Active Voice: Lie, Sit, Stand or Ambulate? The Measurement of Body Postures

By David R. Bassett, Jr., Ph.D., FACSM

Viewpoints presented in SMB commentaries reflect opinions of the authors and do not necessarily reflect positions or policies of ACSM.

Dr. David R. Bassett, Jr., is an exercise physiologist and researcher at the University of Tennessee in Knoxville, Tenn. His research focuses on the validity and reliability of objective measurements of physical activity, cross-sectional studies of the relationship between physical activity and health, and behavioral interventions.

This commentary presents Dr. Bassett’s views on the topic related to a research article which he and colleagues authored. Their research article appears in the October 2014 issue of Medicine & Science in Sports & Exercise® (MSSE).

There are many ways to group activities that people perform in everyday life in order to quantify and describe their physical activity so that it is easily reported and understood. For instance, activities are often grouped by intensity level (sedentary, light, moderate and vigorous). But another method is to group activities by the body postures people assume when performing them (lying down, sitting, standing and ambulating). “Postural allocation” refers to the amount of time people spend each day in different postures. Researchers are becoming convinced that the amount of time spent sitting may influence our health.

The activPAL™ (PALtechnologies, Glasgow, Scotland) is a miniature device worn on the thigh. Roughly the size of a USB thumb drive, it has a miniature tri-axial accelerometer that senses dynamic accelerations produced by human movements, as well as static acceleration due to gravity. The accelerometer in this device can detect the angle of incline of the thigh, allowing it to distinguish between lying/sitting, standing and ambulating. However, since the thigh is horizontal when sitting and lying down, it cannot differentiate these two body postures.

One could ask why it is important to distinguish lying down from sitting. One reason is that lying down usually represents sleep, and sleep has health-enhancing, restorative powers. Prolonged sitting, on the other hand, is viewed as deleterious to health - this applies more to individuals who do not perform leisure time physical activity than to those who perform aerobic activity for hours on end.

In our study, recently reported in MSSE, we examined whether wearing two activPAL™ devices, one each on the thigh and torso, would allow four body postures (lying down, sitting, standing and ambulating) to be accurately classified. Fifteen healthy adults performed a routine consisting of lying down, sitting, standing, sweeping (floors), treadmill walking at 3 mph and treadmill running at 6 mph. We then used the data from both activPALs to predict a person’s body posture. When both activPALs were horizontal, that was taken to represent lying down. When the thigh device was horizontal and the activPAL™ on the torso was vertical, that was taken to represent sitting. When both activPAL™ devices were vertical, that was taken to indicate the upright position. This method of classifying body postures showed good agreement with direct observation.

The notion that sedentary behavior is deleterious to health is not a new idea. Around 1700, Bernardino Ramazzini, the founder of occupational medicine, studied people in various jobs and noted that scurrying messengers were healthier than sedentary tailors and bookkeepers. He referred to the latter two classes of workers as “chair workers.”

Ralph Paffenbarger, Jr., MD, DrPH, devised a questionnaire for the College Alumni Health Study, which began in 1960. His famous questionnaire is best known for its physical activity index (PAI), computed from distance walked, flights of stairs climbed, and sports and recreation. However, Paffenbarger also had the foresight to ask people how much time they spent during a usual 24-hour day, engaged in sleeping/reclining, sitting, light activity (e.g. office work, strolling, standing with little motion), moderate activity and vigorous activity. Some researchers are skeptical about people’s ability to accurately recall these behaviors, so they have turned to wearable activity monitors for more accurate answers.

There is growing evidence that desk jockeys, long-distance commuters and television-viewing couch potatoes are at increased risk for metabolic syndrome and premature death. Interventions are needed to identify practical, low-cost and acceptable ways of reducing time spent in these sedentary pastimes, or converting them to more active ones. Measurement tools are needed to quantify the magnitude of behavior changes induced by these interventions.