<p>Preparatory actions prior to the intended movements significantly contribute to better performance. Preceding feedforward control, known as preparatory muscle activity (PMA), is crucial for utilizing energy without loss in the main movements. Therefore, the present study attempted to clarify the role of PMA in vertical jumps with preparatory actions such as a one-step motion. Fourteen healthy participants performed vertical jumps with a one-step motion while postural muscle activity was recorded using electromyography (EMG). Kinematics were captured using a 3D motion analysis system. A 500 ms window preceding the onset of the thrust (t0) was defined as the preparatory phase. For each muscle, we detected the EMG activation and deactivation onsets with reference to t0 and integrated the amplitude over that window. Center of mass (COM) velocity, COM anterior–posterior sway range (COM y range), and the change in ankle plantar flexion from heel contact to take‑off (ankle pf rate) were treated as jump‑relevant parameters. Earlier activation onset of tibialis anterior before t0 was associated with larger ankle pf rate, whereas later activation onset of biceps femoris (BF) and medial gastrocnemius (MG) was also associated with larger ankle pf rate. The smaller the amplitudes of erector spinae, rectus femoris, vastus lateralis, MG, and soleus during the preparatory phase, the greater the COM velocity. These results indicate that timing and amplitude modulation of PMA are linked to mechanical conditions that favor converting horizontal approach energy into vertical thrust. Therefore, PMA may contribute to both postural equilibrium and favorable neuromuscular conditions for converting horizontal approach energy into vertical propulsion.</p>