The primary purpose of the study was to determine the relationships between %HRR vs. %VO₂R and %HRR vs. %VO₂max during maximal elliptical crosstrainer (ECT) exercise. A secondary aim was to compare the %HRR vs. %VO₂R and %HRR vs. %VO₂max relationships between maximal ECT and treadmill (TM) exercise. Adult subjects (n = 48) completed a maximal exercise test on the ECT, with a subgroup (N = 24) also performing a maximal exercise test on the TM. Continuous HR and VO₂ data were analyzed via linear regression to determine y-intercept and slope values for %HRR vs. %VO₂R and %HRR vs. %VO₂max. Student t-tests were used to determine whether the mean y-intercept and slope values differed from the line of identity (y-intercept = 0, slope = 1). For each group, both the y-intercept and slope for %HRR vs. %VO₂R fit the line of identity. Conversely, for all groups both the y-intercept and slope for %HRR vs. %VO₂max were significantly different (p < 0.001) from the line of identity (y-intercept ≠ 0, slope ≠ 1). In comparing the regressions of %HRR vs. %VO₂R between exercise modes, there were no significant differences (p > 0.05) for either y-intercept (ECT = 0.3 vs. TM = -0.3, p = 0.435) or slope (ECT = 1.01 vs. TM = 1.00, p = 0.079) values. In agreement with previous research on TM and cycle exercise, it was found that %HRR is more closely aligned with %VO₂R, rather than %VO₂max during ECT exercise. Additionally, it was found that the regressions of %HRR vs. %VO₂R and %HRR vs. %VO₂max were equivalent between the ECT and TM.