February 2016

Biomechanists explore effect of obesity on falls

2obesity_iStock73702875Obese patients have an increased risk for falls and fall-related injury, not just because of the loads involved, but also because of the strategies they use to recover from a trip or slip. Findings from a growing body of research will help in developing interventions to minimize that risk.

By Hank Black

People who are obese fall and injure themselves at a higher rate than those who are normal weight,1,2 and scientists and clinicians are working to identify the specific biomechanical variables involved in their falls in hopes of finding ways to prevent them. Identification of mediating factors as well as clinical interventions to help obese individuals safely recover from slips, trips, and other falls are also key research goals.

The combination of two major demographic trends in the US and other developed countries—aging populations and increasing incidence of obesity—are driving increased attention to this problem. About 35% of the US population is considered obese, up from only 13% in the 1960s.3 The unintentional-fall death rate for adults 65 years and older increased more than 40% from 2004 to 2013.4 A direct link is seen between obesity (body mass index [BMI] of more than 30 kg/m2) and the risk of occupational injury due to falls.5,6

The consequences of falls among older individuals are well known. People 65 years and older make up 13% of the US population but account for three-fourths of all deaths caused by falls.7 An estimated 31% of individuals 60 years and older are obese.8 In recent years, obesity has started receiving major attention from the scientific community. In June 2013, the American Medical Association (AMA) House of Delegates adopted a resolution reclassifying obesity as a disease state requiring advances in treatment and prevention.9 Even so, obesity is not yet included as a risk factor in many falls-prevention websites, including that of the Centers for Disease Control and Prevention.

Some research suggests interventions should focus on tasks that challenge mediolateral stability, such as unipedal stance and walking on a narrow path.

“Epidemiological research indicates there is a higher rate of falling among those who are obese, but we don’t understand why,” said Michael L. Madigan, PhD, professor of biomedical engineering at Texas A&M University in College Station, TX. “We are trying to understand the underlying mechanisms and factors and develop interventions to target those factors.”

He was among several researchers whose work on falls was presented at the most recent annual meeting of the American Society of Biomechanics (ASB), held in August 2015 in Columbus, OH. Madigan was senior author of ASB presentations10,11 that found increased rates of falls for obese subjects after laboratory-induced trips and slips.

“We found obesity does not appear to cause more frequent slips or trips, but it does negatively affect the ability to recover balance and prevent a fall compared with normal-weight people,” he said.

Strategies for fall recovery

2obesity_iStock75072815MDAfter tripping—by stubbing a toe on an exposed tree root, for example—most people instinctively try to recover balance by stepping, Madigan said.

“Two stepping strategies are commonly used. In the lowering strategy, you immediately lower the stubbed foot to the ground and then step over the root with the contralateral foot,” he said. “The elevating strategy is employed when the foot hitting the root is not put to the ground but instead is lifted over the root.”

Successful trip recovery is achieved by slowing the forward rotation of the body/trunk12 and maintaining sufficient hip height to allow subsequent steps.13 These accomplishments require activation of the knee extensors, hip extensors, and ankle plantar flexor muscles. In a 2012 study of lab-induced trips of middle-aged and older women living in the community,14 normal-weight fallers tended to be able to complete a recovery step attempt before falling into a support harness, while the majority of obese fallers were not. The study indicated these falls reflect obesity-related differences in reaction and response times, onset of support limb loading, and recovery limb power generation.14

“Recovery for either elevating or lowering strategy requires not so much strength as power, when you look at force recovery on the leg,” said study coauthor Noah J. Rosenblatt, PhD, assistant professor in the Department of Podiatric Surgery and Applied Biomechanics at Rosalind Franklin University of Medicine and Science in Chicago. “There’s not necessarily a relationship between lower extremity strength and recovery. It’s more the ability to coordinate your muscles and the timing of their firing.”

To safely study trips and slips in a laboratory setting, participants wear a harness supported from the ceiling so no one falls to the ground. Madigan’s study of trips10 recruited 21 young adults, 11 of whom were obese. While walking purposefully and looking straight ahead on a 10-m experimental walkway, the participants were tripped near midswing with a 7 cm-high obstacle. The resulting fall rate was 30% among obese adults and 0% among their nonobese counterparts.

The higher rate of obese-adult falls may have resulted from factors that contribute to trunk kinematics, the authors stated. These include an anterior shift of the trunk center of mass (COM) among obese adults,15 which could increase the gravitational moment that rotates the body forward after tripping; greater trunk mass,16 which increases trunk momentum; and relatively low trunk and lower extremity strength,17,18 which could reduce the ability to slow trunk momentum.

Obese and nonobese participants who recovered balance and avoided falling into the harness tended to use similar recovery strategies, but there were differences between the obese fallers and the other participants. In particular, all three fallers employed a lowering strategy, while the 15 of 16 individuals who recovered used an elevating strategy. Madigan said, though, that there wasn’t enough data to determine whether the choice of strategy contributed to these falls. Fallers also exhibited a larger trunk angle and angular velocity, and a lower hip height at the instant of foot strike of the first recovery step. Madigan said he hopes to use these preliminary results to plan a larger-scale study that would provide more definitive results.


Slips cause more than 25% of fall-related injuries in older adults11 and 40% to 50% of occupational fall-related injuries.19 Slips happen where there is too little friction or traction between a person’s footwear and a walking surface.20

As people get older and frail, they walk slower, with smaller and wider steps and a stooped posture, said Thurmon E. Lockhart, PhD, professor of biomedical engineering and biomedical design at Arizona State University in Tempe.

“It’s a kind of shuffling along flat-footed, because the elderly want to veer away from problems that would initiate a slip, especially if they have experienced a previous fall,” he said.

The same types of adaptations, however, actually make an obese person more prone to initiate a slip, according to a 2012 study by Lockhart and colleagues.21

“Importantly, we found that obese people are more likely than the nonobese to slip in a sideways, transverse direction,” Lockhart said.

At the recent ASB meeting, Madigan and colleagues addressed obesity and age in lab-induced slips and confirmed that obese individuals experienced a higher rate of falls, with 32% falling compared with 10% of nonobese participants.19 After adjusting for age, gender, and gait speed, the researchers found obese participants who slipped were more than eight times more likely than nonobese individuals to experience a fall.


The degradation of postural balance is thought to be a major reason for most types of falls, according to Lockhart, though most of the research in this area has focused on aging.22-27 Hue et al found a strong relationship between weight and postural instability and that young adults who are obese swayed at a faster velocity than a person of normal weight.28

Lockhart said that, for 20 years, most researchers have assessed balance control with linear measures such as COP speed, COP ranges, and sway density parameters. More recently researchers have found that nonlinear spatiotemporal measures may capture more subtle aspects of postural control.29 An example is estimating the time required for the COP to reach the boundary of the base of support if it were to continue on its instantaneous trajectory and velocity.

“Traditional measures ignore the time-dependent stability and complexity of postural fluctuations,” he said. “Together, both linear and nonlinear analyses provide a more complete understanding of adaptive strategies used in postural control than either could provide alone.”

Falls and fractures

Also at the ASB meeting, Crenshaw et al looked at fall biomechanics in an attempt to answer why the incidence of wrist fractures in women plateaus or declines after age 75 years, when hip-fracture incidence begins to rise.30 They evaluated 125 community-dwelling women aged 65 to 91 years using laboratory assessments and prospective tracking of falls over one year. Although not analyzing specifically for obesity, the study included participants who had a BMI greater than 30 kg/m2.

Using motion-capture technology, the investigators assessed anterior and posterior falls and direction-specific fall risk. Older age was associated with impaired recovery strategies for both anterior and posterior falls. Because recovery from an anterior fall includes protective arm movements, this impairment led to fewer wrist fractures but increased the risk of other serious injury. Meanwhile, the increased rate of posterior falls with increasing age leads to more fractures of the pelvis or hip.

The study has important implications for the clinic, one of the study’s coauthors said in an email. Kenton R. Kaufman, PhD, PE, professor of biomedical engineering and director of the Biomechanics/Motion Analysis Laboratory at Mayo Clinic in Rochester, MN, suggested therapists focus on tasks that challenge mediolateral stability, such as unipedal stance, obstacle crossing, and walking on a narrow path.

“If it is possible to use compensatory step training, focus on upper extremity reactions, not solely on foot placement,” Kaufman added.


2obesity_iStock21886591Weight-loss programs, strength training, balance training, and other activity regimens have been shown to help lower the risk of falls in obese individuals older than 65 years.31

Controlled whole-body vibration training has been established to enhance neuromuscular performance and reduce risk of falls among elderly adults.32 In a study presented at the recent ASB meeting, Munoz et al found that a six-week course of training significantly improved maximum isometric knee extensor strength as well as dynamic gait stability among young obese adults, which the authors suggest could help reduce the risk of falls.33

Other novel modalities that have shown promise for preventing or reducing falls in obese people include yoga34 and horseback riding.35

“If improving a person’s weight or balance doesn’t sufficiently lower their risk of fall, we focus as a compensatory strategy on making the living environment safe and accessible,” said American Occupational Therapy Association spokesperson Katie Riley, OTD, OTR/L, associate professor of clinical occupational therapy and director of Occupational and Speech Therapy Hospital Practice at the University of Southern California in Los Angeles.

And Mitchell et al, in a large, Australian population study of obese and older community-living people, determined that sleeping medications, sedentary behavior, and chronic health conditions were the strongest mediators of falls.36 It and other studies suggested weight reduction through increased physical activity or bariatric surgery as potential ways to prevent falls.37,38 In fact, Handrigan et al have found that weight loss in people with a BMI greater than 40 kg/m2 is more important than maintaining strength for improving balance control.39

While general exercise programs may reduce fall risk and rate of falling by about 30%,32 those improvements may not be able to keep up with the rate at which the population of older adults—and the subpopulation of obese older adults—is increasing. Task-specific training (TST) may help prevent falls in obese older adults more effectively than do more general programs, Madigan said.40

“This type of training has shown some promise among people over the age of 65 years in our work and that of others. The evidence that this is a beneficial fall prevention strategy is growing, although it’s not currently being used extensively outside the laboratory,” he said.

TST allows individuals to experience and practice recovery strategies that are associated with a low risk of falling, said Rosenblatt, who also studies TST.

“It’s critical that a person take a recovery step while sensing how to do that, and task-specific training allows you to sense how to do that,” Rosenblatt said. “Basically, if you want to learn to throw a football, you practice with a football, not a baseball.”

Diminished cognitive abilities may be expected with aging, of course, but possibly also with increased weight. Birdsill et al found an increased relationship between body fat, brain functioning, and volumes of gray and white matter in the brain in older females.41

Jeffrey Hausdorff, PhD, professor in the Sackler School of Medicine at Tel Aviv University in Israel and director of the Center for the Study of Movement, Cognition and Mobility, was senior author of a five-year prospective study that linked fall risk to executive function, which is responsible for allocating attention among several tasks, prioritizing activities, and the like.42

“We had shown that it’s hard enough for some people to walk in a controlled environment, but in normal life we have many distractions while walking. If executive functioning is impaired, you are more likely to fall,” Hausdorff said.

To more fully assess this relationship, and with hopes of including obesity as a variable, his group is using functional MRI and functional near-infrared spectroscopy (fNIRS), which allows a recording of an individual’s brain activation during a walk.

A role for orthoses

Ankle foot orthoses (AFOs) can help improve postural stability and other variables associated with risk of falling (see “Can AFOs help prevent falls?” August 2012, page 16), but can be problematic for obese patients with regard to fitting issues and durability.

Eric Weber, LCPO, FAAOP, a national orthotics specialist with Hanger Clinic in Seattle, WA, said the introduction of composite materials such as carbon fiber or fiberglass in custom-made AFOs give more support to obese individuals than conventional thermo­plastics.

“AFOs made with polypropylene or other thermoplastics are more susceptible to break down over time by compressive and tensile forces, becoming more flexible, decreasing any restriction on the joint, ultimately resulting in fracture or failure of the brace, and in a fall,” Weber said. “When building custom AFOs for heavier people, the posterior strut itself is what’s replacing the stress points around the ankle. I expect to see more designs of the posterior composite in the future because it provides increased flexibility without limiting the ankle motion.”

In fact, he said, some designs allow for the posterior strut to be removed and replaced with different and/or stiffer struts. Others are incorporated into the laminate itself so the entire orthosis flexes and extends as a unit.

“The advantage is that so much of the stress, which can result in fracture points, can now be distributed through the posterior of the leg,” Weber said. “I believe heavier people who have been dissatisfied with previous materials will find themselves more likely to use braces with these composite materials.”

Vibrating insoles, he said, can be beneficial to balance by providing haptic feedback for an insensate lower limb, which can be an issue in obese patients with diabetes or vascular disease.

“Orthotists must be careful in adapting inserts,” Weber said. “The insert may be haptic in feedback but not contour exactly to the plantar surfaces of the foot and cause undue pressures within a smaller area. The neuropathic foot requires a very accommodating insert that spreads out the plantar pressure evenly.”

Hank Black is a freelance writer in Birmingham, AL.

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