Force of contraction

Yes, muscle contractions are a all or nothing stimulation. The more a muscle is exercised to a certain motion, it gains more neurological recruitment because the body is learning how to deal with the activity and adapting to its work load. With continued recruitment, there a point at which most athletes call a "plateau" meaning there is no further increase in force the muscle produces, but this can be over come by changing the frequency of the activity. There is only no further increase in force at the moment the muscle cannot exert more force during an action but in the future your muscle will be able to do it next time with a bit more ease.

No not all motor units are activated in the stretch reflex. However all muscle fibers from the motor unit do get activated.
More motor units recruited....the greater the force of contraction because more muscle fibers are being activated. Smallest motor nerves get recruited first, which stimulate the smaller muscles like the slow twitch fibers.

No increase in force when all the motor units and muscle fibers associated with it are all activated and therefore reached your maximal force of contraction.

Hi water2014, welcome to the forum.

What textbook are these questions coming from?

Human Anatomy & Physiology Ninth Edition by Elaine N. Marieb and Katja Hoehn

Chapter? I have the solutions manual.

Im guessing 10 but Im not 100 positive since I can't find it in the book.