As you exercise more intensely (increase the force of muscle contractions) which

Muscle cells are roughly cylindrical, with diameters between 10 and 100 µm but up to a few centimeters long. Each cell is embedded in a basal lamina of collagen and large glycoproteins. Between the fiber and the basal lamina are large numbers of satellite cells, that are important in the growth and repair of the fiber. The fiber itself contains specialized structures for excitation-contraction coupling to ensure that a contractile stimulus (received at the synapse) is rapidly and evenly communicated to the whole fiber. Contractile and performance characteristics vary, but are closely linked to the myosin heavy chain isoform expressed by the fiber. Force production occurs in the myofibrils, which are chains of sarcomeres running from one end of the fiber to the other. Energy for contraction comes from metabolism of fats and sugars.