After a skeletal muscle fiber is treated with a membrane permeable drug that speeds up the action of the SR Ca 2-ATPase, how would contraction differ after multiple and frequent action potentials

the twitch would be shorter

Explanation:

In muscle cells, the calcium (Ca2+) pump ATPase membrane transport proteins maintain a low cytosolic Ca2+ concentration in order to promote muscle contraction. Muscle relaxation occurs when the sarcoplasmic reticulum reabsorbs Ca2+ through the Ca2+-ATPase protein, thereby reducing intramuscular Ca2+ levels. In muscle contraction, intracellular Ca2+ levels are raised due to the release of Ca2+ from the sarcoplasmic reticulum (SR) via Ca2+-release channels. In consequence, it is expected that a specific drug that speeds up the action of (Ca2+) pump ATPase protein should shorten the contraction-relaxation cycle by increasing the velocity of muscle relaxation.

i think its the first one