The role of skeletal muscle contractile duration throughout the whole day: reducing sedentary time and promoting universal physical activity in all people

Abstract

A shared goal of many researchers has been to discover how to improve health and prevent disease, through safely replacing a large amount of daily sedentary time with physical activity in everyone, regardless of age and current health status. This involves contrasting how different muscle contractile activity patterns regulate the underlying molecular and physiological responses impacting health-related processes. It also requires an equal attention to behavioural feasibility studies in extremely unfit and sedentary people. A sound scientific principle is that the body is constantly sensing and responding to changes in skeletal muscle metabolism induced by contractile activity. Because of that, the rapid time course of health-related responses to physical inactivity/activity patterns are caused in large part directly because of the variable amounts of muscle inactivity/activity throughout the day. However, traditional modes and doses of exercise fall far short of replacing most of the sedentary time in the modern lifestyle, because both the weekly frequency and the weekly duration of exercise time are an order of magnitude less than those for people sitting inactive. This can explain why high amounts of sedentary time produce distinct metabolic and cardiovascular responses through inactivity physiology that are not sufficiently prevented by low doses of exercise. For these reasons, we hypothesize that maintaining a high metabolic rate over the majority of the day, through safe and sustainable types of muscular activity, will be the optimal way to create a healthy active lifestyle over the whole lifespan.

Keywords: metabolic regulation; muscle activity; skeletal muscle.

Figure 1. Volume of total daily sitting duration

A, quantitative data for the daily time spent sitting of a typical sedentary person as measured with a device that measures upright vs sitting time and also the time engaged in ambulation at different intensities. From this, the cumulative daily time spent sitting was approximately 11.5 h, which was caused by dozens of sitting bouts. B, the relative volume of time occupied by each sitting bout, the number of daily sitting bouts in a sedentary person, and the relatively small influence that a bout of recommended MVPA has on either sedentary parameter (bout number or total sedentary time). The total volume of the square represents the total awake time. The round balls represent individual sitting bouts that fill up approximately 68% of the waking day. Because these sitting bouts are spread throughout the entire day and often without consistent patterns due to variations in lifestyles of different people, the sitting bouts in this graph are not depicted in any order. Instead the panel emphasizes how sedentary time fills up the waking day in variable ways. The theme when interpreting these data is the realization that there is an amount of sitting time spread throughout the whole day in various bout lengths and various patterns that creates an enormous challenge in reducing sedentary time and requires highly focused efforts on this problem. And, because of the need to specifically eliminate as many sedentary bouts and as much sedentary time as is safely possible, innovative behavioural strategies will produce results far different from the longstanding MVPA recommendations.