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.
If you have pulled anything, it's likely the body of the muscles will be sore and tender. So poke around, follow the muslces, and find the tendons. Maybe you pulled something around the rib/sternum connections, which will be little stretch or tear in the webbing of connections. You're looking for a hot spot.