Muscle contraction is activated by delivery of calcium from the internal membrane system of muscle cells, the sarcoplasmic reticulum (SR), to the myofibrils. Muscle relaxation ultimately depends on the sequestering of calcium back into the SR. In general, skeletal muscle is the most efficient in holding on to its calcium and calcium movements across the surface membrane are minimal during muscle activity. Cardiac muscle is less efficient and its activity is accompanied by significant movements of calcium across the surface membrane. Understanding of the calcium movements during muscle activation has relied heavily on structural information using the electron microscope and Lee Peachey was at the forefront in establishing structure function correlations. He talked the physiologists language and thus caught their attention.

Four stages can be detected in the contribution of EM to the unraveling of the calcium story. First the membrane systems involved were identified, and quickly sorted into two types on the basis of structural characteristics dictating specific functions.