what is cross arm?

Myosin is a polymerised protein, i.e., polymer of meromyosins

Meromyosin has two parts: Globular head with a short arm (Heavy meromyosin; HMM) and a tail (Light meromyosin; LMM)

The head and the arm project outwards at a regular distance and angle from each other at the surface of the polymerised myosin and are together called the cross arm.