= Solution
The enzyme <adenylate kinase> rapidly interconverts the adenine <nucleotides>:
$$
2\mathrm{ADP}\rightleftharpoons\mathrm{ATP}+\mathrm{AMP},\qquad K=\frac{[\mathrm{ATP}][\mathrm{AMP}]}{[\mathrm{ADP}]^2}.
$$
For approximately fixed conditions and near-equilibrium activity ratios,
$$
\frac{[\mathrm{AMP}]}{[\mathrm{ATP}]}=K\left(\frac{[\mathrm{ADP}]}{[\mathrm{ATP}]}\right)^2.
$$
\b[The AMP:ATP ratio responds quadratically to ADP:ATP.] Doubling ADP:ATP therefore quadruples AMP:ATP. In working <skeletal muscle>, <ATP> is also buffered by <creatine kinase> and continuing <ATP> production, so its fractional change is relatively small. The initially smaller <AMP> pool can undergo a much larger fractional increase, providing a sensitive signal of energetic stress to regulators such as <AMP-activated protein kinase>. This concerns sensitivity of relative changes; it does not mean that the absolute <AMP> concentration exceeds <ATP>.
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