Consider the barbell curl. You start with the muscle in a stretched position, This puts the actin and myosin configuration in thier weakest arrangement, but the relative force exerted by the weight on the muscle in this position is also modulated by the torque it exerts on your forearm.
The force exerted by the weight is always acting in the same direction, downwards towards to floor, this means that as you move the weight through the motion, the stress exerted on your muscle from the torque in your forearm varies exactly as the Sin function.
The force and torque is maximal when perpendicular to your forearm, thus half way through the motion when your forearm is parallel to the floor is the only time during the entire motion that your really lifting the amount of weight on the barbell.
However, at 90degrees, the actin and myosin fibres are approx half way through thier ratcheting system, and this is also the configuration at which they are at thier strongest.
As you move up past 90 dgerees, the torque and therefore tension exerted once again diminishes as sin 90 -> sin 180, meanwhile the relative strength of the actin and myosin also decreases again as the maximum contracted position is reached.
One way around this is to move your torso as you curl your arms, always keeping your forearms parallel to the floor thus always keeping the maximum torque on your muscles. This way, each configuration of the actin and myosin arrangment is exposed to the same degree of tension, which is pretty much the maxmium.
This machine is an excellent example for the pec fly.
In this machine, the force is pretty much always perpendicular to your forearm, thus maximising the actin/moysin multiplied by tension stimulus.
Compared to the traditional pec fly laying down with dumbells, the maximal force in only exerted in the starting position. As you move your arms up, the force decreases as the sin function once again.
I would predict that keeping the tension constant throughout the range of motion to be superior for mass gains.