When skeletal muscles contracts the Z lines that are at the end of every sarcomere became closer to each other due to the fact that the I bands that are attached to the Z lines and contains the myosin filaments move closer to each other interacting with the actin filaments and shorten the the H zone that is in the middle of the sarcomere. In other words, It all becomes compact during a contraction.
Diffusion has a membrane which allows even large molecules to pass through it whereas osmosis allows only water molecules as it has a partially permeable membrane.
