During muscle contraction, myosin motors anchored to thick filaments bind to and drive sliding motions of actin thin filaments. These motors rely on energy derived from ATP, supplied, in part, by diffusion from the surrounding cytoplasm to the interior of the lattice of actin and myosin filaments. The radial spacing of filaments in this lattice may change or remain constant during contraction. If the lattice is isovolumetric, it must expand when the muscle shortens. If, however, the spacing is constant or has a different pattern of axial and radial motion, then the lattice changes volume during contraction, driving fluid motion and assisting in the transport of molecules between the contractile lattice and the surrounding intracellular space. Advective transport coupled to diffusion has the potential to substantially enhance metabolite exchange within the crowded lattice of contractile proteins. Time-resolved x-ray diffraction of contracting muscle reveals a pattern of sarcomere volume changes that result in significant fluid flow. That flow drives advective transport of substrates, augmenting flux relative to diffusion alone. Akin to "breathing," advective-diffusive transport in sarcomeres is sufficient to promote metabolite exchange and may play a role in the regulation of contraction itself.