The StepWatch™ 3 Activity Monitor (SAM) (OrthoCare Innovations, Washington, D.C.) was used to measure PA. The SAM is a waterproof microprocessor-controlled biaxial PA monitor that combines acceleration, position, and timing information to count steps taken every minute. It is designed to capture step-based ambulation, which accounts for the majority of PA energy expenditure (31 (link)), and provides steps, a unit of measurement that is easily interpretable. Because the monitor is worn just above the ankle it is not susceptible to motion of soft tissue which can induce significant errors in belt worn devices. Also, because of directional sensitivity, the SAM is not affected by off-axis accelerations and will not pick up vehicle vibration and similar events that may be counted by other monitors. Numerous studies have shown that it is accurate in lean and obese individuals at “slow” (1 mph) and “purposeful” (2–3 mph) walking speeds and with a variety of gait styles, with accuracy typically exceeding 98% (32 ).
Study staff at each site were trained and certified before PA assessment began. During the pre-surgical research visit, the monitor was programmed with sensitivity settings appropriate to the subject’s height, cadence, and gait speed using accompanying software (32 ) and set to immediately record data. To ensure technical reliability of the SAM and appropriateness of settings, a verification light on the monitor was set to flash for the first 100 steps; study staff confirmed that the monitor blinked one time per step while observing subjects walk at their normal pace as well as their slowest and fastest paces. To maximize compliance, subjects were encouraged to wear the monitor continuously, for seven consecutive days following their clinic visit or until the day of surgery. However, assuming that not all subjects would find night-wear tolerable, subjects were instructed that they could wear the monitor during waking hours only. Based on other researchers’ experience (1 , 26 (link)) we felt that assessing wear time from self-report of exact wearing practices would result in poor quality data. However, in order to prompt subjects to make-up an assessment day for days in which monitor wear time might be insufficient, the PA diary included the question: “How many hours were you awake today, but not wearing the SAM? This includes the time between when you got out of bed in the morning and put on your SAM, the time between when you took off the SAM and got into bed, and the time that you removed the monitor during the day.” Choices were less than an hour, 1–<3 hours, 3–<5 hours, and 5 or more hours. On days subjects selected “5 or more hours” the PA diary prompted them to wear the monitor for an additional day. However, 327 (30.0%) received the monitor three to eight days before surgery so they could not make-up an assessment day. Subjects did not record whether they chose to wear the monitor continuously.
LABS-2 subjects were excluded from PA if their clinic visit was within three days of their scheduled surgery, they exclusively used a wheelchair, they had a health-related reason other than obesity that limited walking (e.g. multiple sclerosis), a temporary injury that affected walking (e.g. sprained ankle), or a medical condition that could be exacerbated by wearing the monitor (e.g. edema), or no monitors were available for distribution.
Study staff at each site were trained and certified before PA assessment began. During the pre-surgical research visit, the monitor was programmed with sensitivity settings appropriate to the subject’s height, cadence, and gait speed using accompanying software (32 ) and set to immediately record data. To ensure technical reliability of the SAM and appropriateness of settings, a verification light on the monitor was set to flash for the first 100 steps; study staff confirmed that the monitor blinked one time per step while observing subjects walk at their normal pace as well as their slowest and fastest paces. To maximize compliance, subjects were encouraged to wear the monitor continuously, for seven consecutive days following their clinic visit or until the day of surgery. However, assuming that not all subjects would find night-wear tolerable, subjects were instructed that they could wear the monitor during waking hours only. Based on other researchers’ experience (1 , 26 (link)) we felt that assessing wear time from self-report of exact wearing practices would result in poor quality data. However, in order to prompt subjects to make-up an assessment day for days in which monitor wear time might be insufficient, the PA diary included the question: “How many hours were you awake today, but not wearing the SAM? This includes the time between when you got out of bed in the morning and put on your SAM, the time between when you took off the SAM and got into bed, and the time that you removed the monitor during the day.” Choices were less than an hour, 1–<3 hours, 3–<5 hours, and 5 or more hours. On days subjects selected “5 or more hours” the PA diary prompted them to wear the monitor for an additional day. However, 327 (30.0%) received the monitor three to eight days before surgery so they could not make-up an assessment day. Subjects did not record whether they chose to wear the monitor continuously.
LABS-2 subjects were excluded from PA if their clinic visit was within three days of their scheduled surgery, they exclusively used a wheelchair, they had a health-related reason other than obesity that limited walking (e.g. multiple sclerosis), a temporary injury that affected walking (e.g. sprained ankle), or a medical condition that could be exacerbated by wearing the monitor (e.g. edema), or no monitors were available for distribution.