Physical Function and Physical Activity in Obese Adults After Total Knee Arthroplasty

Obese patients are more likely to have osteoarthritis and total knee arthroplasty (TKA). This investigation sought to evaluate physical function, activity level, and quality of life (QOL). Obese participants near 1-year postsurgical follow-up appointment were recruited. Evaluation included QOL and activity questionnaire, medical histories, anthropometrics, strength, and aerobic capacity. Sixty participants completed assessments. Obese TKA patients have physical performance limitations and low physical activity levels 1 year after surgery and completion of postoperative rehabilitation.

Key points

  • Obese patients are more likely to have osteoarthritis and total knee arthroplasty (TKA). This investigation sought to evaluate physical function, activity level, and quality of life (QOL) in obese TKA patients.

  • Obese adults who had TKA and were near 1-year postsurgical follow-up appointment were recruited. Evaluation included QOL and activity questionnaire, medical histories, anthropometrics, strength, and aerobic capacity.

  • Obese TKA patients have physical performance limitations and low physical activity levels 1 year after surgery and completion of postoperative rehabilitation.


Obesity is associated with several health-related conditions detrimental to general health and, specifically, the musculoskeletal system. Excess body weight directly and indirectly stresses the joints, particularly the knees, which increases the deterioration of the protective soft tissues in the joint structures. Thus, obesity is a strong risk factor for development of osteoarthritis (OA) and obese individuals have a substantially higher rate of total knee arthroplasty (TKA) than healthy weight individuals.

Although the associations between obesity and OA and obesity and incidence of TKA are reasonably well described, the effects of obesity on TKA rehabilitation and long-term functional recovery are not sufficiently understood. All patients, regardless of body weight, seem to experience decreased pain and improved function following TKA. However, there is some evidence indicating worse long-term outcomes among obese patients. For example, morbidly obese patients are more than 5 times more likely than healthy weight patients to experience complications and device failure within 5 years of TKA. Obese individuals, accordingly, have higher reported need for additional surgical revision to adjust or repair the TKA and, although functional abilities improve following TKA, outcomes are significantly poorer when compared with healthy weight patients. Functional abilities, such as completion of activities of daily living and exercise capacity, and factors that influence functional capacity, including persistent pain, muscle weakness, and balance impairments, are not well researched after completion of initial rehabilitation programs. Thus, the long-term benefit of the TKA intervention among obese adults requires further consideration.

The purpose of this investigation was to evaluate baseline physical function, physical activity levels, and quality of life in obese patients 1 year following TKA as part of an individually tailored exercise intervention designed to improve physical function among obese adults.

Materials and methods


Sixty obese (body mass index [BMI] >30 kg/m 2 ) patients who were 10 to 18 months post-TKA volunteered to complete surveys, a functional assessment, and 16 weeks of home-based exercise. Patients were required to have medical clearance to participate in exercise testing and intervention. Patients were identified and recruited from surgical follow-up clinics at an orthopedic clinic based at a large health science center. Before consent, each participant was prescreened for eligibility by phone by a research nurse ( Fig. 1 ). As approved by Institutional Review Boards for Human Subjects Research at the University of Tennessee Health Science Center and the University of Memphis, participants were informed of all procedures, potential risks, and benefits associated with the study, and written informed consent was obtained for each participant. Final eligibility was verified through health and medical history and a physical activity survey at the initial clinic visit.

Fig. 1

Consort diagram.



Height and weight were collected without shoes using a calibrated digital clinic scale and a wall-mounted stadiometer. Body composition was measured using standardized skinfold measures developed by Jackson and Pollock and described in detail by the American College of Sports Medicine (ACSM). Skinfolds were measured with a Lange skinfold caliper (Beta Technology, Santa Cruz, CA, USA) at the chest, abdomen, and thigh in men; and triceps, suprailiac crest, and thigh in women. Sum of skinfolds measured were used to calculate percentage body fat using 2-stage predictive equations by first calculating body density and then body fat percentage. BMI was calculated by dividing kilograms of body weight by height in meters squared. Waist and hip circumference were measured using a (Gulick II tape, North Coast Medical Inc, Gilroy, CA) measure at the narrowest point between the umbilicus and the xiphoid process and the widest point between umbilicus and the knee, respectively.

Physical function

Heart rate, blood pressure, and respiration were measured following a 5-minute quiet, seated rest period. Walk endurance was measured using the 6-minute walk test (6-MW), which consists of continuous walking at a self-selected walk pace in accordance with the American Thoracic Society guidelines. During the 6-MW, participants were encouraged to walk as quickly as possible for 6 minutes on the designated walk path. Participants could stop and rest as needed during the test; however, the test timer did not stop. The distance walked (meters) in 6 minutes was recorded for each participant. Expected normative walk distances were based on previously published predictive equations in age-matched and gender-matched healthy adults.

Passive and active range of motion at the knee was measured by a licensed physical therapist using a goniometer. Participants were instructed to flex and extend the knee actively as far as possible and passive measurements made by the physical therapist were recorded at the terminal range in degrees. Knee extension strength was measured using a handheld dynamometer (Chantillion DFE, AMETEK Test & Calibration Instruments, Largo, FL). The participants were seated with the knee positioned at 60° and the dynamometer was placed 20 cm below the tibial tuberosity. The participant was instructed to extend his or her lower leg against the dynamometer as hard as possible. Three trials were performed on each leg with 1-minute rest between trials. Predicted strength values were based on previously published gender-based normative ranges in healthy adults.

Self-reported knee function and health-related quality of life

The Western Ontario and McMaster University Osteoarthritis Index (WOMAC) was used to assess pain, function, and quality of life. WOMAC scoring yields 1 composite (0–96) and 3 subscales: pain (0–20), stiffness (0–8), and physical function (0–68). Responses to each question range from 0, indicating none, to 4, indicating severe; thus higher scores on the composite or subscales indicate worse pain, stiffness, and/or function. The composite and subscales have reasonable validity (Spearman correlations from 0.63 to 0.67) and reliability (Cronbach’s alphas from 0.86 to 0.95).

Health-related quality of life was assessed using the Medical Outcomes Study short form 36 version 2 (SF36). The survey is a widely applied generic health questionnaire previously used in obese and osteoarthritic populations. Scores were calculated for 8 health domains (mental health, role physical, physical function, vitality, social function, bodily pain, role emotional, and general health) and 2 summary measures: the physical component scale and mental component scale. Survey responses were summed to generate raw scores in each subscale and health domain, and used to create general population norm-based scoring (T-scores with a population mean of 50 and a standard deviation [SD] of 10 using previously described methodology). Scores were considered poor if the normalized scores (T-scores) were less than 37, which corresponds to the lowest 10th percentile of the general population.

Physical activity levels

Physical activity was assessed using the Nation Health and Nutrition Examination Survey (NHANES) Physical Activity Questionnaire (PAQ) version 2009 (available at ). The survey consists of questions about daily, leisure time, and sedentary activities. Weekly time spent doing vigorous or moderate physical activity was calculated from responses to daily (work) and leisure time activities. Sedentary time was collected and reported in average hours per day. Screen time was evaluated separately as television and computer time. Responses were coded as less than 1 hour, and hourly increments up to 5 hours or more per day.

All testing was completed by the same trained clinical exercise physiologist, with the exception of lower extremity range of motion and the WOMAC index, which were completed and reviewed by the same licensed physical therapist.

Data analysis

Study data was managed using REDCap electronic data capture tools. REDCap (Research Electronic Data Capture, Vanderbilt University, Nashville, TN) is a secure, Web-based application designed to support data capture for research studies, providing

  • 1.

    An intuitive interface for validated data entry

  • 2.

    Audit trails for tracking data manipulation and export procedures

  • 3.

    Automated export procedures for seamless data downloads to common statistical packages

  • 4.

    Procedures for importing data from external sources.

All data reduction, processing, and analysis for this project were generated using SAS software, version 9.2 of the SAS system for Windows (SAS Institute, Cary, NC, USA). The outcomes considered in this article are primarily descriptive with statistical analysis consisting of means and SDs for each measure. Reference ranges were constructed from population-based normative equations and references when available.


Patient Description

This study included 39 women and 21 men with an average age of 64.7 plus or minus 8.6 years old (mean ± SD) and 14.0 plus or minus 2.6 months postsurgery ( Table 1 ). On average, participants had a normal resting heart rate (76.5 ± 6.8 beats per minute), borderline hypertensive systolic blood pressure (131.1 ± 8.9 mm Hg), and prehypertensive diastolic blood pressure (81.1 ± 4.5 mm Hg). Three participants (5.0%) were current smokers. All had medical clearance and none were currently receiving medical treatment of an acute medical issue.

Table 1

Participant characteristics

Total (N = 60) Men (n = 21) Women (n = 39)
N (%) N (%) N (%)
Smoker 3 (5.0) 0 (0) 3 (5.0)
Mean ± SD Mean ± SD Mean ± SD
Age (y) 64.7 ± 8.7 63.3 ± 7.3 65.5 ± 9.3
Height (cm) 165.8 ± 10.6 175.9 ± 8.6 160.3 ± 7.0
Weight (kg) 103.0 ± 18.3 113.0 ± 18.5 97.6 ± 16.0
BMI (kg/m 2 ) 37.4 ± 5.5 36.4 ± 4.7 38.0 ± 5.9
Body fat (%) 34.8 ± 3.5 44.6 ± 2.8
Skinfolds (mm) 135.0 ± 20.1 121.0 ± 17.7 142.6 ± 17.1
Waist to hip 0.91 ± 0.10 0.98 ± 0.09 0.87 ± 0.08
Composite (0–96) 20.5 ± 15.2 16.1 ± 12.4 23.0 ± 16.2
Function (0–68) 14.9 ± 11.6 11.3 ± 9.1 16.8 ± 12.4
Pain (0–20) 3.5 ± 3.4 2.8 ± 2.7 3.9 ± 3.6
Stiffness (0–8) 2.6 ± 1.5 2.0 ± 1.4 2.9 ± 1.5


Participants were on average 165.8 plus or minus 10.6 cm tall, weighed 103.0 plus or minus 18.3 kg, and had a BMI of 37.4 plus or minus 5.5 kg/m 2 (all patients had a BMI higher than 30 kg/m 2 indicating obesity) (see Table 1 ). Mean sum of skinfolds were 135.0 plus or minus 20.1 mm on average, which corresponds to an estimated body fat percentage of 41.2 plus or minus 5.6%. Men, relative to women, had lower body fat (34.8 ± 3.5% vs 44.6 ± 2.8%), BMI (36.3 ± 4.7 kg/m 2 vs 38.0 ± 5.9 kg/m 2 ), skinfolds (121.0 ± 17.7 mm vs 142.6 ± 17.1 mm), and higher waist to hip ratios (0.98 ± 0.09 vs 0.87 ± 0.08), respectively (see Table 1 ).

Knee Range of Motion

Active knee extension ranged from 8° of hyperextension to lacking 18° of full extension. The average active knee extension was lacking 2.6°plus or minus 3.9° of extension on TKA knee and lacking 2.4°plus or minus 5.4° of extension on the nonsurgery knee. Active knee extension limb difference was 3.7°plus or minus 3.3° with a range of 0° to 14°. Active knee flexion ranged from 80° to 154°. The average active knee flexion was 117.8°plus or minus 10.5° on TKA knee and 121.1 plus or minus 11.6° on non-TKA knee. Active knee extension limb difference was 8.2 plus or minus 6.6° with a range of 0 to 31°.

Knee Extensor Strength

Knee extension strength ranged from 8.5 kg to 45.6 kg. The average knee extension strength was 26.5 plus or minus 8.5 kg on TKA knee and 26.1 plus or minus 7.7 kg on non-TKA knee. Knee extension strength difference was 3.4 plus or minus 4.4 kg with a range of 0.2 to 11.2 kg. Percent of predicted strength based on gender-based normative data were 84.2% plus or minus 20.4%. Nineteen patients (31.7%) failed to reach 75% of their predicted knee extension strength values.

Walk Performance

Total distance walked during 6 minutes ranged from 122.0 m to 489.0 m, with a mean of 312.3 plus or minus 77.2 m. Percent of predicted walk distance, based on the gender-based prediction equation, averaged 71.7% plus or minus 16.6%. Thirty-five patients (58.3%) failed to reach 75% of their predicted walk distance during the 6-MW. Average peak heart rate during the 6-MW was 105.2 plus or minus 12.6 beats per minute. Average peak rating of perceived exertion was 10.6 plus or minus 2.9 on the Borg scale. The average rating of perceived exertion scores corresponds to very light to light exertion values on the 6 to 20 Borg scale.

Self-Reported Knee Function

WOMAC composite scores (see Table 1 ) were 20.5 plus or minus 15.2 points of a possible 96 with a minimum of 2 and maximum of 71. The function subscale scores were 14.9 plus or minus 11.6 points of a possible 68 with a minimum of 0 and maximum of 50. Pain subscale scores were 3.5 plus or minus 3.4 points of a possible 20 with a minimum of 0 and a maximum of 15. Stiffness subscale scores were 2.6 plus or minus 1.5 points of a possible 8 with a minimum of 0 and a maximum of 6.

Self-Reported Health-Related Quality of Life

Participants had a mean T-score of 40.3 plus or minus 9.3 on the SF36 Physical Component Scale ( Table 2 ), including 18 participants (30.0%) who scored in the lowest 10th percentile of the general population. Only 8 participants (13.0%) scored average or better (≥40 th percentile) on the physical component scale. Normative scores on the physical function subscale were 37.2 plus or minus 10.0, with 28 participants (46.6%) scoring in the lowest 10th percentile of the general population. In addition, only 7 (11.7%) scored at or above the 40th percentile, indicating physical performance was within normal limits on the physical function subscale. Role physical subscale (42.8 ± 10.9) and bodily pain subscales (45.9 ± 9.1) were slightly below the normative values. Participants had a mean slightly above the population normal (55.5 ± 9.0) on the Mental Component Scale. Social functioning, mental health, and general health subscales were all with the normative ranges (50.5 + 8.8, 53.5 ± 9.6, and 49.6 ± 7.6, respectively).

Oct 6, 2017 | Posted by in ORTHOPEDIC | Comments Off on Physical Function and Physical Activity in Obese Adults After Total Knee Arthroplasty
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