Despite growing clinical use of left bundle branch pacing (LBBP), data regarding the fundamentals of this pacing modality, including chronaxie and rheobase, are scarce.The purpose of this study was to calculate strength-duration curves with chronaxie and rheobase values for LBBP and left ventricular septal pacing (LVSP), and to analyze battery current drain and presence of selective LBBP at very short pulse duration (PD).A group of 141 patients with permanent LBBP were studied. LBBP and LVSP capture thresholds were assessed at 6 different PDs to calculate the strength-duration curves. Battery current drain at these PDs and presence of selective LBBP were determined. For comparison of strength-duration curves between His-bundle pacing (HBP) and LBBP, source data from our previous work based on 127 patients with HBP were obtained.The chronaxies for LBBP and LVSP were very similar (0.38 vs 0.39 ms), and the rheobases were identical (0.27 V). The chronaxie for LBBP was lower than for HBP (0.38 vs 0.53 ms; P <.001), whereas rheobases were similar (0.27 vs 0.26 V). A narrow zone of selective capture was present in 19% and 41% of patients at PD of 0.06 and 0.03 ms, respectively. When pacing with the safety margin of +1 V, the lowest battery current drain was achieved with PD of 0.2 ms.The obtained strength-duration curves for LBBP and LVSP provide insights to optimal programming of left bundle branch area pacing devices with regard to PD, voltage amplitude, battery longevity, and selective capture.