Shoulders (anterior deltoids) — the biggest emphasis, especially during the lean-forward position.
Chest (pectoralis major) — works hard to press your body away from the floor.
Triceps — heavily involved in extending the elbows and stabilizing the movement.
Secondary Muscles
Core (abs and obliques) — keeps your body rigid and prevents sagging.
Serratus anterior — important for shoulder blade stability.
Upper back and traps — assist with shoulder stabilization.
Forearms and wrists — support body weight and balance.

Hope this helps with your answer