The title of this post reflects the findings reported by Davy Vancampfort and colleagues  (open-access) who concluded that various diagnoses - "schizophrenia, bipolar disorder or major depressive disorder" - under the label of severe mental illness were associated with decreased physical activity and increased sedentary behaviour(s).
Under the auspice of a "global systematic review and meta-analysis", authors settled on some 70-odd studies that met their search criteria comprising over 35,000 people diagnosed with one of the conditions described and nearly 3000 asymptomatic (asymptomatic for severe mental illness) controls. Vancampfort et al describe assessing for "co-primary outcomes" that "were the mean time (min) per day that people with severe mental illness and healthy controls (in case-control studies) engaged in physical activity, or were sedentary." They found "23 study estimates of physical activity were based on objective measures, three utilized objective and subjective measures and 57 were based on self-report questionnaires"; something important in the context of the technology available to measure activity these days.
Results: Those diagnosed with a severe mental illness (SMI) were "more sedentary than age- and gender-matched controls from the general population, spending a mean of 476 min per day (or almost 8 hours) during waking hours in sedentary behavior." Further: "people with severe mental illness are significantly less physically active and spend only an average of 38.4 min per day in moderate or vigorous physical activity." When tweeting about this article, one of the authors - Brendon Stubbs - also linked to another important paper  published on the same day as their own, observing that: "Increasing physical activity is a simple, widely applicable, low cost global strategy that could reduce deaths and CVD [cardiovascular disease] in middle age." It's not difficult to see the connection(s) between the two findings.
"Our data documented that higher body mass index, lower cardiorespiratory fitness, and antidepressant or antipsychotic prescription might constitute barriers for engaging in physical activity." One of the value-added bits to the Vancampfort data were the discussions on some of the barriers to engaging in physical activity in those with SMI. Combined with observations such as: "Those who are single or unemployed, those with a low educational level and men are less physically active" it's not too difficult to see what areas might need to be 'tackled' if physical activity levels are to be improved. Indeed, other data (see here) has even talked about what types of exercise might be best suited to what labels (minus any sweeping generalisations).
And since I've mentioned the topic of depression in this post, I might also draw your attention to another paper recently published by Joseph Firth and colleagues  - one of the authors on the Vancampfort paper - talking about how technology might provide a useful backdrop to intervention for depressive symptoms. Specifically how the delivery of smartphone apps might be something useful to consider in the context of depression. Indeed, if one assumes that physical activity levels might also be lower in relation to something like depression, one could forsee a time when one or more smartphone apps might either prompt the need for more physical activity or even potentially offer a tailor-made physical activity schedule complementary to other intervention options...
Music to close and Sammy singing Mr Bojangles...
 Vancampfort D. et al. Sedentary behavior and physical activity levels in people with schizophrenia, bipolar disorder and major depressive disorder: a global systematic review and meta-analysis. World Psychiatry. 2017. Sept 21.
 Lear SA. et al. The effect of physical activity on mortality and cardiovascular disease in 130 000 people from 17 high-income, middle-income, and low-income countries: the PURE study. Lancet. 2017. Sept 21.
 Firth J. et al. The efficacy of smartphone-based mental health interventions for depressive symptoms: a meta-analysis of randomized controlled trials. World Psychiatry. 2017 Oct;16(3):287-298.