Muscle origin and insertion are terms that describe where the muscle attaches to the bones  or connective tissues, but these are relative terms. Generally, the origin is where the muscles "originates" on the body (usually a bone, but not always) of the stationary part. The insertion in the other hand is where the muscle attaches on the bone of the moving lever across a single, or multiple, joint lines. When the muscle contracts, it pulls the insertion to the origin.

Some authors don't use the aforementioned  nomenclature, but use a slightly different one. They refer to the origin as the proximal attachment. This is generally considered the least movable part or the part that attaches closes to the midline or center of the body while the insertion is the distal attachment. This generally considered the most movable part or the part that attaches farthest from the midline or center of the body.In any event consider the following picture.

Figure 1

Figure 2

The insertions are represented by blue and the origins by red. Now, let's talk about an example- "subscapularis" and think about the previously mentioned definition [origin Vs. insertion]. Just imaging the muscle fibers, crossing laterally and inserting at the lesser tubercle of the humerus [take a quick look at Figure 1]. Picture this; subscapularis [the muscle] pulling the insertion [the lesser tubercle], toward the origin [the scapula]. The movement is an internal rotation of the glenohumeral joint. As a practice, do the same with all the muscles and have some fun!!!




Note: For practical purposes use these pictures [above] to learn the origins and insertions of interest.