Reliability and validity of the beats above baseline index: a new measure of task difficulty

The Beats Above Baseline Index (BABI) is mathematically based on the integral of effort (estimated by instantaneous heart rate increase) across the time interval for task completion. This is equivalent to the number of heart beats occurring during a task minus the number of beats that would have occurred during the same time interval at the baseline rate. We assessed the test-retest reliability and construct validity of the BABI using 19 normal subjects accomplishing a stair-climbing task. Subjects completed the task under normal conditions (N), with a knee immobilizer and cane (CANE), and using crutches (CRUTCH). Reliability of the BABI was high within (ICC 0.84, 95% CI 0.70-0.92) and between sessions (ICC 0.91, 95% CI 0.77-0.97). Individual BABI values for all 19 subjects conformed to the underlying construct, namely N < CANE < CRUTCH. In contrast, two commonly used alternative measures, namely maximum change in heart rate and time for task completion, showed inferior concordance to the construct.