Final answer:
The two main factors contributing to passive muscle force are titin elasticity and collagen stiffness, which provide tension in muscles during stretching.
Step-by-step explanation:
The two main factors that contribute to passive force in muscles are titin elasticity and collagen stiffness. Passive force is the tension generated by a muscle when it is stretched but not actively contracting. The elasticity of the protein titin, which is interconnected with myofilaments within the muscle cells, plays a vital role in passive stretching. Collagen, another protein present within the connective tissue of muscles, contributes to passive force due to its stiff nature, which resists overstretching and protects muscle integrity. Neither the active processes such as myosin activation or ATP hydrolysis, nor the other options like the position of tropomyosin or sodium influx directly contribute to the passive force generated in muscles.