There were two main findings of this study. First, the refined C2RM was a significant improvement over the 2006 C2RM and it closely reflected the measured kcal kg-1 hr-1 values, averaged over the entire bout, for all three conditions as hypothesized. Second, it was found that the refined C2RM predicted EE significantly better than the 2006 C2RM during transitional minutes from walking to resting.
Examining the first minute of the 0-s condition, there was a significant difference between the 2006 and refined C2RMs and measured EE. These differences resulted from the lag in oxygen uptake at the beginning of a bout (i.e., the "oxygen deficit"), as shown in Figure 1 . The predicted values of the first minute were similar to the measured steady state values, showing that both models accurately predict steady state EE values when the walking bouts start in synchronization with the ActiGraph clock. In contrast, during the first and last transitional minutes of the 20-s and 40-s conditions, the 2006 C2RM significantly over-predicted the measured kcal kg-1 hr-1. The main reason is that the transitional minutes included some "0" counts in the beginning and end of the walking bouts, which increased the coefficient of variation (CV) within those minutes, causing the models to use the intermittent lifestyle equation. This led to an over-prediction of EE, which is especially evident in the first transitional minute of the 20-s bout and the last transitional minute of the 40-s bout.
The refined C2RM predicted EE more accurately than the original 2006 C2RM because it eliminated the over-predictions at the beginning and end of a walking bout without affecting the steady state values. Short (< 1 min), intermittent, walking bouts interspersed with inactivity would be classified as lifestyle activity, which was intentional. We chose not to identify bouts lasting less than 60 seconds as "walking" because brief periods of walking are often performed within the context of intermittent lifestyle activities (e.g., lawn-mowing, golf, and garden work). Thus, there is a risk that these lifestyle activities would be misclassified as walking, using that approach.
Based on the Bland-Altman plots, the refined C2RM provided a closer mean estimate than the 2006 C2RM. The one exception was for the 0-s condition where the 2006 and refined C2RM had similar mean errors; we expected this since the walking bout started exactly on the minute of the ActiGraph clock. For the 20-s and 40-s conditions the mean error was closer to zero for the refined C2RM due to it not being influenced by the transitions between walking and rest. In addition, for all walking conditions, the refined C2RM had tighter 95%PIs around the mean, indicating that it provided a more accurate individual prediction during structured walking bouts. An improvement in the individual prediction is an important step forward to obtaining more accurate predictions of free-living physical activity.
Recently Rothney and colleagues found that the 2006 C2RM significantly over-predicted EE measured by doubly-labeled water (DLW) and whole-room calorimetry by 6.0% and 10.2%, respectively . Although we cannot be certain of what is causing this overestimation (since DLW does not capture minute-by-minute EE), the Rothney study did show the 2006 C2RM to overestimate moderate PA by 36.9 min compared to the whole-room calorimeter . We hypothesize that the overestimations by the 2006 C2RM were likely due to the presence of multiple transitional minutes during their testing which would result in the misclassification of walking as intermittent, lifestyle activity, thus causing EE and time spent in moderate activity to be overestimated. Further, in a separate study we have shown that the refined C2RM significantly improved estimated EE and time spent in moderate activity during 6 hours of free-living measurement, compared to indirect calorimetry . At this time it is difficult to quantify how much the refined C2RM improves upon the 2006 C2RM. During continuous walking bouts of long duration we would expect little improvement in the prediction of EE, since it is only the transitional minutes at the start and end of the bout that are being misclassified. However, individuals generally perform walking bouts of short duration which are mixed with intermittent lifestyle activities . It was these shorter bouts of walking that were being misclassified resulting in an overestimation of EE and moderate activity. Future research should investigate the implications of these shorter bouts to be able to quantify the improvement of the refined C2RM compared to the 2006 C2RM in free-living environments.
A major strength of the current study is that we examined the time course of changes in measured and predicted kcal kg-1 hr-1, rather than simply examining mean EE across the entire activity bout. This allowed us to examine the walk-to-rest transitions in detail. In addition, we were able to examine walking at a self-selected pace, which makes the results applicable to real-life settings.
The main limitation of this study is that we only validated the refined C2RM for walking bouts of 8 minutes duration, although we believe that continuous walking bouts as short as 1 minute can be identified using this technique. Future research is needed to determine the accuracy of the new, refined approach over longer time periods in free-living individuals, using doubly labeled water. Another limitation is that the 2006 C2RM was developed using the ActiGraph 7164, while the current study used the ActiGraph GT1M. Although different models were used, we do not feel it substantially influenced the results as it has been shown that the 7164 and GT1M give comparable count values during treadmill walking and running .
In conclusion, the 2006 C2RM significantly over-predicted EE during walking when the walking bout did not start in synchronization with the ActiGraph clock. The over-prediction of EE in the first and last minutes of the walking bouts for the 20-s and 40-s condition was due to the high CV during the transitional minutes. However, we observed that a refined C2RM  eliminated the over-predictions of EE that are likely to occur at the beginning and end of continuous walking bouts, and should therefore be used in future studies.