It is important to avoid isostatic stretches because they can

a. increase body and muscle temperature
b. extend a joint beyond its regular range of motion
c. exceed the stretchable limits of the tissues involved
d. stretch the muscle to its farthest point

The answer to this question would be: b. extend a joint beyond its regular range of motion
The isostatic stretch can pose risk to the muscle tissue because it might go more than the joint limit. When the muscle stretched too far, it might go strain, causing pain and make the muscle became unusable. It might be damage the tendon too causing a sprain.