When you activate your muscle, it will not produce a constant force over time, for a variety of reasons.
One key reason is that the maximal force that a muscle can produce is dependent on the muscle length.
Your bicep, for instance, can produce a maximal amount of force when the elbow is flexed at around 90 degrees. At more extended and flexed positions, maximum force is less.
from the wiki article: A generic example of a muscle force / length relationship
The active force component of muscle force generation is related to actin-myosin cross-bridges pulling past one-another. When they have pulled too far past each other (contracted position), their ability to produce tension reduces. Similarly when the components are too far away from each other, limited elements are in contact and the amount of tension generated is reduced.
When you get to really stretched positions, there is an increasing "passive" contribution which is really connective tissue increasing in resistance to further strain.
The dependency of muscle force on muscle length has implications in performance and training, and will be recalled in many discussions.