Each tray had five slots to securely hold activity monitors in place and eliminate movement during orbital shaking. Prior to data collection, we performed shaker testing using 4 GT3X+ and GENEA monitors to establish inter-unit reliability for each monitor type. Intra-monitor coefficient of variation was less than 1.6% for both monitors. This is similar to previous reports of intra-monitor reliability for ActiGraph? and GENEA monitors using MEMS capacitive sensors [7,8]. A single GT3X+ and GENEA were initialized to collect data at a sampling frequency of 80 Hz and were oscillated during 10 trials. Each trial lasted 10 min (five frequencies �� 2 min each) and consisted of monitor oscillation at 0.7, 1.3, 2.3, 3.3 and 4.0 Hz on a fixed radius of 5.08 cm [9,10].

These frequencies are similar to those observed during ambulation at speeds ranging between 1.5 to 16 mph [11]. The activity monitors were randomly positioned to a different slot prior to each trial and no device was oscillated in the same slot more than one time. Figure 1A depicts the mechanical shaker used in the study.Figure 1.(A) Orbital mechanical shaker use for shaker testing, (B) Wrist worn GT3X+ and GENEA monitors.2.2. Human TestingEight participants (mean �� SD: age = 23.8 �� 5.4 years; Body Mass Index = 22.7 �� 1.4 kg?m2) were recruited from the University of Massachusetts, Amherst and the surrounding community. The University of Massachusetts, Amherst Institutional Review Board, approved the experimental protocol and all participants provided written informed consent.

Participants visited the Physical Activity and Health Laboratory to perform the human testing protocol. Participants wore activity monitors at the wrist on two Velcro? wristbands while performing the activity protocol. The monitors were positioned such that one was distal to the other when the arm was straight and pointing downwards on the side of the body. We minimized residual confounding due to placement effect by counterbalancing proximal/distal monitor placement. Figure 1B illustrates monitor placement on the wrist.The lab-testing protocol included treadmill and simulated free-living activities. Participants walked at 2.0 and 3.5 mph and ran at 5.5 and 7.5 mph on a treadmill for 2 min each. These were followed by 2 min of seated computer-work vacuuming, cleaning the room and throwing a ball.

The activities were selected to cover a wide range of dynamic acceleration between 0 and 6 g. Start and stop times for all activities were recorded.2.3. Data AnalysesInter-monitor comparisons during shaker testing used data from Anacetrapib the 2nd minute of each two-minute trial. Linear mixed models with likelihood ratio tests (p < 0.05) were used to compare mean triaxial vector magnitude of raw acceleration between the GT3X+ and GENEA at each oscillation frequency.