The major problem is that strength is only developed at the angle of tension and can lead to non symmetric developed joints. From the Greek meaning same tone. The idea is that the muscle appears to contract with the same tension throughout the exercise. The phase where the muscle shortens as the weight is moved. The phase where the muscle lengthens as the weight is moved.
Exercises utilizing free weights and dumbbells are constant load.
The resistance of the weight does not change, however the amount of tension available and amount of strength available in your muscles changes during the exercise. Certain weight lifting machines have been designed to cause the resistance to change during the lift to compensate for the changes in available strength in the muscle throughout the range of motion. Examples are the bench press, leg press and shoulder press machines in our weight room which utilize a lever system to vary the resistance. Nautilus is also variable resistance. It uses a cam shaped like a nautilus shell to vary the resistance during the lift.