Why does muscle tissue have relatively constant AMP + ADP + AMP?
Adenosine triphosphate (ATP) is often thought of as the energy currency of cells. It is not "used up" per se, but energy is released from the conversion of ATP to ADP (adenosine diphosphate), and yet more can be obtained by removing another phosphate to make AMP (adenosine monophosphate). 

Here's a chemical schematic of ATP, you can see the three phosphates to the left.



Your slides state that the amount of (ATP + ADP + AMP) remains constant in the muscle, which makes sense because ATP is not used up, but converted to ADP. So as cells use energy, they do not actually reduce the total concentration of (ATP + ADP + AMP). Energy is required to add the phosphates back onto an ADP or AMP molecule - the process is called oxidative phosphorylation in cellular respiration (although this is not the only way).

I hope I didn't misunderstand your question.