Increasing BMI has an adverse effect on balance, muscle strength and gait, especially vertical ground reaction forces. The NHANES I and Epidemiologic Follow-up (NHEFS) studies revealed that obesity at baseline increased upper and lower body disability across 20 years of follow-up 5;8
. More recently, Jenkins 9
found that functional impairment in older adults increased as BMI increased. 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 10
As body weight increases there is an increase in both fat mass and fat free mass 11
. The relationship between fat free mass and BMI is stronger in males, suggesting that an increase in BMI in females is due predominately to an increase in fat mass. Although these data imply that obese people have greater strength than their non-obese counterparts, the opposite actually is true. Specifically, when strength is represented as a function of body weight, both obese males and females are weaker, irrespective of age 12
. In older adults aged 60–80 years, mean knee strength in obese males is 65% of body weight compared to 77% for controls, and 50% of body weight for obese females compared to 62% for non-obese females.
Muscle weakness in older adults is of great consequence. It is the second leading cause of falls in the elderly, accounting for 17% of all falls 13
. Falls are the leading cause of death by injury in older adults. Three-quarters of deaths due to falls occur in adults older than 65 years. Only one-half of those older adults hospitalized due to falls will be alive after 1 year. By 2030, deaths due to falls will reach 280,000 Americans annually14
Falls occur due to a loss in balance; hence, it is common to use measures of balance to identify people who are more susceptible to falls. Kejonen et al. 15
found significant correlations between poor balance and high BMI in women, but not in men, once again suggesting that muscle weakness is a mediator of poor balance and falls. Within older adults with a BMI above 30 kg/m2
, Jadelis et al.16
found that for a given amount of knee strength, the more severe the obesity, the worse the balance. This suggests that obesity, independent of strength, is a risk factor for poor balance and falls.
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 non-obese counterparts with an odds ratio of 5.6 17
. Similarly, obese men and women exert greater plantar pressure during both standing and walking 18–22
. Hills et al. found a significant correlation (r = 0.81) between mid-foot peak pressure and BMI 19
(). Gravante et al. 23
found greater mid-foot 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 resulting in its collapse. Considering that the medial longitudinal arch is critical in distributing loads to both the rearfoot and forefoot, it is not surprising that foot aliments are common among obese people.
Figure 2 Relationship between midfoot peak pressure and BMI19.
Abnormal gait is also characteristic of obese people. Messier et al 24
found that a severely overweight population walked with bilateral abducted forefeet or a more toed-out stance that was 276% greater than a normal weight group. Chodera and Levell 25
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, indicating a need for balance and suggesting that balance is more important than direction24
In addition to the greater forefoot angles, severely obese people have more rearfoot motion.24
More specifically, greater touchdown angle, more pronation range of motion, and a faster pronation velocity are typical of severely obese gait. This excessive rearfoot motion may cause injury and discomfort and have a negative effect on mobility.
Liu and Nigg 26
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 had a strong influence on the propulsive peak. As upper body wobbling mass increased, vertical force propulsive peaks increased. In contrast, an increase in lower body wobbling mass showed a strong influence on impact peak forces. These results suggest that obese individuals would exert greater forces during gait, due to their greater wobbling mass.
Empirical data support Liu and Nigg’s model. Messier et al 27
found a strong positive association between BMI and peak ground reaction forces (r = 0.76, p = 0.0001) in older adults with knee OA. This was also the case in a study by Browning and Kram 28
, who found that obese people exerted 60% greater vertical ground reaction forces compared to normal weight people ().
Figure 3 Vertical ground reaction forces during the stance phase of gait. Obese individuals exert 60% more vertical ground reaction force than normal-weight individuals28.
Obesity is also related to a fear of falling and injury risk 29;30
. Austin et al 29
followed 1,282 community-dwelling women aged 70–85 years for 3 years and found that fear of falling at baseline was independently associated with obesity, and obesity also was associated with the new-onset of fear of falling 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% 30
Obese adults make adjustments to help stabilize their larger mass and reduce fall risk. DeVita et al. 31
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, assist in propulsion. The greater mass in obese people requires more ankle plantar flexor torque to perform these tasks.
Obese people also try to reduce the load on the knee by shortening stride length and reducing the knee extensor torque. In an obese cohort, the greater the BMI, the shorter the stride length and the lower the knee joint extensor/flexor torque, with an actual shifting from an overall extensor torque to a dominant flexor torque at high BMIs. This switch results in knee stability being provided by the hamstrings rather than the quadriceps. In lean subjects, the relationships between BMI and stride length and knee torques do not exist, indicating that lower BMI values have little effect on gait 31
. In summary, during gait obese adults exert greater forces than normal weight adults. As obesity worsens this compensatory strategy increases, and minimizing these loads is attempted by shortening stride length. Taken together, these results suggest that adjusting gait mechanics without reducing body weight does not eliminate the detrimental effects obesity has on the lower extremity.