<prompt> Post-tetanic potentiation is due to -Hyperpolarization of muscle fibresRapid K effluxIncreased availability of CaRapid Na influx  <response>: Ans. is c i.e., Increased availability of Ca Repetitive stimulation enhances force development due to rise in intracellular Ca, a phenomenon called posttetanic potentiation. It is due to increased phosphorylation of myosine light chain with increased number of cross bridges. 