Asymmetrical Lower Extremity Power Deficit as a Risk Factor for Injurious Falls in Healthy Older Women

To the Editor: In older people, falls often cause injuries, which may lead to hospitalization, institutionalization,1 and mortality.2 Frequent fallers often have poorer lower extremity power (LEP) and asymmetrical deficit when compared to nonfalling controls.3 Asymmetrical LEP deficit, which also exists in healthy older women,4 may complicate postural control when moving and thereby increases the risk of falls. The aim of this study was to determine whether the incidence of injurious falls was higher in healthy older women with asymmetrical LEP deficit. Four hundred three healthy women aged 63 to 75 from the Finnish Twin Study on Aging participated in the reported assessments after signing a written informed consent. The ethics committee of the Jyväskylä Central Hospital Board approved the study. LEP was measured on both sides using the Nottingham power rig,5 unless the physician observed contraindications for testing.4 Body height and weight, chronic disease burden, lower limb pain, and habitual physical activity were assessed.4 Data on falls and related injuries were collected prospectively for 1 year using monthly fall calendars and telephone follow-up.6 A fall was defined as unintentionally coming to rest on the ground, floor, or other lower level, other than as the consequence of sudden onset of acute illness or overwhelming external force.7 Falls resulting in fracture, bruise, laceration, or pain were considered injurious. Two dichotomous variables were created: at least one injurious fall versus no injurious falls and recurrent (two or more) injurious falls versus no or one injurious fall. The average of LEP and difference between the legs was calculated. Asymmetrical LEP deficit was defined as a difference in LEP at the far end of the distribution: participants belonging to the tertile with the largest LEP difference (>17 W) and participants with LEP measured in one leg only (n=20, because of contraindications for power testing). The sample was treated as a set of individuals; all continuous variables were adjusted for the dependency between the twin sisters using a random-effects general linear model.4 Group differences were tested using analysis of variance and Mann–Whitney U tests. Logistic regression was used to estimate the relative risks (RRs) and 95% confidence intervals (CIs) for falls, adjusting for important determinants of LEP and falls (age, height, and weight) and other potential confounders (number of diseases, physical activity, and lower limb pain). Statistical significance was set at P<.05. No differences between those with (n=146) and without (n=257) asymmetrical LEP deficit were observed in mean age±standard deviation (68.5±3.4), body height (158.6±5.7 cm) and weight (70.0±10.4 kg), number of diseases (2.3±1.0), and mean LEP of two legs (93.0±28.5 W) or in the proportions reporting only light walking once or twice a week (27%) and reporting no pain in the lower limbs (35%). During follow-up, 111 participants reported a total of 171 injurious falls. Of those with asymmetrical LEP deficit, 22% reported a single injurious fall, and 12% recurrent injurious falls. For those without asymmetrical LEP deficit, the corresponding figures were 18% and 5%. The RR for at least one injurious fall was 1.7 and for recurrent injurious falls was 2.4 in those with asymmetrical LEP deficit compared with those without (Table 1). Adjusting for important determinants of LEP and falls (Table 1) and potential other confounders (data not shown; confounders did not predict injurious falls) did not change the estimates. Analyses excluding those with LEP measured in one leg rendered RRs of 1.5 (95% CI=1.0–2.4) and 2.1 (95% CI=1.0–4.6), respectively. This prospective study showed that asymmetrical LEP deficit is an independent and strong risk factor for injurious falls, especially for recurrent injurious falls. The most likely explanation is that the relative weakness of one limb decreases the ability to maintain postural stability and counterbalance significant perturbations. The risk associated with asymmetrical LEP deficit is independent of and similar to that of established risk factors, including being aged 80 and older, the presence of specified chronic diseases, and gait balance and functional deficits.8 It is known that recurrent falling may eventually lead to serious injuries such as hip fracture that are associated with high risks of disability and mortality.9 The costs of fall-related injuries are substantial, and healthcare utilization is high.8 The causes of asymmetrical LEP deficit are unclear, but one explanation is the persisting effects of unilateral disease and injury.10 Returning symmetrical muscle function should be stressed in rehabilitation after disease or injury. Research on asymmetrical deficits in muscle strength and power and its consequences may help to develop improved prevention strategies for injurious falls. This study was funded by the Finnish Ministry of Education, the Juho Vainion Foundation, the Finnish Cultural Foundation and the Academy of Finland. The results of this study were presented at the Congress of the European College of Sports Sciences 2005 in Belgrade, Serbia and Montenegro. Financial Disclosure: Erja Portegijs and Satu Pajala: Funded by the Finnish Ministry of Education, the Juho Vainion Foundation, and the Finnish Cultural Foundation. Sarianna Sipilä: Funded by the Finnish Ministry of Education. Sarah Lamb, Markku Alen, Jaakko Kaprio, and Markku Koskenvuo: None. Taina Rantanen: Funded by the Finnish Ministry of Education and the Academy of Finland. Author Contributions: Erja Portegijs: formulation of the study question, analysis of the data, and writing of the manuscript. Sarianna Sipilä and Sarah Lamb: participated in the design and finalization of the manuscript. Satu Pajala: managed the fall surveillance of the study and participated in the analyses and finalization of the manuscript. Markku Alen: participated in finalization of the manuscript with a special focus on the medical aspects of the study. Jaakko Kaprio: participated in the design and implementation of the study and finalization of the manuscript. Markku Koskenvuo: participated in the design and implementation of the study. Taina Rantanen: participated in the design and implementation of the study, organization of funding and finalization of the manuscript as the project leader. Sponsor's Role: None of the funders had further interest or involvement in the design, methods, subject recruitment, data collection, and preparation of the manuscript.