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Physical activity and sedentary behavior patterns using accelerometry from a national sample of United States adults

Background: This study described the patterns of accelerometer-determined physical activity and sedentary behavior among adults using a nationally representative sample from the United States.

Methods: Using 2003-2006 National Health and Nutrition Examination Survey (NHANES) data, 7931 adults at least 18 years old wore an ActiGraph accelerometer for one week, providing at least 3 days of wear for 8 hours/day. Cut-points defined moderate to vigorous physical activity (MVPA; >2020 and >760 counts/minute), vigorous physical activity (>5999 counts/minute), and sedentary behavior (<100 counts/minute). Latent class analysis (LCA) was used to estimate patterns of physical activity and sedentary behavior. All estimates were weighted to reflect the United States population.

Results: For weighted percent of MVPA out of total wearing time, 5 classes were identified from least to most active: 65.3% of population (weighted mean 9.3 minutes/day), 24.9% (32.1 minutes/day), 3.2% that was low on the weekdays but much higher on the weekends (52.0 minutes/day), 5.9% (59.9 minutes/day), and 0.7% in the highest class (113.6 minutes/day). Using the lower MVPA threshold, 6 classes emerged with each class ranging in population from 1.2% to 43.6%. A vigorous activity class could not be derived due to low prevalence. For weighted percent of sedentary behavior out of total wearing time, 5 classes were identified from most to least sedentary: 6.3% of population (weighted mean 660.2 minutes/day), 25.1% (546.8 minutes/day), 37.7% (453.9 minutes/day), 24.0% (354.8 minutes/day), and 7.0% (256.3 minutes/day). Four of the classes showed generally similar results across every day of the week, with the absolute percents differing across classes. In contrast, the least sedentary class showing a marked rise in percent of time spent in sedentary behavior on the weekend (weighted mean 336.7-346.5 minutes/day) compared to weekdays (weighted mean 255.2-292.4 minutes/day).

Conclusion: The LCA models provided a data reduction process to identify patterns using minute-by-minute accelerometry data in order to explore meaningful contrasts. The models supported 5 or 6 distinct patterns for MVPA and sedentary behavior. These physical activity and sedentary behavior patterns can be used as intervention targets and as independent or dependent variables in future studies of correlates, determinants, or outcomes.

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Alex’s Notes: In 2012, 69% of adults were overweight or obese. Although the US government released its first physical activity guidelines in 2008, the sheer magnitude of fatties in the US suggests that either the guidelines aren’t working, or that Americans aren’t following them. Obviously diet plays a role as well, but let’s ignore that for a moment. The guidelines themselves were based on observational studies (at least partly) that relied almost exclusively on self-reported physical activity. An alternative method for collecting data is accelerometry, which can provide second-by-second information on activity levels.

The current study sought to describe patterns of accelerometer-assessed physical activity and sedentary behaviors among US adults.

The data was collected from the NHANES 2003-2006 cohort of persons who wore an accelerometer on their hip for seven consecutive days. Ultimately this resulted in a study population of 7,931 individuals over 18 years old.

The accelerometer recorded 1-minute epochs of activity and the researchers defined sedentariness as less than 100 counts/minute. Sedentary bouts were time periods of ≥30 minutes of unbroken sedentary behavior. Vigorous physical activity was greater than 6000 counts/minute and can sadly be ignored due to so few persons engaging in it.

Light-intensity physical activity wasn’t assessed for whatever reason, and moderate intensity physical activity (MVPA) used two different cutoff points. The first was between 2020-5998 counts/minute as represented walking or running. The second was ≥760 counts/minute and represented lifestyle activities. I personally fail to see how anything below walking (~2000 counts/minute) could be considered MVPA, and I wouldn’t even classify walking at that level. Regardless, bouts of MVPA were defined as ≥10 minutes of MVPA, with an allowance for dropping 20% below the threshold for less than 5 minutes.

The physical activity habits of US adults were divided into distinct classes, which are basically points along a continuum from “least” whatever to “most” whatever.

Starting with the unsurprising and disappointing sedentary time of the cohort, five distinct classes of sedentariness were found. Those who were the most sedentary (class #1) spent an incredible 82.3% (11 hours) of their waking hours doing nothing. This was stable across all days of the week. On the opposite end of the extreme, those who were least sedentary (class #5) spent 35% (4 hours) of their waking hours doing nothing, which is still high for my liking, but fair when we consider time spent commuting to/from work and sitting down to eat several times per day.

Overall, one-third of the cohort spent at least nine waking hours per day being sedentary, with over two-thirds spending 7.5 or more waking hours per day sedentary.

Moving now towards the (in my opinion) more realistic MVPA cut-points that represents walking or running (2k-6k counts/minute). With the depressing mood set by the levels of sedentariness, it is only fitting we continue down the dark path of American activity levels by revealing that 65.3% of the cohort (class #1) had absolutely zero MVPA. Another 24.9% (class #2) met the current recommendations of the ACSM and averaged 32 minutes per day. Interestingly, those in class #3 averaged 50 minutes of MVPA per day, but there was a clear “weekend warrior” activity pattern with Saturday and Sunday averaging 80-90 minutes/day compared to 40-50 minutes on weekdays. Another interesting outcome was that the most active individuals (classes #4 & 5) were most active on the weekdays and relaxed on the weekends.

If we lower our standards and include basic activities of daily living as MVPA (seriously, I cannot wrap my head around this), then things look much better. The least active class (#1) still gets 34 minutes/day of MVPA, and 43% (class #2) average 84 minutes/day. Regardless of MVPA classification, females had higher representation of the two least active categories. Sedentariness was roughly even across sex. Activity levels were also reduced as age increased.

Bottom line

The accelerometer under counts some activities, such as bicycling and weight lifting, and misses other activities, such as swimming, because the monitor was not waterproof and participants were told to remove it for any water-based activity. Thus, if anything, MVPA is understated and sedentariness overstated. Nonetheless, two-thirds of adults are sedentary for at least 7.5 waking hours per day, and two-thirds of adults also do not engage in any moderate-intensity physical activity.

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