Sensorimotor and psychosocial determinants of 3-year incident mobility disability in middle-aged and older adults

Abstract

Objective: to identify sensorimotor and psychosocial determinants of 3-year incident mobility disability.

Design: prospective.

Setting: population-based sample of community-dwelling older persons.

Participants: community-living middle-aged and older persons (age: 50–85 years) without baseline mobility disability (n = 622).

Measurements: mobility disability, defined as self-reported inability to walk a quarter mile without resting or inability to walk up a flight of stairs unsupported, was ascertained at baseline and 3-year follow-up. Potential baseline determinant characteristics included demographics, education, social support, financial condition, knee extensor strength, visual contrast sensitivity, cognition, depression, presence of chronic conditions and history of falls.

Results: a total of 13.5% participant reported 3-year incident mobility disability. Age ≥75 years, female sex, knee extensor strength in the lowest quartile, visual contrast sensitivity <1.7 on the Pelli-Robson chart or significant depressive symptoms (CESD score >16) were independent determinants of 3-year incident mobility disability (ORs 1.84–16.51).

Conclusions: low visual contrast sensitivity, poor knee extensor strength and significant depressive symptoms are independent determinants of future onset of mobility disability.

Introduction

Mobility is defined as the ability to walk safely and independently in one's natural environment [1] and thus, is a pre-requisite for one's independence in activities of daily living (ADLs and IADLs) [2], preserving social interactions and for maintaining overall quality of life [3]. In older persons, disability usually occurs first in mobility and sets a downward spiral of progressively more severe disablement [4].

It is generally accepted that detecting mobility disability during an early or pre-clinical stage is likely an important opportunity for implementing preventive measures [5]. However, the pathway to this initial stage of disablement process in older persons, unless preceded by a catastrophic event, is not well understood. The incidence of mobility disability increases with ageing even when not accompanied by overt co-morbid conditions [6] and compared with older men, the rate of incidence is higher in older women [7]. Several studies have independently shown that poor lower limb muscle strength [8], body mass index (BMI) >35 [9] and lower education in women [10] are a significant predictor of mobility disability. Although motor control studies have demonstrated the impact of poor visual contrast sensitivity on measures of balance control during various physical tasks (e.g. during stepping [11] or walking [12]), the evidence for the association between mobility disability and visual impairment is not available. Similarly, although many studies have reported a relationship between depressive symptoms and advanced stage of disability (disability in ADL) [13], how depression modulates or contributes to the initiation of disablement process is not known.

Since the available evidence is predominantly sporadic from independent studies focusing on specific parameters, the purpose of this study was to perform a comprehensive analysis to identify sensorimotor (visual contrast sensitivity and knee extensor strength) and psychosocial (depression, cognition, perceived financial status and social support) factors that are independent determinants of future incident mobility disability.

Methods

Participants

The InChianti study population represents the population living in the Chianti region of Tuscany, Italy. The study design and data collection have been described previously [14]. The study protocol was approved by the ethical committee of the Italian National Institute of Research and Care of Aging and complies with the Declaration of Helsinki. All participants signed an informed consent. The present study used the 2001–02 data as baseline data and the 2004–05 data as the 3-year follow-up. A total of 873 participants between the ages of 50 and 85 years attended baseline interview session. In the final analysis, the participants who had a complete data set for the interview and the physiological measures and with no severe cognitive impairment [Mini-Mental State Examination score (MMSE) <18 [15]] or self-reported mobility disability at baseline were included (n = 622). InChianti participants over the age of 85 years were excluded as majority of them reported mobility disability at baseline leaving a very small sample size representing that the age group.

Outcome measures

Mobility disability was identified using the established approach (self-reported inability to walk a quarter mile without resting or inability to walk up a flight of stairs unsupported [16]). Mobility disability was ascertained at baseline and at the 3-year follow-up. The potential determinants of incident mobility disability were measured at baseline. Demographic variables included age, sex and BMI (kg/m²). Education was recorded as the years of formal education. Social support was determined by the number of persons completely available, if need be, from the following nine categories of people: spouse, son, daughter, son-in-law, daughter-in-law, other relatives and other non-relatives. The question ‘do you think your current income is adequate for your needs?’ was developed for the InChianti study to determine self-perceived financial condition. The options were adequate, barely adequate and inadequate. The replies were dichotomised into inadequate (barely adequate and inadequate) and adequate.

Center for Epidemiological Studies Depression scale (CES-D) [17] and MMSE [18] quantified depressive symptoms and global cognition, respectively. Knee extensor strength was measured using a hand-held dynamometer that was placed on the anterior side of the shin 10 cm above the lateral malleolus. Three trials were performed bilaterally and the maximum value of the weaker leg was recorded [19]. The Pelli–Robson chart [20] was used to assess binocular visual contrast sensitivity. The presence of chronic disease/s was recorded from participants' self-report and medical records and self-reported history of fall/s in the previous year was noted.

Statistical analysis

The baseline differences between the participants who reported 3-year incident mobility disability and those who did not report it were determined using a general linear model or a Chi-square test, as appropriate, for the demographics, psychosocial and sensorimotor characteristics, presence of chronic diseases and a history of falls in the previous year. The variables that were different between the two groups (P < 0.1) were included in the binary logistic regression analysis to identify significant independent determinants of 3-year incident mobility disability. For this analysis, age was classified into three groups (50–64, 65–74 and 75–85 years) and the age group 50–64 years was used as the reference group. Low education was identified as education less than the high school level [21]. Poor cognition and significant depressive symptoms were defined as MMSE score <24 [15] and CES-D scale score >16 [17], respectively. The suggested lowest normal Pelli–Robson test score for older persons is 1.5 logMAR [22]. However, at baseline there were only 11 participants with score <1.5 logMAR. Therefore, a cut-off level of 1.7 logMAR, which is a reported average score in older persons [23], was used. Since clinically relevant cut-off points are not clear in literature for knee extensor strength, the sex-specific lowest quartile was used as a demarcation point. Data were analysed using IBM SPSS version19. A P < 0.05 was considered statistically significant.

Results

Out of the 873 participants initially included, 677 participants did not report mobility disability at baseline. Out of these 677 participants 622 participants attended the follow-up and were included in the final analysis. Of these 622 participants, 3.5% had a history of stroke, 2.9% had myocardial infarction, 4.5% reported angina pectoris, 47.7% had hypertension, 11.4% had diabetes and 2.5% had peripheral arterial disease; 1.7% reported hip replacement, 21.5% had either hip or knee pain requiring medication and 4.6% reported severe deafness. A total of 82 participants were between the ages of 50–64 years, 320 participants were between 65 and 74 years and 220 participants were 75–85 years old. The 55 participants who did not come at the follow-up were older [72.5 (7.0) versus 74.3 (5.8) years, P = 0.005]. However, they were not different in sex distribution (P = 0.497).

A total of 84 participants reported mobility disability at the 3-year follow-up (total 13.5%, men 8.7%, women 17.6%). The gender-specific distribution of incident mobility disability in each age group was as follows: age group 50–64 years—men 0%, women 2.3%, age group 65–74 years—men 6.0%, women 9.9%, age group 75–85 years—men 16.2%, women 33.9%. Those who reported mobility disability at the follow-up were older and had higher proportion of women, less formal education, more depressive symptoms, lower knee extensor strength and worse visual contrast sensitivity at baseline (P < 0.10) (Table 1).

Table 1.

Baseline differences in the demographic, sensorimotor and psychosocial characteristics between those with and without 3-year incident mobility disability

Variable	No incident mobility disability (n = 538/622)	Incident mobility disability (n = 84/622)	F or X2 and P-value
Age (years)	70.7 (70.1–71.3)	76.4 (74.9–78.9)	49.203, <0.001
Sex (women %)	51.3	70.2	10.484, 0.001
BMI (kg/m2)	26.3 (26.0–26.6)	26.8 (26.1–27.6)	1.438, 0.231
Years of education	6.5 (6.3–6.8)	5.3 (4.5–6.1)	7.266, 0.007
Social support	2.7 (2.5–2.9)	2.4 (1.8–2.8)	1.840, 0.175
Perceived inadequate financial condition (%)	52.4	62.9	2.754, 0.252
Cognition (MMSE)	26.8 (26.6–27.1)	25.5 (25.1–28.2)	14.391, <0.001
Depressive symptoms (CESD)	13.2 (12.5–13.8)	16.8 (15.2–18.5)	16.890, <0.001
Contrast sensitivity (Pelli–Robson score)	1.69 (1.69–1.70)	1.66 (1.66–18.6)	22.550, <0.001
Knee extensor strength (kg)	17.6 (17.2–18.1)	13.7 (12.8–14.8)	48.923, <0.001
Presence of chronic disease (%)	55.9	53.6	0.166, 0.683
History of falls in previous year (%)	18.2	25.0	2.162, 0.142

The group differences were identified using general linear model or Chi-square analysis, as appropriate. Accordingly, mean (95% confidence interval) and % values are provided.

BMI, body mass index, MMSE, Mini-Mental State Examination, CES-D, Center for Epidemiological Studies Depression scale.

The results of the model 1 binary logistic regression analysis assessed for demographic characteristics demonstrated that women and those 75 years and older were significantly more likely to report incident mobility disability. Next, the variables that were different between those who reported and those who did not report incident mobility disability (Table 1) were dichotomised using known cut-off points and included in the final model of binary logistic regression analysis (model 2). The final model demonstrated that participants with significant depressive symptoms (CES-D > 16) [17] were almost twice as likely to report incident mobility at the follow-up. Further, those with poor visual contrast sensitivity (Pelli–Robson score <1.7) [20] were 2.4 times as likely and who were in the lowest quartile of knee extensor strength (men <17.7 kg, women <11.7 kg) were 2.8 times as likely to report 3-year incident mobility disability. Even in this fully adjusted model, being ≥75 years of age or being a woman was highly indicative of incident mobility disability. The MMSE scores <24 or not having at least high school level education were not independently associated with incident mobility disability (Table 2).

Table 2.

The predictors of 3-year incident mobility disability

Variable	Model 1			Model 2
	B (SE)	P-value	OR (CI)	B (SE)	P-value	OR (CI)
Age group (overall)		<0.001			<0.001
Age group 1 (65–74 years)	1.959 (1.028)	0.067	6.66 (0.87–50.57)	1.756 (1.039)	0.091	5.79 (0.75–44.34)
Age group 2 (75–85 years)	3.371 (1.019)	0.001	29.10 (3.94–214.05)	2.804 (1.036)	0.007	16.51 (2.17–125.71)
Sex (women)	0.799 (0.266)	0.003	2.22 (1.32–3.73)	0.708 (0.287)	0.014	2.03 (1.15–3.56)
Low educationa				0.037 (0.433)	0.931	1.03 (0.44–2.42)
Poor cognitionb				0.050 (0.317)	0.874	1.05 (0.56–1.95)
Significant depressive symptomsc				0.614 (0.268)	0.022	1.84 (1.09–3.12)
Poor contrast sensitivityd				0.866 (0.384)	0.024	2.37 (1.12–5.04)
Knee extensor strength lowest quartilee				1.002 (0.268)	<0.001	2.77 (1.64–4.69)
Constant	−4.965 (1.025)	<0.001	0.01	−5.189 (1.068)	<0.000	0.01

Model 1 included only demographic variables (age group, sex). In addition to demographics, model 2 also included baseline variables that were different (P < 0.100) between the participants with and without 3-year incident mobility disability. Odds ratios (OR) and respective 95% confidence intervals (CI) are reported.

^aLow education: education less than high school level (n = 521 had low education).

^bPoor cognition: Mini-Mental State Examination score <24 (n = 88 had poor cognition).

^cSignificant depression: Center for Epidemiological Studies Depression scale score >16 (n = 204 had significant depression).

^dPoor contrast sensitivity: Pelli–Robson Score <1.7 (n = 44 had poor contrast sensitivity).

^eKnee extensor strength lowest quartile: men <17.7 kg , women <11.7 kg (n = 142 , 72 males and 70 females, had strength in the lowest quartile.

Discussion

Using the population-based data, this comprehensive study demonstrated independent sensorimotor and psychological determinants of 3-years incident mobility disability. The results demonstrated that the middle aged and older persons with lesser knee extensor strength, poor visual contrast sensitivity or significant depressive symptoms were highly likely to report incident mobility disability in 3 years.

To our knowledge, this is the first study that demonstrated independent role of visual contrast sensitivity in predicting incident mobility disability. The contrast sensitivity that declines with increasing age has been shown to be associated with poor balance control and falls in the elderly [24]. Further, poor contrast sensitivity is also associated with the need to use handrails while using the staircase [25] suggesting that optimum contrast sensitivity is necessary for visual processing while negotiating stairs. Considering that one of the two questions that are commonly asked for ascertaining mobility disability specifically focuses on the stair climbing ability without using support, it is clear why poor contrast sensitivity can be the independent determinant of incident mobility disability.

Optimum vision is also critical when one independently ventures in complex community environments. It is suggested that compared with those with normal vision, persons with poor vision (poor visual field and contrast sensitivity) travel less on foot independently and avoid activities [26]. Particularly, decreased ability to detect edges due to poor contrast sensitivity may impose challenge while encountering steps, curbs and pavements commonly found in the community. When asked about ability to walk a quarter mile, it is highly likely that participants envision walking in the community or neighbourhoods rather than walking in an inert experimental environment. Therefore, visual contrast sensitivity could be a determining factor whether they choose to or are able to walk a quarter mile independently. Although the cut-off level for the normal Pelli–Robson test score of older persons is suggested to be 1.5 logMAR [23], our results indicate that the cut-off point of 1.7 logMAR is useful for predicting mobility disability in future. Therefore, middle-aged and older persons with common vision pathologies that particularly impact contrast sensitivity in the early stages (e.g. cataract, glaucoma) should be specifically monitored for maintaining mobility.

Very few studies have investigated the role of lower limb strength for predicting self-reported incident mobility disability using a prospective study design. Visser et al. [27] have demonstrated similar findings in an older cohort of a narrower age-range (70–79 years). Our results indicate that compared with the rest of the participants, the participants in the lowest quartile were twice as likely to report mobility disability at the 3-year follow-up in the much broader age spectrum of 50–85 years.

The incidence of mobility disability in our population-based sample was twice in women compared with men even after adjusting for other possible confounding variables. When we used the quartiles of the overall strength data (i.e. for all 622 participants disregarding the sex), it was noted that those in the lowest quartile of the overall strength data (knee extensor strength <13 kg) were twice as likely to report incident mobility disability (data not presented). More importantly, the association of the sex and incident mobility disability disappeared after adjusting for the overall knee extensor strength. When data were explored further, it was noted that only 6.5% men had the knee extensor strength <13 kg (lowest overall quartile cut-off). In contrast, 40.1% women's knee extensor strength was <13 kg suggesting that higher incidence of mobility disability in women can be explained by innate lower muscle strength. Our findings raise the question whether implementing sex-specific strength-related thresholds are appropriate for clinical decision-making.

The relationship between depressive symptoms and prevalence [13] and recovery [28] of ADL-related disability has been examined. Our findings demonstrated that the presence of significant depressive symptoms may play a crucial role even in the onset of disablement process. To understand clinical impact of these findings intervention studies are warranted to investigate whether timely intervention of depressive symptoms can avert or significantly postpone the initiation of disablement process in older persons.

Vincent et al. [9] have shown that when adjusted for other covariates the association of BMI and mobility disability exists only for BMI >35. In the present cohort only 10 out of 622 participants had BMI >35. Therefore, possibly, the relationship with BMI was not extracted. The evidence also suggests the relationship of poor cognition and ADL-related disability. However, this relationship has been more frequently established for executive function and not for the global cognition as measured by MMSE [29]. The data were re-analysed by lowering the MMSE cut-off point to 21 (instead of 24 as relatively fewer participants were high school graduates [30]). This re-allocation of MMSE cut-off level did not alter the results. Similarly, in the fully adjusted model, low education was not independently associated with incidence of mobility disability. Previous studies have identified low education as a predictor of adverse health-related outcomes [21]. However, many of these studies focused primarily from the socio-economic perspectives. It is possible that inclusion of cognition, depression, vision and strength addressed the possible risk factors associated with low education thus, rendering the low education as not significant.

The limitation of this study is that the population primarily comprised Caucasian older people living in small towns with the low level of education and therefore, may not completely represent a racially mixed and possibly, highly educated older cohort living in metro/city areas. Additionally, the mobility disability assessment was performed using a dichotomous variable and not a continuous variable. However, the method of mobility disability assessment used in this study is the most commonly used approach primarily in the longitudinal studies of ageing.

Further, the fall history, perceived social support and a total number of chronic diseases were not found to be predictors of mobility disability. However, no fall-related diary was maintained by participants to confirm self-reported falls in the previous year. Further, the reliability of the social support outcome measure used in this study has not been established and the diagnosis of chronic disease/s was not re-established in the study. Therefore, it is possible that these measures were rendered methodologically less robust.

In conclusion, when several potential determinants are taken into consideration, poor visual contrast sensitivity, low knee extensor strength and significant depressive symptoms were independent determinants of the 3-year onset of mobility disability. Adults over the age of 50 years should seek timely intervention if they have these sensorimotor deficits or depression related problems. Identifying modifiable sensorimotor and psychological factors that may contribute to incident mobility disability can have direct impact on primary prevention of the onset of mobility disability. The incidence of mobility disability was significantly higher in women and in those ≥75 years of age even after adjusting for multiple factors. Therefore, the mobility status of particularly older women should be monitored on a routine basis.

Key points.

• Older persons with vision pathologies affecting contrast sensitivity should be monitored for maintaining mobility.

• Higher incidence of mobility disability in women may be explained by innate lower muscle strength.

• Having significant depressive symptoms is an independent predictor of the initiation of the disablement process.

Conflicts of interest

None declared.

Funding

The InCHIANTI study was supported as a ‘targeted project’ (ICS 110.1\RS97.71) by the Italian Ministry of Health and in part, by National Institute on Aging Contracts N01-AG-916413, N01-AG-821336, N01-AG-5-0002 and NIA Grant R01 AG027012 and the Intramural Research Program, National Institute on Aging, NIH.