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.
Pulled muscles really aren't damage to a muscle, per se. While you -can- get actual muscle tears, almost always you get injury at the tendon, specifically at the weakest points where muscle meets tendon, and tendon meets bone. A tweaked muscle just needs to chill out and relax.