The study was a randomized placebo-controlled trial investigating mechanisms by which chronic β2-adrenergic stimulation enhances muscle force and power output during maximal cycle ergometer exercise in young men. Eighteen trained men were assigned to an experimental group (oral terbutaline 5 mg∙30 kgbw(-1) twice daily; TER, n=9) or a control group (placebo; PLA, n=9) for a four-week intervention. No changes were observed with the intervention in PLA. Isometric muscle force of the quadriceps increased (P≤0.01) by 97±29 N (mean±SE) with the intervention in TER compared to PLA. Peak and mean power output during 30-s of maximal cycling increased (P≤0.01) by 32±8 and 25±9 W, respectively, with the intervention in TER compared to PLA. Maximal oxygen consumption (V̇o2 max) and time to fatigue during incremental cycling did not change with the intervention. Lean body mass increased by 1.95±0.8 kg (P≤0.05) with the intervention in TER compared to PLA. Change in single fiber cross-sectional area of MHCI (1205±558 µm(2))(P≤0.01) and MHCII fibers (1277±595 µm(2))(P≤0.05) of the vastus lateralis muscle, was higher for TER than PLA with the intervention, whereas no changes were observed in MHC isoform distribution. Expression of muscle proteins involved in growth, ion handling, lactate production and clearance increased (P≤0.05) with the intervention in TER compared to PLA, with no change in oxidative enzymes. Our observations suggest that muscle hypertrophy is the primary mechanism underlying enhancements in muscle force and peak power during maximal cycling induced by chronic β2-adrenergic stimulation in humans.