Developing a Physiological Understanding of High Duration Activity

Overview

When muscles are not contracting, the local energy demand by muscle and use of specific fuels used to produce energy by oxidative metabolism are minimal. The time people spend sitting inactive (sedentary time) typically comprises more than half of the day. This sedentary behavior is associated with elevated risk of diabetes, cardiovascular diseases, some cancers, and multiple conditions leading to poor aging. From a progressive series of experiments, the driving goal is to develop a physiological method for sustaining contractile activity via oxidative metabolism over more time than is possible by traditional exercise (hours, not minutes per day). Developing a physiological method suitable of prolonged muscular activity for ordinary people (who are often unfit) requires gaining fundamental insights about muscle biology and biomechanics. This also entails a careful appreciation of the ability to isolate specific muscles in the leg during controlled movements, such as the soleus muscle during isolated plantarflexion. This includes quantifying specific biological processes that are directly responsive to elevated skeletal muscle recruitment. The investigators will focus on movement that is safe and practical for ordinary people to do given their high amount of daily sitting time. This includes developing methods to optimally raise muscle contractile activity, in a way that is not limited by fatigue, and is feasible throughout as many minutes of the day as possible safely. This also requires development of methodologies to quantify specific muscular activity, rather than generalized body movement. There is a need to learn how much people can increase muscle metabolism by physical activity that is perceived to them as being light effort. It is important to learn if this impacts systemic metabolic processes under experimental conditions over a short term time span in order to avoid confounding influences of changes in body weight or other factors.

Physical activity/inactivity will be carefully measured with objective devices. Wearable devices most commonly include accelerometers capable of capturing various types of movement and body posture. The intensity of muscle activation (the soleus and other leg muscles) will be measured in some participants in the developmental studies with EMG, with the limb motion quantified with goniometry. Skeletal muscle and whole body metabolism will be evaluated, especially after isolated local contractile activity focusing on the slow oxidative soleus muscle. Blood chemistry will also be investigated in this comprehensive series of studies to understand how replacing sedentary time with low effort muscular activity can be enhanced. Glycemia will be evaluated in the postprandial period in the morning after an overnight fast when there can be standardized control of carbohydrate ingestion. This includes a standardized oral glucose tolerance test with careful experimental assessment of posture and muscular recruitment during the testing periods. One phase of this study is particularly interested in assessing the acute responses that occur immediately as a result of contractile activity, while also evaluating in another phase how this may be impacted by a change in the sedentary lifestyle. This includes assessing new approaches for improving metabolism throughout the day by reducing the amount of time sitting inactive (i.e. sedentary time). Importantly, because the potential immediate benefits of muscle contractile activity are directly dependent on the duration of activity, the investigators aim to develop in a series of experiments how much muscular activity time can be performed comfortably and safely by anybody instead of sitting inactive with low muscle metabolism regardless of age, fitness, body type, and other conditions commonly limiting effectiveness of traditional exercise prescriptions.