When studying isolated single muscle fibers, which of the following are an advantage : disadvantage?  a) experimental interventions are very easy : Mixture of fiber types b) all physiological mechanisms are present : produces correlative data; hard to identify mechanism c) only one fiber type is present : prone to damage at physiological temperatures d) possible to study myofibrillar properties : may lose important intracellular constituents e) fatigue due to central nervous system is eliminated : drugs cannot be applied rapidly because of diffusion gradients   How does the Na+K+ ATPase help prevent the loss muscle excitation during normal exercise? a) lowering extracellular K+ b) lowering extracellular K+ and hyperpolarizing the fiber c) raising extracellular K+ d) raising extracellular K+ and hyperpolarizing the fiber  e) lowering extracellular K+ and depolarizing the fiber    In muscle that has been exercising, as the level of phosphocreatine (PCr) reaches low levels, the levels of ATP ____ and the levels of ADP______.  a) increase : increase  b) increase : decrease c) decrease : increase  d) decrease : decrease  e) stay the same   In comparison to slow-oxidative and fast-glycolytic fiber types, the fast-oxidative-glycolytic fibers typically have:  a) the lowest resistance to fatigue  b) the lowest levels of oxidative enzymes  c) intermediate fiber sizes (CSA) d) very low volumes of mitochondria  e) the highest force producing capacity   During high frequency stimulation, an accumulation of K+ is more likely to be a problem in the ____. a) sarcolemma  b) t-tubules  c) sarcoplasm d) endoplasmic reticulum    Which of the following is correct?  a) Type I muscle fibers have intermediate oxidative capacity. b) Type IIa muscle fibers have the high oxidative capacity. c) Type IIb muscle fibers have the highest oxidative capacity. d) Type I muscle fibers have the highest glycolytic capacity.  e) Type IIa muscle fibers have intermediate oxidative capacity.