What kind of energy is used for the flexing of an arm muscle1.electrical energy2.gravitational potential energy3.mechanical energy

Cellular respiration consists of different process as glycolysis. Krebs cycle and electron transport so that an energy in the form of ATP extracting in the process of cellular respiration in the food you eat. All organism undertake the first step which is glycolysis. 
Cellular respiration goes on all the time, it involves an intake oxygen and glucose in order to create energy. Respiration is important because without it you would'nt  be alive because you need oxygen for you to live.

Yes. it has a situation where living cells are not respiring, for example, the microbial cells that live in anaeboric environments in certain environments will use fermentation rather than respiration to produce ATP. An example is yeast cell in beer mash.