The aim of the current study was to define the MDC thresholds for Zephyr Bioharness (ZB) and Fitbit Charge (FC-HR) heart rate variables, and to establish the internal responsiveness of ZB and FC-HR devices in terms of their heart rate, respiratory rate variables and number of steps taken variables, in a healthy sample of males and females at resting, sub-maximal activity and throughout recovery phases. A total of sixty participants (30 females, 48 ± 15) and (30 males, 48 ± 15) were recruited using stratified convenience and snowball sampling approaches from our university student, staff and faculty population. At rest, MDC90 values of ± 4.89 and ± 5.15 were noted for ZB and FC-HR devices respectively. Throughout the recovery, thresholds of ±8.14 for ZB and ± 8.74 for FC-HR were reported. During the sub-maximal phases, scores of ± 8.18 and ± 9.35 were established for ZB and FC-HR respectively. From a resting to a sub-maximal state, the Standardized Response Means for ZB and FC-HR heart rate variables exhibited large responsiveness of SRMZB-HR=4.42; SRMFC-HR=4.04. In addition, the ZB respiratory rate variable showed the largest effect size of SRMZB-RR= 2.55. Similarly, large effect size of SRMFC-ST=1.76 were reported for the FC-HR steps taken variable. From a sub-maximal to a recovery state, he Standardized Response Means for ZB and FC-HR heart rate variables demonstrated effect sizes of SRMZBHR= -3.10; SRMFC-HR= -3.12). Furthermore, the ZB respiratory rate variable displayed similar internal responsiveness measures of SRMZB-RR= -2.45. To conclude, Similar MDC heart rate thresholds can be expected using the ZB and FC-HR devices, and that the two devices demonstrated large internal responsiveness in our healthy male and female cohort when transitioning from resting to submaximal states and vice-versa. Our findings suggest that the ZB and FC-HR devices can be used in monitoring performance improvements or deterioration over time, and would allow qualified personnel in assessing the effectiveness of exercise programs in healthy individuals.