Midlife Predictors of Old Age Functioning: Insights from Longitudinal Cohort Studies

Addressing the challenges posed by aging populations is a primary focus of public health efforts worldwide. Central to this effort is the enhancement of functional ability among older adults, as good functional ability promotes maintaining independence, autonomy, and a high quality of life. Moreover, supporting healthy aging not only benefits individuals and communities but also carries profound implications for the demands placed on social and healthcare systems.

The process of developing and maintaining functional ability throughout older age is influenced by various factors experienced across one's lifetime. This study seeks to contribute to the understanding of these factors. Through the adoption of a life course perspective, the aim is to investigate the effects of mid-life predictors, including education level, occupational physical activity, and the presence of low back pain on functioning in old age. Instead of a single-point assessment, this study incorporates the developmental trajectories of low back pain and occupational physical activity from mid-working life to retirement, providing a broader history of exposure to these factors. Furthermore, the mechanisms through which educational attainment influences functioning are investigated.

This study utilizes data from three different cohort studies that have tracked individuals from their mid-working years to old age. These cohorts include the Finnish Health 2000 and 2011 Studies (with an 11-year follow-up period), the Finnish Longitudinal Study of Ageing Municipality Employees (FLAME) (spanning from 1981 to 2009), and the Health and Retirement Study (HRS) in the USA (covering the period from 1992 to 2016). At baseline, the participants included in the analysis were aged 55 years and older in the Finnish 2000 Study, 44 to 58 years in FLAME, and 50 and older in the HRS.

The overall outcome of the study, functioning, comprises both objectively measured and self-reported data. In Study I, the maximum walking speed of Finnish older adults aged 55 years and older was measured at baseline and again at an 11- year follow-up. The outcome variable, the decline in maximum walking speed, was calculated as the difference between these two measurements. In Studies II and III, the outcome variables were self-reported mobility limitations and disability. These outcomes were assessed 28 years after the baseline in FLAME (in 2009) and 24 years after the baseline in HRS (in 2016). The outcomes were assessed with self-reported limitations in walking and stair climbing collected through questionnaires (FLAME) or phone interviews (HRS). Any level of difficulty was considered as having mobility limitations. Similarly, any level of difficulty in performing basic activities of daily living was considered as having a disability. Data on the main predictors (education level, occupational physical activity, and low back pain), and the covariates were collected through questionnaires (Finnish Health 2000 Study and FLAME) or interviews (HRS).

Study I focused on educational disparities in walking speed decline over an 11-year follow-up among Finnish general population aged 55 years and older. General linear model was employed to investigate variations in the decline of maximum walking speed across different education groups. Mediation analyses, utilizing the product method, were carried out to dissect the overall effect of education on the decline in maximum walking speed into direct and indirect effects mediated through intermediaries. The findings indicated that lower education levels were associated with a more rapid decline in maximum walking speed, with the differences between education groups widening as individuals aged. Additionally, the results indicated that higher lifetime exposure to physical workload and higher body mass index among individuals with lower education levels contributed significantly to the educational differences observed in mobility decline.

The results of Study II also underscore the role of midlife physical workload in increasing the risk of functional limitations in old age. The study investigated the developmental profiles of occupational physical activity in Finnish and U.S. aging workers, and their associations with mobility limitations and disability two decades later. Repeated measurements of self-reported occupational physical activity were collected during approximately an 11–12-year period using harmonized data from FLAME and HRS. Latent class growth analysis was used to identify the profiles, and risk ratios with 95% confidence intervals for the associations of the profiles and later life mobility limitations and disability were estimated using Poisson loglinear regression analyses. The results showed that perceived occupational physical activity remained relatively stable among the workers during the follow-up, particularly among workers reporting high or low occupational physical activity at baseline, whereas people reporting moderate level occupational physical activity had higher fluctuation in their exposure level. Notably, consistently high occupational physical activity significantly increased the risk of self-reported mobility limitations in old age.

In Study III, low back pain emerged as another crucial determinant of old age functioning. Using data from FLAME, the study investigated low back pain trajectories from midlife to retirement among Finnish municipal workers, and the association of these trajectories with mobility limitations and disability at old age. Latent class growth analysis was used to identify the low back pain trajectories and adjusted logistic regression analysis to investigate the associations of the derived LBP trajectories with the outcomes. The results showed three distinct trajectories of low back pain: high-decreasing, intermediate-stable, and low. Members of the high-decreasing and the intermediate-stable trajectories exhibited markedly more mobility limitations and disability in old age compared with those in the low trajectory. Moreover, individuals working in physically demanding occupations (“blue-collar” workers) were more likely to belong to the high-decreasing low back pain trajectory than those working in physically lighter occupations, suggesting that low back pain may be an important mediator between physical workload and late-life functional limitations.

In summary, the results of the study showed that all the investigated mid-life predictors - educational level, occupational physical activity, and presence of low back pain, were significantly associated with old age functioning, thus highlighting their importance for healthy aging. This study pinpointed several modifiable risk factors acting during a sensitive period spanning from mid-working life to old age that profoundly impact functioning beyond retirement. The research underscores the importance of mitigating physical work demands, in particular among less educated individuals, and managing mid-life low back pain to enhance functional ability and ensure quality of life in old age. These insights contribute to understanding the sensitive period in late working life as pivotal for interventions aimed at supporting sustained functional independence among the aging population.