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To investigate the mediating role of social network size and perceived quality in the associations of physical activity with quality of life and depressive symptoms in middle-aged and older adults. We analyzed information of 10,569 middle-aged and older adults from waves 2 (2006–2007), 4 (2011–2012), and 6 (2015) of the Survey of Health, Ageing, and Retirement in Europe study (SHARE). Data on physical activity (moderate and vigorous intensities), social network (size and quality), depressive symptoms (EURO-D scale), and quality of life (CASP) were self-reported. Sex, age, country of residency, schooling, occupational status, mobility, and baseline values of the outcome were used as covariates. We created mediation models to test the mediating effect of social network size and quality in the association between physical activity and depressive symptoms. Social network size partly mediated the association between vigorous physical activity and depressive symptoms (7.1%; 95%CI: 1.7–12.6) as well as the association between moderate (9.9%; 1.6–19.7) and vigorous (8.1%; 0.7–15.4) physical activity and quality of life. Social network quality did not mediate any of the associations tested. We conclude that social network's size, but not satisfaction, mediates part of the association of physical activity with depressive symptoms and quality of life in middle-aged and older adults. Future physical activity interventions among middle-aged and older adults should consider increasing social interactions to facilitate benefits for mental health-related outcomes. [ABSTRACT FROM AUTHOR]

• Moderate to vigorous levels of physical activity were independently associated with a lower incidence of mild cognitive impairment. • Handgrip strength was associated a lower hazard for incident mild cognitive impairment and dementia. • Handgrip strength partly mediated the association between moderate to vigorous levels of physical activity and mild cognitive impairment. We analyzed the mediating role of handgrip strength in the association between moderate to vigorous physical activity and incident mild cognitive impairment and dementia. We used prospective data from 14 European countries participating in the Survey of Health, Ageing, and Retirement in Europe. 19,686 participants free of dementia and mild cognitive impairment (64.9 ± 8.7 years) were followed up for a mean of 10.2 years. Moderate to vigorous physical activity was self-reported, and handgrip strength was assessed with a dynamometer. Mild cognitive impairment was defined as 1.5 standard deviations below the mean of the standardized global cognition score, while dementia was determined by physician diagnosis. Gender, age, country, education, presence of chronic diseases, depressive symptoms, limitations in activities of daily living, body mass index, and baseline cognitive levels were used as covariates. Cox proportional hazards as well as mediation models were used. Moderate to vigorous physical activity for at least 1 day per week was independently associated with lower incident mild cognitive impairment (HR: 0.85; 95%CI: 0.74–0.98). A 10 % increase in handgrip strength was associated a 6 % lower hazard for incident mild cognitive impairment (0.94; 0.92–0.97) and 5 % lower hazard for incident dementia (0.95; 0.93–0.98). Handgrip strength partly mediated the association of moderate to vigorous physical activity with mild cognitive impairment (Coefficient: 0.03; 95%CI: 0.01–0.05; 17.9 %). Physical activity is independently associated with a lower incidence of mild cognitive impairment. [ABSTRACT FROM AUTHOR]