Four types of animal studies from the 1970s to 1990s established links between active and passive tension and hypertrophy: (1) tendon-cutting studies increasing overload on the soleus muscle found substantial hypertrophy; (2) limb immobilization studies found muscle size decreased less when immobilized in stretched versus shortened positions; (3) weight-attachment studies on chicken wings found hypertrophy of stretched muscles; (4) spring-loaded bar studies produced similar hypertrophy via continuous stretching.