Recent research has focused on the detrimental effects of a sedentary lifestyle. Researchers have hypothesized that breaking up sedentary bouts of behavior with physical activity will improve the health of sedentary individuals. However, the mechanisms that result in health improvements have not been elucidated. Therefore, the primary aim of this study quantify the total energy expenditure of three different durations of physical activity within a 30-minute sedentary period and to examine the potential benefits of interrupting sedentary behavior with physical activity for weight control. Our results demonstrate that standing up and walking at a usual, self-selected pace for one minute during a 30-minute sitting period resulted in an additional 3.0 kilo-calorie net expenditure compared with 30 minutes of sitting. When extrapolated for a full 5 day, eight-hour per day working week, if an individual stood up and walked for 1-minute every hour, they would theoretically expend an additional 120 kilocalories per week compared with sitting for 8 hours. Further, standing up and walking at a normal, self selected pace for two and five minutes once every hour during an eight hour work day would result in 296 and 660 additional kilocalories per week, compared with sitting. This level of energy expenditure is likely to have an important impact on weight maintenance, or even possibly weight loss.
These results have tremendous public health relevance. In 2003, Hill et al suggested that most of the population weight gain that we have seen over the past few decades could be eliminated by some combination of increasing energy expenditure and reducing energy intake by 100 kilocalories per day . Based on the data in this study, if individuals who have sedentary occupations, stood up and walked for at least 5 minutes every hour (walk to the water fountain, to a colleagues desk, etc.), they would attain this theoretical threshold to prevent weight gain and potentially positively impact chronic disease.
Data from this study showed that if an individual took a 1-minute break from sitting every half hour, they would theoretically expend 48 more kilocalories compared with sitting, and an individual who stood up and walked for 2 minutes every hour would expend an additional 59 kilocalories per day, a caloric difference of 11 kilocalories, with no difference in time. Therefore, our data do not lend direct support to the results of Healy et al.  who demonstrated that, independent of total time spent being sedentary and time spent in moderate-to vigorous-physical activity, more breaks in sedentary time were associated with a more beneficial waist circumfe-rence, body mass index, triglyceride level and 2-h post load glucose level. Our data, showing an extrapolated difference of 11 kilocalories per day expenditure between a 1-minute break every half hour and a 2-minute break every hour suggest that the more beneficial metabolic profile associated with more frequent breaks may not be due to differences in energy expenditure. However, it should be noted that breaks in the Healy et al study were at least 1-minute in length with an average duration of 4.5 minutes, were light intensity, and were frequent (average of 86 breaks during the day) . Together these data suggest that muscle contractions involved in standing up and sitting down may significantly contribute to the beneficial associations seen with more frequent breaks by Healy et al .
During the walking breaks in bouts 2, 3 and 4 average oxygen consumption of all participants was 5.0 ml/kg/min, 12.7 ml/kg/min and 38.7 ml/kg/min. The average walking speeds of all participants were 2.2 mph, 2.4 mph, and 2.5 mph. The compendium of physical activities  documented "walking when gathering things at work ready to leave" as 3.0 METS or 10.5 ml/kg/min, "walking less than 2.0 mph on a firm surface" as 2.0 METS or 7.0 ml/kg/min, and "walking 2.5 mph on a flat surface" as 3.0 METS or 10.5 ml/kg/min. Although our calculations of ml/kg/min from compendium data assumed a resting metabolic rate of 3.5 ml/kg/min while measured resting metabolic rate for this study was 3.1 ml/kg/min, our walking results are comparable to the compendium of physical activities.
There are a few important strengths and limitations to take into account while considering the results of this study. First, the primary dependent variable, net energy expenditure, was assessed using indirect calorimetry and calculated using measured resting metabolic rate. Second, although the study population was small, the observed power was calculated at over 0.8 for all repeated measures performed in this study. Third, the focus of the current study was focused on disrupting sedentary behavior within a regular working day. Current results are delimited to those 20-39 years of age. However, it is likely that the increase in measured energy expenditure associated with disrupting sedentary behavior will carry over to individuals over the age of 39 years. Finally, although data was collected in a lab-setting, this intervention allowed the participants to choose a sedentary desk activity and walk at their own pace during breaks, to better mimic the daily activities of an individual with a sedentary job.
These data do not address a number of very important considerations that should be taken into account in future investigations. First, during this intervention, participants were asked to interrupt their sedentary time with walking behavior. In real world situations, this may or may not happen. For instance, an individual may choose to break up their sedentary time by standing to talk with a colleague or walking to the break room to get a snack. In these situations, the energy expenditure would most likely be less, and if the individual consumed calories during this time, i.e. with a snack, the net consumption may well outweigh the net expenditure. Understanding more about how individuals will choose to break up sedentary time will, in addition to the results of this paper, provide good insight into designing interventions to break up sedentary time in order to positively impact health.