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To put it as simply as possible, reciprocating loads are loads resulting from any rapid 'back and forth' motion such as a piston moving rapidly down the cylinder, stopping, and then moving rapidly back up the cylinder. They are really caused by the inertia of the moving object (piston in this case).
When a piston is forced to either suddenly stop or start moving, its inertia resists that change. This is felt as a strain on the bearings at the big end of the crankshaft and at the top of the connecting rod. The higher the RPM, the faster the piston must change direction from down to up or up to down. Reciprocating loads increase rapidly as RPM increases.