what case is "manus" in Seneca's Bath house description?

What case is the word “manus” in Seneca’s description of a noisy bathhouse
cum fortiores exercentur et manus plumbo graves iactant, cum aut laborant aut laborantem imitantur, gemitus audio,
Nominative plural “hands throw heavy lead weights”
Thank you

My take: accusative plural depending on iactant.

Yeah, while being modified by the adjective graves (also an accusative plural).