July 2018

PERSPECTIVE: Biomechanics Research

By Karen Mickle, BSc (Hons), PhD

Dr. Mickle is a postdoctoral research fellow at the Institute of Health and Sport, Victoria University, Melbourne, Victoria, Australia. She has disclosed no relevant relationships.

LER: Your research on the role of lower-extremity muscle strengthening in preventing injury, especially falls in older people, has delivered valuable observations as well as recommendations for lower-extremity practitioners. What is the context for your work?

KM: Falls are the leading cause of injury, disability, hospitalization, and death in older adults, with 1 in 3 people 65 years and older falling at least once a year. Many aspects of declining physical functioning, including muscle strength, are regarded as an inevitable consequence of aging, and we know that age-related loss of muscle mass is one of the main determinants of frailty.

This decline in muscle strength usually appears around the 6th decade, and is accompanied by slower walking speed and an increased risk of disability and falls. Muscles within the feet, including those that control the toes, also suffer from atrophy and an associated loss of strength with aging. My research, and that of others, has shown that, compared with younger counterparts, older adults have as much as 38% less toe flexor muscle strength17-19 and that the size of their individual toe muscles is reduced by as much as 45%.19

Figure 3. With the Toe Strength Dynamometer (Human Locomotion), the patient sits in a chair with the heel and forefoot flat on the floor. The examiner places the plastic card beneath the second through fifth toes and pulls the device in a straight line (arrow) while the patient resists by gripping with the toes. Strength is recorded and the process is repeated beneath the hallux. Strength measurements are compared bilaterally.

Consider that, during most activities of daily living, the foot is the only source of direct contact with the ground. The foot therefore plays a critical role in maintaining stability and balance, as well as providing sensory feedback regarding body position and movement. If the toes cannot adequately push down on the ground, they receive little or none of the sensory information from the contacting surface. That input helps most people manage the body’s ever-swaying center of mass. Reduction in toe strength is therefore likely to have a profound effect on the ability of older adults to walk safely, which could be critical for balance recovery or when attempting to take corrective steps to maintain balance in a near-fall situation.

LER: What did you and your colleagues uncover regarding foot-related risk factors for falling?

KM: Not surprisingly, we found that fallers had significantly less strength of the hallux and lesser toes (a reduction of > 20%) than nonfallers. As quadriceps and ankle strength did not differ between fallers and nonfallers, reduced toe strength recorded for fallers was deemed unlikely to be a marker of generalized lower-limb muscle weakness. Furthermore, hallux strength was found to be a stronger predictor of falls than other, more commonly measured falls risk factors, such as age, gender, falls risk score (which includes assessments of visual contrast sensitivity, proprioception, knee extension strength, reaction time, and postural sway standing on a foam rubber mat), and quadriceps strength. In fact, each unit increase (by percentage of body weight) in hallux strength decreased the odds of sustaining a fall by 7%.20

LER: How are your findings translatable to potentially better care for patients?

KM: In response to what we found, I developed a foot-strengthening exercise program and conducted a randomized clinical trial to determine whether the program could increase toe strength in older people. After baseline testing, 85 men and women (age, 60 to 90 years) were randomized to a supervised progressive-resistance “toe-training” program (n = 43) or home-based exercise (n = 42) for 12 weeks. An additional 32 participants were recruited as controls (no intervention). Participants in the toe-training group attended 3 group-exercise classes a week for 12 weeks. During each class, an exercise physiologist led participants through a series of 10 foot exercises, which progressively increased in level of difficulty throughout the intervention.

Participants in the toe-training group significantly increased their hallux and lesser-toe strength (as much as 36%) compared to baseline, whereas there was no significant change in toe strength in the home-based exercise or control groups.13 Additional benefits of the toe-training program for participants in the intervention group included a significant increase in single-leg balance time with eyes open (+5 seconds) and closed (+3 seconds), as well as significant improvement in the general foot-health domain of the Foot Health Status Questionnaire.

Despite these promising findings, we need further funding to run a full clinical trial to determine whether performing these foot exercises can actually prevent falls in older people. However, I believe we should encourage older people, particularly those considered at high risk of falling, to perform foot-strengthening exercises. The presence of hallux valgus and lesser-toe deformities has also been associated with poorer performance on balance and functional tests21 and is an independent predictor of falls—such that people with a lesser-toe deformity have been found to be twice as likely to fall as people without such a deformity.22

LER: Your research has also examined the association of toe deformities and muscle weakness. What have you learned from your and others’ work?

KM: Toe deformities are highly prevalent in older adults. Reports indicate that 35% to 74% of older men and women having some degree of hallux valgus and approximately 60% of older adults exhibit a deformity in at least 1 of toes 2 to 5. My research has shown that the intrinsic foot muscles of older adults with hallux valgus and lesser-toe deformities are significantly smaller than those of older adults without toe deformities.21

Regrettably, we are unable to determine whether muscle weakness is a cause or an effect of these deformities. However, other researchers have found that performing the short-foot exercise over a 2-month period can increase hallux flexor strength and the cross-sectional area of the abductor hallucis muscle (the muscle that abducts and flexes the big toe) in patients with pes planus.22 I believe that strengthening the intrinsic toe muscles might reduce the incidence and severity of toe deformities in older adults.

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