What is motor unit and why do they vary in size?
First describe the common action, using the correct terms from the descriptions in figure 11.2 in your book. Then find out what the origins and insertions of the muscles are and figure out how the two complement each other in performing the particular movement you have identified that they accomplish in common. In other words, how do their motion and relative effect differ? Finally, try to explain why you need both of them. Do not concern yourself about other things that either of the muscles do, just those that affect the common action.
The two muscles: Teres major/latissimus dorsi