The Acute Effects of Sitting Time on Physiological and Psychological Function in Older Adults

Until relatively recently, sedentary behaviour was considered as the 'lower end' of the physical activity spectrum. However there is accumulating evidence from both epidemiological and observational studies which shows that sedentary behaviour per se rather than just low physical activity is also a major modifiable risk factor for chronic disease and predicts cardiovascular and all-cause mortality, independent of the amount of physical activity: A followup of the Australian Diabetic (AusDiab) cohort study showed a 46% increased risk of all-cause and an 80% increased risk of cardiovascular disease mortality in those watching four or more hours of television (TV) per day, compared with those watching TV for <2 h per day. In the Canadian Fitness Survey, a dose response relationship was shown between sitting time and cardiovascular disease and all-cause mortality. Significantly poorer long term mortality outcomes were found in those reporting spending most of the day sitting compared with those reporting less time sitting and this was consistent across all levels of self reported sitting time. Crucially, sitting time mortality relationships were apparent even among those who were physically active at other time points in their day. Although there is evidence supporting the longer term detrimental health effects of prolonged bouts of sedentary behaviour (sitting) across the age range, limited information exists with respect to the acute (or 'last bout') effects of sedentariness on the physiological and psychological health of older people.

Interestingly the current UK physical activity guidelines state that "while there is sufficient evidence to support a recommendation to reduce sedentary behaviour in older adults, it is not currently possible to suggest a specific time limit". There is the need to be able to characterise sitting time in older adults in terms of acute functional and cognitive effects, as well as identify the main effects that older people perceive as being important after periods of sitting. Thus there is a possibility of generating specific advice to older people, but this requires the elucidation of dose response relationships between sitting and health outcomes to be defined during carefully controlled studies. Although these studies have yet to be conducted, it is possible to select 'key' outcome measures based upon established evidence derived from epidemiological and smaller scale observational studies (viz recent studies reporting a significant negative association between sedentariness and physical and psychological impairment in older adults) as well as from other 'models' of physical inactivity such as bed rest and limb unloading. One such outcome is skeletal muscle power output. The ability to quickly produce sufficient muscle power is of paramount importance for controlling body movements during mobility related activities in older people, such as standing up from a chair or stair climbing. Muscle power output is therefore a functionally relevant outcome of the utmost importance to the maintenance of physical independence. In addition, older people are less able to maintain core temperature, partly due to a reduced muscle mass yet it is known that reducing muscle temperature by only 1˚C can result in a decline of power output of up to 26%. Work by the current study group has recently shown a reduction in muscle power (45%), sit to stand performance velocity (9%), gait speed and maximum quadriceps strength after only 45 minutes sitting in a cool environment (15 degrees Celsius). It is not known if sitting (and consequent muscle inactivity) in normal temperature environments leads to reduced power in older people. However, older people talk of more difficulty getting out of a chair when they have been sitting for long periods, so muscle power may be implicated. In older people with joint pain, the benefits of keeping moving and not just sitting are clear.

Although work on the effect of cold environments on cognition (dual task cost in walking speed and executive function using the trail making test) did not show any significant changes after 45 minutes exposure to 15 degrees Celsius in older people, little is known about effects on mood, well being or other cognitive and psychological outcomes of sitting. Associations between sitting and tiredness and depression have been shown in a longitudinal study of a middle aged Australian cohort in those who spent a total of >9 h/day sitting compared with those who spent <6 h/day sitting. However little is known about the effects of acute bouts of sitting. Although there is a body of evidence on the acute effects of physical activity, no published data reporting acute effects of sitting, or on dose response effects on psychological outcomes was found.

Thus, the current understanding of the acute effects of sitting time is severely limited. The proposed study will inform the development, implementation and evaluation of future interventions to reduce sedentary behaviour in older people by identifying key elements of the immediate (acute) benefits of breaking up long periods of sitting, aiding engagement with older adults about immediate individual level benefits of reduced sitting and identifying outcome measures sensitive to changes in sitting time. Alongside views of older people, it will provide practical messages that can ensure more effective take up of an intervention among older people and effective advice for the professionals that work with older people. For example, for professionals working in residential home settings or on hospital wards/ intermediate or transition care settings (post acute illness or surgery), the results of this research will give valuable information on desirable time periods for encouraging mobilisation.