Which is the sequence of events in muscle contraction?

I. Use of ATP
II. Formation of cross bridges
III. Release of calcium ions from the sarcoplasmic reticulum
IV. Actin filament moves towards the centre of the sarcomere

A.    I    →    II    →    III    →    IV
B.    III    →    II    →    IV    →    I
C.    IV    →    I    →    II    →    III
D.    II    →    IV    →    I    →    III

B

There were several G2 comments on the answer choices for the sequence of events in muscle contraction. As release of calcium is the first stage, B is the only correct answer. The placement of ATP at the end of the list was confusing and many candidates chose A, making the assumption that use of ATP must be at the beginning of the sequence.