Spring washers, sometimes called disc springs, are a subtype of washers. They lend their mechanical capabilities to the unique profile of the material: when subject to a load, the irregularities of the washer compress with a proportionate resistance to return to their predeflected shape.
You should never stretch a pulled muscle to the point of pain or you risk re-injuring it. Do not overstretch the pulled muscles. Too much stretching can retear injured muscle fibers. Gently stretching a pulled muscle may relax the surrounding muscle fibers, which may be tight in response to the injury.
The cord-like structure of the tendon is not the only conformation a tendon can take. The muscles of the skull are held together by tendons with the same basic building blocks (collagen), but they take a different shape. Go ahead and feel your head until you locate a dense tendon cord like the Achilles.