The NHANES I and Epidemiologic Follow-up (NHEFS) studies revealed that obesity at baseline increased upper and lower body disability across 20 years.5;9
More recently, Jenkins10
found that functional impairment in older adults increased with BMI. In the Cardiovascular Health Study, an adjusted odds ratio of 2.94 for self-reported mobility-related disability was found for those in the highest versus the lowest quintile of fat mass.11
As body weight increases, both fat mass and fat-free mass increase.12
The relationship between fat-free mass and BMI is stronger in men, suggesting that increased BMI in women is due predominantly to an increase in fat mass. Although these data seem to imply that obese people are stronger than non-obese counterparts, the opposite is true. Specifically, when strength is represented as a function of body weight, both obese men and women are weaker, irrespective of age.13
In obese men aged 60–80, mean knee strength is 65% of body weight, as compared to 77% for controls; for their female counterparts, mean knee strength is 50% of body weight, compared to 62% for nonobese women.
Muscle weakness in older adults can have dire consequences. It is the second leading cause of falls in the elderly, accounting for 17%.14
Falls are the leading cause of death by injury in older adults, and 75% of deaths due to falls occur in adults over 65 years. Only one-half of older adults hospitalized due to falls will be alive after 1 year. By 2030, 280,000 Americans will die annually from falls.15
Loss of balance causes falls, so balance measures are used to identify people who are more susceptible. Kejonen et al.16
found significant correlations between poor balance and high BMI in women but not men, once again suggesting that muscle weakness is a mediator of poor balance and falls. Jadelis et al.17
found that for older adults with a BMI above 30 kg/m2
and a given amount of knee strength, the more obese, the worse the balance, suggesting that obesity, independent of strength, is a risk factor for poor balance and falls.
Not without reason, then, obesity is related to a fear of falling and injury risk.18;19
Austin et al.18
followed 1,282 community-dwelling women aged 70–85 for 3 years and found that obesity was independently associated with fear of falling at baseline and with the onset of this fear in women who were symptom-free at baseline. In a sample of over 42,000 adults, the odds of sustaining an injury were greater among those with excess weight. As BMI category increased from overweight (25.0 kg/m2
≤BMI ≤29.9 kg/m2
) to class III obesity (BMI ≥ 40.0 kg/m2
), the odds of sustaining an injury, including those related to falls, rose from 15% to 48%.19
Obese adults make adjustments to help stabilize their larger mass and reduce fall risk. DeVita et al.20
compared obese and lean adults and noted that the obese group increased ankle torque during walking but showed no difference in knee or hip torque. Specifically, the ankle plantar flexors act eccentrically to control the forward motion of the leg throughout stance, to stabilize body mass, and at toe-off to assist in propulsion. Their greater mass requires more ankle plantar flexor torque to perform these tasks.
Obese people try to reduce the load on their knees by shortening their stride and reducing knee extensor torque. In an obese cohort, the greater the BMI, the shorter the stride and the lower the knee-joint extensor/flexor torque, actually shifting from an overall extensor torque to a dominant flexor torque at high BMIs. This switch results in the hamstrings, rather than the quadriceps, providing knee stability. In lean subjects, no relationship exists among BMI, stride length, and knee torques, indicating that lower BMI values have little effect on gait.20
Liu and Nigg21
examined the effects of rigid and soft tissue mass on impact forces during running. They termed the soft tissue mass wobbling mass
. Their spring-damper-mass model consisted of upper- and lower-body rigid and wobbling masses. A computer simulation found that upper-body wobbling mass had no effect on impact forces but strongly influenced the propulsive peak. As upper-body wobbling mass increased, vertical force propulsive peaks increased, suggesting that obese individuals exert greater forces during gait due to their greater wobbling mass.
Empirical data support Liu and Nigg’s model. Messier et al.22
found a strong positive association between BMI and peak ground reaction forces (r = 0.76, p = 0.0001) in older adults with knee OA. A study by Browning and Kram23
found that obese people exerted 60% greater vertical ground reaction forces compared to normal weight people ().
Obese exert 60% more force than normal weight individuals.
Abnormal gait is characteristic of obese people. Messier et al.24
found that a severely overweight population walked with bilaterally abducted forefeet, or a stance that was 276% more toed-out than that of a normal weight group. Chodera and Levell25
suggested that the feet have different functions, with the more abducted forefoot responsible for balance and the less abducted foot responsible for direction. In severely obese people, the amount of abduction is significantly greater in both feet relative to a normal weight control group, suggesting that balance is more important than direction.24
In addition to greater forefoot angles, severely obese people have more rearfoot motion: typically, greater touchdown angle, more pronation range of motion, and faster pronation. This excessive rearfoot motion may cause injury and discomfort and negatively affect mobility.
Plantar fasciitis and heel pain are commonly associated with obesity. In a case-controlled study, obese subjects were 5 times more likely to have heel pain than their nonobese counterparts, with an odds ratio of 5.6.26
Similarly, obese men and women exert greater plantar pressure while both standing and walking.27–31
Hills et al. found a significant correlation (r = 0.81) between midfoot peak pressure and BMI28
(). Gravante et al.32
found greater midfoot weight-bearing area in obese men and women versus a control. The additional pressure on the medial longitudinal arch could have a detrimental effect on the plantar ligaments, causing them to collapse. Considering that the medial longitudinal arch is critical in distributing loads to both the rearfoot and forefoot, it is not surprising that foot ailments are common among the obese.
In summary, obese adults exert greater forces than normal weight adults during gait. As obesity worsens they try to minimize these loads by shortening their stride. However, adjusting gait mechanics without reducing body weight does not eliminate obesity’s detrimental effects on the lower extremities.
What role obesity plays in the degenerative process is unclear. The multiphasic nature of articular cartilage permits it to withstand compressive stresses as high as 20 MPa, or 3000 lb/in2
A densely woven collagen fibrillar network, water (normally < 80% by wet weight), and ionic species of Na+, Ca++, and Cl− provide a unique combination of material properties that prevents such high loads from crushing the tissue. Cartilage has little permeability, resulting in large interstitial fluid pressures inside the tissue during compression. This pressurized fluid accounts for most of the load-bearing capability and protects proteoglycans and chondrocytes from dangerously high stresses and strains. Moderate running increases cartilage matrix synthesis and may have a protective effect on the joint.34 35
Peak knee-joint compressive forces in humans during long-distance running range between 10–14 times body weight.36
In spite of these high forces, most studies have shown no relationship between running and OA.37
Compressive loads during walking are less than half those found during running.38
Hence, obesity’s cumulative effect on knee-joint loads during daily activities must play the critical role in the disease process.