Immediate Energy Sources

There are 3 components of immediate energy sources in muscle: ATP, Creatine Phosphate, and Myokinase. All three are water-soluble, so they reside in the aqueous part of the cell near the myosin and actin (contractile proteins of muscle).

ATP (adenosine triphosphate) is degraded by enzymes called ATPases, and the process usually involves combining with water; therefore, is referred to as hydrolysis. The chemical products of hydrolysis are ADP and Pi, where Pi is an inorganic phosphate. The standard free energy of ATP is 11 kcal/mol.


Creatine phosphate (CP) is 5-6x more abundant in resting muscle than ATP. It serves as a reserve of phosphate energy to regenerate ATP by rephosphorylising ADP.

CP + ADP ----creatine kinase----> ATP + C

The resulting Creatine is then rephosphorylated with mitochondrial creatine kinase which accesses mitochondrial ATP.

The enzyme adenylate kinase, also known as myokinase, can generate 1 ATP and 1 AMP from 2ADPs.

ATP and CP together are known as phosphagen. Exisiting ATP can't sustain maximal contraction for more than 2 seconds; and even with the assistance of CP and myokinase, other energy sources are needed to kick in if contraction lasts over 5-15 seconds.