If isolated myofibrils were placed in a flask containing a solution of calcium and AMPPNP (a nonhydrozyable analog of ATP) you would expect to find the myofibrils arrested in which stage of the myosin power stroke cycle?
a. just completed the power stroke
b. all locked up in cross bridges
c. all cross bridges broken
d. in the high energy cocked head conformation

Muscle contraction is explained by the most accepted sliding filament theory given by Huxley and Huxley. During the process of muscle contraction actin filament slide over the myosin filament by the formation of cross-bridges. In this process the length of I band reduces whereas the length of A band remains the same. Also we should note that the actin and myosin filament remains unchanged.

The release of acetylcholine at the neuro-muscular junction stimulates the release of calcium ions from endoplasmic reticulum into the sarcoplasm. During the relaxed condition troponin-tropomyosin complex always block the myosin binding site on the surface of actin.

Once calcium is released it binds to the TpC component of troponin causing the shift of troponn-tropomyosin complex. Now actin can attach and slide over myosin.

The myosin head binds to actin forming the cross bridge and the pulling of myosin head towards M-line along with actin is called the power stroke. But myosin head also has a site for binding of ATP. Binding of ATP causes the detachment of myosin head from actin filament. Now since its the analog of ATP i.e AMPPNP present in the solution, cross-bridges will be broken. And only if ATP undergoes hydrolysis, the angle of myosin head changes into high energy cocked head conformation. Since AMPPNP is non-hydrolyzable we can rule out the fourth option.