When performing an exercise through a range of motion, not all of the force produced by the muscles goes toward movement. As you approach the endpoints, muscles waste more force.
Depending on the angle of pull for the muscle, some force created during a contraction contributes toward stabilizing or destabilizing. These pull the bone either into or away from the joint.
Force most efficiently converts to torque when the angle of the force applies perpendicularly, equaling 90° to the long axis of the bone. This position also represents the ideal medium length according to the length-tension relationship.
This gives yet another piece of evidence in favor of compound over isolation exercises, since these by nature limit the need for force at the endpoints.
Much of the force gets applied in parallel along the length of the bone and pulls it away from the joint when at an acute angle of less than 90°. This force destabilizes the joint.
For instance, this happens at the top of a curl when your forearm begins to smash against your biceps. The force here will actually pull the forearm away from the humerus.
Due to changes in leverage since gravity always pulls downward, you would never load this position heavily with a free weight. This only happens if you alter your position away from standing, such as by using a preacher curl bench or a machine.
Much of the force again get applied in parallel along the length of the bone and directs into the joint when at an obtuse angle of more than 90°. This force stabilizes the joint.
A good example involves the elbow again. When the elbow extends fully, the tendon of the biceps brachii inserts into the radius at a low angle. Most of the force generated by the biceps brachii directs into the elbow rather than moving the forearm.
In some scenarios, it makes sense to keep the joints stable. Imagine pushing a heavy object across the ground. We will extend our arms to push on an object so we can transfer force from the powerful lower body. This would fail to stimulate the triceps that extend the elbow much though.
Avoiding Stabilizing and Destabilizing Forces
The angle of insertion allows more force to work toward moving the segment at the midpoint between these two extremes. This maximizes the perpendicular force and minimizes the parallel force.
Your body adapted to not expect much resistance at the endpoints of any movement. When dealing with a free weight, leverage improves in these positions, requiring far less force from our muscles.
Multi-joint, compound movements inherently prevent excessive wasting since they limit the range of motion possible for the involved joints.
This also explains why moving through a full range of motion for many exercises is a poor idea. Not only do you risk safety since the muscles fail to support the joints as well at the endpoints, you make the movement more inefficient as well.
Focus on the midpoint of any good exercise to avoid excessive stabilizing and destabilizing forces. Keep in mind that some movement serves a purpose though.