Final answer:
The strength and effort used by muscles vary depending on the task. When picking up a pencil, fewer muscle fibers are activated compared to picking up a full gallon container of milk, resulting in a lower force being exerted.
Step-by-step explanation:
The strength and effort used by muscles can be varied based on the task at hand. When picking up a pencil, the motor cortex of the brain signals only a few neurons of the biceps, causing only a few muscle fibers to respond. This results in a lower force being exerted. On the other hand, when picking up a full gallon container of milk, the motor cortex signals all neurons in the biceps, causing all muscle fibers to participate in generating force. This results in a higher force being exerted.