The ability of a muscle group to perform repeated contractions over a period of time sufficient to cause muscular fatigue or to maintain a specific percentage of maximum voluntary contraction for a prolonged period of time is called what?

Muscle endurance

Explanation:

Muscular endurance is the ability of a muscle or collections of muscles to satiate or endure repeated contractions against a resistance for an extended period of time. It is one of the components of muscular contractions, muscular fitness, along with muscular strength and power.

During exercise or  training fitness , muscular endurance implies  to the number of repetitions of a single exercise you can underg without needing to stop and rest.

it is very expensive

explanation:

cellular respiration oxidizes food into energy in the form of atps.

glycolysis (6 c hexose) > 2x atps are used > 6c hexose biphosphate > 2 x 3c triphosphate >

2 x 3c pyruvate (2x nadh produced from nad) and (4x atp produced)

pyruvate actively pumped mitochondrial matrix.