<prompt> The force of a twitch is less than that in a tetanus, BECAUSE:One action potential does not release sufficient Ca to saturate the troponin binding sitesCa reuptake into the sarcoplasmic reticulum is so rapid that there is insufficient time for all the troponin Ca -binding sites to be occupiedCrossbridge cycling is not fast enough to allow expression of the maximum possible force before Ca dissociates from troponinAll of the above  <response>: There is enough elasticity in the structures transmitting force crossbridges, thick and thin filaments, Z-disks, tendons that the 10 nm shoening half sarcomere produced by one cycle will not produce much force. Many cycles are needed to fully stretch these elastic elements. Ca must remain high as a result of a train of action potentials in a tetanus for some time to allow the full force to be developed. A is not correct because sufficient Ca is released after the transit of one action potential to saturate essentially all of the troponin binding sites and enable all the crossbridges to attach and cycle. Other factors must explain the low twitch force. B is not correct because Ca reuptake is a very rapid process. Neveheless, the very high affinity of troponin for Ca ensures that it is saturated before the myoplasmic Ca falls significantly. 