Sympathetic nervous system (SNS) and parasympathetic nervous system (PNS) influences on cardiac rhythm at the onset of exercise, a time of rapid autonomic adjustments, are clinically important areas of investigation. Continuous wavelet transform (CWT) involves time- frequency- based heart rate variability (HRV) anal-ysis allowing investigation of autonomic influences on cardiac rhythm during short durations of exercise. Therefore, the purpose of this study was to characterize SNS and PNS influences on cardiac rhythm at the onset of isometric exercise in healthy young adults. CWT analysis was retrospectively applied to R- R interval data (elec-trocardiogram) previously collected from 14 healthy young adults (26 ± 2 years) who performed 30- s, one- legged, isometric, calf exercise at 70% maximal voluntary contraction (MVC; 70% MVC trial) or rested (0% MVC trial). Absolute and normal-ized low- frequency (aLF, nLF; 0.04– 0.15 Hz) and high- frequency (aHF, nHF; 0.15–0.4 Hz) bands and LF/HF were used to analyze one 30- s baseline period and six 5- s time windows during the 30- s exercise (70% MVC) or rest (0% MVC). Statistical analysis involved two- way analysis of variance with post- hoc analysis. aHF, aLF, LF/HF, nHF, and nLF displayed a trial- time interaction (all p≤ 0.027). In the 70% compared to the 0% MVC trial, aHF and nHF were lower after 5– 30 s (all p≤ 0.040), aLF was lower after 20– 30 s (all p≤ 0.011) and LF/HF and nLF were higher after 5– 20 s (all p≤ 0.045). These results indicate the reduction of the PNS influence on cardiac rhythm begins sooner than the augmentation of the SNS influence at the onset of isometric exercise in healthy young adults.