Natural Science Forum / Biology / Biology / October 2004

Muscle, like most tissues, stores very little ATP.  However, muscle
does have a special energy storage compound, creatine phosphate, that
can be used to quickly generate ATP when needed.  With vigorous
exercise, the ATP may start to be depleted within 2 seconds and be
essentially gone in 10.  The creatine phosphate will last ten seconds
and be essentially gone in one minute.  Continued high levels of
activity beyond that result in a combination or aerobic and anaerobic
glycolysis from either glucose or, more likely, glycogen stores.

Muscle may contain creatine in concentrations of perhaps 4 g/kg dry
mass or about 30 mmol/kg dry mass.  Of course, muscle is mostly water,
so the concentration in live muscle is much smaller, on the order of a
perhaps 10 mM. Values in different muscles in different individuals
under different circumstances may differ  by a factor of 2 or 3.

One source is

 Soderlund K, Hultman E.
 ATP content in single fibres from human skeletal muscle after
    electrical stimulation and during recovery.
 Acta Physiol Scand. 1990 Jul;139(3):459-66.

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=2
239349&dopt=Abstract

which says:

The ATP content was measured in type I and type II fibres from human
vastus lateralis muscle at rest, after electrical stimulation and
during recovery. At rest the mean values were 25.2 +/- 4.02 and 25.9
+/- 3.62 mmol kg-1 dry muscle (mean +/- SD) for type I and type II
fibres respectively. Normal distribution curves were found for both
types I and II fibres. After intermittent electrical stimulation for
83 s (1.6 s stimulation, 1.6 s pause) with occluded blood flow, the
force generation decreased to 22% of the initial value and the muscle
tissue showed a mean decrease in ATP to 14.8 and in phosphocreatine to
5.44 mmol kg-1 dry muscle; lactate increased to 128.9 mmol kg-1 dry
muscle. The ATP content in isolated fibres was equally decreased in
both fibre types to 16 mmol kg-1 dry muscle. In 11% of the fibres the
ATP content was lower than 10 mmol kg-1 dry muscle. After 15 min rest
with intact blood circulation ATP was completely resynthesized in type
I fibres and to 91% in type II fibres.