Final answer:
The body regulates muscle force through the recruitment of muscle fibers, affected by the number of active myofibers and the frequency of action potentials. Torque plays a role in the actual force exerted by muscles, which varies with joint angles and the positions of pivot points in real systems.
Step-by-step explanation:
The body controls the amount of force used by a muscle through the recruitment of muscle fibers, or myofibers. When performing a task that requires little force, such as lifting a pencil, the motor cortex sends a signal to only a few neurons of the biceps, resulting in a minimal number of myofibers responding. In contrast, lifting a heavy object like a piano requires the motor cortex to signal all of the neurons, maximizing myofiber participation and thus producing maximum force possible by that muscle.
Moreover, the force produced by a muscle can be slightly increased by raising the frequency of action potentials, which leads to calcium flooding the tropomyosin. This increased frequency can enhance muscle force.
In terms of torque and forces on muscles, the actual force exerted by muscles, like the biceps when lifting a weight, is not identical to the weight of the object. This is because of the concept of torque, where the force needed is also a function of the distance from the pivot point, in this case, the elbow joint, and the angle at which the force is applied.
Lastly, the force a muscle must exert to maintain a position, like holding up a book, can vary depending on the joint angle, which changes the mechanical advantage and the length of the lever arm in real systems of muscles, bones, and joints.