1.18 Nm/kg) (pvalue 0.01).

The higher support moment was primarily due to an increase in the knee extensor moments (correlation between knee and support moment = 0.75), which increased from squat 60 (0.67 Nm/kg) to squat 80 (0.82 Nm/kg) (p-value 0.01) in obese subjects, but was not different for normal weight subjects (p-value 0.18). (Figure 2). Knee extensor moments were not different between obese and normal weight groups (table1). No differences were seen in support moments between obese and normal weight subjects. (Figure 3) No differences were seen in hip, knee and ankle range of motion between obese and normal weight subjects for squat and lunge activities (table 2). There was no significant relationship between BMI and moments for lunge and squat levels, adjusted r-square=0.03 p-value 0.33. Likewise, fitness level as estimated by VO 2 max., waist circumference and waist to hip ratio did not show any significant relationships with joint moments. DISCUSSION The purpose of this study was to analyze the biomechanics of obese and normal weight individuals, as measured by hip and knee extensor moments, while performing squat and lunge exercises. The results suggests that obese subjects experience higher biomechanical stress than normal weight subjects while performing basic rehabilitation exercises, although BMI did not show any significant correlations with any of the extensor moments. The hip, knee and ankle extensor moments were not different in obese and normal weight subjects during the squat activity, but the overall support moment was higher in obese for squat 70 and 80. The lunge activity did show differences at all levels at the hip but no differences in the knee, ankle and support moments. The range of motion also did not show any differences between obese and

normal weight groups, but the obese group had higher correlations between range of motion and moments. Previous studies have reported similar knee extensor moments during squat activity, in normal weight subjects (Salem, 2003). Support moment has been used to characterize squat and stoop lifting techniques (Hwang, 2009), but the magnitudes reported were higher as compared to our normal weight subjects. Similar hip and knee moments in obese and normal weight during squat in our study could be due to similar trunk flexion. Increased support moment during squat, with main contribution from the knee joint moment indicates overall higher biomechanical stress in obese subjects. On the other hand, lunge activity did show differences in hip moments at all three levels. It has been shown that hip extensor moments increase as forward trunk lean is increased while performing the lunge activity (Farrokhi, 2008). Clinician s progress rehabilitation protocols by increasing the difficulty of the exercise; by increasing the depth of the squat or increasing the distance between feet during lunge. Biomechanical stresses at the knee significantly increased as the difficulty level of squat increases in obese subjects, but were not different in normal weight subjects. On the other hand, stresses at the hip joint were significantly higher in obese subjects during all levels of lunge. This should be taken in consideration while prescribing exercises to obese subjects, especially in subjects with joint pathologies like osteoarthritis. Although there were no differences in range of motion between obese and normal weight subjects, stronger correlations between moments and range of motion were seen in obese subjects. The higher support moment and higher correlation might point to the obese group being

less efficient in holding the position and having less flexibility in how they were able to accomplish this. Surprisingly, BMI or anthropometric measures did not show any significant correlations with any of the extensor moments. A possible explanation for this would be that the obese subject used similar strategies to normal weight subjects while holding the position. Obviously, the waist circumference was different between both groups; but it did not seem to restrict range of motion. To conclude, there are trends in data suggesting increased biomechanical stresses in obese subjects while performing basic rehabilitation exercises, which might be confirmed with more subjects. Further analysis is being performed to explore the strategies used by the two groups.

Knee Extensor Moments (Nm/kg) Figure 1: Shows the skeletal model of an obese female subject during squat activity (left), placement of markers(center) and lunge activity (right). 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 Ankle Hip Knee 0.2 0 S60 S 70 S80 Figure 2: Within groups, the support moment between squat 80 was greater than squat 60 in obese (p-value 0.01) and not different for normal weight subjects (p-value 0.08).

Hip Extensor Moments (Nm/kg) 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 Obese Control Lunge 1 Lunge 1.1 Lunge 1.2 Figure 3: For the lunge, hip extensor moments were greater in obese than normal weight control subjects for level 1, 1.1 and 1.2 (p-value 0.004, 0.003 and 0.007 respectively). Moments (Nm/kg) Hip Knee Ankle Obese Normal Obese Normal Obese Normal Squat 60 0.22 (.24) 0.12 (.17) 0.67 (.10) 0.59 (.22) 0.28 (.16) 0.19 (.10) Squat 70 0.29 (.28) 0.17 (.18) 0.73 (.12) 0.66 (.23) 0.31 (.19) 0.20 (.13) Squat 80 0.37 (.30) 0.24 (.18) 0.82 (.12) 0.75 (.26) 0.34 (.19) 0.20 (.11) Lunge 1 1.32 (.27) 0.96 (.39) 0.53 (.15) 0.64 (.30) 0.42 (.20) 0.45 (.26) Lunge 1.1 1.41 (.28) 1.07 (.38) 0.53 (.16) 0.56 (.29) 0.43 (.20) 0.42 (.25) Lunge 1.2 1.48 (.32) 1.14 (.39) 0.50 (.22) 0.52 (.24) 0.47 (.21) 0.40 (.22) Table 1: Represents the mean (standard deviation) of hip, knee and ankle extensor moment for different levels of squat and lunge exercises in obese and normal weight subjects. ROM Hip Knee Ankle Obese Normal Obese Normal Obese Normal Squat 60 64.7 (19.2) 62.3 (22.5) 59.6 (7.9) 57.5 (6.7) 119.6 (5.8) 119.3 (7.3) Squat 70 75.6 (23.2) 71.4 (25.0) 68.4 (8.5) 66.2 (7.5) 121.8 (6.3) 122.7 (7.3) Squat 80 85.0 (24.2) 82.4 (24.9) 78.3 (9.3) 75.3 (7.5) 124.1 (5.7) 125.5 (6.8) Lunge 1 98.4 (12.2) 89.1 (20.9) 83.6 (12.7) 86.7 (9.3) 109.3 (6.9) 117.2 (9.6) Lunge 1.1 102.4 (12) 91.4 (19.9) 88.0 (11.5) 85.9 (11.1) 109.2 (7.4) 114.1 (9.5) Lunge 1.2 102.4 (8.3) 92.7 (17.7) 88.3 (13.4) 86.5 (10.0) 109.2 (8.9) 112.8 (9.3) Table 2: Represents the mean (standard deviation) of hip, knee and ankle range of motion for different levels of squat and lunge exercises in obese and normal weight subjects.

References Escamilla RF. Knee biomechanics of the dynamic squat exercise. Med Sci Sports Exerc. 2001;33(1):127-41. Farrokhi S, Pollard CD, Souza RB, Chen YJ, Reischl S, Powers CM. Trunk position influences the kinematics, kinetics, and muscle activity of the lead lower extremity during the forward lunge exercise. J Orthop Sports Phys Ther. 2008 ;38(7):403-9. Flanagan SP, Biomechanical Attributes of Lunging Activities for Older Adults J Strength Cond Res. 2004 August; 18(3): 599 605. Flanagan SP, Salem GJ, Wang MY, Sanker SE, Greendale GA. Squatting exercises in older adults: kinematic and kinetic comparisons. Med Sci Sports Exerc. 2003;35(4):635-43. Lai PP, Leung AK, Li AN, Zhang M. Three-dimensional gait analysis of obese adults. Clin Biomech (Bristol, Avon). 2008;23 Suppl 1:S2-6 Gilleard W, Smith T. Effect of obesity on posture and hip joint moments during a standing task, and trunk forward flexion motion. Int J Obes (Lond). 2007;31(2):267-271. Hwang S, Kim Y, Kim Y. Lower extremity joint kinetics and lumbar curvature during squat and stoop lifting. BMC Musculoskelet Disord. 2009; 2,10-15. Larsson EU, Mattsson E. Functional limitations linked to high body mass index, age and current pain in obese women. Int J Obes Relat Metab Disord. 2001;25(6):893-899. Salem GJ, Salinas R, Harding FV. Bilateral kinematic and kinetic analysis of the squat exercise after anterior cruciate ligament reconstruction. Arch Phys Med Rehabil. 2003 Aug;84(8):1211-6. Sibella F, Galli M, Romei M, Montesano A, Crivellini M. Biomechanical analysis of sit-to-stand movement in normal and obese subjects. Clin Biomech (Bristol, Avon). 2003;18(8):745-750.